Evaluation of ERA-40 and ERA-interim re-analysis incoming surface shortwave radiation datasets with mesoscale remote sensing data

In this paper an evaluation of the European Center for Medium Range Forecasting 40-year re-analysis (ERA-40) and interim re-analysis (ERA-Interim) solar irradiation datasets is presented. The goal of the study is to evaluate the accuracy of the downward solar surface radiation fluxes of ERA-40 and ERA-Interim. A positive result of these examinations would justify the use of ERA-40 and ERA-Interim, respectively, to investigate long-term changes of anomaly patterns. Unlike other recent studies that operate on a global scale, this study concentrates on regional aspects of climate research. As reference, high-resolution remote sensing data derived from METEOSAT using the Heliosat method are used. The study area covers Germany and adjacent countries. Structures were found which indicate that ERA-40 does not properly represent cloud and fog occurrence. Especially during the solar season May, June, July (MJJ) large deviations of up to 40 and 60 W/m(2), respectively, compared to the reference dataset were found. During the solar seasons of August, September, October (ASO) and November, December, January (NDJ) the deviations are negligible for both re-analysis datasets as well as for ERA-40 during the season of February, March, April (FMA). The results lead to the conclusion that in and around Germany, the accuracy of the parameter surface solar irradiation downward (ssrd) of ERA-40 and ERA-Interim re-analysis is limited, especially during the season of MJJ and in ERA-Interim also during the season of FMA.