Mean bias deviation of the Heliosat algorithm for varying cloud properties and sun-ground-satellite geometry

Reflectances measured by the 'visible' channel of Meteosat are converted to global radiation at the ground by the Heliosat-algorithm. This algorithm is based on the inverse relationship between reflectance and transmittance. This relationship is, however, complicated by two factors: 1) the absorptance of the atmosphere is varying, and 2) the reflectivity is different in different directions. These two factors are again depending on the state of the atmosphere ( mainly clouds) and the sun-ground-satellite geometry. The performance of the Heliosat-method is here tested against ground data from Bergen ( Norway), Geneva ( Switzerland) and Lyon ( France), and analysed in light of cloud properties as observed from ground, and different sun-ground-satellite geometries. Situations where modelled and measured radiation differ are identified, and possible causes are suggested. Some of the suggestions are supported by calculations with a radiative transfer model.