Influence of three-dimensional cloud effects on satellite derived solar irradiance estimation—First approaches to improve the Heliosat method

Three-dimensional (3D) cloud effects which influence the surface solar irradiance are investigated by using the radiative transfer model SHDOM (spherical harmonics discrete ordinate method). The calculations are motivated by improving the Heliosat method which is used for the estimation of surface solar irradiance from satellite data. The modelled results lead to the assumption that it is possible to find a correction for moderate viewing and solar geometries for the current Heliosat method. Based on these results an empirical study has been done, comparing ground measurements with Heliosat derived data. It will be shown that a correction of the current Heliosat method for 3D cloud effects is possible. For two out of four sites the proposed corrections decrease the relative root mean square error (rRMSE) of the irradiance derived by the Heliosat method in a range of about 2%. The use of a site specific correction for different locations shows that the adaptability of the corrections depend on the dominant local cloud regimes, which governs e.g. the existence of certain cloud types.