2011-08-31: Evaluation of model predicted top-of-atmosphere radiation and cloud parameters over Africa

In a paper recently published in the Journal of Climate, Wouter Greuell, Erik van Meijgaard, Jan Fokke Meirink present an evaluation of KNMI's regional climate model RACMO for Africa and the surrounding oceans. They compared model output for July 2006 with satellite data by simultaneously looking at cloud properties retrieved from SEVIRI data and at top-of-atmosphere fluxes measured by GERB.
The simulations show generally accurate positioning of the various cloud regimes, but also some important model-observation differences (see figure), which the authors tried to reduce by altering model parameterizations. These differences are: 1) Top-of-atmosphere albedo differences in clear-sky regions like the Sahara and southern Africa. These differences were considerably reduced by prescribing the surface albedo from MODIS satellite data. 2) A considerable overestimation of outgoing long-wave radiation within the continental ITCZ caused by the fact that modeled cirrus clouds are far too thin. 3) Model underestimation of cloud cover, condensed water path and albedo of the stratocumulus fields off the coast of Angola. These underestimations were reduced by suppressing the amount of turbulent mixing above the boundary layer, by prescribing droplet radii derived from SEVIRI data and by assuming in-cloud horizontal homogeneity for the radiation calculations. 4) Model overestimation of the albedo of the trade wind cumulus fields over the Atlantic Ocean, likely caused by a model overestimation of condensed water path. In general, the analyses demonstrate the power of the simultaneous evaluation of the top-of-atmosphere fluxes and cloud properties.

The full article can be found here.
Satellite observations
Model calculations
Model Bias