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Radar, passive microwave, and lightning characteristics of precipitating features in the tropics

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RADAR, PASSIVE MICROWAVE, AND LIGHTNING CHARACTERISTICS
OF PRECIPITATING FEATURES IN THE TROPICS
A Dissertation
by
ERNEST RICHARD TORACINTA
Submitted to the Office of Graduate Studies of
Texas A&M University
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
December 2000
Major Subject: Atmospheric Sciences
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RADAR, PASSIVE MICROWAVE, AND LIGHTNING CHARACTERISTICS
OF PRECIPITATING FEATURES IN THE TROPICS
A Dissertation
By
ERNEST RICHARD TORACINTA
Submitted to Texas A&M University
in partial fulfillment of the requirements
for the degree of
DOCTOR OF PHILOSOPHY
Approved as to style and content by:
Edward J.'
(Co-Chair of Committee)
Richard E. Orville
(Co-Chair of Committee)
Michael I. Bi§
(Member)
Thomas Over
(Member)
JBfenjamin Giese
(Member)
Gerald North
(Head of Department)
December 2000
Major Subject: Atmospheric Sciences
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ABSTRACT
Radar, Passive Microwave, and Lightning Characteristics
of Precipitating Features in the Tropics. (December 2000)
Ernest Richard Toracinta, B.S., University of Massachusetts at Lowell;
M.S.. Texas A&M University
Co-Chairs of Advisory Committee: Dr. Edward J. Zipser
Dr. Richard E. Orville
The first part of this study is a systematic comparison o f the distributions of
mesoscale convective systems (MCSs) and lightning between 35°N and 35°S over a full
year beginning in June 1995. MCSs are classified according to their 85 GHz brightness
temperature (Tb), while the lightning flashes are grouped into clusters. The land bias
among the lightning clusters is much stronger than among the MCSs. Ocean and land
regions typically contain 18% and 60%, respectively, of the lightning cluster population.
MCSs are more evenly distributed between land and ocean with roughly 40% and 42%
occurring over land and ocean, respectively. Over land, MCSs with strong ice scattering
signatures and high flash rate lightning clusters are relatively numerous.
lightning/ice scattering relationship is less clear.
The ocean
When normalized for 85 GHz ice
scattering intensity, lightning is more likely in a land MCS than in an ocean MCS. It is
inferred that differences in lightning probability are related to differences in ice
microphysics between land and ocean storms.
The second part of the study uses measurements from the Tropical Rainfall
Measuring Mission (TRMM) satellite to quantify relationships between active/passive
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microwave and lightning for tropical convective systems. Continental systems typically
have greater magnitudes of reflectivity at higher altitudes than ocean systems.
Normalized by reflectivity heights, continental storms consistently have higher 85 GHz
Tb than ocean storms. It is inferred that greater supercooled water contents aloft in the
continental systems contribute to the T0 differences.
Normalized by Tb or reflectivity heights, lightning is much more likely in continental
than oceanic storms. Vertical profiles of reflectivity add information to the non-unique
lightning/brightness temperature relationships. Lightning systems tend to have greater
reflectivity values and smaller decreases of reflectivity with height above the freezing
level than systems without lightning.
Relationships between the TRMM observables are consistent with what is known or
hypothesized about the microphysical differences between systems with or without
lightning. Inferences made from these observations should be addressed with numerical
model techniques.
In addition, this quantitative database provides a substantial
observational framework for use with numerical models.
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This dissertation is dedicated first and foremost ad majorem Dei gloriam. I would
also like to dedicate this work to my parents and stepparents whose unwavering love and
encouragement through the years helped make it all possible.
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ACKNOWLEDGMENTS
I would like to thank my advisor, Dr. Edward Zipser, for supporting and mentoring
me during my graduate years at Texas A&M. I would also like to thank my committee
members, especially Dr. Richard Orville for serving as a committee co-chair.
I would like to thank my colleagues, Dan Cecil and Steve Nesbitt, for their
assistance in working with the TRMM data set. Thanks also to Chris West, Neil Smith,
and Jerry Guynes for computer programming and hardware support. I am very grateful
to the office staff in the Department of Atmospheric Sciences and to Dodie Guffy in the
Working Collection for their invaluable assistance. They are simply the greatest.
The Lightning Imaging Sensor and Optical Transient Detector data were made
available from the Global Hydrology Resource Center (GHRC) at the Global
Hydrology and Climate Center, Huntsville, Alabama.
Many thanks to Dr. Dennis
Boccippio for answering my questions about the lightning sensors and data.
The
TRMM Microwave Imager and Precipitation Radar data were made available from the
TRMM Science Data and Information System.
I would like to express deep gratitude to my family, my godparents, many dear
friends, and especially Catherine Miller.
Their love, encouragement, patience, and
prayers have been a source of strength and consolation for me during these years of
study at Texas A&M.
This research was made possible through funding by a NASA Earth System Science
Fellowship as well as a NASA TRMM grant.
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TABLE OF CONTENTS
Page
ABSTRACT...........................................................................................................................
iii
DEDICATION.......................................................................................................................
v
ACKNOWLEDGMENTS....................................................................................................
vi
TABLE OF CONTENTS.......................................................................................................
vii
LIST OF FIGURES..............................................................................................................
ix
LIST OF TABLES.................................................................................................................
xv
CHAPTER
I
INTRODUCTION......................................................................................................
1
II OTD LIGHTNING AND SSM/I ICE SCATTERING MESOSCALE
CONVECTIVE SYSTEMS IN THE TROPICS.....................................................
5
Introduction.......................................................................................
5
Data and Methods............................................................................. 7
Results................................................................................................ 13
Discussion.......................................................................................... 39
Summary............................................................................................ 46
m
MICROWAVE AND LIGHTNING SIGNATURES OF CONVECTIVE
SYSTEMS USING TRM M .....................................................................................
48
Introduction....................................................................................... 48
Data and Methods............................................................................ 50
Results............................................................................................... 55
Discussion......................................................................................... 87
Summary and Conclusions............................................................. 94
IV CONCLUSIONS........................................................................................................ 97
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viii
Page
REFERENCES........................................................................................................................ 101
APPENDIX A ......................................................................................................................... 114
APPENDIX B ......................................................................................................................... 127
APPENDIX C ......................................................................................................................... 137
VITA........................................................................................................................................ 138
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ix
LIST OF FIGURES
FIGURE
1
2
3a
3b
4a
4b
5a
5b
6a
Page
Spatial domain consisting of 19 regions classified as ocean,
land (bold), or a mixture of land and ocean (italic). Several regions
(e.g., Subtropical South America) have coastlines as one or more
of their boundaries. The North Pacific, Central Pacific, and South
Pacific each have areas on either side of the 180° meridian. The
ellipse delineates the approximate area in which the lightning
data are compromised due to the SAA (see text)...................................................
8
Example OTD lightning flashes (denoted by plus (+) symbols) and
corresponding lightning clusters (denoted by ellipses)..........................................
11
June-July-August (JJA) class 1 and class 2 OTD lightning
clusters by lightning flash count: 1-3 flashes (dots),
4-24 flashes (plus)....................................................................................................
17
June-July-August (JJA) class 3 and class 4 OTD lighting clusters by
cluster flash count: 25-75 flashes (small plus), > 75 flashes (large
plus)............................................................................................................................
18
June-July-August (JJA) class 1 and class 2 MCSs by minimum
85 GHz PCT: 225-191 K (dots), 190-156 K (plus)................................................
19
June-July-August (JJA) class 3 and class 4 MCSs by minimum
85 GHz PCT: 155-121 K (small plus), < 120 K (large plus).............................
20
September-October-November (SON) class 1 and class 2 OTD
lightning clusters by lightning flash count: 1-3 flashes (dots),
4-24 flashes (plus).....................................................................................................
24
September-October-November (SON) class 3 and class 4 OTD
lightning clusters by lightning flash count: 25-75 flashes (small plus),
> 75 flashes (large plus)...........................................................................................
25
September-October-November (SON) class I and class 2 MCSs by
minimum 85 GHz PCT: 225-191 K (dots), 190-156 K (plus)............................. 26
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FIGURE
6b
September-October-November (SON) class 3 and class 4 MCSs by
minimum 85 GHz PCT: 155-121 K (small plus), < 120 K
(large plus)....................................................................................................................
Page
27
7a
December-January-February (DJF) class 1 and class 2 OTD lightning
clusters by lightning flash count:1-3 flashes (dots), 4-24 flashes (plus)................ 29
7b
December-January-February (DJF) class 3 and class 4 OTD lightning
clusters by lightning flash count: 25-75 flashes (small plus),
> 75 flashes (large plus)..............................................................................................
30
8a
December-January-February (DJF) class 1 and class 2 MCSs by
minimum 85 GHz PCT: 225-191 K (dots), 190-156 K (plus)................................ 31
8b
December-January-February (DJF) class 3 and class 4 MCSs by
minimum 85 GHz PCT: 155-121 K (small plus), < 120 K
(large plus)...................................................................................................................
32
March-April-May (MAM) class 1 and class 2 OTD lightning clusters
by lightning flash count: 1-3 flashes (dots), 4-24 flashes (plus)............................
35
March-April-May (MAM) class 3 and class 4 OTD lightning clusters
by lightning flash count: 25-75 flashes (small plus),
>75 flashes (large plus)...............................................................................................
36
March-April-May (MAM) class 1 and class 2 MCSs by minimum
85 GHz PCT: 225-191 K (dots), 190-156 K (plus)..............................................
37
March-April-May (MAM) class 3 and class 4 MCSs by minimum
85 GHz PCT: 155-121 K (small plus), < 120 K (large plus).................................
38
June 1995 - May 1996 “intense” lightning clusters. Plotting symbols
correspond to the number o f flashes in a cluster and increase in size
beginning with the smallest plus: 45-60 flashes, 61-90 flashes,
91-155 flashes, > 155 flashes.....................................................................................
40
June 1995 - May 1996 “intense” MCSs. Plotting symbols
correspond to the minimum 85 GHz PCT and increase in size
beginning with the smallest plus: 175-141 K, 140-121 K ,120-101 K,
< 100 K .........................................................................................................................
41
Map of study regions..................................................................................................
53
9a
9b
10a
10b
1la
I lb
12
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XI
FIGURE
13
14
15
16
17
18
Page
Cumulative distribution of precipitation features by total volumetric
rainfall (mm km2 hr'1) for Africa (solid), South America (dotted), west
Pacific (dashed), and east Pacific (dot-dashed). Volumetric rainfall
threshold ( 103 mm km2 hr'1) is indicated................................................................
57
Frequency distribution of oceanic (upper) and continental (lower)
precipitation features. The oceanic and continental distributions are
contoured and shaded, respectively. Contour and shading values are
1, 10, 25, 50, 75, 100, 200, 300,400, and 500. Bin sizes are 1 km for
the ordinate (maximum 20 dBZ height) and abscissa (maximum 30 dBZ
height).........................................................................................................................
59
Relative frequency distributions of 30- and 40 dBZ heights for land
and ocean precipitation features with maximum 20 dBZ heights from
13-14 km.....................................................................................................................
60
Frequency distribution of oceanic (upper) and continental (lower)
precipitation features. The oceanic and continental distributions are
contoured and shaded, respectively. Contour and shading values
are 1, 10, 25, 50, 75, 100,200, 300,400, and 500. Bin sizes are 1 km
for the ordinate (maximum 20 dBZ height) and abscissa (maximum
40 dBZ height)............................................................................................................
61
Frequency distribution of oceanic (upper) and continental (lower)
precipitation features. The oceanic and continental distributions are
contoured and shaded, respectively. Bin sizes are 5 K for the ordinate
(minimum 37 GHz PCT) and 10 K for the abscissa (minimum
85 GHz PCT). Contour and shading values are 1,5, 10,50, 100,
250, and 500...............................................................................................................
63
Median of the minimum 85 GHz PCT for oceanic (upper) and
continental (lower) precipitation features. The oceanic and
continental distributions are contoured and shaded, respectively.
Contour and shading intervals are every 25 K beginning at 100 K.
Bin sizes are 1 km for the ordinate (maximum 20 dBZ height) and
abscissa (maximum 30 dBZ height). Bins containing fewer than
five precipitation features are not plotted................................................................ 65
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FIGURE
19
Median of the minimum 37 GHz PCT for oceanic (upper) and
continental (lower) precipitation features. The oceanic and
continental distributions are contoured and shaded, respectively.
Contour and shading intervals are every 10 K beginning at 220 K.
Bin sizes are 1 km for the ordinate (maximum 20 dBZ height)
and abscissa (maximum 30 dBZ height). Bins containing fewer
than five precipitation features are not plotted.......................................................
Page
66
20
Subset of precipitation features with maximum 20 dBZ heights
from 13-14 km. Land and ocean distributions of maximum 30 dBZ
height versus minimum 85 GHz PCT (a); land and ocean
distributions of maximum 30 dBZ height versus minimum
37 GHz PCT (b)......................................................................................................... 67
21
Median of the minimum 85 GHz PCT for oceanic (upper) and
continental (lower) precipitation features. The oceanic and
continental distributions are contoured and shaded, respectively.
Contour and shading intervals are every 25 K beginning at 100 K.
Bin sizes are I km for the ordinate (maximum 20 dBZ height) and
abscissa (maximum 40 dBZ height). Bins containing fewer than
five precipitation features are not plotted................................................................ 68
22
Median of the minimum 37 GHz PCT for oceanic (upper) and
continental (lower) precipitation features. The oceanic and
continental distributions are contoured and shaded, respectively.
Contour and shading intervals are every 10 K beginning at 220 K.
Bin sizes are 1 km for the ordinate (maximum 20 dBZ height) and
abscissa (maximum 40 dBZ height). Bins containing fewer than
five precipitation features are not plotted................................................................ 69
23
Geographic locations of precipitation features with lightning (a)
and without lightning (b)........................................................................................... 70
24
Relative frequency distribution of precipitation features over
Africa (contoured) and South America (shaded) with lightning
(left panels) and without lightning (right panels). Contour and
shading values are 0 .0 5 ,0 .1, 0.5, 1,5, and 10 percent. Bin sizes
are 10 K and 5 K for the abscissa and ordinate, respectively,
in (a) and (b), and 1 km for the abscissa and ordinate in (c)-(f).......................... 73
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FIGURE
25
Page
Relative frequency distribution of precipitation features over
east Pacific (contoured) and west Pacific (shaded) with lightning
(left panels) and without lightning (right panels). Contour and
shading values are 0.05,0.1, 0.5, 1,5, and 10 percent.
East Pacific features with lightning are indicated by “plus” symbols
due to the small sample size. Bin sizes are 10 K and 5 K for
the abscissa and ordinate, respectively, in (a) and (b), and 1 km
for the abscissa and ordinate in (c)-(f)....................................................................
75
26
Fraction of oceanic (upper) and continental (lower) precipitation
features with lightning. Bin sizes are 10 K on the abscissa (minimum
85 GHz PCT) and 5 K on the ordinate (minimum 37 GHz PCT).
Bins containing fewer than five precipitation features are not plotted................ 78
27
Fraction of oceanic (upper) and continental (lower) precipitation
features with lightning. Bin sizes are 1 km on the abscissa
(maximum 30 dBZ height) and the ordinate (maximum 20 dBZ height).
Bins containing fewer than five precipitation features are not plotted................ 80
28
Median vertical reflectivity profiles for three subsets of land
and ocean precipitation features selected from the following ranges
of minimum 85 GHz PCT: 250-260 K (a), 200-210 K (b), and
150-160 K (c). Land profiles are indicated with bold lines.
Profiles for lightning features are indicated with solid lines................................. 81
29
Distribution of precipitation features with lightning by flash rate
relative to the minimum 85 GHz PCT and maximum 7 km reflectivity
for Africa (a), South America (b), and combined west and
east Pacific regions (c). The flash rate is proportional to the
symbol size................................................................................................................. 84
30
Distribution of precipitation features with lightning by flash rate
relative to the minimum 37 GHz PCT and maximum 7 km reflectivity
for Africa (a), South America (b), and combined west and
east Pacific regions (c). The flash rate is proportional to the
symbol size................................................................................................................. 85
31
Frequency distribution of continental (a) and oceanic (b)
precipitation features without lightning. Bin size is 2 dBZ for
the ordinate (maximum 7 km reflectivity) and 10 K for the
abscissa (minimum 85 GHz PCT). Contour values are 1, 5, 10,25,
50,75, 100, 125, and 150......................................................................................... 86
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XXV
FIGURE
32
Page
Frequency distribution of continental (a) and oceanic (b)
precipitation features without lightning. Bin size is 2 dBZ for
the ordinate (maximum 7 km reflectivity) and 5 K for the
abscissa (minimum 37 GHz PCT). Contour values are 1,5, 10, 25,
50,75, 100, 125, and 150........................................................................................ 87
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XV
LIST OF TABLES
TABLE
1 Frequency distributions of OTD lightning clusters (upper row) and
SSM/I MCSs (lower row, bold) for land, ocean, and mixed land
and ocean regions....................................................................................................
2
3
4
5
6
Page
21
June 1995 - May 1996 OTD lightning and SSM/I MCS
summary statistics. Regions are sorted bythe flash-to-MCS ratio...................
43
Summary characteristics of precipitation features in Africa (AF),
South America (SA), west Pacific(WP), and east Pacific (EP).......................
56
Summary characteristics of precipitation features that do not meet
the total volumetric rainfall threshold (103 mm km2 hr'1). Values in
parentheses correspond to precipitation features that meet the total
volumetric rainfall threshold................................................................................
58
Summary of precipitation feature and lightning statistics for
Africa (AF), South America (SA), west Pacific (WP), and
east Pacific (EP).....................................................................................................
71
Sample sizes and lightning statistics for three subsets of precipitation
features selected by the minimum 85 GHz PCT. Regions are
Africa (AF), South America (SA), west Pacific (WP), and
east Pacific (EP)......................................................................................................
82
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1
CHAPTER I
INTRODUCTION
A considerable effort has been put forth during the last half century to study and
understand precipitating systems in the Tropics, which account for two-thirds of the global
rainfall and are a major source of latent heat for the atmosphere (Simpson et al. 1986).
The spectrum of precipitating cloud types in the tropics covers broad range of spatial and
temporal scales, ranging from single cumulonimbus clouds to large, long-lived mesoscale
convective systems (MCSs), with varying contributions to total tropical rainfall (Mohr et
al. 1999).
Mohr and Zipser ( 1996a,b) and Zolman et al. (2000) have quantified the
distribution of MCSs in the Tropics using 85.5 GHz (3.5 mm wavelength; 85 GHz
hereafter) passive microwave data from the Special Sensor Microwave/Imager (SSM/I)
and their studies reveal the ubiquitous nature of tropical MCSs.
When classified by
minimum 85 GHz brightness temperature, which is used as a proxy for convective
intensity, the strongest MCSs tend to occur over land, but the tropical oceans also contain
large numbers of MCSs with low 85 GHz brightness temperatures.
Tropical precipitating systems are also a major component of the global electric circuit
with approximately 75% of the global lightning occurring between 30°N and 30°S
(Christian et al. 1999). While the charge separation processes leading to lightning are not
fully understood, it is widely accepted based on a wealth of laboratory evidence together
in situ observations of thunderstorms, that mixed (liquid and ice) phase cloud
This dissertation follows the style of the Monthly Weather Review.
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2
microphysics have an essential role.
Specifically, non-inductive collisions involving
millimeter-sized ice (graupel or hail) and smaller ice crystals in the presence of
supercooled liquid water at temperatures between 0°C and -40°C represent the most viable
mechanism to explain robust cloud electrification (Reynolds et al. 1957; Takahashi 1978;
Jayaratne et al. 1983; Saunders et al. 1991; Williams 1989; and others).
