Model clouds over oceans as seen from space: Comparison with HIRS/2 and MSU radiances

Radiation observations are a key element in the evaluation of the 40-yr rea nalysis at the European Centre for Medium-Range Weather Forecasts. This pap er uses the High-Resolution Infrared Radiation Sounder/2 (HIRS/2) and Micro wave Sounding Unit (MSU) observations on board the National Oceanic and Atm ospheric Administration satellites, to assess the characteristics of the cl oud fields produced by the forecasting system over midlatitude and tropical oceans. Infrared and microwave radiation have different sensitivities to c louds and are therefore complementary. Observed and model-generated radianc es, as well as HIRS/2-derived cloud parameters, are compared. The model clouds are shown to be well distributed, with realistic seasonal cycles. However, deficiencies are identified and discussed: the cloud radia tive impact may be too low in the midlatitudes, the frequency of occurrence of high clouds is overestimated in the intertropical convergence zone, and the stratocumulus off the west coast of the continents is underestimated. The methods described here provide a framework for assessing the impact of forthcoming improvements to the cloud scheme.