Validation of cloud liquid water path retrievals from SEVIRI on METEOSAT-8 using CLOUDNET observations

METEOSAT-8 is the first European satellite with the potential to provide accurate spatial distributions of cloud properties that can be used to examine daily variations in cloud properties with a 15 minutes temporal resolution. Although there has been progress in quantifying the accuracy of cloud property retrievals from NOAA-AVHRR, little research has been done on the validation of these retrievals from METEOSAT-SEVIRI.

Within the Climate Monitoring Satellite Application Facility (CM-SAF) KNMI developed a cloud properties retrieval algorithm, based on narrowband visible and near-infrared radiances from NOAA-AVHRR and METEOSAT-SEVERI. This algorithm provides Cloud Optical Thickness (COT), droplet effective radius and Cloud Liquid Water Path (CLWP) over Europe. The CloudNET research project, supported by the European Commission, provides quasi continuous measurements of ground-based cloud properties for the development and implementation of cloud remote sensing algorithms. The accuracy of the ground-based measurements is superior to current satellite remote sensing techniques, which makes these observations the appropriate data source for assessing the accuracy of METEOSAT-8 cloud property retrievals.

This paper quantifies the accuracy of SEVIRI retrieved CLWP values over Europe during the summer months. A four-month dataset of about 5000 CLWP retrievals from SEVIRI is compared to microwave radiometer (MWR) LWP observations at two CloudNET stations. The bias and accuracy are determined for the instantaneous and daily SEVIRI CLWP retrievals. The magnitude of the bias differs per CloudNET station, but never exceeds 10 g m-2. The difference of 60 g m-2 for instantaneous CLWP retrievals is acceptable, taking into account validation uncertainties such as the accuracy of the CloudNET MWRs (about 35 g m-2). The comparison of daily median CLWP from SEVIRI and MWR, taking advantage of the 15 minutes sampling frequency of METEOSAT-8, led to an improvement of the accuracy to about 25 g m-2.