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Estimating surface albedo over Saudi Arabia
Renewable Energy 34 (2009) 1607–1610
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Estimating surface albedo over Saudi Arabia A.H. Maghrabi*, Z.A. Al-Mostafa King Abdulaziz City for Science and Technology, Astronomy and Geophysics Research Institute, Astronomy Department, P. O. Box 6086, Riyadh 11442, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2008 Accepted 13 November 2008 Available online 22 January 2009
Using ten years of solar radiation data, the surface albedo of twenty-four sites in the Kingdom of Saudi Arabia was calculated, and annual, seasonal, and geographical variations were investigated. The selected sites encompass a wide range of atmospheric conditions. The mean annual albedo values range from 0.15 to 0.54, and show high variability between different sites and even at individual sites. The differences between the maximum and minimum albedo range from 0.02 to 0.44. The average albedo over the entire kingdom is 0.31 0.05. Seasonal investigations revealed that the lowest albedo values occurred during the summer and the highest in the winter. In examining the variations of the calculated albedo with the altitude, sites were divided into three groups: low altitude (0–500 m), middle altitude (500–1000 m), and high mountain sites (higher than 1000 m); the mean albedo values for each category are 0.32, 0.31, and 0.28, respectively. In studying the effects of latitude on albedo values, the sites were also divided into three groups: low latitudes (15–20), middle latitudes (20–25), and high latitudes (>25), for which the mean monthly albedo values are 0.25, 0.30, and 0.31, respectively. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Solar energy Global solar radiation Energy budget Clear sky Albedo
1. Introduction Part of the incident energy striking a surface may be absorbed, reflected, and transmitted. The surface properties associated with these three processes are absorptivity, reflectivity, and transmissivity. The fractions of the total incident energy associated with these properties are termed as absorptance, reflectance, and transmittance. However, when the sun is the source of the incident energy, the term albedo (a) is commonly used instead of reflectance. In general, albedo strongly depends on the surface properties of the material that the sunlight strikes, as well as the incident direction and the hemispherical distribution of the incoming radiation. Studies of atmospheric heat balance have shown that 17.5% of the incident radiation is absorbed by the atmosphere, 47.5% is absorbed by the earth, and that the surface albedo of the earth reflects 35% back into space [1]. A reliable estimate of surface albedo is an important factor in evaluating the total insolation of a building or solar energy collecting device, and for studies dealing with the energy balance of the atmosphere. In order to more accurately account for the diverse effects of physical, agronomic and biological processes, changes of surface temperature, and climate variation, more information is needed about surface albedo. A number of ground-based, aircraft, and satellite measurements of albedo over forested areas and non-forested vegetation have been reported in literature ([2], and references therein). * Corresponding author. Tel.: þ966 1 481 4302; fax: þ966 1 481 3521. E-mail address: [email protected] (A.H. Maghrabi). 0960-1481/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2008.11.025
The present paper is a continuation of our work dealing with the atmosphere above Saudi Arabia [3–5]. The aim of the present study is to calculate the surface albedo over Saudi Arabia and to investigate its variability. 2. Observational data The data utilized in the present study were taken from the Saudi Arabian Solar Radiation Atlas, published by the Saudi Arabian National Center for Science and Technology [6] (presently King Abdulaziz City for Science and Technology KACST). The data given in the atlas include mean monthly values of global solar radiation on the horizontal surface, the duration of sunshine, and the number of daylight hours for the period between 1971 and 1981. All values presented in the atlas represent daily totals based on ten years of homogeneous data. 3. Data reduction Following the procedure of Alnaser [7], the surface albedo a can be calculated from the following equations:
Hm ¼ H 0 1 a 0:25 S=S0o þ 0:6 1 0:25 S=S0o 0
(1)
where H is the monthly average of daily global irradiation that strikes the surface, obtained either by Hay’s [8] modification of Angstrom’s equation [9]