Many researchers have investigated the microphysical relationships
between
occurrence or absence of lightning and the structure and environment of tropical
convective systems, often with the aid o f remote sensing (radar or high frequency passive
microwave) techniques (Williams et al. 1992; Rutledge et al. 1992; Zipser 1994; Petersen
et al. 1996, 2000; Black and Hallett 1999; Cecil and Zipser 1999; Boccippio et al. 2000a;
Williams et al. 2000; Toracinta and Zipser 2001). Information from radar and passive
microwave instruments is useful in this regard because they take advantage of the
interaction of microwave energy with liquid and ice phase hydrometeors.
Radar
reflectivity is strongly related to the size (diameter to the sixth power in the case of
Rayleigh scatterers) of the precipitation particles in a sampled volume.
Hydrometeor
phase is also an important component of reflectivity measurements owing to the near
order-of-magnitude difference in the dielectric constants of ice and liquid water which
translates to nearly 7 dB difference when sampling ice versus liquid phase particles.
Measured top-of-atmosphere brightness temperatures (Tb) are the integrated result of
emission and scattering processes that act to modulate upwelling radiation along the
optical path to the radiometer. In the remote sensing of precipitating systems the emission
sources are primarily cloud liquid water, rain, and melting phase hydrometeors (Wilheit
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3
1986; Mugnai et al. 1990; Vivekanandan et al. 1991). At 37- and 85 GHz, the frequencies
used in this and many other studies of convective systems, scattering o f upwelling
radiation is primarily due to precipitation-sized ice hydrometeors present above the
emitting rain layer (Wilheit et al. 1982; Wu and Weinman 1984; Spencer et al. 1989). The
resulting reduction of the observed brightness temperatures, termed the ice scattering
signature, is a function of the particle size distribution, bulk density, number
concentration, and geometric depth of the scattering layer (Vivekanandan et al. 1990,
1991). Ice scattering at 85 GHz (3.5 mm) is due to relatively small precipitation-sized ice
(0(0.5 mm)) and is typically the dominant signal when ice phase precipitation is present.
At 37 GHz (8 mm wavelength), ice scattering is due to the presence o f larger (O(mm))
graupel or frozen raindrops in convective cores.
The first part of this study, in Chapter II, compares data from the OTD (Optical
Transient Detector) and SSM/I, both of which independently measure properties of
convective systems that depend upon the presence o f precipitation-sized ice in the cloud.
The similarities and differences in the distributions of lightning and 85 GHz ice scattering
MCSs are examined in the Tropics over a full year with the specific goal of quantifying
the lightning/ice scattering relationships.
The second part of the study, Chapter HI, focuses on the broader spectrum of
precipitating systems using simultaneous multi-parameter data from the TRMM (Tropical
Rainfall Measuring Mission) satellite (Simpson et al. 1986; Kummerow 1998).
The
specific goal is to provide a substantial quantitative observational database of these
properties, organized in a way that facilitates the evaluation of the existing hypothesis
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4
regarding microphysical differences between systems with and without lightning,
lightning. The results are summarized and recommendations for further work given in
Chapter IV.
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5
CHAPTER II
OTD LIGHTNING AND SSM/I ICE SCATTERING MESOSCALE
CONVECTIVE SYSTEMS IN THE TROPICS'
Introduction
Mesoscale convective systems are major contributors to global rainfall and
therefore a key component in the maintenance of the global atmospheric circulation via
latent heating. It is estimated that two-thirds of global rainfall occurs in the Tropics and
that MCSs account for a significant portion (-50-80%) of that rainfall amount, although
they represent a relatively small fraction of the total population of tropical precipitating
systems (Simpson et al. 1988; Mohr et al. 1999; Nesbitt et al. 2000). Mohr and Zipser
(1996a,b) and Zolman et al. (2000) have quantified the distribution of MCSs in the
Tropics
using
85
GHz
passive
microwave
data
from
the
Special
Sensor
Microwave/Imager (SSM/I) and their studies reveal the ubiquitous nature of tropical
MCSs. At 85 GHz, the single scatter albedo for ice is near unity for all but very light rain
rates (Spencer et al. 1989).
Thus, volume scattering due to precipitation-sized ice
particles becomes a dominant mechanism at 85 GHz making it particularly useful for
studying convective systems. Used in this context, “precipitation-sized” or “large” ice are
qualitative references to particles large enough to effectively scatter 85 GHz radiation
(i.e., particles with diameter -0 .5 mm or larger in at least one direction). Mohr and Zipser
( 1996a,b) and Zolman et al. (2000) show that 85 GHz ice scattering MCSs of various size
* Reprinted with permission from "Lightning and SSM/I Ice Scattering Mesoscale Convective Systems in
the Global Tropics” by E. Richard Toracinta and Edward J. Zipser, 2001, Journal o f Applied Meteorology,
(accepted). Copyright 2000 by the American Meteorological Society.
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6
are found over land and ocean throughout the Tropics. When classified by minimum 85
GHz brightness temperature, the most intense MCSs tend to occur over land, but the
tropical oceans also contain large numbers of MCSs with low 85 GHz brightness
temperatures.
While MCSs tend to occur throughout the Tropics with little land or ocean bias,
satellite-based studies of global lightning clearly show a preponderance o f lightning over
land with an order of magnitude difference between land and ocean lightning (Orville and
Henderson 1986; Goodman and Christian 1993; Christian et al. 1999). Regional studies
also indicate that tropical continental MCSs are much more prolific lightning producers
than their oceanic counterparts (Williams et al. 1992; Rutledge et al. 1992; Zipser 1994;
Petersen and Rutledge 1998).
Our current understanding of the cloud electrification
processes requisite for lightning in deep convective clouds is built upon a significant body
of laboratory research focused on cloud microphysics and electrical charge separation
(Reynolds et al. 1957; Takahashi 1978; Jayaratne et al. 1983; Keith and Saunders 1990,
Saunders et al. 1991; and many others). Specifically, these studies reveal that significant
electrical charge transfer occurs during non-inductive collisions between small and large
ice particles (i.e., graupel or hail) in the presence of supercooled liquid water in the mixed
phase region of the cloud (where 0°C > T > -40°C). The magnitude and sign of charge
transferred during individual ice-ice collisions, which are governed by several
environmental parameters (i.e., temperature, supercooled liquid water content, ice particle
size and differential velocity), are sufficient to account for observed space charge densities
and vertical charge structure in thunderstorms. Thus, charging via non-inductive ice-ice
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7
collisions is widely accepted as a viable mechanism for robust electrical charge separation
in thunderstorms (Williams 1989).
A qualitative comparison of the distributions of 85 GHz ice scattering MCSs and
lightning in the Tropics points to an apparent paradox. That is, although oceanic MCSs
generally have weaker ice scattering (warmer minimum 85 GHz brightness temperatures)
than continental MCSs, the fraction of oceanic MCSs with moderate to strong ice
scattering is in striking disproportion to the occurrence o f oceanic lightning. Yet, 85 GHz
ice scattering and the charge separation processes that lead to lightning are both inherently
related to the presence of precipitation-sized ice hydrometeors aloft in the cloud. Until
recently, a quantitative investigation of the relationship between 85 GHz ice scattering and
lightning has not been practicable on a global scale.
However, since spring 1995 the
satellite-borne OTD (Buechler et al. 1996) has mapped global lightning. In addition, 85
GHz data from the SSM/I has been used to quantify MCS distributions in the Tropics for
the 1995/96 La Nina year (Zolman et al. 2000). The purpose of this study is to compare
data from the OTD and SSM/I, which independently measure properties o f convective
systems that depend upon the presence o f precipitation-sized ice in the cloud. We will
examine the similarities and differences in the distributions of lightning and 85 GHz ice
scattering MCSs in the Tropics over a full year with the specific goal of quantifying the
lightning/ice scattering relationships.
Data and Methods
The area of interest in this study, shown in Figure 1, is similar to that used by Zolman
et al. (2000). It consists of 19 geographical regions in the Tropics and Subtropics (35°N -
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ion of the copyright owner. Further reproduction prohibited without permission.
India & East Asia
North
Atlantic
..Central
Pacific
Tropical S
jL r —Atlantic
E ast ...
Pacific
Tropical
Indian
i. America
j
ubtrop.
.America
South P^cjfic
-150
-90
South
Atlantic
-30
ubtroi
South Indian
Australia
150
Figure l. Spatial domain consisting of 19 regions classified as ocean, land (bold), or a mixture of land and ocean (italic).
Several regions (e.g., Subtropical South America) have coastlines as one or more of their boundaries. The North Pacific, Central
Pacific, and South Pacific each have areas on either side of the 180° meridian. The ellipse delineates the approximate area in
which the lightning data are compromised due to the SAA (see text).
00
9
35°S) that are classified as land, ocean, or a mixture of both land and ocean. While many
of the regions are the same as those in Zolman et al. (2000), several boundaries have been
modified for more robust land and ocean classifications (e.g., Tropical Africa). Note that
for several regions, the coastlines are used as boundaries between land and ocean. For the
purpose o f examining a full annual cycle, the temporal domain consists of four threemonth periods beginning with June 1995 and extending through May 1996.
The lightning data are from the OTD, which is mounted aboard the polar orbiting OVl (formerly Microlab-1) satellite at approximately 735 km altitude and 70° inclination
from the equatorial plane. The OTD operates as a staring optical imager, similar in design
to a television camera, measuring radiance centered at the 777.4 nm oxygen emission line;
radiance events exceeding the average background radiance by a threshold value are
considered lightning candidates. The OTD detects total lightning (intra-cloud and cloudto-ground) with an approximate detection efficiency of 56% (Boccippio et al. 2000b). In
the current study, no detection efficiency correction has been applied to the lightning data.
The instrument swath width and average ground-pixel resolution are approximately 1300
km x 1300 km and 11 km, respectively. Approximately 55 days are required for the OTD
to precess through the diurnal cycle.
The OTD is prone to interference from high-energy radiation, which is particularly
concentrated over the South Atlantic Anomaly (SAA; Pinto et al. 1992), an area bounded
approximately by 10°E - 90°W and 5°S - 50°S (denoted by the ellipse in Figure 1).
Excessive interference over the SAA causes substantial viewtime loss in this region.
Although most of the spurious events have been removed by filtering (Boccippio et al.
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10
2000b) and legitimate lightning data remain over the SAA, the loss of viewtime as well as
a small number of residual artifacts seriously compromise the data for several regions in
the current study. These are Subtropical South America and the South Atlantic as well as
the southern portions of Tropical South America and the Tropical Atlantic.
Convection is cellular in nature and MCSs are typically viewed as groups or clusters
of convective cells. As a product of convective cells, lightning is spatially correlated to
parent convection and tends to occur in distinct spatial clusters (Krider 1988).
This
tendency has been exploited using the OTD lightning data alone to determine the degree
to which the OTD flashes in each of the study regions occur in distinct clusters. That is,
the OTD lightning flashes in each orbit are grouped into lightning clusters, an MCS
analog, using a 30-km distance criterion. Any number o f flashes meeting this distance
criterion during the OTD viewtime is considered a cluster. The clusters are analogous to
MCSs in that for many larger clusters, the corresponding lightning flashes occur over
mesoscale distances (0(100 km)), the spatial scale of the MCS definition presented by
Houze (1993). A systematic comparison using several distance thresholds revealed that
30 km was adequate to preserve lightning features on mesoscale distances and minimize
potentially misleading clustering of meteorologically isolated lightning occurrences.
Figure 2 shows an example of OTD lightning flash locations along the western coast of
Central America along with the lightning clusters into which they are grouped. In each
geographical region, the total number of lightning clusters is tabulated over each o f the
four three-month periods. The cumulative distribution functions (CDFs) of clusters by
flash count and the cluster totals for each region are given in tables in appendix A.
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11
Pacific Ocean
:
:
:
+
:...............................................:..............................................i.............................................. : 10
-92
Figure 2.
-90
-88
-86
Example OTD lightning flashes (denoted by plus (+) symbols) and
corresponding lightning clusters (denoted by ellipses).
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12
Since 1987 the SSM/I has flown aboard several DMSP (Defense Meteorological
Satellite Program) F-series satellites. The F-13, which pertains to this study, was placed
into a sun-synchronous orbit at 98° inclination in March 1995. It crosses the equator twice
each day at approximately 0543 and 1743 local time. The SSM/I radiometer measures
upwelling radiance using three dual polarized channels: 19.35, 37, and 85 GHz, as well as
a 22.235 GHz vertically polarized channel. The instrument swath width is approximately
1400 km and the ground resolution at 85 GHz is 13 km x 15 km.
Mohr and Zipser (1996a) use SSM/I 85 GHz polarization-corrected temperature data
(PCT; Spencer et al. 1989) from 1992/93 to define an MCS as a contiguous area of at least
2000 km2 (ten 85 GHz pixels) with PCT < 250 K and having at least one 225 K PCT
pixel. The 250 K threshold is used to delineate areas of light to moderate precipitation (3
mm hr'1) based on the work by Spencer et al. (1989). The 225 K criterion in the MCS
definition further constrains the precipitating systems to those most likely containing
moderate to strong convection (McGaughey et al. 1996). The current study utilizes the 85
GHz MCS data set compiled by Zolman et al. (2000) who apply the Mohr and Zipser
(1996a) MCS definition to the 1995/96 SSM/I data set.
Henceforth, unless stated
otherwise, the term “MCS” will refer to a specific system defined using the SSM/I 85
GHz ice scattering signature according to Mohr and Zipser (1996a).
Snow cover acts to scatter upwelling radiation at microwave frequencies and can yield
85 GHz brightness temperatures that are similar to those from precipitating systems
(Grody 1991, Ferraro et al. 1994). To exclude snow artifacts from the MCS database,
Mohr and Zipser (1996a,b) devised a snow screen that systematically eliminates MCSs
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13
over high elevation regions (i.e., the Himalayas and Andes mountains) where, depending
on elevation and season of year, permanent or semi-permanent snow cover is deemed
likely to exist. This snow screen has also been applied to the 1995/96 MCS data set. The
remaining MCSs are sorted into the regions shown in Figure I and the regional MCS
totals are tabulated for each three-month period. In addition, the ratios of total lightning
clusters to total MCSs and total lightning flashes to total MCSs are computed for each
region. As with the lightning clusters, the regional MCS totals and CDFs by minimum 85
GHz brightness temperature are listed in the tables in appendix A.
Lastly, we recognize that lightning flashes originate from many convective systems
not meeting the 85 GHz MCS definition. In fact, using data from the TRMM satellite
(Tropical Rainfall Measuring Mission; Simpson et al. 1988; Kummerow et al. 1998) in
select regions of the Tropics, Nesbitt et al. (2000) find that non-MCS precipitating
systems contribute 47-63% of the total lightning from precipitating systems within those
regions. We feel, however, that comparisons between OTD lightning and SSM/I 85 GHz
MCSs are a suitable means to determine the relative regional and seasonal differences in
the lightning and MCS distributions. A forthcoming work using data from TRMM will
consider lightning and ice scattering signature relationships for the broader spectrum of
tropical precipitating systems.
Results
It is important to view and interpret the results of the lightning cluster and MCS
distributions in light of the physics of 85 GHz ice scattering and the charge separation and
cloud electrification processes. With regard to charge separation and cloud electrification,
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14
there is a wealth of evidence indicating that ice phase microphysics is an essential
component of the charge separation process. Laboratory studies consistently show that
substantial electric charge transfer occurs during non-inductive collisions between graupel
and small ice particles in the presence of supercooled cloud liquid water (Reynolds et al.
1957; Takahashi 1978; Jayaratne et al. 1983; among others). The sign and magnitude of
charge transferred are functions of several parameters including the environmental
temperature, amount of supercooled liquid water present, as well as the ice crystal size
and impact velocity. The non-inductive ice-ice charging theory for cloud electrification is
also corroborated by in situ measurements showing significant cloud electrification in
regions where supercooled cloud liquid water, millimeter-sized graupel, and large
concentrations of small ice particles coexist (Dye et al. 1986, 1988; Willis et al. 1994); the
charges measured on individual hydrometeors are sufficient to account for the observed
thunderstorm charge densities (Weinheimer et al. 1991; Stolzenburg and Marshall 1998).
Thus, laboratory and observational studies form the basis for a conceptual model of
thunderstorm electrification and charge structure in which updrafts loft ice particles and
supercooled liquid water to the mixed phase region (0°C > T > -40°C) where ice particle
growth occurs.
Differential particle motions result in ice-ice collisions and charge
separation; convection and gravitational sedimentation separate charged particles into
several distinct regions of opposite charge in the upper and low to mid levels of the
thunderstorm (Williams 1989).
Ice microphysics is also fundamental to microwave remote sensing of the atmosphere.
In the microwave spectrum, the volume absorption coefficient increases with frequency
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15
and is essentially negligible for ice relative to liquid water.
The volume scattering
coefficient also increases with frequency, but unlike the absorption coefficient, the values
for ice exceed those for liquid water as the rain rate increases beyond about 12 mm h r'1
(Spencer et al. 1989). The net result is that volume scattering due to ice particles becomes
a dominant mechanism at higher microwave frequencies, especially above 60 GHz
(Wilheit 1986). Measured top-of-atmosphere (TOA) brightness temperatures, at 85 GHz
for instance, represent an integrated effect of several processes that modulate the
upwelling radiance through the depth of the atmospheric column.
For precipitating
systems, the primary emission source is the layer of liquid and melting phase
hydrometeors below the freezing level. Above the freezing level, the intervening ice layer
acts to reduce the brightness temperatures by scattering radiation out of the radiometer
field of view. In addition, the presence of cloud liquid water above the freezing level is an
emission source that can partially offset the effects of ice scattering (Adler et al.
199l;Vivekanandan et al. 1991).
The relationship between 85 GHz ice scattering signatures and ice hydrometeor
vertical profiles is non-unique; the magnitude of the ice scattering is largely determined by
the optical depth x of the ice layer, which in turn is a function of the ice bulk density, size
distribution, number density, and geometric depth o f the layer (Vivekanandan 1990,
1991). Thus, it is possible that a single TOA 85 GHz brightness temperature could result
from various physically viable combinations of these parameters. However, observations
and modeling studies consistently show strong ice scattering associated with a deep layer
of numerous large ice particles, which in the case of deep convection are likely to be
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16
millimeter-sized graupel or hail, representing a large optical depth (Wilheit et al. 1982;
Wu and Weinman 1984; Hakkarinen and Adler 1988; Vivekanandan 1990, 1991; Schols
et al. 1999; and others).
The purpose of the current study is not to derive explicit
quantitative microphysics information from the MCS and OTD lightning comparisons, but
rather to infer possible differences in cloud microphysics based on what is known about
85 GHz ice scattering and charge separation related to lightning. With that in mind, a
qualitative comparison of the distributions of lightning clusters and MCSs is presented in
this section and complemented quantitatively with the tables located in appendix A.
Figures 3a and 3b are maps of OTD lightning clusters for June-July-August (JJA).
The lightning clusters are divided according to flash count into the following four class
intervals: 1-3 flashes, 4-24 flashes, 25-75 flashes, and >75 flashes. The upper bounds of
the first three class intervals represent the approximate 70th, 95th, and 99th percentiles,
respectively, of the total lightning cluster population in each of the three-month periods.
The first two class intervals are plotted as dots and small pluses (+) in Figure 3a, while the
remaining two class intervals are plotted as medium and large pluses, respectively, in
Figure 3b. The corresponding maps of SSM/I MCSs for JJA are shown in Figures 4a and
4b. The MCSs are grouped into four class intervals according to the minimum 85 GHz
PCT, following Zolman et al. (2000): 225-191 K, 190-156 K, 155-121 K, < 120 K. The
plotting convention is similar to the OTD clusters with the first two class intervals plotted
as dots and small pluses, respectively, in Figure 4a and the remaining two class intervals
plotted as medium and large pluses in Figure 4b.
