Validation of cloud liquid water path retrievals from SEVIRI using one year of CLOUDNET observations

The accuracy and precision are determined of cloud liquid water path (LWP) retrievals from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard METEOSAT-8 using one-year of LWP retrievals from microwave radiometer (MWR) measurements of two CloudNET stations in Northern Europe. The MWR retrievals of LWP have a precision that is superior to current satellite remote sensing techniques, which justifies their use as validation data. The Cloud Physical Properties (CPP) algorithm of the Satellite Application Facility on Climate Monitoring (CM-SAF) is used to retrieve LWP from SEVIRI reflectances at 0.6 and 1.6 μm.

The results show large differences in the accuracy and precision of LWP retrievals from SEVIRI between summer and winter. During summer, the instantaneous LWP retrievals from SEVIRI agree well with those from the MWRs. The accuracy is better than 5 g m-2 and the precision is better than 30 g m-2, which is similar to the precision of LWP retrievals from MWR. The added value of the 15-minute sampling frequency of METEOSAT-8 becomes evident in the validation of the daily median and diurnal variations in LWP retrievals from SEVIRI. The daily median LWP values from SEVIRI and MWR are highly correlated (corr. > 0.95) and have a precision better than 15 g m-2. In addition, SEVIRI and MWR reveal similar diurnal variations in retrieved LWP values. The peak LWP values occur around noon. During winter, SEVIRI generally overestimates the instantaneous LWP values from MWR, the accuracy drops to about 10 g m-2 and the precision to about 30 g m-2. The most likely reason for these lower accuracies is the shortcoming of CPP, and similar one-dimensional retrieval algorithms, to model in-homogeneous clouds. It is suggested that neglecting cloud in-homogeneities leads to a significant overestimation of LWP retrievals from SEVIRI over
Northern Europe during winter.