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INTERNATIONAL JOURNAL OF CLIMATOLOGY, VOL. 17, 1357±1367 (1997)
DAILY AVERAGES OF NET RADIATION MEASURED AT OSU,
NIGERIA IN 1995
OLUWAGBEMIGA O. JEGEDE{
Deutscher Wetterdienst, Meteorologisches Observatorium Lindenberg, D-15864 Lindenberg, Germany.
Received 27 August 1996
Revised 21 October 1996
Accepted 3 April 1997
ABSTRACT
A series of the daily averaged net all-wave radiative ¯ux measurements recorded at a tropical location in West Africa (Osu,
Nigeria; 743 N, 458 E) for the period 31, December 1994 to 19, December 1995 is presented in this study. The preliminary
data set showed the annual trend as bimodal, with peak values of about 140 W m72 day71 in March and November, and a
prominent minima of about 70 W m72 day71 in August, which is at the peak of the monsoon season. The mean for the year
was 10365 3013 W m72 day71. Day-to-day ¯uctuations of the daily averages increased markedly during the wet season
(which is from April to October). The annual pattern is also investigated by the Fourier series expansion method. #1997 by
the Royal Meteorological Society. Int. J. Climatol., 17: 1357±1367 (1997)
(No. of Figures: 5
KEY WORDS:
No. of Tables: 2
No. of References: 14)
net radiation; Osu, Nigeria; daily averages; statistical analysis; Fourier series.
1. INTRODUCTION
The radiative energy balance at the Earth's surface minus the ground heat ¯ux, represents the available energy for
driving the land-surface energy exchange processes. As such, net radiation is a fundamental quantity for surface
energy balance studies. The Bowen ratio energy balance method (BREB) requires net radiation to estimate the
atmospheric ¯uxes of both sensible heat and latent heat (see Spittlehouse and Black, 1980). Net radiation is also
useful as an index for the classi®cation of atmospheric stability in applied air pollution dispersion models.
Recently, regional evaporation problems have renewed interest in the spatial distribution of net radiation.
It has now become a widely acceptable practice to measure the net radiation using a single net radiometer
(Fritschen and Simpson, 1989; Wallace et al., 1990; Halldin and Lindroth, 1992; El Bakry, 1994). The design of
net radiometers have undergone several modi®cations since they were ®rst introduced in 1933. The ®rst types
produced were the non-shielded and non-ventilated types. Consequently, signi®cant measurement errors arose
from the use of such instruments through convective heat loss due to the sensitivity of the absorbing material to
wind speed. Polyethylene (Lupolene) is commonly used as a shielding material because of its good all-wave
transmission property. Improvements both in the design and method of calibration for speci®c net radiometers
have shown that systematic errors of about 10 per cent can now be expected even from the commercially
produced instruments (Halldin and Lindroth, 1992).
Correspondence to Dr. O. O. Jegede, Deutscher Wetterdienst, Meteorologisches Observatorium Lindenberg, D-15864 Lindenberg, Germany.
e-mail: mol-guest@dwd.d400.de
{On leave from the Department of Physics, Obafemi Awolowo University, Ile-Ife, Nigeria.
Contract grant sponsor: International Programmes in the Physical Sciences of Uppsala University Sweden
Contract grant sponsor: Alexander von Humboldt Foundation (Germany)
CCC 0899-8418/97/0121357-11 $17.50
# 1997 by the Royal Meteorological Society
1358
O. O. JEGEDE
At present in the tropical parts of Africa, there is dearth of net radiation data. The scanty radiation data available
for the subregion have been obtained mainly from some speci®c projects (e.g., HAPEX±Sahel experiment, see
Goutourbe et al., 1994) undertaken at a few research stations and over a limited period. In order to be able to model
land-surface energy exchange processes as accurately as possible for these areas, it is essential to develop a radiation
data bank on a long-term basis. This has been started recently at Obafemi Awolowo University, Ile-Ife, Nigeria
(Adedokun, 1992). Radiation data (global, diffuse, longwave, net, photometric ¯ux) are now collected more or less
on a continuous basis and these have formed a useful data base for the location.