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Figure 3a. June-July-August (JJA) class 1 and class 2 OTD lightning clusters by lightning flash count: 1-3 f
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17
- ____________ ■___________________ ■ " «
«*■-------------- 1 .
■-
. .
30
. .
90
— ------------« *
F
- t- •
150
lash count: 1-3 flashes (dots), 4-24 flashes (plus).
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Figure 3b. June-July-August (JJA) class 3 and class 4 OTD lightning clusters by lightning flash count: 25-75 flash
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18
sh count: 25-75 flashes (small plus), > 75 flashes (large plus)
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+: +
++
:+ \
* 4 V
' 4-
-U
i • ■•
. +*
■ *
«>_■»»*■
H
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. . - .•rtl. 'tiv T V
+ .- 4« ■
150
*rr^
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. .
90
■30
30
Figure 4a. June-July-August (JJA) class I and class 2 MCSs by minimum 85 GHz PCT: 225-191 K (dots), 190-15<
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19
— :—:------ r - v t r -------------------- :--------- z i ----------------------------------
30
90
l
/r r . «
-y *
t
150
-191 K (dots), 190-156 K (plus).
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Figure 4b. June-July-August (JJA) class 3 and class 4 MCSs by minimum 85 GHz PCT: 155-121 K (small plus), <
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20
30
90
150
21 K (small plus), < 120 K (large plus).
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21
Comparing the lightning cluster and MCS maps, the most obvious and striking
differences are the land versus ocean distributions of MCSs and lightning clusters. The
lightning clusters in Figures 3a and 3b show a strong land bias, consistent with previously
published studies of global lightning (Orville and Henderson 1986; Goodman and
Christian 1993; Christian et al. 1999). Table 1 shows that oceanic and continental regions
contain roughly 15% and 66%, respectively, of the total JJA lightning cluster population,
with the remaining clusters occurring in the mixed land/ocean regions.
It is also
noteworthy that a large majority (> 75%) of oceanic lightning clusters have low flash rates
(i.e., 1-3 flash category) while Figure 3b indicates that the ocean regions are essentially
devoid of the higher flash rate clusters (see also Table A -l).
Table 1. Frequency distributions of OTD lightning clusters (upper row) and SSM/I MCSs
(lower row, bold) for land, ocean, and mixed land and ocean regions.
JJA
SON
DJF
MAM
Land
Ocean
Mixed
66%
15%
19%
41%
40%
19%
63%
15%
22%
37%
42%
21%
58%
21%
21%
37%
41%
22%
56%
18%
26%
38%
45%
17%
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22
In contrast to the lightning cluster distribution, the JJA MCSs are nearly evenly
divided between land and ocean, with oceanic and continental MCSs representing 40%
and 41%, respectively, of the total JJA MCS population. The distributions in Figures 4a,b
and the CDFs in Table A-2 show that while roughly half (48-62%) of the MCSs in a given
ocean region have relatively modest ice scattering (PCT > 190 K), oceanic MCSs with
moderate to strong ice scattering are far more frequent than the higher flash rate clusters
(those with > 3 flashes).
Many o f the oceanic MCSs occur in the Inter-Tropical
Convergence Zone (ITCZ), just north of 5°N, a feature that is virtually absent in the
lightning data.
MCSs also occur with greater frequency than lightning clusters in the
Tropical Indian and the Central Pacific. The discrepancy is somewhat less pronounced in
subtropical ocean regions, including the North Pacific and the South Pacific Convergence
Zone (SPCZ), which extends across much of the South Pacific region.
Over land the spatial distribution of MCSs and lightning clusters is qualitatively quite
similar. Specific areas common to both lightning clusters and MCSs include, in North
America, western Mexico along the Sierra Madre Mountains (26°N, 106°W) as well as the
south-central and southeastern U.S. There are large numbers of lightning clusters relative
to MCSs in central and northern Mexico (26°N, 102°W) and the southern U.S., which are
indicative of the ubiquitous summertime thunderstorms.
In some cases these
thunderstorms may be too small to fill an SSM/I 85 GHz pixel (13 km x 15 km), much
less meet the 2000 km2 MCS criterion. It is also possible that some convective systems
may not meet the MCS definition if the intensity (magnitude of the 85 GHz ice scattering)
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23
is not adequately resolved by the SSM/I 85 GHz footprint, resulting in a warmer PCT than
is actually occurring.
The largest spatial density of both MCSs and lightning clusters during JJA occurs in
Central America, with Tropical Africa leading all regions in terms o f MCS and lightning
cluster intensity, represented by the CDFs of minimum 85 GHz PCT and cluster flash
count, respectively (see Tables A -l, A-2).
The MCSs and lightning clusters in the
Tropical Africa region are spatially well correlated, including the spine along the
mountains in western Saudi Arabia and Yemen (16°N, 44°E). The correlation extends
through much of India and East Asia with the exception of the elevated terrain of the
Himalayas and the Tibetan Plateau (30°N, 90°E) where application o f the snow screen has
removed the 85 GHz data.
Figures 5a and 5b are the maps of OTD lightning clusters for September-OctoberNovember (SON), while the 85 GHz MCSs for this period are shown in Figures 6a and
6b. Compared with the JJA distributions shown previously, the seasonal cycle is evident,
particularly for subtropical land regions, with a substantial decrease in the occurrence of
MCSs and lightning clusters in the Northern Hemisphere and increase in the Southern
Hemisphere. One exception is western Saharan Africa (25°N, 0°E), where there is an
increase in lightning clusters from JJA, but still few MCSs. These lightning clusters are
most likely small convective systems, which may have low 85 GHz PCT but not over an
area large enough to meet the MCS definition. Table 1 shows that as with the previous
season, the oceans contain a significantly larger fraction of MCSs (42%) than lightning
clusters (15%). This is particularly noticeable in much of the ITCZ, SPCZ, and southern
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Figure 5a. September-October-November (SON) class 1 and class 2 OTD lightning clusters by lightning flash c<
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24
■i------------- — s p t --------------- «=_--------------------------------- iw r g is_iL it ■‘vj*30
90
150
Dy lightning flash count: 1-3 flashes (dots), 4-24 flashes (plus).
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Figure 5b. September-October-November (SON) class 3 and class 4 OTD lightning clusters by lightning flash
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25
30
90
150
iters by lightning flash count: 25-75 flashes (small plus), > 75 flashes (large plus).
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•
-150
-«.
' •
■
I
-90
/
■ « >
■
-30
■
•
►
»
30
Figure 6a. September-October-November (SON) class 1 and class 2 MCSs by minimum 85 GHz PCT: 225-191 K (d
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5 GHz PCT: 225-191 K (dots), 190-156 K (plus).
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9
Figure 6b. September-October-November (SON) class 3 and class 4 MCSs by minimum 85 GHz PCT: 155-121 K i
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27
€
t
85 GHz PCT: 155-121 K (small plus), < 120 K (large plus).
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28
portions of the Tropical Indian. A weak U C Z signature does appear during this period in
the lightning data with clusters extending westward off the coast of western Africa and the
northern portion of the East Pacific (Figure 5a). In the North Atlantic, both MCSs and
lightning clusters now extend across the ocean basin to the northwestern Africa coast, due
in part to the more southerly intrusion o f synoptic scale extra-tropical disturbances as well
as tropical disturbances occurring during the latter half of the unusually active 1995
tropical cyclone season.
Qualitatively, the MCS and lightning cluster distributions suggest a fairly consistent
inverse relationship over land between the magnitude of the 85 GHz ice scattering (as
measured by the minimum PCT) and the intensity of the lightning flash rate. MCSs with
strong ice scattering (PCT < 155 K; Figure 6b) and high flash rate lightning clusters (> 25
flashes; Figure 5b) are fairly numerous in continental regions such as the northern portions
of South America (10°N, 70°W). much of Tropical Africa, and the eastern portions of
Subtropical Africa (25°S, 30°E). This relationship may not be as strong over the tropical
oceans, however. Oceanic regions such as the Central and East Pacific, as well as the
Tropical Indian, that contain moderate to strong ice scattering MCSs have a few if any of
the higher flash rate lightning clusters.
During the boreal winter months (December-January-February; DJF), most of the
lightning clusters and MCSs, shown in Figures 7a,b and Figures 8a,b, respectively, occur
south of the Equator. The overall trends in the land and ocean distributions are similar to
the previous seasons: oceanic and continental regions contain 41% and 37%, respectively,
of the MCSs versus 21% and 58% of the lightning clusters (see Table 1). In general, the
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Figure 7a. December-January-February (DJF) class 1 and class 2 OTD lightning clusters by lightning flash count
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ttv
r— —J
■
•
•+
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* s +.
±
.-*1
)?■
b
J
T
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150
ling flash count: 1-3 flashes (dots), 4-24 flashes (plus).
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Figure 7b. December-January-February (DJF) class 3 and class 4 OTD lightning clusters by lightning flash count
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30
by lightning flash count: 25-75 flashes (small plus), > 75 flashes (large plus).
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~
t
+«
• 1 . i . + . # ,i / i i H
m
+■
2 . * +'
,
Figure 8a. December-January-February (DJF) class 1 and class 2 MCSs by minimum 85 GHz PCT: 225-191 K (dots)
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31
30
90
150
z PCT: 225-191 K (dots), 190-156 K (plus).
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Figure 8b. December-January-February (DJF) class 3 and class 4 MCSs by minimum 85 GHz PCT: 155-121 B
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32
:Iz PCT: 155-121 K (small plus), < 120 K (large plus).
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33
significant seasonal changes tend to occur in MCS or cluster frequency rather than
intensity. Boccippio et al. (2000a) and Williams et al. (2000) found a similar trend for
lightning-defined storms in the Tropics on diurnal and annual time scales. There are
several noteworthy features in the DJF data. For instance, the ITCZ is still evident in the
MCS data (primarily in Figure 8a) near 5°N latitude, although the numbers of MCSs have
decreased from the previous period. In the corresponding lightning data, particularly in
Figure 7a, the ITCZ remains fairly coherent in the Tropical Atlantic extending o ff the
coast of Africa and in the northern East Pacific. The SPCZ is also a distinguishable
feature over the Central and South Pacific, although more pronounced in the MCS than
lightning data.
Over land, lightning clusters occur in northern India (25°N, 80°E) and Saudi Arabia
(25°N, 45°E) where MCSs are virtually absent. Also, in eastern Tropical Africa, a line of
lightning clusters extends northeastward through Ethiopia (10°N, 40°E; Figure 7a). These
clusters are most likely attributed to sub-MCS sized storms, possibly single cumulonimbi,
resulting from orographic lift along the high terrain of the Ethiopian Plateau and the
Ahmar Mountains.
The South America wet season is well represented by the large
number of MCSs occurring over the continent during this season; among land regions
Tropical South America is second only to Madagascar in terms of the spatial density of
MCSs during this period (see Table A-6). Much of the corresponding OTD lightning data
in South America (Figures 7a,b) is contaminated by the SAA and should not be
interpreted literally.
However, results from other studies suggest that the wet season
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34
MCSs are more monsoonal in nature with much less lightning than those occurring during
SON (Mohr et al. 1999; Boccippio et al. 2000a; Petersen et al. 2000).
The seasonal cycle is again evident in the lightning cluster (Figures 9a,b) and MCS
(Figures 10a,b) distributions for March-April-May (MAM), primarily over the continents
with a shift in MCS and cluster frequency from the Southern to the Northern Hemisphere.
One feature that appears unique to this transitional season, and much more prominent in
the MCS data, is the double ITCZ (Komfield et al. 1967; Waliser and Gautier 1993)
located in the Central and East Pacific along 5°N and 5°S.
Also noteworthy are the
differences in MCS and cluster frequency between the Southern Hemisphere autumn
(MAM; Figures 9a,b, I0a,b) and spring (SON; Figures 5a,b, 6a,b) seasons over
Subtropical Africa and Australia. While lightning clusters consistently outnumber MCSs
over land, the ratio of lightning clusters to MCSs, shown in Tables A-3 and A-7, is much
smaller during MAM than during SON over both of these land regions. This suggests that
during the Southern Hemisphere spring, there are numerous small lightning-producing
storms relative to MCSs in these regions, whereas the autumn is characterized by an
overall decrease in convective activity.
The general trends during MAM are similar to the previous seasons, with a small
fraction (18%) of lightning clusters and nearly half (45%) of the MCSs occurring over the
oceans (Table 1). The moderate to strong ice scattering MCSs and higher flash rate
clusters are both relatively numerous over land regions including, for instance, the south
central U.S. and the northern Gulf of Mexico (30°N, 92°W), over much o f Tropical Africa,
as well as northeastern India and Bangladesh (25°N, 90°E). In contrast, once again there
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Figure 9a. March-April-May (MAM) class I and class 2 OTD lightning clusters by lightning flash count: 1-3 i
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ig flash count: 1-3 flashes (dots), 4-24 flashes (plus).
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Figure 9b. March-April-May (MAM) class 3 and class 4 OTD lightning clusters by lightning flash count: 25-75 flash<
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36
ing flash count: 25-75 flashes (small plus), > 75 flashes (large plus).
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Figure 10a. March-April-May (MAM) class 1 and class 2 MCSs by minimum 85 GHz PCT: 225-191 K (dots),
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PCT: 225-191 K (dots), 190-156 K (plus).
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-150
-90
-30
30
Figure 10b. March-April-May (MAM) class 3 and class 4 MCSs by minimum 85 GHz PCT: 155-121 K (small plus
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38
w
>CT: 155-121 K (small plus), < 120 K (large plus).
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appears to be a different relationship between ice scattering and lightning over much of
the tropical oceans where, MCSs with moderate to strong ice scattering are far more
frequent than the higher flash rate lightning clusters. For example, in the East and Central
Pacific and the Tropical Indian, MCSs with minimum PCT < 190 K account for a roughly
half (42-55%) of the are MCSs in those regions, while the lightning clusters with four or
more flashes represent less than 20% of the corresponding cluster distributions.
Discussion
The results from the MCS and lightning cluster distributions presented in the previous
figures lead to two important inferences regarding the relationship between MCS ice
scattering and lightning over land and ocean. The first is that strong ice scattering MCSs
produce the bulk of the lightning and the land bias in lightning is simply due to a land bias
in strong ice scattering MCSs. This is based upon the fairly consistent observation of a
good spatial agreement, primarily over land, between lightning clusters with high flash
rates and MCSs with moderate to strong ice scattering (low 85 GHz PCT). To examine
this more closely, we consider the distribution of “intense” MCSs, which Mohr and Zipser
(1996b) singled out as having an unusually extensive area o f strong ice scattering.
According to their definition, an intense MCS has an enclosed area o f at least 2000 km2
with 85 GHz PCT < 200 K and a minimum 85 GHz PCT < 175 K. The distribution of
intense MCSs for the 1995/96 data is presented in Figure 1la; these represent the 4% of
the total MCS population. For comparison, Figure 1 lb shows the distribution of intense
lightning clusters, which are defined here as the 98th percentile o f the cluster data set by
flash count. The 98th percentile of intense clusters was chosen to give approximately the
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Figure 11a. June 1995 - May 1996 "intense" lightning clusters. Plotting symbols correspond to the number o f fit
61-90 flashes, 91-155 flashes, > 155 flashes.
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40
30
90
150
to the number o f flashes in a cluster and increase in size beginning with the smallest plus: 45-60 flashes,
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Figure 1lb. June 1995 - May 1996 "intense" MCSs. Plotting symbols correspond to the minimum 85 GHz P<
120-101 K, < 100 K.
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rimum 85 GHz PCT and increase in size beginning with the smallest plus: 175-141 K, 140-121 K,
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42
same sample size as the intense MCSs. The figures clearly indicate that while a land bias
exists in both distributions, it is much more pronounced among the intense lightning
clusters than the intense MCSs.
Approximately 82% of the intense clusters are
continental compared with 62% of the intense MCSs. With few exceptions (excluding the
SAA region) the spatial distribution of intense MCSs and lightning clusters is in quite
close agreement over land. Intuitively, one might expect this since 85 GHz ice scattering
since 85 GHz ice scattering and the processes leading to lightning share a common
dependence upon ice microphysics, i.e., a significant concentration of large graupel and
small ice particles sustained through a deep layer above the freezing level by convective
drafts resulting in a large optical depth (low 85 GHz brightness temperatures) and robust
charge separation (Wilheit et al. 1982; Wu and Weinman 1984; Vivekanandan et al. 1990,
1991; Williams 1989; and others).
A second inference can be drawn from the discrepancy between lightning and ice
scattering over the oceans, namely, given similar 85 GHz ice scattering signatures, there
appears to be a significant difference in the probability of lightning between continental
and oceanic MCSs.
In other words, the continental MCS appears to be a much more
efficient lightning producer than its oceanic counterpart for a similar degree o f microwave
ice scattering. Support for this is presented in Table 2, which is a summary o f MCS and
cluster statistics over the June 1995 - May 1996 period with the regions sorted by the
flash-to-MCS ratio.
Note first that the MCS distribution shows little land or ocean
preference; the same result occurs when the MCSs are normalized by the region areas (not
shown). In contrast, land and ocean are cleanly stratified according to the flash density
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Table 2. June 1995 - May 1996 OTD lightning and SSM/I MCS summary statistics. Regions are sorted by
the flash-to-MCS ratio.
Total MCSs
Total
Clusters
Total
Flashes
Flash
Density
(x 10'3 km'2)
Flash-toMCS ratio
1.0
1120
3774
37180
3.7
33
Tropical Africa
1.9
6491
22706
189283
10
29
Subtropical Africa
0.5
933
4653
26936
5.4
29
North America
1.2
2733
7422
59201
4.9
22
India and East Asia
1.8
3591
10289
69382
3.9
19
Madagascar
0.2
554
1061
8341
4.1
15
Central America
0.3
1301
3079
14641
4.9
11
South Indian
1.5
881
1222
8955
0.6
10
Maritime Continent
2.2
9965
15157
70154
3.2
7
North Atlantic
1.1
1133
1824
7497
0.7
7
North Pacific
2.4
2022
2026
7235
0.3
4
South Pacific
2.6
3249
2962
11047
0.4
3
East Pacific
2.4
2870
1656
6260
0.3
2
Tropical Indian
1.6
3774
1984
5272
0.3
1
Central Pacific
2.0
5373
2044
4488
0.2
1
Area
(x 107 km2)
Australia
Region
44
and flash-to-MCS ratio, which suggests dramatic differences in terms of lightning
production: typical land/ocean differences are roughly order of magnitude with Tropical
Africa clearly dominating all regions in terms of flash density. One could reasonably
argue that differences in lightning production over land and ocean are related to the fact
that the intense MCSs found over the oceans typically do not attain the same degree of ice
scattering intensity as those over land, which the MCS data clearly indicate. While this
may be valid, an important question remains: what differences exist between the vertical
hydrometeor profiles of the many continental and oceanic MCSs with similar 85 GHz ice
scattering but vastly different lightning signatures? One speculation is that the typical
oceanic MCS, with relatively weak updrafts through the mixed phase region, may contain
many small ice particles and relatively small amounts of supercooled liquid water, which
in turn constrains graupel growth to mostly small (< 3 mm diameter) sizes.
During
hurricane flights, Black and Hallett (1986) find limited supercooled liquid water (< 0.5 g
m '3) in all but the strongest eyewall updrafts. Assuming a terminal velocity on the order
of 2-3 m s’1 for small graupel (Bohm 1989), the typical oceanic updraft may be
sufficiently strong to loft many small graupel and ice crystals through a considerable
depth of the cloud, which would contribute to an increased optical depth and 85 GHz ice
scattering. However, the weak updrafts fail to replenish the limited supply of supercooled
cloud liquid water, which would be quickly scavenged by the many small ice particles.
