RADIATION AND CLOUD RADIATIVE PROPERTIES IN THE EUROPEAN CENTER FOR MEDIUM RANGE WEATHER FORECASTS FORECASTING SYSTEM

The successive versions (EC1, EC2, and EC3) of the radiation scheme used in the European Centre for Medium Range Weather Forecasts (ECMWF) operational model, including the version which became operational on May 2, 1989, are reviewed and their results are compared to results of more detailed radiation models made available thanks to the Intercomparison of Radiation Codes Used in Climate Models (ICRCCM) program. For clear-sky conditions, the shortwave H2O absorptivity is overestimated in EC1-EC2, which leads to too large shortwave atmospheric absorption (by up to 20%) and too small downward shortwave radiation at the surface (by 5-10%). EC1 and EC2 both significantly underestimate the longwave radiative cooling, with main errors in the lower troposphere with EC1, and between 700 and 300 hPa with EC2. Therefore EC1 and, to a smaller extent, EC2 underestimate the outgoing longwave radiation at the top of the atmosphere. In cloudy conditions, EC1 shows an exaggerated sensitivity to small amounts of scatterer, a problem corrected in EC2. Due an unrealistic model cloud embedded in EC1-EC2, these schemes cannot properly represent both the shortwave planetary albedo and outgoing longwave radiation for realistic cloud liquid water contents. EC3 corrects most of these deficiencies and gives results in better agreement with those of more detailed models.