Angular dependence of the albedo estimated in models for solar radiation derived from geostationary satellites

Abstract

Many models for derived solar radiation from geostationary satellites images are based upon the determination of the cloud index as a measure of the transmittance of the atmosphere. The estimation of the cloud index relies in the proper normalization of the planetary albedo within the dynamic range, which is the range between the upper bound reflectivity (associated to clouds) and the lower bound reflectivity (referred as ground albedo). The angular influence of the albedo can be noticeable in geostationary satellites due to its large viewing angle. This paper presents a study of the dependence of the ground albedo with the scattering angle for different geostationary satellites (GOES, Meteosat Prime, Meteosat IODC and Meteosat Second Generation). The ground albedo is determined by different satellites in quite different geographical points and a methodology that fits the local angular dependence is evaluated. The robustness of ground albedo estimations is proved by comparing solar radiation estimated in Thesalonica during the same period using two different positions of Meteosat images with ground data. The combination of the information observed by each satellite has improved the global irradiance retrievals in Thesalonica. Finally, an illustrative comparison of ground albedo with well-known meteorological products (MODIS and CM-SAF) is done in a qualitative way.