Thus, graupel growth by accretion is limited and the resulting ice particle spectrum above
the freezing level is relatively narrow and homogenous with small differential velocities.
The combined effects of low supercooled liquid water content and small graupel sizes in
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45
the mixed phase region would not be conducive to charge separation necessary for
lightning. By comparison, we propose that the typical tropical continental MCS has a
much different vertical structure of ice microphysics: vigorous updrafts maintain
supercooled liquid water sufficient to grow large graupel in the mixed phase region where
efficient charge separation can occur. At the same time, numerous smaller graupel and
cloud ice particles extend to heights similar to those in the oceanic cloud and contribute to
significant 85 GHz ice scattering. These models are consistent with previous regional and
case studies of tropical convective systems which demonstrate that oceanic systems
typically have weaker convective updrafts (Zipser and LeMone 1980; Jorgensen and
LeMone 1989; Lucas et al. 1994; Petersen et al. 1999) and smaller magnitudes o f radar
reflectivity in the mixed phase region (Rutledge et al. 1992; Zipser and Lutz 1994;
Petersen et al. 1996, 1999; Nesbitt et al. 2000) than continental systems.
To the first order, a large optical path through millimeter-sized ice (i.e., graupel or
frozen rain) yields strong 85 GHz ice scattering and is also conducive to robust charge
separation via the non-inductive collision mechanism. To that extent, and to that extent
only, one would expect some relationship between the magnitude of the scattering
signature and the lightning flash rate, and the observations reveal such a relationship.
However, it is also somewhat surprising that the same ice scattering signature (minimum
85 GHz PCT) gives different flash rates over land and ocean. While lightning flash rate
and the 85 GHz ice scattering signature are both useful measures o f convective vigor, the
non-unique relationship between brightness temperatures and hydrometeor vertical
profiles means that the 85 GHz channel alone provides very limited diagnostic
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46
information on the microphysical structure of convective systems with comparable ice
scattering signatures but different lightning characteristics.
Nonetheless, the observed
relationships documented in this study do provide an important set of facts that can be
used with data from the TRMM satellite, which makes simultaneous radar, passive
microwave, and lightning measurements, to better quantify these relationships.
Summary
A systematic comparison of the distribution of SSM /I 85 GHz MCSs and OTD
lightning has been conducted over three-month seasonal periods from June 1995 through
May 1996 for 19 geographical regions between 35°N and 35°S in order to document the
observed relationship between 85 GHz ice scattering signatures and lightning.
The
lightning flash data are grouped into lightning clusters, while the MCSs are classified
according to their minimum 85 GHz PCT. In each of the four periods, there is a much
stronger land bias among the lightning clusters than among the MCSs. For a given threemonth period the land, ocean, and mixed land and ocean regions contain 56-66%, 15-21%,
and 19-26%, respectively, of the total cluster population, whereas the same regional
groupings contain 37-41%, 40-45%, and 17-22% of the total MCS population. Generally,
among land regions MCSs with moderate/strong ice scattering (85 GHz PCT < 190 K)
and moderate to high flash rate lightning clusters (four or more flashes) are both numerous
with Tropical Africa tending to dominate the land and ocean regions in terms of ice
scattering intensity and lightning flash rates.
This result is somewhat intuitive since
strong 85 GHz ice scattering and robust charge separation in are both fundamentally
related to ice microphysics, i.e., a large number density of precipitation-sized ice particles
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47
through a deep layer of the convective cloud. The implication is that strong convective
systems with low 85 GHz PCT are the primary lightning producers in the Tropics and that
the observed land bias in the lightning data is simply due to a greater fraction of these
strong systems over land. The ice scattering and lightning relationship appears to be
different over the oceans, however. Oceanic MCSs with moderate to strong ice scattering
occur with much greater frequency than the moderate to high flash rate lightning clusters.
In addition, the lightning flash densities and flash-to-MCS ratios computed for each region
show order-of-magnitude or larger differences between land and ocean suggesting that
continental and oceanic convective systems with similar 85 GHz ice scattering signatures
are significantly different in terms of lightning production. We propose that for a given
range of 85 GHz brightness temperatures the continental MCS will have a much greater
probability of lightning than its oceanic counterpart and this is related to previously
document differences in vertical updraft velocities and supercooled liquid water content,
and thus the microphysics profiles in the mixed phase region (0°C > T > -40°C), between
continental and oceanic systems. While a more detailed investigation is beyond the scope
of the current study, work is already underway using data from the TRMM satellite.
TRMM carries a unique instrument suite including a multi-channel microwave
radiometer, the Lightning Imaging Sensor (LIS), and the Precipitation Radar (PR).
Simultaneous measurements with this ensemble will facilitate high-resolution case studies
across a broad spectrum of tropical convective systems to help elucidate the differences in
their microphysical structure and validate the lightning/ice scattering relationships
observed with SSM/I and OTD.
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48
CHAPTER m
MICROWAVE AND LIGHTNING SIGNATURES OF CONVECTTVE
SYSTEMS USING TRMM
Introduction
A considerable effort has been put forth during the last half century to study and
understand precipitating systems in the Tropics, which account for two-thirds of the global
rainfall and are a major source of latent heat for the atmosphere (Simpson et al. 1986).
The spectrum of precipitating cloud types in the tropics covers broad range of spatial and
temporal scales, ranging from single cumulonimbus clouds to large, long-lived mesoscale
convective systems, with varying contributions to total tropical rainfall (Mohr et al. 1999).
Tropical precipitating systems are also a major component of the global electric circuit
with approximately 75% of the global lightning occurring between 30°N and 30°S
(Christian et al. 1999). While the charge separation processes leading to lightning are not
fully understood, it is widely accepted, based on a wealth of laboratory evidence together
in situ observations of thunderstorms, that mixed (liquid and ice) phase cloud
microphysics have an essential role.
Specifically, non-inductive collisions involving
millimeter-sized ice (graupel or hail) and smaller ice crystals in the presence of
supercooled liquid water at temperatures between 0°C and -40°C represent the most viable
mechanism to explain robust cloud electrification (Reynolds et al. 1957; Takahashi 1978;
Jayaratne et al. 1983; Saunders et al. 1991; W illiams 1989; and others).
W hile precipitating cloud systems tend to occur throughout the tropics, lightning has a
strong land bias (Orville and Henderson 1986; Goodman and Christian 1993; Christian
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49
1999; Toracinta and Zipser 2001).
This has led many researchers to investigate the
microphysical relationships between occurrence or absence of lightning and the structure
and environment of tropical convective systems, often with the aid of remote sensing
(radar or high frequency passive microwave) techniques (Williams et al. 1992; Rutledge et
al. 1992; Zipser 1994; Petersen et al. 1996, 2000; Black and Hallett 1999; Cecil and
Zipser 1999; Boccippio et al. 2000a; Williams et al. 2000; Toracinta and Zipser 2001).
Information from radar and passive microwave instruments is useful in this regard
because they take advantage of the interaction of microwave energy with liquid and ice
phase hydrometeors. Radar reflectivity is strongly related to the size of the precipitation
particles in a sampled volume. In the case of Rayleigh scatterers (small in comparison
with the wavelength of incident radiation), reflectivity is related to the sum of particle
diameters to the sixth power.
At PR wavelengths (2.2 cm), this would apply to
precipitation particles on the order of 3 mm diameter or smaller in the sampled volume.
For larger diameter particles the scattering coefficient is a damped oscillating function
which oscillates about the geometric area of the scatterer (see Rinehart 1991, for instance).
Hydrometeor phase is also an important component of reflectivity measurements owing to
the near order-of-magnitude difference in the dielectric constants of ice and liquid water
which translates to nearly 7 dB difference when sampling ice versus liquid phase particles.
For downward looking radiometers, measured brightness temperatures (Tb) are the
integrated result of emission and scattering processes that act to modulate upwelling
radiation along the optical path to the radiometer. In the remote sensing of precipitating
systems the emission sources are primarily cloud liquid water, rain, and melting phase
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50
hydrometeors (Wilheit 1986; Mugnai et al. 1990; Vivekanandan et al. 1991). At 37- and
85 GHz, the frequencies used in this and many other studies of convective systems,
scattering of upwelling radiation is primarily due to precipitation-sized ice hydrometeors
present above the emitting rain layer (Wilheit et al. 1982; Wu and Weinman 1984;
Spencer et al. 1989). The resulting reduction of the observed brightness temperatures,
termed the ice scattering signature, is a function of the particle size distribution, bulk
density, num ber concentration, and geometric depth of the scattering layer (Vivekanandan
et al. 1990, 1991).
Ice scattering at 85 GHz (3.5 mm) is due to relatively small
precipitation-sized ice (0(0.5 mm)) and is typically the dominant signal when ice phase
precipitation is present.
At 37 GHz (8 mm wavelength), ice scattering is due to the
presence of larger (0(several mm)) graupel or frozen raindrops in convective cores.
The objective of the present study is to compare the observed radar and brightness
temperature properties of large sample of tropical continental and oceanic precipitating
systems using data from the TRMM satellite (Simpson et al. 1986; Kummerow 1998).
The specific goal is to provide a substantial quantitative observational database of these
properties, organized in a way that facilitates the evaluation of the existing hypothesis
regarding microphysical differences between systems with and without lightning. Cecil
(2000) presents a similar quantitative database o f tropical cyclones observed by TRMM.
Data and Methods
The TRM M satellite was launched in late November 1997 into a circular orbit at
approximately 350 km altitude and 35° inclination from the equatorial plane. With this
orbital geometry, TRMM precesses through the diurnal cycle at a given geo-location in
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approximately 47 days. The current study uses data from the TRMM Microwave Imager
(TMI), the Precipitation Radar (PR; Kummerow 1998), and the Lightning Imaging Sensor
(LIS; Christian 1999).
The PR is a 2-meter active phased array radar system operating near 13.8 GHz
(approximately 2.2 cm wavelength). The PR scans across a 215 km swath with 80 vertical
range bins extending to 20 km above the earth ellipsoid.
The vertical and horizontal
resolutions are 250 m and 4.3 x 4.3 km2, respectively at nadir, and the minimum
detectable signal (MDS) is approximately 17 dBZ.
The current study utilizes the
attenuation-corrected reflectivity data from the TSDIS 2A25 version 4.0 algorithm.
Detailed information about the PR algorithms can be found at http://tsdis.gsfc.nasa.gov.
The TM I is a conical scanning multi-channel passive microwave radiometer
measuring upwelling radiance at 10.65, 19.35, 21.3, 37.0, and 85.5-GHz (85 GHz
hereafter) at a 49° angle from nadir. Each of the TMI frequencies is horizontally and
vertically polarized except 21.3 GHz, which is vertically polarized. The ground footprints
of the 37- and 85 GHz channels are 1 6 x 9 km2 and 7 x 5 km2, respectively, with the major
axes oriented along-track. Kummerow et al. (1998) explain that during the time of a full
TMI scan, the TRMM sub-satellite point advances approximately 14 km which results in
an along-track undersampling at 85 GHz and a slight oversampling at 37 GHz (see their
Figure 2). Although the full TMI swath width is 759 km, the current study focuses on
data within the 215 km wide PR swath in order to take advantage of the instrument
overlap there.
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52
At TM I frequencies, the ocean surface has a much lower emissivity than land surfaces
such that large brightness temperature gradients appear along coastlines. In addition, the
emissivity of land surfaces can vary due to the presence of surface water bodies (i.e.,
lakes) and changes in soil moisture and vegetation. At oblique viewing angles like that
used by the TMI, water surfaces are highly polarized. Spencer et al. (1989) make use of
the 85 GHz polarization information to derive a polarization-corrected temperature (PCT)
which largely eliminates emissivity discontinuities between land and ocean and over
varying land surfaces. Their 85 GHz PCT, which is utilized in the current study, is given
by:
PCT = 1.8 Tg5v - 0.8 Tgsh
(1)
where Tg5V and Tgsh are the vertically and horizontally polarized 85 GHz brightness
temperatures. A similar empirical technique was used to derive a 37 GHz PCT (Nesbitt,
personal communication), which is given by:
PCT = 2 . 2 T 37v - 1 . 2 T 37h
(2)
where T 37v and T37h are the vertically and horizontally polarized 37 GHz brightness
temperatures. This 37 GHz PCT relationship is used in the current study.
The LIS consists of a staring optical imager and onboard electronics designed to detect
and record transient optical radiance events associated with lightning. Radiance events
that exceed a threshold value above the continuously averaged background are considered
lightning candidates.
The LIS instrument instantaneous field of view (FOV) is
approximately 600 km x 600 km and the ground resolution is 4 km at nadir. LIS detects
total (intra-cloud and cloud-to-ground) lightning with typical view times on the order of
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53
80 seconds. Although LIS validation is still under investigation, the lightning detection
efficiency is currently estimated at 70-90% (Christian 1999; Boccippio et al. 2000b); no
correction has been applied for the detection efficiency in the present study.
Nesbitt et al. (2000) present an analysis of the distribution and characteristics of
discrete precipitation features using TRMM data for two tropical continental (.Africa and
South America) and oceanic (west Pacific and east Pacific) regions during August October 1998. These regions, shown in Figure 12, were chosen to ensure a large sample
of tropical continental and tropical oceanic precipitating systems.
The precipitation
features are defined using a combination of the PR 2A25 near-surface reflectivity and
TMI 85 GHz PCT data, which ensures inclusion of the precipitating area as well as
associated non-precipitating cloud (i.e., anvil) with an optical depth sufficient for
significant brightness temperature depressions at 85 GHz. The present study makes use of
the Nesbitt et al. (2000) precipitation feature database and a summary of their
experimental design is given here.
35
25
-east;
Pacific
Africa
Soilth^ v ^
Afnericai
Figure 12. Map of study regions.
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54
The PR and TM I data are first matched to a common grid by interpolating the TMI
brightness temperatures to the 4.3 x 4.3 km2 PR grid using a nearest neighbor technique.
With the TMI pixel spacing approximately 14 km in the along-track direction, the
interpolation typically resulted in two or three PR pixels being matched with the same
TMI brightness temperature. Once the PR and TMI fields are combined, the precipitation
features are defined as four or more contiguous pixels (-75 km2) having either a near­
surface reflectivity > 20 dBZ or an 85 GHz PCT < 250 K. The 20 dBZ near-surface
reflectivity criterion is sufficiently above the PR minimum detectable signal (-17 dBZ)
while ensuring that areas of light rainfall are included in the system. The 250 K PCT
threshold, which was first used by Mohr and Zipser (1996a) to define mesoscale
convective systems from SSM/I 85 GHz data, denotes areas of significant 85 GHz ice
scattering.
Surface snow cover can scatter microwave radiation to produce 85 GHz brightness
temperatures similar to those associated with precipitating systems (Grody 1991; Ferraro
et al. 1994). In the Tropics, this is a concern in high elevation regions such as the Andes
Mountains where permanent or semi-permanent snow cover may exist.
Actual
precipitating areas with 85 GHz PCT below 250 K are not likely to occur without
appreciable radar echo at the surface or aloft. Hence, Nesbitt et al. (2000) apply the
criteria of near-surface reflectivity or 6 km reflectivity greater than 15 dBZ to ensure that
snow artifacts are removed from the data base.
LIS lightning flash totals and flash densities are computed for each region in order to
compare relative differences in lightning occurrence. Lightning flashes are also assigned
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55
to individual precipitation features on a spatial coincidence basis and the lightning flash
rate is computed for each feature. While most areas of LIS instrument coverage are
viewed for approximately 80 seconds, a small fraction of LIS flashes have view times less
than 60 seconds which have been filtered from the data set to ensure more robust flash
rate statistics. Also, typical LIS view times give a minimum detectable flash rate around
0.7 m in'1. In the following comparisons of systems with and without lightning, those
without lightning are understood to have either no flashes or very low flash rates.
The approach to characterizing the precipitation features in this database is to use
single value parameters (minimum 85- and 37 GHz PCT, maximum height of threshold
reflectivities, and lightning flash rate) for each system.
Also, the vertical reflectivity
profiles are computed using the maximum reflectivity at each vertical level from
anywhere in the system.
In any given storm, these values may not be coincident.
However, based on theory and past observations, we are making an implicit assumption
that the relationships between them are sufficiently robust for describing and comparing
the relative strength of the convective systems and the relationship to lightning.
Results
Table 3 shows that precipitation features occur with greater frequency in the ocean
regions than in the land regions even when normalized by the respective region areas. The
large majority of features in each of the four regions do not have lightning, although the
likelihood of a precipitation feature with lightning is more than 40 times greater over land
than ocean. The regions are stratified by the 10th percentile minimum 85 GHz PCT with
coldest features in Africa and the warmest in the east Pacific. Land systems have much
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56
stronger 85 GHz ice scattering with a 30-50 K difference in the 10th percentile minimum
PCT. Similarly, land features have deeper 30 dBZ reflectivity than the oceans with Africa
features having the highest 30 dBZ echo heights of the four regions.
Table 3. Summary characteristics of precipitation features in Africa (AF), South America
(S A), west Pacific (WP), and east Pacific (EP).
Region
AF
SA
WP
EP
Region area (xlO6 km2)
12
11
11
8.5
Number of systems
12788
12584
28550
17317
System density (xlO'3 km'2)
1.1
1.1
2.6
2.0
% of systems with lightning
15
12
0.3
0.2
90th %-tile system size (km2)
1276
998
536
814
10th %-tile Min 85 GHz PCT (K)
197
212
241
251
Median 30 dBZ height (km)
4.5
4.5
3.25
1.5
90th %-tile 30 dBZ height (km)
9.0
7.75
6.0
5.04
Significant land/ocean differences also occur in the distributions of precipitation
feature volumetric rainfall, which are shown in Figure 13. The two land and the two
ocean regions have very similar rainfall distributions with the oceanic features producing
lower rainfall totals than continental features. The bulk of the total volumetric rainfall
over land and ocean is contributed by a small fraction of precipitating systems, as early
researchers have noted (Riehl 1954) and recent satellite-based studies confirm (Mohr et al.
1999; Nesbitt et al. 2000).
In order to concentrate on systems producing significant
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57
1.0
0.8
a 0.6
u.
i
0.4
.Amca
S. America
,W. Pacific
,E. Pacific
0.2
0.0
Total Volumetric Rainfall (mm kmA2 hm-1)
Figure 13. Cumulative distribution of precipitation features by total volumetric rainfall
(mm km2 h r 1) for Africa (solid), South America (dotted), west Pacific (dashed), and east
Pacific (dot-dashed). Volumetric rainfall threshold (103 mm km2 hr'1) is indicated.
rainfall, the population of features was reduced to those exceeding an arbitrary volumetric
rainfall threshold (103 mm km2 hr'1). Applying the threshold reduces the land and ocean
populations by approximately 67% and 84%, respectively. Table 4 compares summary
characteristics for features that do and do not exceed the rainfall threshold. In summary,
the bulk of the non-threshold precipitation features in each region are shallow
precipitating systems (by 30 dBZ echo height), which contribute little to lightning flash
totals and have little or no 85 GHz ice scattering (at 85 GHz resolution). Therefore, these
systems will be excluded from further analysis and unless stated otherwise, subsequent
use of the term “precipitation feature” will refer only to the subset of systems that exceed
the rainfall threshold.
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58
Table 4.
Summary characteristics of precipitation features that do not meet the total
volumetric rainfall threshold (103 mm km2 h r'1). Values in parentheses correspond to
precipitation features that meet the total volumetric rainfall threshold.