The net radiation data set which is reported in this paper has been acquired over a period of one complete year
(January to December 1995), as an aspect of a continuous and intensive ®eld study of both the mean wind and
temperature structure in the atmospheric surface layer (SL) at a tropical location within West Africa. It is the
main objective in this paper to present the temporal variations of the daily averaged net radiation at Osu; Nigeria
(743 N, 458 E), using the preliminary data collected during the experiment of 1995.
2. METHODS OF ANALYSIS
To analyse the data set, we have determined the frequency distribution for the daily averages of the net radiation
Rn. Also, statistical measures of the data (mean, maximum, minimum, standard deviation, and the standard error)
have been estimated. The annual trend of the daily averages of the net all-wave radiation ¯ux, Rn (t) recorded at
the location is modelled by the Fourier series expansion method (see Festa et al., 1992). The Fourier series can be
expressed as:
2ti
2ti
Rn …t† ˆ hRn i ‡ Ai cos
‡ Bi sin
; i ˆ 1; 2; . . .
…1†
N
N
where hRni, is the mean for the total number of days (in this case, N ˆ 350) and t is the day number. The Fourier
coef®cients Ai and Bi in equation (1) are given by
N
2P
2ti
‰Rn …t† ÿ hRn iŠ cos
Ai ˆ
N tˆ1
N
…2†
N
P
2
2ti
‰Rn …t† ÿ hRn iŠ sin
Bi ˆ
N tˆ1
N
In the whole of the data series, there are some 17 days (21±26 March; 17±23 November; 3±6 December) of
missing data due to problems encountered at different times with the ®eld power supply and datalogging
program. To perform the Fourier analysis, it is essential that the whole data series is presented as unbroken. The
missing data periods were replaced by performing a numerical interpolation between the data edges (Press et al.,
1987). To the values so obtained, is added the numbers randomly generated in such a manner that the additional
values lie within one standard deviation (SD ˆ 3013 W m72 day71). The daily averages of the air temperature
(at 544 m) were processed similarly.
3. DATA AND INSTRUMENTATION
The net radiation data were acquired from the SL ®eld measurements conducted at Osu between 31 December
1994 and 19 December 1995. The experimental site, which spans an area of about 85 hectares, is within the
tropical rain forest belt of West Africa. The location had been deforested some 15 years earlier as part of a project
that was later abandoned. The surface cover was fallow bush (about 2 m high) and changed in texture from dry
twigs during the dry season to the leafy green (elephant) grass in the wet season.
A 50-m meteorological mast installed at the site was instrumented at ®ve different levels (up to 22 m) with the
following devices: ®ve cup anemometers, a wind vane, and four aspirated platinum resistance thermometers to
measure the wind and temperature pro®les in the SL. Also installed on the mast was a single level measurement
of net radiation. The net radiometer was positioned at a highest of 4 m so that it was well above the bush to give
INT. J. CLIMATOL., VOL. 17: 1357±1367 (1997)
# 1997 Royal Meteorological Society
1359
NET RADIATION IN NIGERIA
an area average. The measuring instruments were connected by cables to a datalogger (Campbell Scienti®c,
model CR10). Measurements were carried out every minute, and 10-minute averages of all variables were stored
for the analysis. A more complete description of the experiment is contained in Jegede (1996a).
3.1. Measurement errors in net radiometry
The attraction of using a single radiometer to measure the net radiation must be tempered with caution because
serious problems exist regarding its adoption as a standard for micrometeorological research. Such drawbacks
include the wavelength-dependent responsivities of its sensing elements, stability of the calibration constants, the
technique used for calibration, and the temperature deviation between environmental air and the air entrapped
inside the dome (Halldin, 1991).
Oliver and Wright (1990) compared the measurements made by a double-dome net radiometer (REBS Q 4)
with the sum of the individual components of the net radiation. They found a large difference between the
longwave and short-wave responsivities of the radiometer. They suggested that the wavelength-dependent
responsivities of such meters can be corrected by a simple addition of 40 per cent of the net longwave radiation.
Halldin (1991), commenting on this study, has argued that such a ®xed correction factor will produce night-time
values that are about 20±25 per cent too negative. He further pointed that such a correction factor should vary
with both the time and the location of the measurement.
