Estimation of Surface Solar Energy Budget over Iran

In this research, three steps were taken to estimate the solar energy balance on the earth's surface. First, the amount of incident radiation on a tilted surface at the top of the atmosphere was calculated. Then, by using MODIS data, the transmittance coefficients of the atmosphere were estimated and the amount of direct radiation, diffuse radiation and global radiation in cloudless sky conditions were estimated. In the next step, based on the cloud transmittance coefficient, the amount of all sky radiation was estimated. Finally, by estimating the actual albedo of the earth's surface, the balance of solar radiation on the earth's surface was evaluated.
The average top of atmosphere radiation in Iran is about 365 Watts per square meter. On a tilted surface, Iran receives 356 Watts per square meter of solar radiation. The difference in the angle of radiation on a tilted surface compared to the flat ground due to the slope of the ground and the difference in the duration of the radiation on a tilted surface compared to the flat ground due to the aspect of slope resulted a 2.5 percent reduction in the amount of radiation in Iran.
In Iran, on a clear and sunny day about one percent of solar radiation is lost by air molecules not reaching the ground. The phenomenon of Rayleigh scattering also prevents about 9% of radiation from reaching the earth's surface. Therefore, about 10% of solar radiation is reduced due to atmospheric gases. The presence of aerosols, water vapor and ozone also affect the transparency of the atmosphere to solar radiation. The effect of these gases can be expressed by the transmission coefficient namely the aerosols transmittance coefficient which is low in desert areas of the country and on the coasts of Oman Sea and Persian Gulf and for Khuzestan Plain. In these areas, between 20 and 40 percent of the solar radiation is prevented from reaching the earth's surface by the aerosols. On the other hand, in the heights of Zagros and Alborz mountains and in the heights of Khorasan and in the north-west of Iran, aerosols do not play a significant role in reducing solar radiation. In Iran, the average reduction of solar radiation due to the presence of aerosols is about 17%.
As expected, water vapor transmission is minimal at high altitudes, and about 10% of solar radiation is prevented from reaching the earth's surface due to atmospheric water vapor. On the shores of the Oman Sea, Caspian Sea, and Persian Gulf, the amount of attenuation due to atmospheric water vapor is about 14%. In Iran, the average reduction of solar radiation due to the presence of water vapor in the atmosphere is about 11%.
The average transmittance of direct surface solar radiation in Iran is about 60%. In other words, the atmosphere prevents about 40% of direct sunlight from reaching the earth's surface. In mountainous areas the transmittance coefficient is the maximum and exceeds 70%. In the southern banks and eastern and central regions of Iran, due to the presence of aerosols and water vapor, the figure is less than 60%. The amount of mean direct radiation in Iran is about 213 Watts per square meter. Diffuse radiation is a small part of the total radiation. The average transmittance of diffuse radiation in Iran is about 10%. Aerosols play an important role in scattering solar radiation. The amount of mean diffuse radiation that reaches the earth's surface in Iran is about 35 Watts per square meter.
This study shows that the global radiation in Iran is 248 Watts per square meter. The average transmittance coefficient of global radiation is 70% and follows the configuration of topography and distance from the sea. Average cloudiness of Iran is about 26% and the average ratio of actual to possible sunshine hours is about 72%. On the shores of the Caspian Sea, the cloudiness exceeds 60%. The average cloud transmittance coefficient in Iran is about 83%. In Iran, clouds contribute about 17% in the reduction of radiation. On a cloudy day, the mean amount of solar radiation that passes through the atmosphere and reaches the surface of the earth on a tilted surface is 205 Watts per square meter. The average albedo of Iran is about 21%. Nearly 80% of the solar radiation that reaches the earth's surface is absorbed by the surface. The amount of net annual solar radiation on the earth's surface in Iran varies between 80 and 220 Watts per square meter.