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Deriving surface global irradiance over the Alpine region from METEOSAT Second Generation data by supplementing the HELIOSAT method

In the framework of the Satellite Application Facility for Climate Monitoring (CM-SAF) an upgraded formulation of the HELIOSAT surface global irradiance retrieval scheme is proposed, which is suitable for real-time application to METEOSAT Second Generation (MSG) satellite data. The new scheme includes image georeferencing, pixel-wise snow-cover detection, special treatment of clouds above snow and correction of terrain effects over the Alpine region. Results show that the mean bias difference between the revised irradiance estimates from satellite and from ground measurements can be substantially reduced by correctly distinguishing between clouds and snow, and by applying the new cloud-index scheme for clouds blue above snow. The increase of the root mean square difference with increasing altitude can be mainly attributed to the increase of the variability of the natural irradiance field. We strongly recommend the use of MSG-based irradiance estimates for locations at more than 4 km distance to the next measurement site.