SIMULATION OF CLOUD RADIATIVE FORCING WITH THE ECMWF MODEL

Radiative fluxes and corresponding cloud forcings derived from Earth Radiation Budget Experiment (ERBE) measurements are compared with similar quantities produced by the T63 version of the European Centre for Medium Range Weather Forecasts (ECMWF) model for January 1986. Monthly mean values and standard deviations of the radiation fields on a global scale are considered as well as regional diagnostics over the areas showing discrepancies between observations and model simulations. Clouds in the storm tracks of the 3 Hemisphere are correctly represented. Shortwave and longwave impacts of clouds in tropical convective areas are overestimated whereas low-level stratiform clouds on the eastern parts of subtropical oceans are missing. Cloud liquid water content diagnosed from the saturation mixing ratio is shown to give too small optical thickness at cold temperatures. This is particularly seen in the underestimation of the shortwave radiative impact of the clouds in the higher latitudes of the Southern Hemisphere.