An Extension of Cloud-Radiation Parameterization in the ECMWF Model: The Representation of Subgrid-Scale Variations of Optical Depth

Abstract

Current climate and forecast models treat clouds as plane-uniform, ignoring subgrid-scale variations of cloud water content in radiative transfer calculations. The plane-uniform assumption is relaxed in a new cloud-radiation parameterization that considers cloud inhomogeneities and heterogeneities associated with convective and stratiform cloudiness. Convective cloudiness is diagnosed from the assumption that the ensemble of convective clouds within a model grid column is in a quasi-steady state. Global forecast experiments with the new parameterization show that the introduction of cloud inhomogeneities and heterogeneities leads to substantially increased and more realistic net downward shortwave radiative fluxes at the top of the atmosphere; over the tropical and subtropical oceans (30°S?30°N) the increase by inhomogeneities and heterogeneities is on average 9 and 7 W m?2, respectively.