Cloud parameter retrievals from Meteosat and their effects on the shortwave radiation at the surface

A method based on Spinning Enhanced Visible and Infrared Imager SEVIRI measured reflectance at 0.6 and 3.9 µm is used to retrieve the cloud optical thickness COT and cloud effective radius re over the Iberian Peninsula. A sensitivity analysis of simulated retrievals to the input parameters demonstrates that the cloud top height is an important factor in satellite retrievals of COT and re with uncertainties around 10% for small values of COT and re; for water clouds these uncertainties can be greater than 10% for small values of re. The uncertainties found related with geometries are around 3%. The COT and re are assessed using well-known satellite cloud products, showing that the method used characterize the cloud field with more than 80% 82% of the absolute differences between COT re mean values of all clouds water plus ice clouds centred in the range from ±10 ±10 µm, with absolute bias lower than 2 2 μm for COT re and root mean square error values lower than 10 8 μm for COT re. The cloud water path CWP, derived from satellite retrievals, and the shortwave cloud radiative effect at the surface CRESW are related for high fractional sky covers Fsc >0.8, showing that water clouds produce more negative CRESW than ice clouds. The COT retrieved was also related to the cloud modification factor, which exhibits reductions and enhancements of the surface SW radiation of the order of 80% and 30%, respectively, for COT values lower than 10. A selected case study shows, using a ground-based sky camera that some situations classified by the satellite with high Fsc values correspond to situations of broken clouds where the enhancements actually occur. For this case study, a closure between the liquid water path LWP obtained from the satellite retrievals and the same cloud quantity obtained from ground-based microwave measurements was performed showing a good agreement between both LWP data set values.