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A.H. Maghrabi, Z.A. Al-Mostafa / Renewable Energy 34 (2009) 1607–1610
Table 1 The mean monthly clear-sky surface albedo values of all stations. City
Alt.a
Lat.b
Long.b
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Abha AlHofuf AlKharj AlNumas Biljuarshy Bisha Derab Hail Khulays Madina Najran Qatif Qurayyat Riyadh Sabya Sakaka Shaqra Sulayyil Tabarjal Tabuk Taif Tayma Turabah Unayzah
2200 160 430 2600 2040 1020 4 1010 60 590 1250 8 2 564 40 574 730 600 3 773 1530 820 1130 724
18.22 25.50 22.13 26.55 19.85 20.02 24.42 27.47 18.73 24.52 17.55 31.33 24.57 21.62 17.17 19.00 25.25 20.47 28.38 21.23 27.63 21.40 26.62 26.07
42.48 49.57 39.43 42.15 41.57 42.60 46.57 41.63 41.40 39.58 44.23 37.35 46.72 40.42 42.62 41.88 45.25 45.57 36.58 40.35 38.48 40.45 37.85 43.98
0.31 0.37 0.25 0.20 0.56 0.54 0.38 0.20 0.36 0.37 0.25 0.38 0.22 0.37 0.57 0.25 0.39 0.42 0.49 0.53 0.26 0.28 0.34 0.33
0.26 0.36 0.32 0.35 0.54 0.50 0.35 0.18 0.35 0.36 0.25 0.35 0.39 0.48 0.52 0.26 0.35 0.59 0.46 0.56 0.27 0.30 0.36 0.36
0.22 0.33 0.37 0.31 0.51 0.43 0.36 0.17 0.22 0.12 0.24 0.33 0.38 0.34 0.64 0.20 0.35 0.28 0.46 0.59 0.25 0.32 0.36 0.30
0.20 0.33 0.36 0.31 0.52 0.42 0.34 0.13 0.18 0.12 0.21 0.32 0.23 0.35 0.46 0.21 0.31 0.29 0.45 0.53 0.25 0.21 0.35 0.29
0.19 0.31 0.26 0.24 0.45 0.35 0.27 0.11 0.14 0.19 0.11 0.31 0.25 0.32 0.20 0.18 0.23 0.28 0.43 0.56 0.21 0.20 0.35 0.22
0.17 0.30 0.27 0.25 0.46 0.35 0.18 0.10 0.15 0.14 0.18 0.24 0.26 0.35 0.21 0.18 0.11 0.22 0.41 0.55 0.20 0.21 0.34 0.25
0.15 0.29 0.27 0.10 0.20 0.34 0.25 0.13 0.14 0.15 0.13 0.25 0.17 0.37 0.25 0.18 0.18 0.26 0.32 0.51 0.21 0.21 0.34 0.23
0.19 0.29 0.25 0.11 0.22 0.46 0.26 0.18 0.15 0.32 0.16 0.31 0.11 0.24 0.46 0.21 0.13 0.22 0.32 0.52 0.22 0.22 0.35 0.22
0.17 0.30 0.23 0.12 0.43 0.48 0.30 0.18 0.23 0.42 0.26 0.34 0.19 0.27 0.51 0.21 0.26 0.29 0.31 0.54 0.21 0.23 0.36 0.24
0.18 0.33 0.22 0.26 0.47 0.48 0.35 0.19 0.32 0.31 0.21 0.32 0.12 0.25 0.53 0.26 0.29 0.44 0.48 0.55 0.21 0.24 0.35 0.24
0.26 0.36 0.22 0.16 0.47 0.50 0.34 0.11 0.38 0.26 0.20 0.35 0.28 0.36 0.50 0.28 0.29 0.61 0.48 0.52 0.21 0.24 0.36 0.28
0.29 0.36 0.22 0.19 0.54 0.52 0.36 0.12 0.37 0.24 0.21 0.36 0.27 0.35 0.64 0.23 0.25 0.53 0.48 0.52 0.24 0.25 0.36 0.31
a b
Alt. is the site altitude in meters. Lat. and Long. are site latitude and longitude respectively in degrees.
H0 Ho A þ BS=S0o
(2)
or by Jain’s formula [10]
H0 ¼ Ho ½A þ ðBS=So Þ
(3)
0
Hm, S and So are monthly averages of measured daily global irradiation, So monthly averages of daily sunshine duration, and monthly averages of the daily maximum possible duration of sunshine hours, on a Campbell-Stokes sunshine recorder, respectively. These values are related as follows:
Hm ¼ Ho A þ BS=S0o S0o ¼ 2=15cos1
h i cos 85 sin Lsin d =ðcos Lcos dÞ
(4) (5)
where L is the latitude of the site in degrees, d the declination angle of the sun in degrees, and Ho the estimation of the monthly average of daily extraterrestrial irradiation as given by Duffie and Beckman [11]. The values of the regression constants in equations (3) and (4) for each month of the year were obtained using Zabara’s equation [12]:
A ¼ 0:395 1:247ðS=So Þ þ 2:680ðS=So Þ2 1:674ðS=So Þ3 B ¼ 0:395 þ 1:384ðS=So Þ2 3:249ðS=So Þ2 þ2:055ðS=So Þ3 4. Results and discussion Table 1 presents the monthly mean clear-sky surface albedo values for the twenty-four stations in Saudi Arabia, calculated using the method described above. 4.1. Annual variations The annual mean, maximum, and minimum values of the surface albedo are presented in Table 2. These values were obtained by averaging the monthly values of a for each station. The lowest
mean value occurred in Hail (0.15), while the highest was recorded in Tabuk (0.54). Although Derab and Al Kharj are not far from the capital Riyadh, their average albedo values differ from that of the latter by 0.3 and 0.7, respectively. These differences may be due to the effects of vegetation in Al Kharj and of the low altitude of Derab. Fig. 1 illustrates the frequency distribution of the calculated mean a values. The surface albedo of eight stations falls within the range of 0.2–0.25, while three stations show values between 0.25 and 0.30, and five stations have values ranging from 0.30 to 0.35. Furthermore, two stations show albedo values less than 0.2, and six stations have values greater than 0.35. The overall averaged surface albedo (mean average for all stations) in Saudi Arabia is 0.31 0.05, which is comparable to the international value used in energy budget calculations and to values reported from elsewhere, such as that of 0.33 in Bahrain [7]. Table 2 The annual mean, maximum, and minimum surface albedo values.