Recent improvements in both the design and the calibration of net radiometers have brought down the
magnitude of the typical errors associated with the use of such meters to acceptable levels. These design features
include introduction of thinner (pressurized) wind shields, ventilated systems, and new sensing materials (e.g.
semiconductors). Measurement errors in net radiometry could be reduced further by ensuring that the air space
between the shields is very dry because water vapour absorbs the longwave radiation.
3.2. The model Q-7 net radiometer
The double-dome model Q-7 net radiometer (manufactured by Campbel Scienti®c, Inc., USA) used in this
investigation was acquired brand new in November 1994. This instrument is a high-output 60-junction thermopile
(inserted between blackened sensor plates) and measures the algebraic sum of the incoming and outgoing allwave radiation. Its low electrical resistance (4 ohms nominal) is to reduce susceptibility to noise. The heavy duty
polyethylene shield requires no pressurization. The time constant of the instrument is approximately 30 s. The
calibration factors as supplied by the manufacturer are wavelength dependent: 992 W m72 m V71 for positive
values (day-times) and 1172 W m72 mV71 for the negative values (night-times). Dew formation inside the
hemispherical domes was prevented by silica gel placed inside the tubing of the support/connecting arm.
Once a week, the radiometer head was dusted using a soft bristle hand-brush and inspected for any sign of
cracks on the wind shields (polyethylene domes). The manufacturer recommends replacement of the shields after
every 3 months of exposure or at any visible sign of ageing, whichever comes ®rst. Due to the very humid nature
of the tropical environment and its high solar intensity, the polyethylene wind shields deteriorated faster and were
replaced about every 2 months. The instrument has an integrated spirit-level, which allows for a levelling better
than 1 . The silica gel in the tube was replaced whenever it changed colour.
Table I. Days with missing net radiation data (partly or
wholly) at Osu station in 1995
Month
March
November
December
# 1997 Royal Meteorological Society
Dates
21st to 26th
17th to 23rd
3rd to 6th
INT. J. CLIMATOL., VOL. 17: 1357±1367 (1997)
1360
O. O. JEGEDE
3.3. The net radiation data set
The net radiation data has been processed with a great deal of care. Because the instrument was acquired
brand-new for the investigations, there is reason to believe that its calibration was constant throughout the period
of measurements (although no reference was available for comparison). Generally, its ®eld performance was
observed to be stable. The data collection procedure was automated, so that the measurement series is very
homogeneous. The number of days with data losses (partly or wholly) due to problems with the power supply or
an interruption of the datalogging program is less than 5 per cent (see Table I) of the whole measuring period.
The presently compiled net radiation data represents the most comprehensive and up to date set from within
tropical West Africa. The data set is available on request from the author.
4. DISCUSSION OF RESULTS
The daily averages, the maximum (daytime) and minimum (night-time) values of the net radiation measured at
the Osu station during 1995 are presented in Table II. For the period, the mean of the daily averages was
10365 3013 W m72 day71. The mean of the daily maximum in the year was 45782 10668 W m72 and
the corresponding minimum was ÿ2386 829 W m72. The choice of the sign is that the positive values
indicate upwardly directed net radiation, whereas the converse is true for the negative values. The frequency
distribution of the daily averages of net radiation (in 20 W m72 day71 classes) is shown in Figure 1.
The time series of the daily averaged net radiation data at the Osu station for 1995 has been plotted in Figure 2,
alongside the curve obtained from the Fourier series. It was found that with i ˆ 2 (i.e. two terms each of sine and
cosine functions), the annual trend for 1995 was well reproduced. Higher order series (e.g. i ˆ 3) did not produce
an improvement. For the Fourier series, the coef®cients are:
hRn i ˆ 10326;
A1 ˆ 1184;
A2 ˆ ÿ454;
B1 ˆ 1021; and
B2 ˆ ÿ1434
(all numbers are in units of W m72 day71).
From Figure 2 it can be observed that the annual variation of the net radiation in 1995 showed a bimodal
distribution, with peak values of about 140 W m72 day71 around March and November and a minima of about
70 W m72 day71 in August, which is at the peak of the monsoon season. Also noticeable from the ®gure are the
large ¯uctuations in the daily averages, especially during the wet months (from about April to October). This
pattern is attributed to the attenuating effects of clouds (and aerosols) on the incoming solar radiation, which vary
vastly both on the spatial and temporal scales, consequently affecting the radiation balance over the area (Jegede,
1996b; Okogbue et al., 1996).
