Comparison of space-based cloud property retrievals with ground-based observations and climate model simulations in West Africa

The still relatively poor representation of clouds in general circulation models introduces major uncertainties in the global radiation budget and hydrological cycles. Satellite observations of cloud properties can provide a valuable contribution to improve this situation. The Spinning Enhanced Visible and Infrared Imager (SEVIRI) onboard the geostationary METEOSAT satellite collects visible, near-infrared and infrared measurements in 12 channels at a nominal spatial resolution of 3x3 km2 and at an unprecedented temporal resolution of 15 minutes. Within EUMETSAT's Satellite Application Facility on Climate Monitoring (CM-SAF), cloud properties (fraction, optical thickness, thermodynamic phase, particle effective radius, and liquid/ice water path) are retrieved from the SEVIRI measurements. These retrievals are particularly suited to study the (daytime only) diurnal cycle of cloud properties.

In this presentation, we focus on the Sahel in West Africa, a region characterized by intense convective cloud systems during the monsoon season from May until September. Our study consists of two parts. First, the SEVIRI retrievals are validated using ground-based observations performed at the ARM Mobile Facility in Niamey in the framework of the African Monsoon Multidisciplinary Analysis (AMMA) project in 2006. Measurements from a passive microwave radiometer (MWR) will be used to validate space-based liquid water path observations. Furthermore, the combination of MWR and multifilter rotating shadowband radiometer measurements allows obtaining simultaneous estimates of cloud optical thickness and effective droplet size, providing additional validation data for the satellite retrievals.

Second, the SEVIRI observations are used to evaluate cloud properties simulated with the RACMO2 regional climate model. A simultaneous evaluation of modeled top-of-atmosphere radiative fluxes will be performed using data from the Geostationary Earth Radiation Budget sensor (GERB). While observed and modeled seasonal cycles are reasonably consistent, the diurnal cycles show pronounced differences. Particularly, the observations do not confirm the simulated early-afternoon maximum in cloud water path.