Abha AlHofuf AlKharj AlNumas Biljuarshy Bisha Derab Hail Khulays Madina Najran Qatif Qurayyat Riyadh Sabya Sakaka Shaqra Sulayyil Tabarjal Tabuk Taif Tayma Turabah Unayzah
Range
Minimum
Maximum
Mean
0.16 0.08 0.15 0.25 0.36 0.20 0.20 0.10 0.24 0.30 0.15 0.14 0.28 0.24 0.44 0.10 0.28 0.39 0.18 0.08 0.07 0.12 0.02 0.14
0.15 0.29 0.22 0.10 0.20 0.34 0.18 0.10 0.14 0.12 0.11 0.24 0.11 0.24 0.20 0.18 0.11 0.22 0.31 0.51 0.20 0.20 0.34 0.22
0.31 0.37 0.37 0.35 0.56 0.54 0.38 0.20 0.38 0.42 0.26 0.38 0.39 0.48 0.64 0.28 0.39 0.61 0.49 0.59 0.27 0.32 0.36 0.36
0.22 0.33 0.27 0.22 0.45 0.45 0.31 0.15 0.25 0.25 0.20 0.32 0.24 0.34 0.46 0.22 0.26 0.37 0.42 0.54 0.23 0.24 0.35 0.27
A.H. Maghrabi, Z.A. Al-Mostafa / Renewable Energy 34 (2009) 1607–1610
1609
0.4 0.38 0.36
Albedo
0.34 0.32 0.3 0.28 0.26 0.24 0.22 0.2
1
2
3
4
5
6
7
8
9
10
11
12
Month Fig. 3. The averaged monthly mean albedo values of all stations.
Fig. 1. Frequency distributions of the calculated mean albedo values.
The maximum values range from 0.20 at Hail to 0.64 at Sabya, while the minimum values range between 0.51 at Tabuk to 0.10 at Hail and Al Numas. However, the differences between maximum and minimum albedo values at individual sites are also noteworthy. As shown in Fig. 2, the lowest difference was 0.02 at the Turabah site, while the highest difference of 0.44 was recorded at Sabya. The average difference between the maximum and the minimum albedo values is 0.25 for the rest of the stations. The existence of such differences between the maximum and minimum albedo values at individual stations may be due to extreme changes in the weather conditions from one season to another. This includes temperature changes and aerosol concentrations in the air. However, such broad range of a values over large areas have been reported previously. Iqbal [13] presented tables of the regional albedo values of sites in the United States and Canada, in which the surface albedo values for the sites in the U.S. vary between 0.14 and 0.66, while those of the Canadian sites range between 0.18 and 0.62. 4.2. Seasonal variations Fig. 3 shows the mean monthly albedo values of all the stations in Saudi Arabia. It demonstrates the characteristic seasonal variation of a values, with highs in winter (December, January and
0.7 0.6
Albedo
0.5 0.4 0.3 0.2 Sabya Turabah
0.1 0
1
2
3
4
5
6
7
8
9
10
11
12
Month Fig. 2. The mean monthly albedo values averaged for the Sabya and Turbah stations.