Region
AF
SA
WP
EP
Number of Systems
8380
(4408)
8632
(3952)
23918
(4632)
14489
(2828)
Median Min 85 GHz PCT (K)
277 (237)
278(244)
284 (243)
283(254)
Median Max 30 dBZ Height (km)
4.25 (7.25)
4.0 (6.5)
3.0 (6.0)
1.25 (5.25)
Number with Lightning
99(1795)
86(1393)
6(87)
2(2 4 )
% of Total LIS Rashes
1 (99)
1(99)
2(98)
3(97)
% of Total Volumetric Rainfall
5(95)
6(94)
16(84)
10 (90)
A simple but useful method to describe and compare the vertical reflectivity structures
of continental and oceanic precipitation features is with the maximum heights of threshold
reflectivity values.
This method is used in Figure 14, which shows the frequency
distributions of land and ocean precipitation features according to their maximum 20 dBZ
and 30 dBZ echo top heights. Nearly half (45%) of oceanic precipitation features and
roughly one-third of the continental features have maximum 20 dBZ echo top heights
from 6-10 km and maximum 30 dBZ echo heights ranging from 5-7 km. The deepest 30
dBZ heights in continental features extend greater extremes than those in oceanic features.
In fact, inspection of the reflectivity height distributions shows that land features typically
have deeper 30 dBZ echo than ocean features with similar 20 dBZ echo heights.
A
quantitative example o f this is shown in Figure 15, which compares the frequency
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0
5
10
Maximum 30 dBZ Height (km)
15
20
Figure 14. Frequency distribution of oceanic (upper) and continental (lower) precipitation
features.
The oceanic and continental distributions are contoured and shaded,
respectively. Contour and shading values are 1, 10, 25, 50, 75, 100, 200, 300, 400, and
500.
Bin sizes are 1 km for the ordinate (maximum 20 dBZ height) and abscissa
(maximum 30 dBZ height).
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60
distributions of 30- and 40 dBZ heights for the subset of land and ocean precipitation
features with maximum 20 dBZ heights from 13-14 km. For features with the same 20
dBZ echo heights, those over land have 30 dBZ echo heights that are typically about 3 km
higher than those over the oceans.
L an d 3 0 dBZ
14
O c e a n 3 0 dBZ
L an d 4 0 dBZ
12
O c e a n 4 0 dBZ
ISJ
8
6
4 -/
2 Ll
0.0
0.1
0 .4
0.2
0 .3
R e la tiv e F re q u e n c y
0 .5
0.6
Figure 15. Relative frequency distributions of 30- and 40 dBZ heights for land and ocean
precipitation features with maximum 20 dBZ heights from 13-14 km.
Figure 16 shows that land features typically have 40 dBZ reflectivities extending to
much higher extremes (> 15 km) than those in ocean features. In sharp contrast, the 40
dBZ echo heights in ocean features are largely confined to below 7 km with nearly onethird of the ocean systems having no 40 dBZ echo. For systems with fixed 13-14 km 20
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61
1ST 137
30 '18
5
10
Maximum 40 dBZ Height (km)
15
Figure 16. Frequency distribution of oceanic (upper) and continental (lower) precipitation
features.
The oceanic and continental distributions are contoured and shaded,
respectively. Contour and shading values are 1, 10, 25, 50, 75, 100, 200, 300, 400, and
500.
Bin sizes are 1 km for the ordinate (maximum 20 dBZ height) and abscissa
(maximum 40 dBZ height).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
dBZ echo heights (Figure 15), land the 40 dBZ heights in land and ocean features
typically differ by about 1 km.
The frequency distributions of the continental and oceanic features by their minimum
37- and minimum 85 GHz PCT in Figure 17 show two striking results. First, land features
dominate the low brightness temperature extremes.
For instance, there are almost no
oceanic features with 85 GHz PCT < 150 K and 37 GHz PCT < 240 K, where strong ice
scattering is occurring at both frequencies. Secondly, the highest frequency of land and
ocean features occurs at the warm end of the Tb parameter space (37 GHz PCT > 275 K
and 85 GHz PCT > 250 BC), which is indicative o f either minimal 85 GHz ice scattering or
possibly 85 GHz PCT reductions due to high cloud liquid water content where ice phase
microphysics is not yet dominant (Spencer et al. 1989).
When the features are normalized by the 85 GHz ice scattering signature, particularly
where appreciable 85 GHz ice scattering occurs (PCT < 250), continental features tend to
have colder 37 GHz PCT than their oceanic counterparts. For example, continental and
oceanic features with minimum 85 GHz PCT between 200-210 K have modal 37 GHz
PCT that differ by 4 K. The difference increases to around 6 K as the 85 PCT decreases.
Along with the observed deeper 30- and 40 dBZ reflectivity in continental features, this
trend is indicative of the presence of larger graupel or hail in the continental systems than
the oceanic systems.
As the 37 GHz PCT decrease below about 255 K, the oceanic features are confined to
a colder, more narrow range of 85 GHz PCT than the corresponding continental features.
An example is the land and ocean systems with minimum 37 GHz PCT from 245-250 K.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
100
150
200
Minimum85 GHz PCT (K)
250
300
Figure 17. Frequency distribution of oceanic (upper) and continental (lower) precipitation
features.
The oceanic and continental distributions are contoured and shaded,
respectively. Bin sizes are 5 K for the ordinate (minimum 37 GHz PCT) and 10 K for the
abscissa (minimum 85 GHz PCT). Contour and shading values are 1,5, 10, 50, 100, 250,
and 500.
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64
These have similar modal 85 GHz PCT (140-150 K), but only the land systems have
minimum 85 PCT extending to much warmer values above 170 K. This may be due to the
presence of larger supercooled water contents aloft in the land systems, which when
present in the ice layer, acts as an emission source to reduce the effects of 85 GHz ice
scattering and increase the observed Tb (Adler et al. 1991; Vivekanandan et al. 1991;
Smith et al. 1992).
To more closely examine the reflectivity and brightness temperature relationships,
Figure 18 compares the median minimum 85 GHz PCT for continental and oceanic
systems as a function of their maximum 20- and 30 dBZ echo heights. The median 85
GHz PCT tend to decrease with increasing 20- and 30 dBZ heights, consistent with
increased 85 GHz ice scattering with increasing optical depth. The trend is similar with
the median 37 GHz PCT shown in Figure 19, although the brightness temperature
decrease is more gradual since the 37 GHz ice scattering requires the presence of larger
graupel or hail particles than at 85 GHz.
Figure 18 also shows that land and ocean features with the same reflectivity heights,
the median 85 GHz PCT are substantially warmer than corresponding ocean values. An
example is shown in Figure 20a for features with 13-14 km 20 dBZ heights where, at any
given 30 dBZ height, the 85 PCT in continental features is 10-25 K warmer than in the
ocean features. The 37 GHz PCT difference is much smaller than at the higher frequency
with less consistency between land and ocean (Figure 20b). This may be due in part to
non-uniform beamfilling effects at 37 GHz, which has a footprint larger than the 85 GHz
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0
5
10
Maximum30 dBZ Height (km)
15
20
Figure 18. Median of the minimum 85 GHz PCT for oceanic (upper) and continental
(lower) precipitation features. The oceanic and continental distributions are contoured
and shaded, respectively. Contour and shading intervals are every 25 K beginning at 100
K. Bin sizes are 1 km for the ordinate (maximum 20 dBZ height) and abscissa (maximum
30 dBZ height). Bins containing fewer than Five precipitation features are not plotted.
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66
20
197
224 236 220 222
230 219
259 2571
233
E 15
£OJ
268
<D
X
N
m
XJ
>S|261
/
A
279
E
3
E
X
5
10
/in
'ill
279
280
281
5
0
5
10
Maximum 30 dBZ Height (km)
15
20
Figure 19. Median of the minimum 37 GHz PCT for oceanic (upper) and continental
(lower) precipitation features. The oceanic and continental distributions are contoured
and shaded, respectively. Contour and shading intervals are every 10 K beginning at 220
K. Bin sizes are 1 km for the ordinate (maximum 20 dBZ height) and abscissa (maximum
30 dBZ height). Bins containing fewer than five precipitation features are not plotted.
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67
frequency by a factor of four and is therefore not as effective in resolving small ice
scattering cores.
14
14
Land
O cean
160
180
200
22 0
Min 8 5 GHz PCT (K)
240
250
Land
O cean
255
2 6 0 2 6 5 2 7 0 275
Min 3 7 GHz PCT (K)
28 0
Figure 20. Subset of precipitation features with maximum 20 dBZ heights from 13-14
km. Land and ocean distributions of maximum 30 dBZ height versus minimum 85 GHz
PCT (a); land and ocean distributions of maximum 30 dBZ height versus minimum 37
GHz PCT (b).
Figures 21 and 22 show the distributions of median minimum 85 GHz PCT and
minimum 37 GHz PCT, respectively, for continental and oceanic features as a function of
their maximum 20- and 40 dBZ echo heights. Among the ocean features, for a given 20
dBZ height the median 85 GHz Tb decrease as the 40 dBZ height increases indicating
greater ice scattering with increasing optical depth. The trend is similar at 37 GHz (Figure
22) with the lowest median PCT (247 K) occurring with the deepest ocean 20- and 40
dBZ reflectivities. For continental features, the median 85 GHz PCT tend to decrease
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0
5
10
Maximum 40 dBZ Height (km)
15
Figure 21. Median of the minimum 85 GHz PCT for oceanic (upper) and continental
(lower) precipitation features. The oceanic and continental distributions are contoured
and shaded, respectively. Contour and shading intervals are every 25 K beginning at 100
K. Bin sizes are 1 km for the ordinate (maximum 20 dBZ height) and abscissa (maximum
40 dBZ height). Bins containing fewer than five precipitation features are not plotted.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
69
20
0
5
10
Maximum 40 dBZ Height (km)
15
Figure 22. Median of the minimum 37 GHz PCT for oceanic (upper) and continental
(lower) precipitation features. The oceanic and continental distributions are contoured
and shaded, respectively. Contour and shading intervals are every 10 K beginning at 220
K. Bin sizes are 1 km for the ordinate (maximum 20 dBZ height) and abscissa (maximum
40 dBZ height). Bins containing fewer than five precipitation features are not plotted.
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70
with increasing 40 dBZ to 5-7 km and then show a slight warming as the 40 dBZ heights
increase toward the extremes.
This 85 GHz Tb warming might also be related to
differences in the amount and/or vertical distribution of supercooled liquid water between
vigorous continental convective systems storms with deep 40 dBZ reflectivity and those
with shallower 40 dBZ echo.
The geographic distributions of precipitation features with and without lightning are
shown in Figure 23a and 23b, respectively, with summary statistics in Table 5. Of the
Figure 23. Geographic locations of precipitation features with lightning (a) and without
lightning (b).
four regions, the east Pacific has the lowest raw count and spatial density o f features,
which are largely concentrated along the Inter-tropical Convergence Zone (ITCZ) around
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71
8°N. A very small fraction of east Pacific features have lightning (-1% ) resulting in a low
regional flash density. The west Pacific contains the largest raw count and spatial density
of features of the four regions.
These are also locally concentrated along the ITCZ
(~7°N), along with a significant concentration extending northward to 15°N and in the
southwestern portion of the region toward New Guinea.
The west Pacific is also
dominated by precipitation features without lightning, which constitute 98% of the
sample.
Table 5. Summary o f precipitation feature and lightning statistics for Africa (AF), South
America (SA), west Pacific (WP), and east Pacific (EP).
Region
AF
SA
WP
EP
Number of Systems
4408
3952
4632
2828
System density (xlO-4 km'2)
3.7
3.6
4.2
3.3
Total LIS flashes
22594
13110
244
77
F ash density (xlO*5 km'2)
188
119
2.2
0.9
% of systems with lightning
41
35
2
1
In both land regions the fraction of features with lightning is more than an order of
magnitude greater than over the oceans; 41% of the features in Africa and 35% of those in
South America have lightning (Table 5). Africa contains by far the highest total lightning
count and flash density of the four regions. Figure 23a indicates that the lightning systems
over Africa have a fairly uniform distribution across the sub-Sahara (5°N-15°N) with a
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72
slightly higher concentration of lightning features in the central portion o f the continent
(5°S-10°S), and relatively few systems in the southeastern portion of the region. Africa
features without lightning are generally found throughout the region with numerous
systems along the western coast (near 5°N, 10°W) and across the sub-Sahara to the central
portion of the continent. In South America, systems with lightning are distributed fairly
uniformly with the exception of northeastern Brazil (near 5°S, 40°W) and the eastern
Brazilian coast. Features without lightning generally increase in frequency of occurrence
from southeast to northwest with a small number of systems along and off the eastern
Brazilian coast and few systems in northeastern Brazil.
While the land and ocean spatial densities of precipitation features given in Table 5
are very similar, the land/ocean differences in lightning feature frequency and thus total
lightning counts are quite large.
Quantitatively, when normalized by the area of each
region, the ratio of land to ocean precipitation feature occurrence is roughly 0.95, yet the
probability of a lightning system differs by a factor o f 25.
In addition, the land flash
densities range from a factor of 50 to two orders of magnitude greater than the
corresponding ocean flash densities.
Figure 24 presents relative frequency distributions of Africa and South America
precipitation features according to their radar reflectivity and microwave brightness
temperature characteristics. The left and right panels compare features with and without
lightning, respectively. Figures 24a and 24b indicate that the features with and without
lightning in Africa and South America are similarly distributed in much o f the brightness
temperature parameter space.
Africa tends to have a greater occurrence of lightning
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
50
100 150 200 250 300
Minimum 85-GHz PCT (K)
50
_
0
20
5
10
15
20
Maximum 30 dBZ Height (km)
„
100 150 200 250 300
Minimum 85-GHz PCT (K)
0
5
10
15
20
Maximum 30-dBZ Height (km)
0
5
10
15
20
Maximum 40-dBZ Height (km)
20
I
0
Figure 24.
5
10
15
20
Maximum 40 dBZ Height (km)
Relative frequency distribution of precipitation features over Africa
(contoured) and South America (shaded) with lightning (left panels) and without lightning
(right panels). Contour and shading values are 0.05, 0.1, 0.5, I, 5, and 10 percent. Bin
sizes are 10 K and 5 K for the abscissa and ordinate, respectively, in (a) and (b), and 1 km
for the abscissa and ordinate in (c)-(f).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
74
features at the colder brightness temperature extremes (85 PCT < 120 K; 37 PCT < 220 K)
than South America. The majority (57%) of Africa and South America features without
lightning have minimum 85 GHz PCT > 250 K and minimum 37 GHz PCT > 270 K.
There is, however, considerable overlap in the brightness temperature parameter space
between systems with and without lightning.
Africa and South America lightning features have very similar maximum 20- and 30
dBZ echo heights with 30 dBZ echo heights reaching at least 5 km in both regions for
lightning to occur (Figure 24c).
However, Africa has a greater fraction o f lightning
features with deep (> 8 km) 40 dBZ echo than South America (Figure 24e).
Of the
systems without lightning, the maximum 20- and 30 dBZ echo tops reach similar height
extremes (Figure 24d), although Africa systems have slightly deeper 30 dBZ reflectivity
for a given 20 dBZ height above about 7 km than those in South America. A similar
though less pronounced trend occurs with 40 dBZ echo heights among the no-lightning
systems (Figure 24f). There is a considerable overlap in the reflectivity parameter space
between features with and without lightning (cf. Figures 24c,d).
For instance,
approximately the same fractions (78%) of continental features with and without lightning
have 20 dBZ echo heights from 6-15 km and 30 dBZ heights ranging from 5-14 km.
The relative frequency distributions of west Pacific and east Pacific features with and
without lightning are shown in Figure 25. The east Pacific lightning features are indicated
by “plus” symbols due to the small sample size. The majority of east and west Pacific
lightning features occupy similar portions of the brightness temperature parameter space
(Figure 25a), although a small fraction of west Pacific lightning features occur at the
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
75
300
300
i
250
cl
CT
250
200
150
50
50
100 150 200 250 300
Minimum 85-GHz PCT (K)
20
_
100 150 200 250 300
Minimum 85-GHz PCT (K)
20
1
15
10
5
0
0
0
10
15
20
5
Maximum 30 dBZ Height (km)
5
10
15
20
Maximum 30-dBZ Height (km)
_ 20
20
1
15
10
5
0
0
Figure 25.
10
20
5
15
Maximum 40 dBZ Height (km)
Maximum 40-dBZ Height (km)
Relative frequency distribution of precipitation features over east Pacific
(contoured) and west Pacific (shaded) with lightning (left panels) and without lightning
(right panels). Contour and shading values are 0.05, 0.1, 0.5, I, 5, and 10 percent. East
Pacific features with lightning are indicated by “plus” symbols due to the small sample
size. Bin sizes are 10 K and 5 K for the abscissa and ordinate, respectively, in (a) and (b),
and 1 km for the abscissa and ordinate in (c)-(f).
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
76
lower Tb extremes. The obvious differences between oceanic and continental systems
with lightning are that systems over land occupy broader brightness temperature ranges
and attain a greater degree o f ice scattering (lower 85- and 37 GHz PCT) than their
oceanic counterparts (cf. Figure 24a).
The east and west Pacific features without lightning have nearly identical brightness
temperature distributions (Figure 25b).
A comparison with Figure 24b shows that
continental and oceanic features without lightning have similar brightness temperature
characteristics with the ocean features skewed slightly toward colder brightness
temperatures relative to those over land.
With little exception, oceanic features with and without lightning have similar 20 dBZ
echo tops, extending to 16-17 km (Figures 25c,d). This is a different relationship than
among the continental features where the lightning and no-lightning features are
distinguished by a 3 km difference in the 20 dB echo height extremes (see Figures 24c,d).
A slightly higher 30 dBZ echo (6 km) is required for lightning over the oceans compared
with systems over land. West Pacific systems without lightning tend to have deeper 30
dBZ reflectivity for a given 20 dBZ height than those in the east Pacific (Figure 25d).
East and west Pacific lightning features generally have 40 dBZ echo heights below 7 km
(Figure 25e), significantly lower than the continental lightning features (see Figure 24e).
Among the oceanic features without lightning (Figure 25f), the east Pacific generally has
shallower 40 dBZ reflectivity as well as the majority of ocean features containing no 40
dBZ echo.
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77
Figure 26 indicates that the lightning probabilities for continental features are non-zero
in virtually every part of the 85- and 37 GHz brightness temperature parameter space.
The lightning probabilities increase rapidly from around 10-15% for 85 GHz PCT > 250
K, to greater than 50% for 85 GHz PCT around 200 K, and 80-100% for 85 GHz PCT <
170 K. Also, the lightning probabilities appear more sensitive to changes in 37 GHz PCT
than 85 GHz PCT. For example, for features with 85 GHz PCT between 210-220 K, the
lightning probabilities increase from 29% to 91% as the 37 GHz PCT decrease from about
280 K to 255 K. For a fixed 37 GHz PCT, say 260-265 K, the lightning probabilities
show a similar increase, but for 85 GHz PCT decreasing from 240 K to 140 K. This is
consistent with the difference in Mie scattering effects at these two frequencies. At 37
GHz, the wavelength (~8 mm) is more than twice that at 85 GHz such that the ice
scattering response at 37 GHz requires larger graupel or hail particles than at 85 GHz.
The lightning probabilities for oceanic features in much of the brightness temperature
parameter space are near zero.
Lightning probabilities reach 10% or greater only for
features with minimum 85 GHz PCT < 180 K and minimum 37 GHz PCT < 265 K. The
greatest oceanic lightning probability (50%) occurs near the oceanic minimum Tb
extremes.
A comparison of continental and oceanic lightning probabilities shows the
striking result that land and ocean systems with very similar 85- and 37 GHz signatures
have very different lightning probabilities.
The differences are typically an order of
magnitude or larger in regions of the brightness temperature parameter space (150 K< 85
GHz PCT < 250 K; 250 K < 37 GHz PCT < 280 K) containing similar sample sizes of
continental and oceanic features.
