Parametrisation of solar radiation at the surface under cloudless conditions and its validation against observation

A parametrisation of global and net solar irradiances at the earth’s surface under clear sky and aerosol-free conditions is developed based on a series of radiative transfer calculations for various atmospheric conditions that may occur in the atmosphere. Instead of parametrising the solar irradiances in a full short wave spectrum as is the common practice, the solar spectrum is divided into three bands in order to simplify the treatment of gaseous absorption. The absorption of solar radiation by water vapour, ozone and carbon dioxide in each band is processed in a manner such that the effect of their overlapping absorption is properly treated. These three species are major contributors to the greenhouse effect and are explicitly treated, while CH4, N2O, and O2 are minor contributors and their effects are implicitly considered. The solar zenith angle, surface albedo and atmospheric albedo due to Rayleigh scattering are also processed explicitly as they have a significant impact on both the global and net solar radiation. The parametrisation and the radiative transfer model are in excellent agreement. The parametrisation is also tested against observations obtained at Gaize on the Tibetan Plateau and three field research sites from the Atmospheric Radiation Measurements (ARM) program. There is good agreement at Gaize, but the errors are larger for the ARM sites, indicating the need for including aerosol effects.