Heliosat 2: an improved method for the mapping of the solar radiation from Meteosat imagery

Abstract : Solar irradiance at the ground level can be determinated from Meteosat images by the means of several methods. The purpose of this work is to present the method Heliosat 2, an improved version of the well known Heliosat 1 method. Heliosat 1 converts observations made by geostationary meteorological satellites into estimates of the global irradiation at ground level. It was implemented in many places and is routinely used to produce maps. It suffers from some drawbacks, which come from several empirically defined parameters inside the method. The accuracy of the estimates is very sensitive to their tuning during the implementation phase. This method cannot cope with changes in spaceborne radiometers. Consequently, parameters have to be tuned at each change. Four points of improvements were identified. Values in images were calibrated, hence large time-series spanning over several changes of sensors and satellite may be processed. It also permits to adopt known models of the physical processes in atmospheric optics; thus removing the need of empirically defined parameters. A new model of clear sky irradiation was used for a better accuracy of the estimates performed by the method. The computation of the ground albedo and the cloud albedo makes use of recognised models of the radiance and the atmospheric transmittance. The relationship between the cloud index and the hourly global irradiation was modified to better represent the cloudy skies. This new method Heliosat 2 exhibits results that are more accurate than those provided by the method Heliosat 1 by approximately 30 %. Comparisons were made with ground measurements obtained at 35 sites in Europe. For daily irradiation we observed a relative mean square error of less than 200 J. cm-2 and 110 J.cm-2 for the monthly mean of daily irradiation.