Dear Ildikó,
thanks for providing plot of the time series of the validation of the CM SAF data sets for time series from Budapest. This results agrees very well to our analysis using the monthly mean data available in the GEBA data set from Budapest. Please find enclosed the corresponding plots for the CLARA and the MVIRI-based data sets.
Unfortunately these plots are a bit messy (designed for internal use..), but on the second figure you can see the multi-year annual cylce from the surface and the satellite measurement. While the MVIRI-based data fits nicely to the surface observations, there is an underestimation of the CLARA data set, in particular during summer, comparable to your results.
In our analysis a temporal trend in the bias after 2001 is not obvious (first figure, middle row). The CLARA data set includes many missing data, so an interpretation can be difficult; the time series of the bias of the MVIRI-based data set suggests something suspicious happening around 1998, when the bias seems to drop suddenly. You can find more information on the GEBA data and their application for validation in the article by Sanchez-Lorenzo et al., 2013.
We are currently preparing for the next release of the Meteosat-based surface radiation data set, which will be released in fall 2014 covering the time period from 1983 to 2013. Hopefully this data set will provide a consistent and homogeneous data record of the surface solar radiation.
Kind regards,
Jörg