Shown in Figure 3 is the diurnal variation of the net radiative ¯ux over the two seasons: wet (April±October)
and dry (November±March), at the Osu station in 1995. There is similarity in both trends except that values of the
daytime net radiation were lower in the wet season compared with the dry season. The maximum value of the
hourly net radiation at Osu is about 500 W m72 (or 350 W m72 for the wet season). This is lower than the shortperiod hourly averaged values published by Wallace et al. (1990) for the southern Sahelian region of Niger in
West Africa. The large difference of the daytime net radiation between the two seasons shows that the convective
clouds are more effective in altering the radiation balance than the turbid air of the dry season. The two effects
can be regarded as almost separate manifestations because the thunderstorms of the wet season continuously
wash out the particulates whereas the surface conditions during the dry season greatly inhibit the formation of
clouds.
Figure 4 shows the annual trend of the daily maximum air temperature (at 544 m) for the same location in
1995. Unlike the time series obtained for the net radiation, the annual temperature course, portrays only a single
peak, which occurs about February (the hottest month of the year at the location), with a minima in August. The
mean for the daily maximum temperature was 2906 C. The coef®cients for the Fourier series (of order 2) ®tted
to the temperature data are:
A1 ˆ 204;
INT. J. CLIMATOL., VOL. 17: 1357±1367 (1997)
A2 ˆ 025;
B1 ˆ 243
and
B2 ˆ 019
# 1997 Royal Meteorological Society
1361
NET RADIATION IN NIGERIA
Table II. Daily mean, maximum (daytime) and minimum (night-time) values of net radiation (W m72) at Osu, Nigeria in 1995
Day
Mean
Maximum
Minimum
Day
Mean
Maximum
Minimum
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
9133
9015
9444
11599
10055
11083
12888
12903
11328
11415
9457
8631
8072
7569
8967
9013
8051
9093
9513
8217
9383
9092
9108
9742
9370
9030
8628
10011
9376
8396
9708
9576
10824
7058
10634
11829
11140
11909
10917
9847
11341
11139
11902
14444
13338
12434
12751
10415
11458
13045
13813
12887
13649
13987
12312
11731
42779
43072
44604
52184
47382
49936
53015
53304
50633
50882
47167
42978
39839
36831
39814
40899
38046
42924
44326
40436
46534
42454
45357
44776
44927
46275
39479
43681
42485
36275
40930
41123
45269
39110
44251
50967
48448
47364
45388
45618
44179
50277
54958
55995
53272
51452
54336
52010
53121
52439
58095
53725
56132
56725
53156
50996
7 3082
7 3312
7 3534
7 4219
7 3486
7 3132
7 3547
7 3738
7 3667
7 4588
7 4303
7 3839
7 3637
7 2941
7 3449
7 3537
7 3777
7 3617
7 3276
7 4122
7 4002
7 3741
7 3762
7 3524
7 3396
7 3612
7 3104
7 3533
7 3230
7 3132
7 3115
7 3519
7 3508
7 2747
7 4248
7 4151
7 3894
7 3446
7 3232
7 3392
7 2816
7 3305
7 3324
7 3974
7 3767
7 3765
7 3695
7 3384
7 3579
7 3792
7 4258
7 4403
7 4862
7 4521
7 3841
7 3697
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
..
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
9697
10774
12071
9492
12516
11496
13726
12881
9977
11118
12081
13698
14885
16288
15058
11996
14604
10628
12710
11297
1268
17370
15701
4048
.....
7583
14877
14859
13473
13492
13473
10874
13092
13107
14150
14005
12385
13113
15012
12272
13614
14102
10804
11827
10568
10923
16355
11893
9762
7615
15684
3582
6047
14517
10845
11794
42779
47990
49889
54259
53387
60452
60283
61365
58041
54767
60386
59671
56165
65669
63561
47788
56915
43092
46717
53129
9285
65134
63231
30947
.....
