Verification of cloudiness and radiation forecasts in the greater Alpine region

The skill of ECMWF operational cloudiness and radiation forecasts is evaluated for the greater Alpine region using ground-based and satellite observations. Ground-based observations are the total cloud cover reported by SYNOP stations. The satellite data we use is the downward surface solar radiation and the top of the atmosphere (TOA) reflected solar radiation from the EUMETSAT Climate Monitoring Satellite Application Facility (CM SAF). A cloud type which is difficult to represent correctly in numerical weather prediction models and which has particular relevance in central Europe is wintertime low stratus. This evaluation shows how it affects forecast skill in mountain areas compared to flat terrain. Results indicate higher skill of the cloud forecast in the Alps compared to the surrounding lowlands throughout the year, with the largest differences in skill occurring in late autumn and early winter. There is also a marked asymmetry in skill between the northern and southern lowlands adjacent to the Alps, which can be attributed to the higher prevalence of low stratus in northern areas. Comparison with other regions shows that cloud forecast skill in Europe is generally high, and that there are large areas of low skill dominated by marine stratus and stratocumulus.