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78
300
10
280
0
100
20
0
26
0
2
0
10
1
0
20
0
5
0
4
0
3
0
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0
3
20
2
36
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27
0
9
2
47
1
36
o
43
0
28
2
37
0
17
0
11
A
A
42
0
41
2
53
17
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5
2
0
54
0
44
0
23
0
24
0
14
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0
0
39
0
30
31
a
43
0
73
2
71
8
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i
54
1
64
2
66
1
71
15
87
8
78
1
84
3
71
0
72
78
38
92
5
88
10
91
14
89
8
90
0
83
92
92
91
26
100 100
23
93
27
95
19
93
17
95
88
83 100
13 38
96 100
13
97
96 100
80
94 100 100 100
83
92 100 100
0
10
0
27
9
85
33
98
0
0
29
90
50
100 100
36
0
8
93
100
E
3
| 240
33
0
27
a
82
4
91
I- 260
O
a.
38
a
0
3
A
W
95 100 100
100 100 100 100 100 100 100
100
220
92
93 100
100 100 100 100
100
80
100 100 100 100
100 100 100
100
200
100
100
150
200
Minimum85 GHz PCT (K)
250
300
Figure 26. Fraction of oceanic (upper) and continental (lower) precipitation features with
lightning. Bin sizes are 10 K on the abscissa (minimum 85 GHz PCT) and 5 K on the
ordinate (minimum 37 GHz PCT). Bins containing fewer than five precipitation features
are not plotted.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
79
Figure 27 shows that the lightning probabilities for continental features are 10% or
greater when the 20 dBZ echo heights exceed 7 or 8 km and the 30 dBZ echo heights are
at least 6 km. The vast majority (> 80%) of continental features with deep (> 12 km) 20
dBZ echo and deep (> 9 km) 30 dBZ echo have lightning. The lightning probabilities
appear more sensitive to changes in 30 dBZ rather than 20 dBZ heights. For instance,
with the 30 dBZ echo height Fixed at 7-8 km, the fraction of continental systems with
lightning increases from 12-52% as the 20 dBZ echo increases from 7 to 12 km. With 20
dBZ heights fixed at 12-13 km, the lightning probability increases from 16-93% as the 30
dBZ depth increases from 6-11 km.
In contrast to land, the lightning probabilities over the oceans are near zero for systems
with 30 dBZ heights below 7 km and are generally less than 25% for features with deeper
30 dBZ reflectivity. Figure 27 also indicates very clearly that land systems consistently
have much greater lightning probabilities than ocean features with the same reflectivity
heights.
The land/ocean lightning probability differences are roughly an order-of-
magnitude in much of the parameter space and decrease toward the reflectivity height
extremes.
Figure 28 presents a comparison of the median vertical reflectivity structures of land
and ocean lightning and no-lightning systems that have similar 85 GHz ice scattering
signatures. Table 6 lists the corresponding sample sizes and lightning statistics. In each
85 GHz PCT range, the continental lightning profile has the greatest reflectivity values
and smallest decreases in reflectivity with height (reflectivity lapse rate). At the warmer
Tb range (250-260 K; Figure 28a), the land lightning/no-lightning reflectivity difference is
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80
I-"
100
20
100
0
0
0
5
0
98 100
50
94
97
0
23
2
47
8
76
20
87
15
93
64
97
92 10O
0
16
1
52
5
74
11
82
20
93
88
0
6
0
27
2
52
3
72
16
83
77
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6
1
9
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2
49
4
73
0
71
0
3
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34
7
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95
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7
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14
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3
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71
95 100
100
10
Maximum30 dBZ Height (km)
92
15
20
Figure 27. Fraction of oceanic (upper) and continental (lower) precipitation features with
lightning. Bin sizes are 1 km on the abscissa (maximum 30 dBZ height) and the ordinate
(maximum 20 dBZ height). Bins containing fewer than Five precipitation features are not
plotted.
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81
20
20
- ■ Land w/ lightning
— _ Land no lightning
— — Ocean no lightning
15
. Land w/ lightning
— Land no lightning
Ocean w/ lightning
— — Ocean no lightning
—
15
E
JC
10
g>
10
<D
X
5
5
0
0
20
30
40
50
MaximumReflectivity (dBZ)
60
20
30
40
50
MaximumReflectivity (dBZ)
20
15
2
o>
— — Land w/ lightning
— — Land no lightning
Ocean w/ lightning
— — Ocean no lightning
10
®
X
5
0
20
30
40
50
60
Maximum Reflectivity (dBZ)
Figure 28.
Median vertical reflectivity profiles for three subsets of land and ocean
precipitation features selected from the following ranges of minimum 85 GHz PCT: 250260 K (a), 200-210 K (b), and 150-160 K (c). Land profiles are indicated with bold lines.
Profiles for lightning features are indicated with solid lines.
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 6. Sample sizes and lightning statistics for three subsets of precipitation features selected by the
minimum 85 GHz PCT. Regions are Africa (AF), South America (SA), west Pacific (WP), and east Pacific
(EP).
250 < 85 PCT < 260 K
200 < 85 PCT < 210 K
150 < 85 PCT < 160 K
Region
Total
% w/
lightning
Flash
total
Total
% w/
lightning
Flash
total
Total
% w/
lightning
Flash
total
AF
555
12
113
207
63
521
142
92
1386
SA
444
10
83
196
64
485
111
95
1393
Land
999
11
196
403
63
1006
253
94
2789
WP
547
0
0
242
1
4
70
10
21
EP
380
0
0
109
0
0
32
22
35
Ocean
927
0
0
351
< 1
4
102
14
56
83
10 dBZ or greater above the freezing level (~5 km). For the colder 85 GHz PCT intervals,
the difference is typically 4-7 dBZ. In each Tb interval, the reflectivity lapse rates above
the freezing level are roughly 4 dBZ km '1 for continental lightning features and 5-6 dBZ
km*1 for the no-lightning features.
The reflectivity lapse rates are similar for the continental and oceanic lightning
features in Figure 28c, with slightly (2-4 dBZ) weaker reflectivity values for the oceanic
cases. The reflectivity profiles for land and ocean features without lightning are very
similar above the freezing level in each 85 GHz PCT interval. It is also notable that the
median profiles in Figure 28b and Figure 28c converge to similar 20 dBZ echo heights.
This is indicative of the 85 GHz ice scattering response to ice particles in the upper cloud
levels and it suggests that the total reflectivity depth of strong convective systems alone is
not a good discriminator between continental and oceanic systems with or without
lightning.
Figure 29 presents distributions of precipitation feature flash rates relative to the
minimum 85 GHz PCT and maximum 7 km reflectivity.
distributions using the minimum 37 GHz PCT.
Figure 30 shows similar
The 7 km level was chosen since it
corresponds to a temperature range (-10°C to -15°C) within the mixed phase region.
The lightning flash rates for the continental features tend to increase with decreasing
85 GHz PCT and increasing 7 km reflectivity (Figures 29a,b). The relationship is similar
for 37 GHz (Figures 30a,b), with the flash rates increasing significantly as the 37 GHz
PCT decreases below about 255 K. The peak storm flash rates are 367 m in'1 for Africa
and 221 m in'1 for South America. Not surprisingly, the majority of ocean features with
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84
100
150
200
250
Minimum 85 GHz PCT (K)
100
100
150
200
250
Minimum 85 GHz PCT (K)
150
200
250
Minimum 85 GHz PCT (K)
300
Figure 29. Distribution of precipitation features with lightning by flash rate relative to
the minimum 85 GHz PCT and maximum 7 km reflectivity for Africa (a), South
America (b), and combined west and east Pacific regions (c). The flash rate is
proportional to the symbol size.
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85
150
200
250
Minimum 37 GHz PCT (K)
300
150
200
250
Minimum 37 GHz PCT (K)
300
60
50
40
|
30
20
150
200
250
Minimum 37 GHz PCT (K)
300
Figure 30. Distribution of precipitation features with lightning by flash rate relative to
the minimum 37 GHz PCT and maximum 7 km reflectivity for Africa (a), South
America (b), and combined west and east Pacific regions (c). The flash rate is
proportional to the symbol size.
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86
lightning have relatively low flash rates (Figure 29c); the peak ocean flash rate is 26 m in'1.
Over land, the high flash rate features typically have 7 km reflectivities greater than
about 45 dBZ, 85 GHz PCT < 130 K, and 37 GHz PCT < 220 K with the greatest
frequency of these systems in Africa. The bulk of the continental and oceanic features
without lightning have 7 km reflectivities < 20 km along with 85 GHz PCT > 250 K
(Figures 31a,b) or 37 GHz PCT > 260 K (Figures 32a,b).
The small fraction of no­
lightning systems over land and ocean with 7 km reflectivities greater than 20 dBZ tend to
overlap with relatively low flash rate features in their respective regions. This suggests
that some of the features without lightning may have non-zero flash rates that are below
the LIS minimum detection level.
N
X
60
N 60
40
% 40
20
x 20
100 150 200 250 300
Minimum85 GHz PCT(K)
100 150 200 250 300
Minimum85 GHz PCT (K)
Figure 31. Frequency distribution of continental (a) and oceanic (b) precipitation features
without lightning. Bin size is 2 dBZ for the ordinate (maximum 7 km reflectivity) and 10
K for the abscissa (minimum 85 GHz PCT). Contour values are 1,5, 10, 25, 50, 75, 100,
125, and 150.
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87
N
co
2,
60
60
m
T3,
N
£> 50
.£>■ 50
0W 40
E
* 30
©
40
w
oo
0<D
E
^ 30
c
3
p
5
(a)
1<0 20
(b)
1 20
(0
150
200
250
300
Minimum37 GHz PCT(K)
150
200
250
300
Minimum37 GHz PCT(K)
Figure 32. Frequency distribution of continental (a) and oceanic (b) precipitation features
without lightning. Bin size is 2 dBZ for the ordinate (maximum 7 km reflectivity) and 5
K for the abscissa (minimum 37 GHz PCT). Contour values are 1.5, 10, 25, 50, 75, 100,
125, and 150.
Discussion
A comparison of the radar reflectivity echo height distributions for oceanic and
continental precipitation features indicate relatively small differences in the overall echo
depths (maximum 20 dBZ heights). Land and ocean features extend to similar heights
with land features reaching the extreme heights with slightly greater frequency than
oceanic features. There are more significant differences in relative system intensity as
measured by the heights of the higher reflectivity thresholds. For a given maximum 20
dBZ echo height, features over land typically have 30 dBZ and 40 dBZ echo heights that
extend several kilometers higher than those in oceanic systems (see Figure 15).
addition, nearly one-third of the oceanic features contain no 40 dBZ echo.
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In
The
88
differences in reflectivity structure indicate that continental precipitation features are
typically more capable of lofting larger hydrometeors (including supercooled rain, frozen
drops, and millimeter-sized graupel) to higher altitudes than most oceanic features.
Others have also documented substantial differences between the reflectivity structures of
continental and oceanic storms (Rutledge et al. 1992; Williams et al. 1992; Zipser and
Lutz 1994). These finding are consistent with well-documented characteristically weak
updraft magnitudes of oceanic storms (Zipser and LeMone 1980; Jorgensen and LeMone
1989; Lucas et al. 1994).
The relatively weak oceanic updrafts have a two-fold effect in terms of the observed
reflectivity structure. First, the bulk of the hydrometeor mass falls out without particles
being carried sufficiently above the freezing level to grow by accretion processes.
Secondly, the weak updrafts limit the supply of supercooled liquid water in the mixed
phase portion of the cloud such that the many small hydrometeors are competing for the
available supercooled liquid water. Jorgensen et al. (1985) find that sharp reflectivity
decreases above the freezing level in hurricane updrafts are coincident with decreases in
the amount of supercooled liquid water above that level, which limits the potential for
particle growth, and they attribute this to the observed weak updrafts.
The observed reflectivity differences between continental and oceanic precipitation
features are consistent, at least in a bulk sense, with the distributions of minimum 85- and
37 GHz brightness temperatures. The most obvious and striking discrepancy is the fact
continental features reach lower 85- and 37 GHz brightness temperature extremes than
ocean features (Figure 17). This result is not particularly unique as the land bias in 85
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89
GHz ice scattering intensity has been previously documented (Mohr and Zipser 1996a,b;
Zolman et al. 2000). However, a more subtle but nonetheless important finding is that
features over land typically have lower 37 GHz PCT for a given 85 GHz PCT than
oceanic features, particularly where appreciable 85 GHz ice scattering is indicated (PCT <
250 K).
Thus, for a given degree of significant 85 GHz ice scattering, the 37 GHz
frequency is most likely responding to the presence of larger graupel or hail particles
present in the continental features. This is corroborated by the observed differences in the
30- and 40 dBZ echo heights between oceanic and continental storms indicating that the
larger graupel or hail particles are more likely to occur and extend to higher altitudes in
the continental cases.
Continental features typically have warmer 85 GHz PCT than oceanic features when
normalized by the radar echo heights. The brightness temperature differences are usually
5-15 K across much of the echo height parameter space, particularly where the minimum
85 GHz PCT are below 250 K indicating significant 85 GHz ice scattering.
Such a
consistent relationship is difficult to explain unless one considers the effect of supercooled
cloud liquid water on the measured 85 GHz brightness temperatures. Several modeling
studies have demonstrated that the presence of cloud liquid water above the freezing level
acts an emission source that can partially mask the effects of 85 GHz ice scattering on
upwelling radiation (Adler et al. 1991; Vivekanandan et al. 1991; Smith et al. 1992). The
net result is that the model retrieved 85 GHz brightness temperatures are warmer than
would otherwise be observed. Since the cloud water droplets are too small to contribute
to reflectivity at the PR wavelength, it is reasonable to infer that a greater supercooled
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90
liquid water content aloft in the continental systems would yield higher 85 GHz Tb than
ocean systems with similar reflectivity heights.
When normalized by the 37 GHz brightness temperatures, particularly where
significant 37 GHz ice scattering is occurring (PCT < 255 K), land systems extend to
much higher 85 GHz brightness temperatures than the ocean systems. The higher 85 GHz
Tb may be indicative of a smaller optical depth at this frequency, which can result from a
shallower layer of ice aloft, the presence of lower density ice particles, a smaller
concentration of scatterers, or combinations of these factors. Here again we suggest that it
is possible for the higher 85 PCT to be related to the presence o f greater supercooled
liquid water contents in the mixed phase regions o f these land systems. The 37 GHz ice
scattering signature suggests that the land and ocean systems have sufficient quantities of
cloud liquid water above the freezing level to grow large graupel. However, cloud liquid
water may extend through a deeper vertical layer and in greater quantities in the land
systems.
As with radar reflectivity and microwave brightness temperatures, lightning in
precipitating systems is inherently related to cloud microphysics.
Laboratory evidence
strongly suggests that non-inductive collisions involving millimeter-sized graupel and
small ice crystals in the presence of supercooled water is a viable mechanism underlying
robust cloud electrification (Reynolds et al. 1957; Takahashi 1978; Jayaratne et al. 1983;
Williams 1989; Saunders et al. 1991). The laboratory results are corroborated by in situ
and radar observations that show the coexistence of large graupel, numerous small ice
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91
particles, and supercooled liquid water in the mixed phase region of highly electrified
storms (Dye et al. 1986, 1988; Willis et al. 1994; Keenan et al. 2000).
The relationships between reflectivity, brightness temperatures, and lightning for the
continental and oceanic features reveal robust trends that can be at least partly understood
in terms of cloud microphysics. For instance, the lightning flash rates generally increase
with decreasing 85- and 37 GHz PCT and increasing 7 km reflectivity. The 7 km level
corresponds to -10°C to -15°C, within the mixed phase region where non-inductive
charge separation would likely occur. Others have noted that rapid cloud electrification in
midlatitude thunderstorms is associated with the presence of millimeter-sized graupel and
high reflectivity (35-40 dBZ) above 6 km (-10°C) (Dye et al. 1986; 1988; 1989). The
relationship between flash rate and the 85- and 37 GHz PCT indicates the microwave
frequencies are responding to an increase in the number and size of liquid and frozen
particles in the mixed phase region of the storm, which is conducive to vigorous charge
separation.
It was noted that it is possible for a continental feature to have lightning almost
without regard to the 85- or 37 GHz brightness temperatures, while oceanic features have
a near zero lightning probability (or are weakly electrified) without significant 85 GHz ice
scattering (PCT < 180 K) occurring. Also, continental features consistently show much
greater lightning probabilities than oceanic features with very similar brightness
temperatures (Figure 27) or very similar reflectivity heights (Figure 28). The lightning
probability difference for land and ocean systems with similar reflectivity heights is, at
first consideration, non-intuitive.
One possible explanation, apart from microphysical
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92
differences between the land and ocean features, has to do convective area. That is, one
would expect the probability of lightning to have some relationship to the size and/or
number of convective cores (i.e., area of deep reflectivity) in a system. It remains to be
seen whether such a relationship exists in the current data set between the land and ocean
features to help explain the different lightning probabilities.
The lightning probability differences for land and ocean storms with similar brightness
temperatures can be at least partially understood by considering that the magnitude of ice
scattering is largely determined by the characteristics o f the ice layer. Sim ilar brightness
temperatures can result from ice layers with quite different depths, ice particle densities,
size distributions, shapes, and number densities. The characteristics of the ice layer that
give similar brightness temperatures may be conducive to charge separation in one system
but not in another.
The vertical profiles of reflectivity provide additional information to the lightning
probability/brightness temperature relationships.
For the same degree of 85 GHz ice
scattering, continental features with lightning tend to have greater reflectivity values and
smaller decreases of reflectivity with height above the freezing level than ocean features
with lightning. Continental and oceanic features without lightning are characterized by
the weakest vertical reflectivity profiles. These relationships can be interpreted in terms
of the cloud liquid water content supplied to the mixed phase regions o f these systems,
which is largely governed by updraft velocity and the number of cloud condensation
nuclei available.
It is well known that maritime air masses contain far fewer cloud
condensation nuclei (CCN) concentrations than continental air masses (Pruppacher and
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93
Klett 1997). For the ocean clouds, this translates to a greater precipitation efficiency, a
shorter residence time for cloud water droplets, and a lower cloud liquid water content
than continental clouds for a given vertical velocity.
Stronger updrafts will also result in a greater cloud liquid water content supplied to the
mixed phase region of the cloud. The supercooled water contributes to graupel growth
through accretion and is a critical component of robust charge separation via the noninductive ice-ice collision mechanism.
While the supercooled cloud droplets are not
contributing to the reflectivity signature, larger (millimeter-sized) supercooled raindrops,
sustained aloft by the updrafts, are highly reflective and together with the growing graupel
yield larger reflectivity values above the freezing level and smaller decreases of
reflectivity with height than in systems without lightning.
While we have no direct
information about updraft velocities in this data set, the fact that a small fraction of
oceanic systems have lightning suggests that these systems have updraft magnitudes
vigorous enough to supply sufficient supercooled liquid water to the mixed phase region
for robust charge separation to occur. However, these processes are much more efficient
in the continental lightning features.
Most of the precipitation features over Africa and South America reflectivity
signatures, 85- and 37 GHz signatures, and lightning flash rates. However, Africa tends to
have a greater occurrence of intense features at the brightness temperature and reflectivity
extremes, which tend to have high flash rates. The occurrence o f these intense systems
over Africa may very well account for the observed regional differences in lightning flash
count and flash density.
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Summary and Conclusions
The bulk radar reflectivity structures and passive microwave 85- and 37 GHz
brightness temperature signatures are presented for a large sample of continental and
oceanic precipitation features with and without lightning from four regions in the Tropics:
Africa, South America, east Pacific, and west Pacific.
The particular focus was on
precipitation features with appreciable rainfall, which account for the bulk of the total
rainfall and lightning flash density in their respective regions. Single value parameters
(minimum 85- and 37 GHz brightness temperature, maximum height of threshold
reflectivities, and lightning flash rate) are used to compare the relative strengths of land
and ocean systems.
