Validation of CM SAF cloud fractions: can cloud cover be reliably derived by satellite data at Hannover, Germany and Lauder, New Zealand? – a comment

Abstract

Abstract. Cloud Fractional Cover (CFC) derived from Spinning Enhanced Visible and Infrared Imager (SEVIRI) on a geostationary satellite and from the Advanced Very High Resolution Radiometer (AVHRR) on polar orbiters, was validated against ground-based observations in Hannover, Germany (three months of data) and Lauder, New Zealand (only AVHRR, two months of data). The ground-based cloud coverage data consists of synoptical data and imagery taken by a Hemispherical Sky Imager. The standard deviation of the differences between the daily mean CFC derived by SEVIRI and synoptical data was satisfactory, but it was 100% larger than the deviations of the differences between SEVIRI and those derived by the hemispherical sky imager. For the instantaneous CFC, clear and completely overcast skies are well detected in the satellite products. During broken cloud coverage the agreement between ground-based data from the hemispherical sky imager and data from satellite is mostly uncorrelated. The standard deviations of the differences between AVHRR and the imager (instantaneous and daily mean data) were smaller than those between SEVIRI and imager. In Lauder, New Zealand, only data from the hemispherical sky imager and AVHRR was available. The standard deviations of the differences were slightly higher than in Hannover, Germany. In addition we found that the SEVIRI algorithm systematically overestimates the cloud coverage. We therefore suggest to refine the parameterization of the cloud contamination factor in the algorithms for the derivation of CFC.