A Consistent Treatment of the Evaporation of Rain and Snow for Use in Large-Scale Models

Abstract

The parameterization of evaporation of rain and snow in large-scale numerical models of the atmosphere is considered. Evaporation coefficients dependent on the precipitation rate are derived following the method of Kessler for both stratiform and convective precipitation and compared with the calculations of more detailed models and observations using passive models. The derived ?bulk? parameterizations are in good agreement with the evaporation rates derived from the microphysical model of Clough and Franks, showing more rapid evaporation of snow than rain. Comparison is made to other recent evaporation parameterizations and the sensitivity of the estimated evaporation rate to the nature of precipitation, and the motion of the air through which it falls is also studied. The impact of the inclusion of different rates for the evaporation of stratiform rain and snow upon climate simulations by the Meteorological Office Unified Model (the Hadley Centre Climate Model) is considered.