The radiative transfer of the solar radiation from the sun to the earth surface is influenced by two main factors, the atmosphere and clouds. Methods to derive the solar irradiance at the surface take two major calculation steps:
Radiative transfer from the sun to the earth surface
The first step assumes a cloud free atmosphere. In the clear sky model, the radiative transfer is only influenced by absorption and scatting at molecules, ozone, water vapor and aerosols. Information about ozone, water vapor and aerosols is taken from climatological data bases (aerosols), reanalysis data of a numerical weather model (water vapor) and other satellite platforms (ozone). The following figure shows examples of the aerosol and water vapor data:
Aerosol optical thicknesses (AOT) and water vapor amounts as used for clear sky model
Clouds have the largest influence on the incoming surface irradiance. They are determined from Meteosat images. A cloud index is calculated as the difference between actual reflectivity of the earth as it is seen by the satellite and a reference image which only includes reflectances of the ground:
Cloud index computed from Meteosat VIS- and IR-channel
Both channels are used to improve cloud detection in the morning and evening hours and the detection of high cirrus and ice clouds.
All information on the transmission of the broad band irradiance is taken together to calculate the irradiance at the earth surface as shown in the next picture:
Influence of atmospheric constituents and clouds on the radiative transfer