February) and lows in the months of June and July. The albedo decreases during March and reaches a minimum in July. In August the albedo starts to increase again, until it reaches a maximum in February. Such a pattern was previously observed in the neighboring country of Bahrain [7] and at some German sites [14]. However, a degree of deviation from this trend is evident at some stations (see Table 1). For example, some stations show higher albedo values in the month of July than in May. In response to this observed trend, we divided the data into four seasonal groups: spring (March, April and May), summer (June, July and August), autumn (September, October and November), and winter (December, January and February). The mean albedo values for spring, summer, autumn, and winter are 0.30, 0.25, 0.31, and 0.36, respectively. These values are consistent with the findings of seasonal variations in albedo values in Saudi Arabia. An increase of 0.11 in the albedo from summer to winter is noteworthy. 4.3. Geographical variations We further investigated the effect of a site’s altitude and latitude on the calculated albedo values. For altitude, the data were divided into the following three groups: low altitude (LA), 0–500 m; middle altitude (MA), 500–1000 m; and upper mountain sites (UM) at altitudes greater than 1000 m. The mean monthly a values for these three groups are 0.32, 0.31, and 0.28, respectively. This shows that albedo increases as altitude decreases. Interestingly, this is the opposite of what Iziomon and Mayer [14] found in their investigations of the surface albedo at three German sites of different altitudes. However, this trend is not uniform over all of the sites in Saudi Arabia. Some low altitude sites, such as Derab, show lower albedo values than do high altitude sites, such as Bisha. The latitudinal variations of surface albedo in Saudi Arabia were studied by dividing the sites into three groups: low latitudes (LL), 15–20; middle latitudes (ML), 20–25; and high latitudes (HL), greater than 25. The mean monthly albedo values for these three groups were 0.25, 0.30, and 0.31, respectively. This indicates that surface albedo generally increases with latitude. However, as in the correlation with altitude, the trend is not completely uniform, as some high latitude stations show lower albedo than do those at lower latitudes (e.g. Hail for the former and Sabya for the latter). Furthermore, some stations located at the same latitude (e.g. Madina and Derab) show different albedo values. This variation is probably due to differences in ground cover and altitude. When two locations have different average solar altitudes, their albedo values will also differ. Additionally, the surface albedos of two locations that have the same latitude but different sunshine conditions may likewise differ.
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A.H. Maghrabi, Z.A. Al-Mostafa / Renewable Energy 34 (2009) 1607–1610
5. Conclusion Using solar radiation data, the clear-sky surface albedo values of twenty-four sites in Saudi Arabia were calculated and their variations were investigated. Mean annual values ranged from 0.15 to 0.54. The albedo values showed high variability between sites and even at individual sites. The average albedo over Saudi Arabia was found to be 0.31 0.05. The lowest albedo values occurred in the summer (0.25), and the highest in the winter (0.36). In investigating the effect of the site’s altitude and latitude on the calculated albedo, we found that albedo values tend to increase at higher latitudes, and to decrease at higher altitudes. Such findings will be of assistance to future energy balance studies and solar energy applications. Acknowledgments We would like to thank Dr. Naif Alabadi from the Energy Research Institute at the King Abdulaziz City for Science and Technology, for providing us with important data. References [1] London J. Study of atmospheric heat balance. Final report contract no. AF. 19-165. NY: New York University; 1957.
[2] Pinker RT, Laszlo I, Goodrich D, Pandithurai G. Satellite estimates of surface radiative fluxes for the extended San Pedro basin: sensitivity to aerosols. Agricultural and Forest Meteorology 2000;105:43–54. [3] Al-Mostafa ZA, Maghrabi AH. The Saudi Arabian’s sky II: the turbidity post the second Gulf War. Adelaide, Australia. In: ISES 2001 solar world congress, vol. 4. 2001, p. 2013–18. [4] Al-Mostafa ZA, Maghrabi AH. The Saudi Arabian’s sky III: the clear sky emissivity over Saudi Arabia, Adelaide, Australia. In: ISES 2001 solar world congress, vol. 4. 2001, p. 2019–24. [5] Al-Mostafa ZA. Calculation of sky turbidity in the kingdom of Saudi Arabia. Journal of the Association of Arab Universities for Basic and Applied Sciences ‘‘JAAUBAS’’ 2005;1:1–11. [6] Saudi Arabian Solar Radiation Atlas. Riydah: Saudi Arabian National Center For Science and Technology; 1983. [7] Alnaser W. Calculation of the surface albedo of Bahrain from solar energy data. Energy 1989;14:551–6. [8] Hay JE. Calculation of monthly mean solar radiation for horizontal and inclined surfaces. Solar Energy 1979;23:301–7. [9] Angstro¨m A. Solar and terrestrial radiation. Q J R Meteorol Soc 1924;50:121–5. [10] Jain PC. Solar irradiation in Zambia, internal report. Trieste, Italy: International Centre for Theoretical Physics, vol. 3. 1988, p. 323. [11] Duffie JA, Beckman WA. Solar engineering of thermal process. New York: Wiley; 1991. [12] Zabara K. Estimation of the global solar radiation in Greece. Solar and Wind Technology 1986;7:267–72. [13] Iqbal M. An introduction to solar radiation. 3rd ed. Toronto: Academic Press; 1983. [14] Iziomon MG, Mayer H. On the variability and modeling of surface albedo and long-wave radiation components. Agricultural and Forest Meteorology 2002;111:141–52.