Continental features tend to have stronger vertical reflectivity profiles (i.e., larger
magnitudes of reflectivity and smaller decreases in reflectivity with height) than oceanic
features. That is, for a given 20 dBZ echo height the layer of 30- and 40 dBZ echo is
deeper in continental than in ocean features. In fact, 30% of the oceanic systems have no
40 dBZ echo, compared with 10% of the systems over land. The weak ocean reflectivity
profiles are consistent with characteristically weak vertical velocities in oceanic storms,
which are insufficient to sustain large liquid or frozen hydrometeors much above the
freezing level (LeMone and Zipser, 1980; Jorgensen and LeMone 1989; Lucas et al.
1994).
The trends in the bulk reflectivity profiles are also consistent with the observed 85and 37 GHz brightness temperature distributions. The continents tend to have a greater
fraction of precipitation features with colder brightness temperatures than the ocean
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95
regions. For a given degree of 85 GHz ice scattering, the continental features typically
have colder minimum 37 GHz PCT than the ocean features, indicating the presence of
larger graupel or hail in the continental systems.
For a given degree of significant 37 GHz ice scattering (PCT < 255 K), continental
and oceanic features have similar modal 85 GHz signatures, with the land systems
extending to much warmer 85 GHz PCT.
Also, when normalized by the radar echo
heights, the minimum 85 GHz PCT for continental features are consistently 5-15 K
warmer than the ocean features. It can be inferred in both instances that the brightness
temperature differences are a result of greater cloud liquid water contents above the
freezing level in the continental features, which acts as an emission source to partially
mask the 85 GHz ice scattering effects.
The lightning distributions are strongly land-biased, as other studies have shown
(Orville and Hendersen 1986; Goodman and Christian 1993; and others), with flash
densities differing by as much as two orders of magnitude between land and ocean. When
the precipitation feature lightning characteristics are considered in relation to the radar
reflectivity and brightness temperature signatures, the following relationships emerge:
(I) when normalized by minimum brightness temperatures, continental features
consistently have much higher lightning probabilities than features over the
oceans. It is inferred that the difference is related in part to greater supercooled
liquid water contents in the continental systems;
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96
(2) when normalized by radar echo heights, continental features consistently have
much higher lightning probabilities than oceanic features, which may be related to
a greater convective area in the continental features;
(3) for a given degree of 85 GHz ice scattering, features with lightning have larger
reflectivity values and smaller decreases of reflectivity with height above the
freezing level than features without lightning;
(4) lightning flash rates generally increase with decreasing 85- and 37 GHz brightness
temperatures and increasing mid-level reflectivity;
(5) there are varying amounts of overlap in the reflectivity height and brightness
temperature parameter space between features with and without lightning, but
typically with the lower flash rate features. Some of the features without lightning
may actually have non-zero flash rates if viewed longer.
The observed relationships between lightning, radar and the 85- and 37 GHz ice
scattering signatures of land and ocean precipitation features are generally consistent with
what is known or hypothesized about the differences in land and ocean lightning
occurrence. The remotely sensed TRMM observables do not provide direct microphysics
information and the inferences based on the observations are best addressed through the
use of numerical cloud model and radiative transfer model techniques.
By the same
token, the data presented here represent an extensive quantitative observational framework
that can be used to help provide constraints on cloud model input to radiative transfer
models and validation for radiative transfer model results.
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97
CHAPTER IV
CONCLUSIONS
The first part of this study presented a systematic comparison of the distribution of
SSM/I 85 GHz MCSs and OTD lightning in the Tropics over three-month seasonal
periods from June 1995 through May 1996 in order to document the observed relationship
between 85 GHz ice scattering signatures and lightning. In each of the four periods, there
is a much stronger land bias among the lightning clusters than among the MCSs. For a
given three-month period the land, ocean, and mixed land and ocean regions contain 5666%, 15-21%, and 19-26%, respectively, of the total cluster population, whereas the same
regional groupings contain 37-41%, 40-45%, and 17-22% of the total MCS population.
Generally, among land regions MCSs with moderate/strong ice scattering (85 GHz PCT <
190 K) and moderate to high flash rate lightning clusters (four or more flashes) are both
numerous with Tropical Africa tending to dominate the land and ocean regions in terms of
ice scattering intensity and lightning flash rates. The implication is that strong convective
systems with low 85 GHz PCT are the primary lightning producers in the Tropics and that
the observed land bias in the lightning data is simply due to a greater fraction of these
strong systems over land.
The ice scattering and lightning relationship appears to be different over the oceans,
however. Oceanic MCSs with moderate to strong ice scattering occur with much greater
frequency than the moderate to high flash rate lightning clusters. It is inferred that for a
given range of 85 GHz brightness temperatures the continental MCS will have a much
greater probability of lightning than its oceanic counterpart and this is related to
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98
previously document differences in vertical updraft velocities and supercooled liquid
water content, and thus the microphysics profiles in the mixed phase region (0°C > T > 40°C), between continental and oceanic systems.
The second part of the study examines the bulk radar reflectivity structures, passive
microwave 85- and 37 GHz brightness temperature signatures, and lightning flash rates
for a broader spectrum of tropical continental and oceanic precipitating systems. The
particular focus was on precipitating systems with appreciable rainfall.
Single value
parameters (minimum 85- and 37 GHz brightness temperature, maximum height o f
threshold reflectivities, and lightning flash rate) are used to compare the relative strengths
of land and ocean systems.
Continental features tend to have stronger vertical reflectivity profiles (i.e., larger
magnitudes of reflectivity and smaller decreases in reflectivity with height) than oceanic
features. That is, for a given 20 dBZ echo height the layer of 30- and 40 dBZ echo is
deeper in continental than in ocean features. The weak ocean reflectivity profiles are
consistent with characteristically weak vertical velocities in oceanic storms, which are
insufficient to sustain large liquid or frozen hydrometeors much above the freezing level
(LeMone and Zipser, 1980; Jorgensen and LeMone 1989; Lucas et al. 1994).
The trends in the bulk reflectivity profiles are also consistent with the observed 85and 37 GHz brightness temperature distributions. The continents tend to have a greater
fraction of precipitation features with colder brightness temperatures than the ocean
regions. For a given degree o f 85 GHz ice scattering, the continental features typically
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
have colder minimum 37 GHz PCT than the ocean features, indicating the presence of
larger graupel or hail in the continental systems.
When normalized by the radar echo heights, the minimum 85 GHz PCT for
continental features are consistently 5-15 K warmer than the ocean features. It can be
inferred that the brightness temperature differences are a result of greater cloud liquid
water contents above the freezing level in the continental features, which acts as an
emission source to partially mask the 85 GHz ice scattering effects.
The lightning distributions are strongly land-biased, as other studies have shown
(Orville and Hendersen 1986; Goodman and Christian 1993; and others), with flash
densities differing by as much as two orders of magnitude between land and ocean. When
the precipitation feature lightning characteristics are considered in relation to the radar
reflectivity and brightness temperature signatures, the following relationships emerge:
(1) when normalized by minimum brightness temperatures, continental features
consistently have much higher lightning probabilities than features over the
oceans. It is inferred that the difference is related in part to greater supercooled
liquid water contents in the continental systems;
(2) when normalized by radar echo heights, continental features consistently have
much higher lightning probabilities than oceanic features, which may be related to
a greater convective area in the continental features;
(3) for a given degree of 85 GHz ice scattering, features with lightning have larger
reflectivity values and smaller decreases of reflectivity with height above the
freezing level than features without lightning;
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100
(4) lightning flash rates generally increase with decreasing 85- and 37 GHz brightness
temperatures and increasing mid-level reflectivity;
(5) there are varying amounts o f overlap in the reflectivity height and brightness
temperature parameter space between features with and without lightning, but
typically with the lower flash rate features. Some of the features without lightning
may actually have non-zero flash rates if viewed longer.
The observed relationships between lightning, radar and the 85- and 37 GHz ice
scattering signatures of land and ocean precipitation features are generally consistent with
what is known or hypothesized about the differences in land and ocean lightning
occurrence.
W hile the remotely sensed TRMM observables do not provide direct
microphysics information, the inferences based on the observations are best addressed
through the use of numerical cloud model and radiative transfer model techniques. By the
same token, the data presented here represent an extensive quantitative observational
framework that can be used to help provide constraints on cloud model input to radiative
transfer models and validation for radiative transfer model results.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
101
REFEREN CES
Adler, R. F., H-Y. M. Yeh, N. Prasad, W-K. Tao, and J. Simpson, 1991: Microwave
simulations of a tropical rainfall system with a three-dimensional cloud model. J.
Appl. Meteor., 30, 924-953.
Battan, L. J., 1959: Radar Meteorology. The Chicago University Press, 161 pp.
Black, R. A., and J. Hallett, 1986: Observations of the distribution of ice in hurricanes. J.
Atmos. Sci., 43, 802-822.
Black, R. A., and J. Hallett, 1999: Electrification of the hurricane. J. Atmos. Sci.. 56,
2004-2028.
Boccippio, D. J., S. J. Goodman, and S. Heckman, 2000a: Regional differences in tropical
lightning distributions. J. Appl. Meteor, (in press).
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APPENDIX A
OTD LIGHTNING CLUSTER AND SSM/I MCS FREQUENCY DISTRIBUTIONS
The tables in this appendix are the quantitative complement to Figures 3-10 in the text.
For each three-month period of study, the tables contain the frequency and cumulative
frequency of either the lightning clusters by flash count or the MCSs by minimum 85 GHz
PCT for each of the geographic regions.
Also included are the spatial densities of
lightning clusters and MCSs as well as the ratios of clusters-to-MCSs. The cluster data
for Tropical South America, Subtropical South America, the Tropical Atlantic, and South
Atlantic have been omitted from the OTD cluster tables since the lightning data for these
four regions are compromised by the S AA (see text).
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115
T able A -1 .
June-July-A ugust (JJA) cluster flash count by frequency (top) and cum ulative percent
(bottom , b old ), cluster density (x lO 5 km'2), and cluster-to-M C S ratio. R egion s are sorted by the fraction
o f clusters in the 1-3 flash colu m n . R egions w ith a sm all sam ple size (few er than 2 0 0 clusters) are below
the bold line.
Cluster flash count
Cluster
Region
1- 3
4 -2 4
2 5 -7 5
>75
density
Cluster/MCS
Tropical Africa
3383
1517
341
105
28
3
63%
92%
98%
100%
2739
1104
211
64
34
3.1
67%
93%
98%
100%
3200
1251
185
45
26
2.4
68%
95%
99%
100%
844
292
44
11
40
2.5
71%
95%
99%
100%
2165
676
82
10
13
1.3
74%
97%
100%
100%
154
41
7
4
14
1.9
75%
95%
98%
100%
304
70
7
1
2
0.4
80%
98%
100%
100%
325
72
0
1
4
1.7
82%
100%
100%
100%
422
78
7
2
2
1.1
83%
98%
100%
100%
535
94
13
1
3
1.2
83%
98%
100%
100%
430
70
11
0
3
0.5
84%
98%
100%
100%
422
50
I
0
2
0.3
89%
100%
100%
100%
84
15
1
0
2
0.5
84%
99%
100%
100%
81
10
0
0
1
0.5
89%
100%
100%
100%
1
0
0
0
0
0.3
100%
100%
100%
100%
North America
India and East Asia
Central America
Maritime Continent
South Indian
East Pacific
North Atlantic
North Pacific
South Pacific
Tropical Indian
Central Pacific
Subtropical Africa
Australia
Madagascar
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
116
T able A -2 . June-July-A ugust (JJA) M C S m inim um 85 G H z PCT (K) by frequency (top) and cu m u lative
percent (bottom , bold ), M C S density (xlO '5 km'2), and 10* percentile m inim um P C T . R egion s are sorted
by the 10* percentile m inim um PC T . R egion s with a sm all sam ple size (few er than 2 0 0 M C Ss) are listed
below the bold line.
Reeion
Tropical Africa
North America
Central America
Tropical S. America
India and East Asia
North Atlantic
Tropical Atlantic
North Pacific
Maritime Continent
Central Pacific
South Pacific
East Pacific
Tropical Indian
Sub. South America
South Indian
225-1 9 1
Minimum PCT (K)
190-156
155-121
< 120
MCS
densitv
10* Percentile
min PCT (K)
9
114
7
125
16
132
6
132
11
134
2
137
7
149
2
154
10
156
405
582
538
238
23%
56%
87%
100%
439
480
300
108
33%
69%
92%
100%
163
189
112
21
34%
73%
96%
100%
293
243
102
58
42%
77%
92%
100%
845
649
360
103
43%
76%
95%
100%
115
77
41
7
48%
80%
97%
100%
578
292
102
27
58%
87%
97%
100%
226
166
44
7
51%
88%
98%
100%
1203
807
199
18
54%
90%
99%
100%
787
578
101
8
7
159
53%
93%
99%
384
104
46
100%
2
2
159
72%
91%
100%
100%
515
346
59
3
4
161
56%
93%
100%
100%
584
300
56
7
6
162
62%
93%
99%
100%
44
19
0
1
147
54%
78%
18
100%
100%
83
18
10
0
1
156
75%
91%
100%
100%
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
117
T able A -2. C ontinued.
Minimum PCT (K)
Region
South Atlantic
Australia
Madagascar
Subtropical Africa
MCS
10UlPercentile
2 25-191
1 9 0 - 156
155-121
< 120
density
min PCT (K)
97
14
7
0
1
178
82%
94%
100%
100%
52
8
I
0
1
188
85%
98%
100%
100%
3
0
0
0
0.2
199
100%
100%
100%
100%
3
0
0
0
0
204
100%
100%
100%
100%
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
118
T ab le A -3 .
Septem ber-O ctober-N ovem ber (S O N ) cluster flash cou n t by frequency (top) and cum ulative
p ercen t (bottom , bold), clu ster density (xlO '5 km'2), and cluster-to-M C S ratio. R egions are sorted by the
fraction o f clusters in the 1-3 flash colum n. R egion s with a sm all sam p le size (few er than 2 0 0 clusters) are
below the bold line.
Cluster
Cluster flash count
Region
Tropical Africa
Australia
North America
India and East Asia
South Indian
Central America
Subtropical Africa
Maritime Continent
North Atlantic
South Pacific
East Pacific
North Pacific
Tropical Indian
Central Pacific
Madagascar
1 -3
4 - 24
25 - 75
>75
density
Cluster/MCS
35
3.7
13
6.5
16
2.3
12
2.6
2
3.2
29
2.1
27
7.5
18
1.5
7
1.5
2
0.9
1
0.3
3
0.9
2
0.3
2
0.3
7
5.2
4259
1801
395
107
65%
92%
98%
100%
943
297
87
36
69%
91%
97%
100%
1356
428
95
26
71%
94%
99%
100%
1579
552
70
13
71%
96%
99%
100%
248
62
14
9
74%
93%
97%
100%
655
202
23
3
74%
97%
100%
100%
1022
268
57
12
75%
95%
99%
100%
3018
857
85
14
76%
98%
100%
100%
594
139
20
5
78%
97%
99%
100%
381
81
10
3
80%
97%
99%
100%
219
38
6
0
83%
98%
100%
100%
609
93
5
2
86%
99%
100%
100%
235
32
1
0
88%
100%
100%
100%
334
32
0
0
91%
100%
100%
100%
78
58
7
2
54%
94%
99%
100%
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
119
T able A -4 . Septem ber-O ctober-N ovem ber (SO N ) M C S m inim um 85 G H z PCT (K ) by frequency (top ) and
cum ulative percent (bottom , bold), M C S density (xlCT5 km'2), and 10* percentile m inim um PCT. R egion s
are sorted by the 10* percentile m inim um PCT. R egions w ith a sm all sam ple siz e (few er than 2 0 0 M C S s)
are listed below the bold line.
Minimum PCT (K)
Region
Sub. South America
Tropical Africa
Tropical S. America
North America
India and East Asia
North Atlantic
Australia
Central America
North Pacific
Tropical Atlantic
Maritime Continent
Central Pacific
East Pacific
Tropical Indian
South Pacific
225 - 191
190- 156
1 5 5 - 121
MCS
10*Percei
< ion
density
min PCT
6
118
9
123
9
133
7
140
5
140
5
140
2
142
14
146
3
150
6
154
11
157
7
158
3
160
6
167
0.2
169
177
122
93
45
41%
68%
90%
100%
466
662
496
153
26%
63%
91%
100%
329
428
223
42
32%
74%
96%
100%
372
291
135
26
45%
80%
97%
100%
401
277
138
30
47%
80%
96%
100%
265
168
64
17
52%
84%
97%
100%
127
46
32
5
60%
82%
98%
100%
196
166
57
9
46%
85%
98%
100%
403
268
91
13
52%
87%
98%
100%
478
262
74
15
58%
89%
98%
100%
1449
919
225
20
55%
91%
99%
100%
761
488
112
5
56%
91%
100%
100%
472
246
57
7
60%
92%
99%
100%
620
234
48
3
69%
94%
100%
100%
366
112
24
2
73%
95%
100%
100%
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120
T able A -4 . C ontinued.
Region
Madagascar
South Indian
Subtropical Africa
South Atlantic
2 2 5 - 191
Minimum PCT (K)
190- 156
155-121
MCS
density
10th Percentile
1
130
I
136
min PCT (K)
11
79%
4
< 120
2
93%
100%
63
23
10
8
61%
83%
92%
100%
104
47
25
6
4
139
57%
83%
97%
98
38
25
100%
2
1
147
60%
83%
99%
100%
11
39%
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121
Table A-5.
December-January-February (DJF) cluster flash count by frequency (top) and cumulative
percent (bottom, bold), cluster density (xlO'5 km’2), and cluster-to-MCS ratio. Regions are sorted by the
fraction of clusters in the 1-3 flash column.
>75
Cluster
density
Cluster/MCS
297
94
25
3.8
92%
98%
100%
1301
505
128
56
20
3.2
65%
91%
97%
100%
259
104
22
3
2
6.5
67%
94%
99%
100%
459
168
38
14
34
1.8
68%
92%
98%
100%
198
76
6
1
9
1.7
70%
98%
100%
100%
1852
630
110
29
52
4.5
71%
95%
99%
100%
302
75
20
10
3
2
74%
93%
98%
100%
2857
802
95
14
17
1.2
76%
97%
100%
100%
532
159
3
0
3
0.6
77%
100%
100%
257
61
6
100%
2
1
0.8
79%
98%
99%
100%
421
83
15
3
2
1.2
81%
97%
99%
100%
424
86
8
4
5
1.8
81%
98%
99%
100%
231
49
5
0
2
0.8
81%
98%
100%
100%
883
184
19
7
4
0.9
81%
98%
99%
100%
454
49
2
2
3
0.5
90%
99%
100%
100%
Cluster flash count
4 -2 4
2 5 -7 5
Region
1 -3
Tropical Africa
3056
1326
64%
Australia
India and East Asia
Madagascar
Central America
Subtropical Africa
North America
Maritime Continent
Central Pacific
East Pacific
North Pacific
North Atlantic
South Indian
South Pacific
Tropical Indian
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122
Table A-6. December-January-February (DJF) MCS minimum 85 GHz PCT (K) by frequency (top) and
cumulative percent (bottom, bold), MCS density (xl0‘5 km 2), and 10* percentile minimum PCT. Regions
are sorted by the 10* percentile minimum PCT. Regions with a small sample size (fewer than 200 MCSs)
are listed below the bold line.