36169
57438
58419
52733
62283
55902
50420
50482
55483
50496
59607
47214
53295
62851
52408
49613
53623
39806
58425
40905
53214
59084
43996
51069
42132
57428
26922
23427
53150
42325
42972
7 2930
7 2548
7 2994
7 2561
7 3662
7 3143
7 3065
7 3305
7 2749
7 2397
7 2238
7 2288
7 2373
7 2554
7 2895
7 2834
7 2869
7 3061
7 2728
7 2749
7 2501
7 2763
7 2848
7 2427
.....
7 2160
7 2525
7 2390
7 1792
7 1723
7 1908
7 2482
7 2064
7 1564
7 2478
7 2732
7 2442
7 2581
7 2093
7 2203
7 2138
7 1916
7 1344
7 2124
7 2110
7 1693
7 2244
7 2081
7 1946
7 1606
7 1350
7 1233
7 1736
7 1863
7 1986
7 1791
# 1997 Royal Meteorological Society
INT. J. CLIMATOL., VOL. 17: 1357±1367 (1997)
1362
O. O. JEGEDE
Table II (continued).
Table II (continued).
Day
Mean
Maximum
Minimum
Day
Mean
Maximum
Minimum
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
10487
11121
12075
13932
5301
8399
10424
15294
14221
12216
15506
12400
14071
9552
14591
11842
11109
16193
12383
9719
12679
12984
16340
10651
14281
9562
7499
14610
12991
10696
14008
12416
12416
13685
8213
11334
855
12873
6303
8744
13515
8271
11611
12194
11161
10426
2303
12604
8009
9320
13240
11893
6514
7167
11949
11204
43787
41123
56152
58767
23441
46972
41411
52240
51098
54805
59627
56799
51208
44664
52671
45910
44500
57840
50872
41917
54859
46885
61300
51170
47637
39572
31633
52734
48485
49439
62563
45765
48821
51796
38083
61750
13165
51170
50758
39929
52126
34619
48995
46818
42960
43762
11978
52972
51971
37293
56625
44090
41533
33858
53955
46940
7 1766
7 1704
7 1391
7 1330
7 1283
7 1340
7 1064
7 1881
7 2024
7 1909
7 1889
7 1792
7 1637
7 1370
7 1651
7 1390
7 1854
7 1957
7 1829
7 1615
7 1320
7 1950
7 1093
7 1409
7 1697
7 1434
7 1837
7 1631
7 1393
7 1936
7 1893
7 1367
7 1188
7 1413
7 2358
7 1814
7 1308
7 2716
7 1687
7 2628
7 1518
7 2491
7 2823
7 3232
7 2301
7 2989
7 1990
7 3130
7 2290
7 2813
7 3595
7 2983
7 1797
7 2878
7 4304
7 1917
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
8127
759
8250
8309
4677
13512
10934
6739
7868
8768
3394
4826
9427
4547
8364
6221
8164
7415
4742
9406
7831
11606
9498
7854
10924
7468
3228
5725
3770
5318
8916
8236
9023
6706
9481
8426
8648
7349
7937
10158
8471
8364
1892
3584
8838
4682
1327
5302
6536
7100
7061
6913
6085
7007
7342
6958
33752
8442
41077
34913
32620
54643
48066
30187
46781
38222
17684
33082
39182
24580
38892
26180
35867
36128
17878
45875
37047
46505
47142
39721
55244
32319
16070
33064
15592
23972
41597
34254
52288
45746
43007
45070
33574
49238
41065
53088
40482
39454
11463
17609
35956
27269
6471
23487
28218
28486
35717
24578
29367
29235
37247
39713
7 2785
7 1718
7 3837
7 1916
7 3476
7 2524
7 2545
7 1149
7 1596
7 2723
7 1066
7 1530
7 3176
7 2319
7 1178
7 1337
7 2789
7 1661
7 2237
7 2919
7 2504
7 2192
7 2000
7 1820
7 1570
7 1339
7 1380
7 1316
7 1103
7 2521
7 3060
7 2687
7 1252
7 1229
7 1221
7 2743
7 2814
7 1122
7 2388
7 2573
7 3181
7 2646
7 961
7 764
7 2427
7 2097
7 378
7 1584
7 1816
7 1622
7 2003
7 1907
7 1266
7 2574
7 2422
7 1547
INT. J. CLIMATOL., VOL. 17: 1357±1367 (1997)
# 1997 Royal Meteorological Society
1363
NET RADIATION IN NIGERIA
Table II (continued).