Contents lists available at ScienceDirect
Renewable Energy journal homepage: www.elsevier.com/locate/renene
Estimating surface albedo over Saudi Arabia A.H. Maghrabi*, Z.A. Al-Mostafa King Abdulaziz City for Science and Technology, Astronomy and Geophysics Research Institute, Astronomy Department, P. O. Box 6086, Riyadh 11442, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2008 Accepted 13 November 2008 Available online 22 January 2009
Using ten years of solar radiation data, the surface albedo of twenty-four sites in the Kingdom of Saudi Arabia was calculated, and annual, seasonal, and geographical variations were investigated. The selected sites encompass a wide range of atmospheric conditions. The mean annual albedo values range from 0.15 to 0.54, and show high variability between different sites and even at individual sites. The differences between the maximum and minimum albedo range from 0.02 to 0.44. The average albedo over the entire kingdom is 0.31 0.05. Seasonal investigations revealed that the lowest albedo values occurred during the summer and the highest in the winter. In examining the variations of the calculated albedo with the altitude, sites were divided into three groups: low altitude (0–500 m), middle altitude (500–1000 m), and high mountain sites (higher than 1000 m); the mean albedo values for each category are 0.32, 0.31, and 0.28, respectively. In studying the effects of latitude on albedo values, the sites were also divided into three groups: low latitudes (15–20), middle latitudes (20–25), and high latitudes (>25), for which the mean monthly albedo values are 0.25, 0.30, and 0.31, respectively. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Solar energy Global solar radiation Energy budget Clear sky Albedo
1. Introduction Part of the incident energy striking a surface may be absorbed, reflected, and transmitted. The surface properties associated with these three processes are absorptivity, reflectivity, and transmissivity. The fractions of the total incident energy associated with these properties are termed as absorptance, reflectance, and transmittance. However, when the sun is the source of the incident energy, the term albedo (a) is commonly used instead of reflectance. In general, albedo strongly depends on the surface properties of the material that the sunlight strikes, as well as the incident direction and the hemispherical distribution of the incoming radiation. Studies of atmospheric heat balance have shown that 17.5% of the incident radiation is absorbed by the atmosphere, 47.5% is absorbed by the earth, and that the surface albedo of the earth reflects 35% back into space [1]. A reliable estimate of surface albedo is an important factor in evaluating the total insolation of a building or solar energy collecting device, and for studies dealing with the energy balance of the atmosphere. In order to more accurately account for the diverse effects of physical, agronomic and biological processes, changes of surface temperature, and climate variation, more information is needed about surface albedo. A number of ground-based, aircraft, and satellite measurements of albedo over forested areas and non-forested vegetation have been reported in literature ([2], and references therein). * Corresponding author. Tel.: þ966 1 481 4302; fax: þ966 1 481 3521. E-mail address: [email protected] (A.H. Maghrabi). 0960-1481/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.renene.2008.11.025
The present paper is a continuation of our work dealing with the atmosphere above Saudi Arabia [3–5]. The aim of the present study is to calculate the surface albedo over Saudi Arabia and to investigate its variability. 2. Observational data The data utilized in the present study were taken from the Saudi Arabian Solar Radiation Atlas, published by the Saudi Arabian National Center for Science and Technology [6] (presently King Abdulaziz City for Science and Technology KACST). The data given in the atlas include mean monthly values of global solar radiation on the horizontal surface, the duration of sunshine, and the number of daylight hours for the period between 1971 and 1981. All values presented in the atlas represent daily totals based on ten years of homogeneous data. 3. Data reduction Following the procedure of Alnaser [7], the surface albedo a can be calculated from the following equations:
Hm ¼ H 0 1 a 0:25 S=S0o þ 0:6 1 0:25 S=S0o 0
(1)
where H is the monthly average of daily global irradiation that strikes the surface, obtained either by Hay’s [8] modification of Angstrom’s equation [9]
1608
