| Academic Open Internet Journal ISSN 1311-4360 |
Volume 17, 2006 |
Study of Long-wave Downward Irradiance at
Cairo, Egypt
S. Abdelaty*
*Physics Department, Faculty of Education Port Said branch, Suez Canal University, Egypt
Abstract
Measurements of daily and monthly
values of incoming
Infrared radiation (I.R.) were carried out at Cairo location (Lat.
30" N, Long. 31 °E) for three years period (1998 - 2000). The Egyptian
Meteorological Authority also measured hourly data for complete one year
through this interval using Epply pyrgeometer. Global and
diffuse solar radiation was recorded for the same period at the same site using Epply pyranometer. All
the meteorological data of the site are collected for the same period. These data were treated using an
advanced computer program to obtain the numerical values of the regression and
the correlation coefficients between
the incoming long wave radiation and the other meteorological data. The ratio of the infrared to the global solar radiation
(I.R./G) ranged between (4.4) in winter
to (2.7) in summer. The ratio of diffuse to global radiation (D / G diffuse fraction) ranged between (0.46) in winter to
(0.33) in summer.
Introduction
The infrared measurements at Cairo began in 1990; the instrument used for measuring the incoming long-wave radiation (I.R.) is the Epply pyrgeometer model EPIR. Measurements of global solar radiation began at the head quarter of the meteorological Authority of Egypt at Cairo in 1956. Also, regular measurements for the diffuse solar radiation began at Cairo in 1980. Global and diffuse solar radiation are measured by Epply pyranometer.
The normal solar radiation
incident on the earth atmosphere has a spectral distribution.
The X - rays and other very short wave radiation of solar spectrum are absorbed
highly in the ionosphere by Nitrogen, Oxygen, and other atmospheric components. Most of the ultraviolet is absorbed by Ozone. Water vapor
absorbs strongly in the infrared bands beyond 2.3
in (Long-wave
radiation). The transmission of atmosphere is very low
due to absorption by HO2 and CO2. The energy in the extraterrestrial solar spectrum is less than 5% of the
total solar spectrum, and energy received at the ground is
relatively small [I]. The flux density of downward long-wave
radiation at the ground must be determined by any study attempted to evaluate the surface net radiation from its components. The
net radiation intensity is of a major
influence in controlling the weather loss from irrigated crops. On the other hand, it is of a great importance in many
fields such as agriculture, hydrology
and meteorology [2], and it is an important element of the energy balance of buildings, greenhouses, solar
collectors and vegetation leaves [3].
The downward long-wave radiative flux on a cloudless night is due mainly to emission by water vapor, carbon dioxide and particulate matter.
Many workers have attempted to correlate empirically the long-wave radiation and weather elements such as water vapor pressure and the screen temperature. For example, [4-6] investigated such correlation under clear sky weather condition. Where cloudy condition were correlated by [7] and [8]. This paper estimates the longwave radiation fraction from global solar radiation. The diffuse fraction were also estimated and compared with the corresponding values of (I.R.) radiation. On the other hand, empirical correlation between the (I.R.) and the weather parameters such as screen temperature (TC 0 ), surface cloud (sc.) and water vapor pressure (e) were also carried out and discussed for daily and monthly data.
Results and discussions
I- Incoming Ions-wave and global solar radiation distribution:
Table (1) with figure (1) shows the monthly variation of the infrared (I.R.), global (G) and the long-wave solar fraction ratio (I.R./ G) at Cairo for three years period. From the table and the Figure, it can be seen that, the infrared and global solar radiation show a systematic variation and have the same trend, where the maximum values occurred in June and the minimum in January and December. The (I.R/G) ratio showed to have a reverse trend as compared with the (I.R.) and (G) components, where such ratio has (4.4) in winter and (2.7) in summer. The annual average values were (18.1, 58.3 and 3.56 for global, Infrared and (I.R/G) respectively, see Table (1).