MCS
10*Percei
< 120
density
min PCT
117
65
6
119
71%
90%
100%
291
318
225
72
11
124
32%
67%
92%
100%
389
452
302
106
7
125
31%
67%
92%
100%
132
133
74
30
18
126
36%
72%
92%
100%
99
58
31
17
2
127
48%
77%
92%
100%
267
195
88
30
12
135
46%
80%
95%
100%
576
546
225
30
13
144
42%
81%
98%
100%
164
58
33
6
2
144
63%
85%
98%
100%
341
223
89
13
5
146
51%
85%
98%
100%
1610
1060
345
39
14
151
53%
87%
99%
100%
759
352
118
13
5
155
61%
89%
99%
100%
226
85
26
2
2
159
67%
92%
99%
100%
235
157
21
2
2
160
57%
94%
100%
100%
688
484
81
4
6
161
55%
93%
100%
100%
184
79
20
I
3
161
65%
93%
100%
100%
Minimum PCT (K)
Region
Australia
Sub. South America
Tropical Africa
Madagascar
North America
Subtropical Africa
Tropical S. America
South Atlantic
Tropical Atlantic
Maritime Continent
South Pacific
South Indian
East Pacific
Central Pacific
North Atlantic
225-191
190 - 156
155 - 12!
252
197
40%
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123
T able A -6 . Continued.
Minimum PCT (K)______________
Region
North Pacific
Tropical Indian
Central America
India and East Asia
2 2 5 - 191
190-156
155-121
316
99
34
< 120
2
70%
92%
100%
100%
650
340
58
2
62%
94%
100%
100%
92
57
17
3
54%
88%
98%
100%
44
11
4
1
73%
92%
98%
100%
MCS
10* Percentile
density
min PCT (K)
2
162
7
163
6
153
0.3
166
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124
Table A-7.
March-April-May (MAM) cluster flash count by frequency (top) and cumulative percent
(bottom, bold), cluster density (xlO'5 km'2), and cluster-to-MCS ratio. Regions are sorted by the fraction of
clusters in the 1-3 flash column. Regions with a small sample size (fewer than 200 clusters) are below the
bold line.
Reeion
India and East Asia
Tropical Africa
North America
South Indian
Madagascar
Maritime Continent
Australia
Central America
Subtropical Africa
North Pacific
East Pacific
South Pacific
Central Pacific
Tropical Indian
North Atlantic
1 -3
Cluster flash count
4-24
2 5 -7 5
> 75
Cluster
densitv
Cluster/MCS
17
4.1
32
3.5
8
2.6
3
1.2
12
1.5
20
2.2
3
1.5
24
3.3
11
3.4
1
0.8
3
0.9
3
0.8
3
0.4
4
0.8
1
1.5
1930
822
197
57
64%
92%
98%
100%
3979
1671
294
81
66%
94%
99%
100%
674
245
45
28
68%
93%
97%
100%
272
99
18
9
68%
93%
98%
100%
170
53
11
2
72%
94%
99%
100%
3267
1018
162
35
73%
96%
99%
100%
243
73
10
4
74%
96%
99%
100%
548
157
19
0
76%
97%
100%
100%
439
118
12
4
77%
97%
99%
100%
226
52
5
3
79%
97%
99%
100%
552
112
20
1
81%
97%
100%
100%
609
125
16
I
81%
98%
100%
100%
445
63
3
0
87%
99%
100%
100%
604
88
5
1
87%
99%
100%
100%
120
25
1
0
82%
98%
100%
100%
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125
Table A-8. March-April-May (MAM) MCS minimum 85 GHz PCT (K) by frequency (top) and cumulative
percent (bottom, bold), MCS density (xlO'5 km‘: ), and 10th percentile minimum PCT. Regions are sorted by
the 10lh percentile minimum PCT. Regions with a small sample size (fewer than 200 MCSs) are listed
below the bold line.
Minimum PCT (K)______________
Region
Tropical Africa
North America
Sub. South America
India and East Asia
Central America
South Indian
South Adantic
Tropical S. America
Maritime Continent
Australia
East Pacific
Tropical Atlantic
Tropical Indian
South Pacific
Central Pacific
MCS
10* Percentile
225 - 191
190-156
155 - 12!
< 120
density
401
593
509
199
9
118
24%
58%
88%
100%
133
109
90
45
3
118
35%
64%
88%
100%
122
145
81
39
5
120
32%
69%
90%
100%
278
201
168
81
4
120
38%
66%
89%
100%
75
80
53
11
1
137
34%
71%
95%
100%
179
92
39
17
2
139
55%
83%
95%
100%
164
66
47
4
2
139
58%
82%
99%
100%
601
465
169
24
11
145
48%
85%
98%
100%
1010
740
283
38
9
147
49%
85%
98%
100%
113
68
26
11
2
148
52%
83%
95%
100%
364
285
96
5
3
150
49%
87%
99%
100%
627
372
112
28
8
151
55%
88%
98%
100%
483
309
74
6
6
157
55%
91%
99%
100%
571
310
81
5
4
157
59%
91%
99%
100%
740
439
92
5
6
161
58%
92%
100%
100%
min PCT
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126
T able A -8. C ontinued.
Region
North Pacific
Madagascar
Subtropical Africa
North Atlantic
225-191
Minimum PCT (K)
190 - 156
155-121
10th Percentile
< 120
MCS
density
2
163
min PCT (K)
243
88
18
4
69%
94%
99%
100%
62
40%
55
31
8
139
76%
96%
6
100%
84
50
30
4
3
144
50%
80%
98%
100%
73
21
0
1
1
178
77%
99%
99%
100%
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APPENDIX B
FUNDAMENTALS OF ELECTROMAGNETIC RADIATION
The principles of electromagnetic radiation presented in this appendix are available in
many introductory textbooks including Kidder and Vonder Haar (1995). Electromagnetic
(EM) radiation consists of alternating electric and magnetic fields, which are oriented such
that the electric Field vector ( E ) is orthogonal to the magnetic field vector ( B ). EM
radiation propagates as a transverse wave with electric and magnetic field oscillations that
are orthogonal to the direction of propagation. Thus, in the right-hand coordinate system,
the curl of the electric and magnetic Field vectors ( E X B ) gives the direction of
propagation.
Three important characteristics used to describe EM waves are the
wavelength (X), the frequency (u), and the wave speed. The wavelength is simply the
distance between wave crests while the frequency is the rate of wave oscillation relative to
a Fixed point. The speed of EM waves in a vacuum is a physical constant given by C =
2.99792458 x 108 m s '1 and the wavelength and frequency are related to the wave speed
according to C = Xu.
The EM spectrum includes a broad range of wavelengths
(frequencies). O f primary interest here is the microwave portion o f the EM spectrum,
which is typically regarded as from approximately 0.3 GHz (1 meter wavelength) to 300
GHz (1 millimeter wavelength).
Another important property of EM radiation is polarization.
Polarization is often
exploited in remote sensing applications since EM waves that differ only in their
polarization can interact differently with matter. Polarization describes the orientation of
the electric (or magnetic) field vector in the plane perpendicular to the direction of
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propagation.
Mathematically, it is customary to use the electric field to describe the
polarization state since the electric field is orthogonal to the magnetic field and thus,
describes it also. The electric field vector can be written as the superposition of two
orthogonal linearly polarized waves that are also orthogonal to the direction of
propagation of the radiation.
The amplitudes of the component waves and the phase
difference between them give the polarization state of the radiation.
If the phase
difference is zero, the radiation is linearly polarized and the electric field vector oscillates
linearly in the plane normal to the direction of propagation. Circularly polarized radiation
results when the component waves have equal amplitudes and a tc/2 phase difference
between them. For unpolarized radiation, such as that from the sun or an incandescent
light source, the electric fields oscillate randomly in the plane normal to the direction of
propagation. This type of EM wave can still be described as the superposition o f two
linearly polarized waves with a random phase difference. The fluctuations of one o f the
superimposed fields are independent of the other and the average amplitudes are equal.
Radiation is partially polarized when its polarization state is somewhere between
unpolarized and completely polarized.
Interaction with matter
Every material with an absolute temperature greater than zero emits radiation.
A
blackbody is defined as one that emits isotropically the maximum amount of radiation at
each wavelength. While no real material is a perfect blackbody, some materials come
close to being blackbody emitters at some wavelengths.
It has also been shown that
radiation contained within a cavity whose walls are thick enough to prevent radiation from
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escaping such that thermal equilibrium is reached is the radiation that would be emitted by
a blackbody. Max Planck observed that when radiation interacts with cavity walls, the
interaction between the radiation and the oscillating atoms in the walls results in an energy
exchange that can be assumed to occur in discrete bundles, or quanta.
The energy
exchange is given by AE = hro, where h = 6.6262 x lO -34/ s is Planck’s constant and u is
the frequency of the atomic oscillation.
With this assumption, Planck derived an
expression for blackbody radiation:
B ,( T ) = ---------^ ---------
where C| and cz are empirical constants.
(B-l)
Equation (B-l) is known as the Planck function
and it says that for a given wavelength, the amount of radiation emitted by a blackbody is
determined only by its temperature. The Planck function also reveals that for a given
temperature there is a corresponding wavelength of maximum blackbody emission (i.e.,
dB,
where — —= 0), which is given by Wien’s Displacement Law:
dX
XmaT = 2S91.9fim -K
(B-2)
Thus, it is possible to estimate the temperature o f an emission source given its emission
spectrum. The total blackbody radiation can be determined by integrating the Planck
function over all wavelengths to obtain:
B(T) = oT a
(B-3)
where a = 5.67 x 10“*W ■m~l ■K ~*is the Stephan-Boltzmann constant.
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130
At centimeter and smaller wavelengths, which includes much of the microwave
portion of the spectrum, and temperatures characteristic of the earth-atmosphere system
the exponential argument in equation (B-l) is much less than unity yielding a useful
approximation to the Planck function known as the Rayleigh-Jeans approximation. Under
these conditions, the blackbody radiance is given by:
B x (T) = ^ ? l-aT
(B-4)
c,
which says that the blackbody radiance at a given wavelength is proportional to the
temperature of the emission source.
The Rayleigh-Jeans approximation to the Planck
function is widely used in microwave remote sensing applications where radiance is
typically presented in terms of the brightness temperature.
As previously mentioned, no real materials are perfect blackbodies.
However, the
emission from any material at a given wavelength can be expressed as that relative to a
blackbody:
E X = £ XBX(T)
(B-5)
where e* is the emissivity, which ranges from zero to unity. Since a blackbody is a perfect
emitter its emissivity is unity for any given wavelength.
blackbody is a perfect absorber with an aborptivity, a
Likewise, by definition a
of unity.
Gustav Kirchoff
determined that any material is exactly as good an absorber as it is an emitter. This is
described mathematically by K irchoff s Law:
ax = e x
(B-6)
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131
This applies for materials in local thermodynamic equilibrium (i.e., can be characterized
by a single thermodynamic temperature), which is typically valid in the atmosphere below
100 km altitude where the collision rates between gaseous species are sufficient to achieve
thermodynamic equilibrium (Kidder and Vonder Haar 1995).
Equation o f radiative transfer
Radiation passing through a volume of atmosphere can be affected by four processes:
absorption, emission, scattering into the beam, and scattering out of the beam. Hence, the
change of radiance with distance can be written
^
= ff„ ( A f e , ( T ) - L l M
+
o , (A ) [ ( £
, ) - L1l(e,<p)]
(B -7 )
where a a(X) is the volume absorption coefficient, Gs(k) is the volume scattering
coefficient. The first term on the right side of equation (B-7). cra(X)Bx(T), represents
emission following K irchoff s Law. The second term, a a(X)L\(0,(p), represent absorption
following Beer’s Law, which states that the rate of decrease in radiation intensity as it
passes through a material is proportional to the intensity of the radiation. The direction of
the propagating radiation is given by the angles 0 and (p. The third term in equation (B-7),
cts(X.)(Lx’}, represents radiation scattered into the direction of propagation. Here, (La.’), is
a directionally weighted average for radiance from all directions. Lastly, a s(A.)Lx(0,cp),
represents the radiation scattered out of the beam.
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132
Two useful terms for satellite meteorology are from equation (B-7). The absorption
and scattering terms are combined to give a volume extinction coefficient, c e. Also, the
single scatter albedo, given by
<B~8>
represent the fraction of extinction due to scattering.
Interaction o f microwave radiation with the atmosphere
Although the Earth emits radiation with a broad spectral peak in the infrared portion of
the electromagnetic spectrum, there is a weak emission signal at microwave frequencies.
While the microwave contribution to the overall radiative flux in the atmosphere is
relatively small, the interaction of microwave radiation with atmospheric constituents,
primarily molecular oxygen (O 2) and water (HjO) in its various phases, has important
applications for remote sensing.
At microwave-band frequencies, Oi and H2O are radiatively active precisely because
they possess permanent dipole moments.
O 2 is a symmetrical linear molecule with
unpaired electrons in its outer orbital. These electrons, having the same spin orientation,
give rise to a permanent magnetic dipole moment in the molecule. The H 2O molecule is
asymmetrical.
leaves two
Covalent bonding between the hydrogen atoms and the oxygen atom
pairs of non-bonding valence electrons
around the
oxygen
atom.
Geometrically, the hydrogen atoms pull away from these pairs of electrons and pull
toward each other. The resulting charge distribution, a net negative charge on the oxygen
atom and a net positive charge on the hydrogen atoms, gives the H 2O molecule a
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133
permanent electric dipole moment. Microwave absorption and emission by H20 and 0 2
takes place via quantum rotational energy transitions in these molecules.
Microwave radiation propagating through the atmosphere also interacts with liquid
and solid precipitation particles. Owing to its permanent electric dipole moment, water is
a dielectric. That is, water molecules tend to align parallel to an applied electric field.
The dielectric property of any material is described by the complex dielectric constant, £.
The complex dielectric constant is related to the complex refractive index (m) of the
material, written m = n - iK where n is the index of refraction and
coefficient of the medium.
k
is the absorption
In the microwave-band, liquid water has a much larger
absorption coefficient and complex dielectric constant than ice (Gunn and East 1954).
Physically, this is due to the fact that H20 molecules in the liquid phase are free to align
with the applied electric field, whereas H20 molecules in the ice phase are iocked in a
crystalline lattice structure, prohibiting alignment with the electric field.
Thus, as
Stephens (1994) points out, ice particles are more effective scatterers o f microwave
radiation than absorbers, while the reverse is true of water drops.
The non-precipitating atmosphere is nearly transparent for most of the microwave
spectrum.
Exceptions are 22.235-GHz and 183-GHz where water vapor absorption
occurs, and 60-GHz and 118-GHz due to absorption by molecular oxygen (Stephens
1994). For cloud and precipitation particles in the atmosphere, the degree of extinction
(scattering and absorption) and emission is also a function of particle size and wavelength.
Mie (1908) gives a complete theory of scattering by spherical particles. Stratton (1941)
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134
and Van de Hulst (1957) present further treatment of particle scattering.
The Mie
scattering cross-section (Qs) and total extinction cross-section (Qt) are given by:
(B-9)
(B -10)
where X is the wavelength o f incident radiation, and a„ and bn are coefficients for the
electric and magnetic dipole (and higher order) moments of the scattered electromagnetic
wave. For an absorbing medium, Qt = Qs + Qa, where Qa is the absorption cross section.
In the series expansion, the a„ and b„ coefficients are expressed in terms of spherical
Bessel functions, and they depend on m, the complex refractive index of the medium, and
x, the size parameter, where
and r is the particle radius. Wiscombe (1980) and Bohren (1987) present computational
methods for determining the Mie coefficients.
If the wavelength of incident radiation is significantly larger than the particle size such
that the size parameter x «
I, the higher order terms in the Mie coefficients can be
neglected. The scattering cross-section (Qs) and absorption cross-section (Qa) become:
(B-l 2)
(B-l 3)
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135
This so-called “Rayleigh approximation” (Stratton 1941; Battan 1959; Gunn and East
1954; and others) is typically valid when x < 0.13. Since cloud particle sizes are typically
on the order of 10 microns, the Rayleigh approximation applies for microwave
frequencies up to approximately 100-GHz (3 mm). Precipitation particles typically range
from hundreds of microns to millimeters in size.
For small droplets, the Rayleigh
approximation is valid for frequencies below about 60-GHz. For larger drop spectra, the
Rayleigh approximation is only valid for frequencies below a few GHz (~ 10 cm
wavelength). Thus, at frequencies used by a few of the currently operating satellite-borne
microwave radiometers (10-GHz to 85.5-GHz), scattering and absorption then become
complex functions of particle size and dielectric constant, and are described using the Mie
cross-sections.
For the purpose of microwave remote sensing of precipitation, Spencer et al. (1989)
use Mie theory to calculate the scattering and absorption coefficients and single-scatter
albedo for a Marshall-Palmer distribution of hydrometeors at several SSM/I frequencies.
Ice has a much larger single scatter albedo than water for these microwave frequencies.
The net result is that extinction and emission occurs at a range of microwave frequencies
from liquid precipitation below the freezing level.
Ice particles above the rain layer,
which contribute little to absorption/emission, will act to scatter upwelling radiation from
below.
The scattering efficiency due to the presence of ice particles increases with
microwave frequency, particle size, number density, and depth of the scattering layer. At
85 GHz, the scattering coefficient increases rapidly with rain-rate such that large
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136
brightness temperature depressions can result over deep convection (W ilheit et al. 1982;
Hakkarinen and Adler 1988; Heymsfield et al 1991; and others).
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137
APPENDIX C
LETTER OF PERMISSION
T E X A S
A & M
D epa rtm en t
of
U N I V E R S I T Y
A t m o s p h e r ic S c ie n c e s
College Station, TX 77843-3150
2 November 2000
American Meteorological Society
45 Beacon Street
Boston, M A 02108-3693
To Whom It May Concern:
I am a doctoral candidate at Texas A&M University and I am in the final stages of completing my
dissertation in the next month. Our library sends our dissertation to University Microfilms Inc. for
preparation o f a microfilm copy o f the document As I am sure you are aware, UMI retains the right to sell
both hard and microfilm copies o f the document
My reason for writing is to request permission to use information from a manuscript I have
submitted and which has been accepted for publication in the Journal of Applied Meteorology. The
manuscript, entitled "Lightning and SSM/I Ice Scattering Mesoscale Convective Systems in die Global
Tropics", and co-authored with Edward J. Zipser, is a chapter in my dissertation. The journal article is
scheduled to appear in the Journal of Applied Meteorology in mid 2001.
I would also like to determine if future publication o f a manuscript taken from other chapters of
my dissertation in an AMS journal will pose any problems. Please inform me o f any necessary action to
prevent future publication complications. If I need to acquire the copyrights to my dissertation in order to
be able to release them to you, I will be happy to do so.
Thank you very much for your effort in resolving these issues.
Sincerely,
Ernest Richard Toracinta
PEHwilSSiON GRANTED BY
AMERICAN METEOKGUJlalUAL bU U ti t
45 BEACON STREET
BOSTON, MA 02108
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138
VITA
Ernest Richard Toracinta was bom on November 8, 1965 in Willimantic, Connecticut.
He spent most of his formative years in southern New England. After graduating from
Nacogdoches High School in Nacogdoches, Texas in 1983, he began undergraduate
studies at Stephen F. Austin State University. He returned to New England to complete
his degree, graduating Magna Cum Laude with a Bachelor of Science in meteorology
from the University of Massachusetts at Lowell in 1993.
Ernest began graduate studies at Texas A&M University in College Station, Texas, in
1993. He completed a Master of Science in meteorology in December 1995 and began
doctoral studies in 1996.
His research experiences included participation in two field
campaigns in Texas and the TRMM-Large Scale Biosphere-Atmosphere (LBA)
Experiment in Rondonia, Brazil in February 1999. Ernest received a NASA Earth System
Science Fellowship in 1997 and was inducted into the Pinnacle Honor Society in 1999.
He can be reached at the address of his father and stepmother:
Rt. 1 Box 56A-1
Gary, Texas 75643
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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