A.H. Maghrabi, Z.A. Al-Mostafa / Renewable Energy 34 (2009) 1607–1610
Table 1 The mean monthly clear-sky surface albedo values of all stations. City
Alt.a
Lat.b
Long.b
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Abha AlHofuf AlKharj AlNumas Biljuarshy Bisha Derab Hail Khulays Madina Najran Qatif Qurayyat Riyadh Sabya Sakaka Shaqra Sulayyil Tabarjal Tabuk Taif Tayma Turabah Unayzah
2200 160 430 2600 2040 1020 4 1010 60 590 1250 8 2 564 40 574 730 600 3 773 1530 820 1130 724
18.22 25.50 22.13 26.55 19.85 20.02 24.42 27.47 18.73 24.52 17.55 31.33 24.57 21.62 17.17 19.00 25.25 20.47 28.38 21.23 27.63 21.40 26.62 26.07
42.48 49.57 39.43 42.15 41.57 42.60 46.57 41.63 41.40 39.58 44.23 37.35 46.72 40.42 42.62 41.88 45.25 45.57 36.58 40.35 38.48 40.45 37.85 43.98
0.31 0.37 0.25 0.20 0.56 0.54 0.38 0.20 0.36 0.37 0.25 0.38 0.22 0.37 0.57 0.25 0.39 0.42 0.49 0.53 0.26 0.28 0.34 0.33
0.26 0.36 0.32 0.35 0.54 0.50 0.35 0.18 0.35 0.36 0.25 0.35 0.39 0.48 0.52 0.26 0.35 0.59 0.46 0.56 0.27 0.30 0.36 0.36
0.22 0.33 0.37 0.31 0.51 0.43 0.36 0.17 0.22 0.12 0.24 0.33 0.38 0.34 0.64 0.20 0.35 0.28 0.46 0.59 0.25 0.32 0.36 0.30
0.20 0.33 0.36 0.31 0.52 0.42 0.34 0.13 0.18 0.12 0.21 0.32 0.23 0.35 0.46 0.21 0.31 0.29 0.45 0.53 0.25 0.21 0.35 0.29
0.19 0.31 0.26 0.24 0.45 0.35 0.27 0.11 0.14 0.19 0.11 0.31 0.25 0.32 0.20 0.18 0.23 0.28 0.43 0.56 0.21 0.20 0.35 0.22
0.17 0.30 0.27 0.25 0.46 0.35 0.18 0.10 0.15 0.14 0.18 0.24 0.26 0.35 0.21 0.18 0.11 0.22 0.41 0.55 0.20 0.21 0.34 0.25
0.15 0.29 0.27 0.10 0.20 0.34 0.25 0.13 0.14 0.15 0.13 0.25 0.17 0.37 0.25 0.18 0.18 0.26 0.32 0.51 0.21 0.21 0.34 0.23
0.19 0.29 0.25 0.11 0.22 0.46 0.26 0.18 0.15 0.32 0.16 0.31 0.11 0.24 0.46 0.21 0.13 0.22 0.32 0.52 0.22 0.22 0.35 0.22
0.17 0.30 0.23 0.12 0.43 0.48 0.30 0.18 0.23 0.42 0.26 0.34 0.19 0.27 0.51 0.21 0.26 0.29 0.31 0.54 0.21 0.23 0.36 0.24
0.18 0.33 0.22 0.26 0.47 0.48 0.35 0.19 0.32 0.31 0.21 0.32 0.12 0.25 0.53 0.26 0.29 0.44 0.48 0.55 0.21 0.24 0.35 0.24
0.26 0.36 0.22 0.16 0.47 0.50 0.34 0.11 0.38 0.26 0.20 0.35 0.28 0.36 0.50 0.28 0.29 0.61 0.48 0.52 0.21 0.24 0.36 0.28
0.29 0.36 0.22 0.19 0.54 0.52 0.36 0.12 0.37 0.24 0.21 0.36 0.27 0.35 0.64 0.23 0.25 0.53 0.48 0.52 0.24 0.25 0.36 0.31
a b
Alt. is the site altitude in meters. Lat. and Long. are site latitude and longitude respectively in degrees.
H0 Ho A þ BS=S0o
(2)
or by Jain’s formula [10]
H0 ¼ Ho ½A þ ðBS=So Þ
(3)
0
Hm, S and So are monthly averages of measured daily global irradiation, So monthly averages of daily sunshine duration, and monthly averages of the daily maximum possible duration of sunshine hours, on a Campbell-Stokes sunshine recorder, respectively. These values are related as follows:
Hm ¼ Ho A þ BS=S0o S0o ¼ 2=15cos1
h i cos 85 sin Lsin d =ðcos Lcos dÞ
(4) (5)
where L is the latitude of the site in degrees, d the declination angle of the sun in degrees, and Ho the estimation of the monthly average of daily extraterrestrial irradiation as given by Duffie and Beckman [11]. The values of the regression constants in equations (3) and (4) for each month of the year were obtained using Zabara’s equation [12]:
A ¼ 0:395 1:247ðS=So Þ þ 2:680ðS=So Þ2 1:674ðS=So Þ3 B ¼ 0:395 þ 1:384ðS=So Þ2 3:249ðS=So Þ2 þ2:055ðS=So Þ3 4. Results and discussion Table 1 presents the monthly mean clear-sky surface albedo values for the twenty-four stations in Saudi Arabia, calculated using the method described above. 4.1. Annual variations The annual mean, maximum, and minimum values of the surface albedo are presented in Table 2. These values were obtained by averaging the monthly values of a for each station. The lowest
mean value occurred in Hail (0.15), while the highest was recorded in Tabuk (0.54). Although Derab and Al Kharj are not far from the capital Riyadh, their average albedo values differ from that of the latter by 0.3 and 0.7, respectively. These differences may be due to the effects of vegetation in Al Kharj and of the low altitude of Derab. Fig. 1 illustrates the frequency distribution of the calculated mean a values. The surface albedo of eight stations falls within the range of 0.2–0.25, while three stations show values between 0.25 and 0.30, and five stations have values ranging from 0.30 to 0.35. Furthermore, two stations show albedo values less than 0.2, and six stations have values greater than 0.35. The overall averaged surface albedo (mean average for all stations) in Saudi Arabia is 0.31 0.05, which is comparable to the international value used in energy budget calculations and to values reported from elsewhere, such as that of 0.33 in Bahrain [7]. Table 2 The annual mean, maximum, and minimum surface albedo values.