2- Incoming long-wave and diffuse index (D / G) distribution:
Measured data for each global (G), diffuse (D) solar radiation and incoming long-wave radiation (I.R,), besides the estimated values of diffuse index (D/G) for the three years were listed in table (2) and the annual monthly average values of IR, G , D and D/G illustrated by Figure (2). Such table and figure shows that the global, diffuse, and I.R. have the same trend. Where the maximum values were in the summer (warm season) and the minimum values were in winter (cold season). For diffuse index (D/G), the picture is completely reversed. Generally, the incoming long-wave radiation is emitted from aerosol, clouds, dust, and other atmospheric pollutants. When the atmosphere is relatively free from such pollutants, the values of diffuse index decreased. On the other hand, existence of such pollutants increased the infrared radiation due to the absorption of the short wave solar radiation. The above-mentioned fact is confirmed by Figure (2).
3-Long-wave radiation and weather parameters correlation:
This section tried to investigate empirical relationships used in estimating the values of I.R. from other weather parameters, such as screen temperature, water vapor pressure, and surface cloud for daily and monthly data. The proposed equation is:
Y = A + BX
Where Y is the incoming longwave radiation (I. R.), A and B are the regression coefficients and X is the element under consideration.
3.a- Daily measurements data.
To evaluate the correlation and regression coefficients, we used daily data for the incoming long-wave radiation (I.R) in MJ/ m2 , water vapor pressure (e) in m. b., and surface cloud (sc). These data have been processed and analyzed using an advanced computer program. The obtained values for the correlation coefficient (R) and the regression coefficients (A and B) were listed in table (3a). From this table, one can notice that, the correlation coefficient between I.R and (TC0) was better as compared with the other weather parameters. Where the (I.R.) to (TC0) correlation coefficients were 96%, 93% and 92% for the considered three years respectively, see table (3a). For (I.R.) to (e) correlation such values were 69%, 59% and 72% for the same years. For clouds, the correlation coefficients were 69%, 58% and 67%. Considering the average values, the (I.R.) to temperature correlation is the best, where its value was 93%. For (e) and (sc.), the corresponding values were 67% and 64% respectively.
3.b- The monthly data.
Values of each correlation and regression coefficient for monthly data were evaluated using the same method of daily data. The obtained results were listed in table (3b). From table (3b) we noticed the following:
- The correlation coefficient for monthly data is better as compared with the daily values, and this may be due to the reduction of the numbers of data.
2. The correlation coefficients for temperature are better as compared with the other parameters.
3. The overall monthly average, correlation coefficients between (I.R) and (TC0) were 97%, while for (e) and (sc.) were 84% and 85% respectively.
Conclusions
Considering our investigated correlation, we can conclude that the incoming long-wave solar radiation is strongly correlated with the ambient air temperature, where the correlation coefficient (R) was 93% and 97% for daily and monthly data respectively. At Cairo, due to the effect of buildings and the highly pollution potential, the (I.R.) to (TC0 ) correlation is of importance for the renewable energy application at the plateau region. On the other hand, our correlation equations can be used with confidence for calculating the (I.R.) fraction of solar radiation from easily measured parameters, such as air temperature, surface cloud, and water vapor pressure.
The incoming long-wave solar radiation data is of great importance when considering the agricultural, hydrology, and architecture applications.
References
1- Arnfield, A. J. "Evaluation of empirical expressions for the estimation of hourly and daily totals of atmospheric long-wave emission under all sky conditions ". Quart. J. R. Met. Soc. 105, pp. 1041-1052 (1979).
2- Marc Aubinet. "Long-wave Sky Radiation Parameter izations" Solar energy. Vol. 53, No. 2, pp. 134 - 147, (1994).
3- Brunt, D. " Notes on radiation in the atmospheres ". Quart, J. R, Met. Soc. 58, 389-418,(1932).
4- ldso, S, B., and R. D. Jackson. "Thermal radiation from the atmosphere". J. Geophysics Res.74, 5397 - 5403, (1960).
