Impact of Changes to the Radiation Transfer Parameterizations Plus Cloud Optical. Properties in the ECMWF Model

Abstract

A new radiation package, shown to correct most of the systematic errors of the operational ECMWF radiation scheme, has been extensively tested in the ECMWF forecast model. Improvements in the clear-sky fluxes and radiative heating/cooling rate profiles mainly stem from a better representation of the transmission functions. New cloud optical properties have been set from detailed narrowband calculations for a more realistic model cloud. Results indicate a greater overall sensitivity of the model to cloud-radiation interactions. Radiative cooling in the subtropics is increased. A decreased radiative cooling in the higher layers of the tropics is caused by larger longwave impact of the high level clouds. An increase in the radiative energy available at the surface and an overall cooling of the troposphere generate larger turbulent heal fluxes. All these changes contribute to a higher level of convective activity, resulting in a more energetic hydrological cycle and a more active model with higher levels of zonal and eddy available potential energy and of eddy kinetic energy at all wavenumbers. The increased contrast in the deposit of radiative energy between land and ocean, as well as between clear-sky and cloudy areas, improves the distribution of diabatic heating. The warm bias in stratospheric temperature is greatly reduced. The divergence in the tropics is larger and does not weaken as much after a few days of integration as with the old operational radiation scheme, thus improving the Hadley circulation. The radiation budget at the top of the atmosphere is now in good agreement with satellite observations. Several other systematic errors of the ECMWF model are also partially corrected.