Abha AlHofuf AlKharj AlNumas Biljuarshy Bisha Derab Hail Khulays Madina Najran Qatif Qurayyat Riyadh Sabya Sakaka Shaqra Sulayyil Tabarjal Tabuk Taif Tayma Turabah Unayzah
Range
Minimum
Maximum
Mean
0.16 0.08 0.15 0.25 0.36 0.20 0.20 0.10 0.24 0.30 0.15 0.14 0.28 0.24 0.44 0.10 0.28 0.39 0.18 0.08 0.07 0.12 0.02 0.14
0.15 0.29 0.22 0.10 0.20 0.34 0.18 0.10 0.14 0.12 0.11 0.24 0.11 0.24 0.20 0.18 0.11 0.22 0.31 0.51 0.20 0.20 0.34 0.22
0.31 0.37 0.37 0.35 0.56 0.54 0.38 0.20 0.38 0.42 0.26 0.38 0.39 0.48 0.64 0.28 0.39 0.61 0.49 0.59 0.27 0.32 0.36 0.36
0.22 0.33 0.27 0.22 0.45 0.45 0.31 0.15 0.25 0.25 0.20 0.32 0.24 0.34 0.46 0.22 0.26 0.37 0.42 0.54 0.23 0.24 0.35 0.27
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0.4 0.38 0.36
Albedo
0.34 0.32 0.3 0.28 0.26 0.24 0.22 0.2
1
2
3
4
5
6
7
8
9
10
11
12
Month Fig. 3. The averaged monthly mean albedo values of all stations.
Fig. 1. Frequency distributions of the calculated mean albedo values.
The maximum values range from 0.20 at Hail to 0.64 at Sabya, while the minimum values range between 0.51 at Tabuk to 0.10 at Hail and Al Numas. However, the differences between maximum and minimum albedo values at individual sites are also noteworthy. As shown in Fig. 2, the lowest difference was 0.02 at the Turabah site, while the highest difference of 0.44 was recorded at Sabya. The average difference between the maximum and the minimum albedo values is 0.25 for the rest of the stations. The existence of such differences between the maximum and minimum albedo values at individual stations may be due to extreme changes in the weather conditions from one season to another. This includes temperature changes and aerosol concentrations in the air. However, such broad range of a values over large areas have been reported previously. Iqbal [13] presented tables of the regional albedo values of sites in the United States and Canada, in which the surface albedo values for the sites in the U.S. vary between 0.14 and 0.66, while those of the Canadian sites range between 0.18 and 0.62. 4.2. Seasonal variations Fig. 3 shows the mean monthly albedo values of all the stations in Saudi Arabia. It demonstrates the characteristic seasonal variation of a values, with highs in winter (December, January and
0.7 0.6
Albedo
0.5 0.4 0.3 0.2 Sabya Turabah
0.1 0
1
2
3
4
5
6
7
8
9
10
11
12
Month Fig. 2. The mean monthly albedo values averaged for the Sabya and Turbah stations.