5- John, A. Dliffie and William A. Beckman. "Solar energy thermal processes" pp. 9 - 11, ( 1974).
6- Orsini, A., Tomasi, C., Calzolari, F., Nardino, M., Cacciari, A. and Georgiadis, T., Atmospheric Research, 61(4), 251 (2002).
7- Sellers, H. B. "Physical climatology, university of Chicago press, Chicago, (1965).
8- Salem, A. I., and Trabea, A. A., "Day light and day night net long-wave radiation calculations at selected sites in Egypt". Cairo third international conference on renewable Energy sources, vol. I pp. 257-266, Dec. 29 (92) / Jan. 2 (1993), Cairo, Egypt, (1993).
Table (1): Average monthly data for G, IR (MJ m -2 day -1) and IR/G ratio for the considered three years at Cairo.
|
Month |
1998 |
1999 |
2000 |
Average over a |
||||||||
|
G |
IR |
IR/G |
G |
IR |
IR/G |
G |
IR |
IR/G |
G |
IR |
IR/G |
|
|
J |
10.3 |
51.6 |
5.01 |
9.8 |
N |
N |
9.8 |
50.6 |
5.16 |
10.1 |
51.1 |
5.11 |
|
F |
13.6 |
51.9 |
3.82 |
13.2 |
54.5 |
4 |
11.6 |
50.8 |
4.38 |
12.8 |
52.4 |
4.09 |
|
M |
18.6 22.2 |
55.4 |
2.96 |
16.7 |
54.6 |
1 3 3 |
17.4 |
50.8 |
2.92 |
17.6 |
53.6 |
3.05 |
|
A |
22.2 |
59.2 |
2 62 |
19.9 |
60.2 |
3.0 |
21.6 |
59.2 |
2.74 |
21.2 |
59.5 |
2.81 |
|
M |
26.1 26.9 25.7 23.6 20.6 16.1 |
62.4 |
2.39 |
22.8 |
61.8 |
2.7 |
23.4 |
61.1 |
2.61 |
24.1 |
61.8 |
2.56 |
|
J |
26.9 |
64.5 |
2.40 |
25.9 |
64.5 |
2.5 |
25.5 |
65.2 |
2 57 |
26.1 |
64.7 |
2.4S |
|
J |
25.7 |
64.6 |
2.51 |
24.9 |
63.7 |
2.6 |
25.3 |
64.1 |
2.53 |
25.3 |
62.1 |
2.53 |
|
A |
23.6 |
63.6 |
2.68 |
22.5 |
62.9 |
2.8 |
23.3 |
64.6 |
2.78 |
23.1 |
63.6 |
2.75 |
|
S |
20.6 |
62.1 |
3.01 |
19.8 |
61.8 |
3.1 |
19.7 |
62.3 |
3.16 |
20.0 |
62.1 |
3.11 |
|
O |
16.1 |
N |
N |
15.8 |
60.9 |
3.9 |
16.3 |
62.6 |
3.82 |
16.1 |
61.5 |
3.82 |
|
N |
11.9 |
N |
N |
11.0 |
56.1 |
5.1 |
11.5 |
56.4 |
4.90 |
11.5 |
56.3 |
4.90 |
|
D |
10.3 |
N |
N |
9..0 |
51.0 |
5.7 |
9.0 |
51.5 |
5.72 |
9.4 |
51.3 |
5.46 |
|
Annual Mean |
|
|
|
|
|
|
|
|
|
18.1 |
58.3 |
3.56 |
Table (2): Average monthly values of G, D (MJ m -2 day -1) and D/G ratio during the same period at Cairo.
