Verification of CM-SAF and MeteoSwiss satellite based retrievals of surface shortwave irradiance over the Alpine region

Verification results from different satellite-based surface shortwave irradiance retrievals and sensitivity runs for key input parameters are presented for the Alpine region. Overall the uncertainty of the hourly retrievals at the Federal Office of Meteorology and Climatology (MeteoSwiss) validated with high-quality surface measurements is comparable with results from the standard Heliosat-3 model, but clearly improved for situations with snow-cover. The sensitivity study reveals that it is recommended to precisely georeference the High Resolution Visible (HRV) and the Spinning Enhanced Visible and Infrared Imager (SEVIRI) channels of the Meteosat Second Generation (MSG) satellites to obtain accurate surface shortwave irradiance estimates. They also confirm the benefit of terrain corrections for sites located in deep Alpine valleys. Monthly mean shortwave irradiance retrievals provided by the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT) Satellite Application Facility on Climate Monitoring (CM-SAF) are verified with the MeteoSwiss products. For the National Oceanic and Atmospheric Administration (NOAA) Advanced Very High Resolution Radiometer (AVHRR)-based product about 60% of all satellite 15 km x 15 km grid cells match the MeteoSwiss product, if locally dependent 90% confidence intervals are applied. This percentage decreases to 52%, if the standard CM-SAF 90% confidence interval of 20 Wm-2 is used. Taking the local spatial variability of the shortwave irradiance field into account therefore allows obtaining more realistic verification results over heterogeneous terrain such as the Alpine region.