February) and lows in the months of June and July. The albedo decreases during March and reaches a minimum in July. In August the albedo starts to increase again, until it reaches a maximum in February. Such a pattern was previously observed in the neighboring country of Bahrain [7] and at some German sites [14]. However, a degree of deviation from this trend is evident at some stations (see Table 1). For example, some stations show higher albedo values in the month of July than in May. In response to this observed trend, we divided the data into four seasonal groups: spring (March, April and May), summer (June, July and August), autumn (September, October and November), and winter (December, January and February). The mean albedo values for spring, summer, autumn, and winter are 0.30, 0.25, 0.31, and 0.36, respectively. These values are consistent with the findings of seasonal variations in albedo values in Saudi Arabia. An increase of 0.11 in the albedo from summer to winter is noteworthy. 4.3. Geographical variations We further investigated the effect of a site’s altitude and latitude on the calculated albedo values. For altitude, the data were divided into the following three groups: low altitude (LA), 0–500 m; middle altitude (MA), 500–1000 m; and upper mountain sites (UM) at altitudes greater than 1000 m. The mean monthly a values for these three groups are 0.32, 0.31, and 0.28, respectively. This shows that albedo increases as altitude decreases. Interestingly, this is the opposite of what Iziomon and Mayer [14] found in their investigations of the surface albedo at three German sites of different altitudes. However, this trend is not uniform over all of the sites in Saudi Arabia. Some low altitude sites, such as Derab, show lower albedo values than do high altitude sites, such as Bisha. The latitudinal variations of surface albedo in Saudi Arabia were studied by dividing the sites into three groups: low latitudes (LL), 15–20; middle latitudes (ML), 20–25; and high latitudes (HL), greater than 25. The mean monthly albedo values for these three groups were 0.25, 0.30, and 0.31, respectively. This indicates that surface albedo generally increases with latitude. However, as in the correlation with altitude, the trend is not completely uniform, as some high latitude stations show lower albedo than do those at lower latitudes (e.g. Hail for the former and Sabya for the latter). Furthermore, some stations located at the same latitude (e.g. Madina and Derab) show different albedo values. This variation is probably due to differences in ground cover and altitude. When two locations have different average solar altitudes, their albedo values will also differ. Additionally, the surface albedos of two locations that have the same latitude but different sunshine conditions may likewise differ.
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5. Conclusion Using solar radiation data, the clear-sky surface albedo values of twenty-four sites in Saudi Arabia were calculated and their variations were investigated. Mean annual values ranged from 0.15 to 0.54. The albedo values showed high variability between sites and even at individual sites. The average albedo over Saudi Arabia was found to be 0.31 0.05. The lowest albedo values occurred in the summer (0.25), and the highest in the winter (0.36). In investigating the effect of the site’s altitude and latitude on the calculated albedo, we found that albedo values tend to increase at higher latitudes, and to decrease at higher altitudes. Such findings will be of assistance to future energy balance studies and solar energy applications. Acknowledgments We would like to thank Dr. Naif Alabadi from the Energy Research Institute at the King Abdulaziz City for Science and Technology, for providing us with important data. References [1] London J. Study of atmospheric heat balance. Final report contract no. AF. 19-165. NY: New York University; 1957.
[2] Pinker RT, Laszlo I, Goodrich D, Pandithurai G. Satellite estimates of surface radiative fluxes for the extended San Pedro basin: sensitivity to aerosols. Agricultural and Forest Meteorology 2000;105:43–54. [3] Al-Mostafa ZA, Maghrabi AH. The Saudi Arabian’s sky II: the turbidity post the second Gulf War. Adelaide, Australia. In: ISES 2001 solar world congress, vol. 4. 2001, p. 2013–18. [4] Al-Mostafa ZA, Maghrabi AH. The Saudi Arabian’s sky III: the clear sky emissivity over Saudi Arabia, Adelaide, Australia. In: ISES 2001 solar world congress, vol. 4. 2001, p. 2019–24. [5] Al-Mostafa ZA. Calculation of sky turbidity in the kingdom of Saudi Arabia. Journal of the Association of Arab Universities for Basic and Applied Sciences ‘‘JAAUBAS’’ 2005;1:1–11. [6] Saudi Arabian Solar Radiation Atlas. Riydah: Saudi Arabian National Center For Science and Technology; 1983. [7] Alnaser W. Calculation of the surface albedo of Bahrain from solar energy data. Energy 1989;14:551–6. [8] Hay JE. Calculation of monthly mean solar radiation for horizontal and inclined surfaces. Solar Energy 1979;23:301–7. [9] Angstro¨m A. Solar and terrestrial radiation. Q J R Meteorol Soc 1924;50:121–5. [10] Jain PC. Solar irradiation in Zambia, internal report. Trieste, Italy: International Centre for Theoretical Physics, vol. 3. 1988, p. 323. [11] Duffie JA, Beckman WA. Solar engineering of thermal process. New York: Wiley; 1991. [12] Zabara K. Estimation of the global solar radiation in Greece. Solar and Wind Technology 1986;7:267–72. [13] Iqbal M. An introduction to solar radiation. 3rd ed. Toronto: Academic Press; 1983. [14] Iziomon MG, Mayer H. On the variability and modeling of surface albedo and long-wave radiation components. Agricultural and Forest Meteorology 2002;111:141–52.