|
Month |
1998 |
1999 |
2000 |
Average over a |
||||||||
|
G |
D |
D/G |
G |
D |
D/G |
G |
D |
D/G |
G |
D |
D/G |
|
|
J |
10.3 |
4.7 |
0.45 |
9.8 |
N |
N |
9.8 |
5.99 |
0.61 |
10.1 |
5.33 |
0.53 |
|
F |
13.6 |
5.5 |
0.40 |
13.2 |
5.8 |
0.44 |
11.6 |
7 03 |
0.61 |
12.8 |
6.10 |
0.48 |
|
M |
18.6 |
6.7 |
0.36 |
16.7 |
6.8 |
0.41 |
17.4 |
9.X8 |
0.57 |
17.6 |
7.79 |
0.44 |
|
A |
22.2 |
7.8 |
0.35 |
19.9 |
8.5 |
0.42 |
21.6 |
10.3 |
0.48 |
21.2 |
8.86 |
0.42 |
|
M |
26 1 |
7.0 |
0.27 |
22.8 |
9.5 |
0.42 |
23.4 |
11.3 |
0.49 |
24.1 |
9 27 |
0.38 |
|
J |
26.9 |
6.7 |
0.25 |
25.9 |
7.8 |
0.30 |
25.5 |
9.62 |
0.38 |
26.1 |
8.05 |
0.31 |
|
J |
25.7 |
7.5 |
0.29 |
24.9 |
7.9 |
0.32 |
25.3 |
9.18 |
0.36 |
25.3 |
8.18 |
0.32 |
|
A |
23.6 |
7.5 |
0.32 |
22.5 |
8.0 |
0.63 |
23.3 |
8.14 |
0.53 |
23.1 |
7.78 |
0.34 |
|
S |
20.6 |
6.7 |
0.33 |
19.8 |
7.3 |
0.37 |
19.7 |
7.83 |
0.40 |
20.0 |
7.27 |
0.36 |
|
O |
16.1 |
5.4 |
0 14 |
15.8 |
6.1 |
0.39 |
16.3 |
6.55 |
0.40 |
16.1 |
6.60 |
0.37 |
|
N |
11.9 |
4.5 |
0.39 |
11.0 |
5.4 |
0.49 |
11.5 |
5.13 |
0.46 |
11.5 |
5.05 |
0.44 |
|
D |
10.3 |
3.9 |
0.38 |
9.0 |
5.0 |
0.56 |
9.0 |
4.48 |
0.50 |
9.4 |
4.44 |
0.47 |
Table (3): Daily and monthly correlation between IR and the weather parameters at Cairo (a) for daily Data and (b) for Monthly data
|
Year |
Element |
(a) |
(b) |
||||
|
R |
A |
B |
R |
A |
B |
||
|
1998 |
T |
0.96 |
-8.2 |
0.94 |
0.99 |
-20.07 |
0.082 |
|
Sc. |
-0.69 |
2.6 |
-0.10 |
-0.94 |
6.25 |
-0.636 |
|
|
e |
0.69 |
2.03 |
0.03 |
0.85 |
4.88 |
0.087 |
|
|
1999 |
T |
0.93 |
-7.9 |
0.04 |
0.95 |
-16.47 |
0.078 |
|
Sc. |
-0.58 |
2.59 |
-0.07 |
-0.78 |
6.62 |
-0.312 |
|
|
e |
0.59 |
2.1 |
0.02 |
0.75 |
4.88 |
0.069 |
|
|
2000 |
T |
0.92 |
-8.2 |
0.02 |
0.95 |
-2189 |
0.094 |
|
Sc. |
-0.67 |
1.9 |
-0.03 |
-0.91 |
6.62 |
-0.312 |
|
|
e |
0.72 |
1.9 |
0.03 |
0.89 |
4.28 |
0.107 |
|
|
Average 90-92 |
T |
0.93 |
-8.13 |
0.04 |
0.97 |
-20.02 |
0.088 |
|
Sc. |
-0.64 |
2.61 |
-0.10 |
-0.85 |
6.63 |
-0.488 |
|
|
e |
0.67 |
2.04 |
0.03 |
0.84 |
4.55 |
0.989 |
|
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