Table II (continued).
Day
Mean
Maximum
Minimum
Day
Mean
Maximum
Minimum
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
9555
8109
8037
11657
8528
11130
8788
9037
9768
7860
6957
6990
8823
8782
7023
10163
8700
8612
9231
8895
9305
8818
9531
11317
12195
7442
6177
10344
8271
7982
10619
9196
15811
10593
8822
14351
5629
8389
10599
11515
8340
12173
13142
11129
7310
7410
6668
8889
8102
11808
10254
12841
11599
9523
10393
15855
45503
46332
47625
61592
37605
43955
52362
45913
52265
40035
36895
44964
41516
46333
45759
46503
43929
41735
41305
48117
40261
39700
39395
49677
59820
50510
25053
53243
41865
32548
50775
63613
64352
55732
55713
53667
31410
29172
46645
47900
38597
56810
58817
42167
39077
31502
37837
43670
45002
58807
35110
47763
55405
40862
41200
58577
7 1926
7 2414
7 1089
7 2672
7 1595
7 1592
7 2084
7 1158
7 1296
7 2109
7 1867
7 2336
7 2445
7 2006
7 2255
7 2643
7 2419
7 3026
7 2828
7 3122
7 3620
7 3359
7 4042
7 3386
7 2652
7 1695
7 1596
7 2791
7 1418
7 1965
7 1763
7 1758
7 1750
7 3118
7 1497
7 2819
7 2724
7 3007
7 1132
7 2557
7 2584
7 2324
7 1719
7 2760
7 1680
7 2378
7 1902
7 2413
7 2115
7 1331
7 2721
7 2561
7 1394
7 2258
7 1776
7 2290
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
..
328
329
330
331
332
333
334
335
336
..
341
342
343
344
345
346
347
348
349
350
9989
11219
10242
8971
12500
12380
9045
15711
8053
12981
6305
10132
11833
15647
9372
10497
5456
11633
9078
13543
9857
10946
11135
13777
12206
12650
12755
13364
13196
13999
13699
14435
14703
13879
13846
.....
13044
12223
13817
13655
13303
12340
13440
12007
12881
.....
11095
10848
12391
11127
11033
10049
10702
9020
11882
10586
40635
54763
41950
45752
50747
48982
46043
64368
32645
49308
37052
59087
44895
62870
48295
43097
26033
50047
40963
51320
51323
45607
52845
53230
52828
55805
50128
54178
55153
54447
51977
54748
55702
53768
53402
.....
52242
51640
53545
51427
52383
51095
54813
53190
53190
.....
42320
47443
51632
54922
50018
44000
47363
40722
49782
47993
7 1383
7 1703
7 1429
7 2039
7 1964
7 1503
7 1712
7 2496
7 2065
7 1678
7 2425
7 1362
7 1210
7 2445
7 2215
7 2389
7 1226
7 1831
7 1550
7 1732
7 2199
7 1666
7 1909
7 1958
7 1870
7 1774
7 2319
7 2550
7 2572
7 2818
7 2363
7 2631
7 2631
7 2615
7 2695
.....
7 2292
7 2249
7 2503
7 2184
7 2013
7 3193
7 2931
7 1927
7 2084
.....
7 1953
7 2233
7 2135
7 2246
7 1855
7 1944
7 2037
7 1975
7 2464
7 2380
# 1997 Royal Meteorological Society
INT. J. CLIMATOL., VOL. 17: 1357±1367 (1997)
1364
O. O. JEGEDE
Table II (continued).
N
Mean
standard deviation
standard error
Mean
Maximum
Minimum
333
10365
3013
165
333
45782
10668
585
333
7 2386
829
045
Figure 1. Histogram of the daily averaged net radiation in (20 W m72 day71) classes at Osu station for 1995
(all in units of C). Unlike the observed net radiation data, the maximum air temperature exhibits considerably
less ¯uctuation on a day-to-day basis and the standard deviation for the period was 279 C. From the data, the
daily maximum temperature course has an amplitude of about 8 C.
The diurnal patterns of both net radiation and air temperature (at 544 m) for the respective months in 1995
are plotted together as time series in Figure 5. It is easy to notice that both the hourly maxima for the net
radiation and temperature follow nearly the same annual trend. The hourly maxima for the net radiation occurred
in March and November, but it is in February and December for the temperature series. Both showed a minima in
July.
6.
CONCLUSIONS
The daily averages, maximum and minimum of the net radiation ¯ux measured during a surface layer ®eld
project at Osu, Nigeria, in 1995 have been presented in this study. The mean for the period was 10365 3013
INT. J. CLIMATOL., VOL. 17: 1357±1367 (1997)
# 1997 Royal Meteorological Society
1365
NET RADIATION IN NIGERIA
Figure 2. Daily averaged net radiation at Osu station (743 N, 458 E), Nigeria, January±December 1995. The dotted curve shows the Fourier
series ®tted to the data
W m72 day71. The annual trend was bimodal, with peak values of about 140 W m72 day71 occurring in March
and November, and a prominent minima of about 70 W m72 day71 is recorded for August. The separate effects
of the monsoon clouds and of the dust-laden environment in the dry season, both of which are highly variable
(spatially and temporally) within West Africa, are believed to be the reason for the observed large variation in the
daily averages of net radiation recorded at our location.
Figure 3. Diurnal variations of the net radiation ¯ux at Osu station for both the dry and wet seasons
# 1997 Royal Meteorological Society
INT. J. CLIMATOL., VOL. 17: 1357±1367 (1997)
1366
O. O. JEGEDE
Figure 4. Same as Figure 2, but for the daily maximum air temperature
Figure 5. Month-by-month trends for the diurnal patterns of the net radiation and air temperature at Osu station
The presently compiled net radiation data are the most comprehensive and up to date are from tropical West
Africa. It is believed that availability of this data set can serve as a baseline for the needs of other researchers
working on related studies in the subregion.
ACKNOWLEDGMENTS
The International Programmes in the Physical Sciences of Uppsala University, Uppsala, Sweden has supported
this study by way of equipment donation. Collaborative efforts of colleagues particularly, Drs T. A. Fasheun and
Z. D. Adeyefa of the Federal University of Technology, Akure, Nigeria within the project is hereby
INT. J. CLIMATOL., VOL. 17: 1357±1367 (1997)
# 1997 Royal Meteorological Society
NET RADIATION IN NIGERIA
1367
acknowledged. The Alexander von Humboldt Foundation, Germany, provided additional assistance by an award
of a fellowship to the author, which was utilized to complete this report. The hospitality of the German Weather
Service in Lindenberg is very much appreciated.
REFERENCES
Adedokun, J. A. 1992. The OAU Solar/Atmospheric Radiation Databank, Unpublished Report, Obafemi Awolowo University, Ile-Ife, Nigeria.
El Bakry, M. M. 1994. `Net radiation over the Aswam high dam lake', Theor. Appl. Climatol., 49, 129±133.
Festa, R., Jain, S. and Ratto, C. F. 1992. `Stochastic modelling of daily global irradiance', Renewable En., 2, 23±34.
Fritschen, L. J. and Simpson J. R. 1989. `Surface energy and radiation balance systems: general description and improvements', J. Appl.
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Okogbue, E. C., Adedokun, J. A. and Jegede, O. O. 1996. `On the estimation of the daily global and diffuse irradiation for Ile-Ife, Nigeria',
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Oliver, H. R. and Wright, I. R. 1990. `Correction of errors associated with measurement of net all-wave radiation with double-domed
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Press, W. H., Flannery, B. P., Teukolsky, S. A. and Vetterling, W. T. 1987. Numerical Recipees: the Art of Computing, Cambridge University
Press, Cambridge, 818 pp.
Spittlehouse, D. L. and Black, T. A. 1980. `Evaluation of the Bowen ratio-energy balance method for determining forest evapotranspiration',
Atmos. Ocean., 13, 98±116.
Wallace, J. S., Gash, J. H. C. and Sivakumar, M. V. K. 1990. `Preliminary measurements of net radiation and evaporation over bare soil and
fallow bushland in the Sahel', Int. J. Climatol., 10, 203±210.
# 1997 Royal Meteorological Society
INT. J. CLIMATOL., VOL. 17: 1357±1367 (1997)
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