Response of the National Center for Atmospheric Research community climate model to improvements in the representation of clouds

Abstract

A new parameterization for the shortwave radiative properties of water clouds and the European Centre for Medium‐range Weather Forecasts (ECMWF) cloud prediction scheme have both been included in the National Center for Atmospheric Research (NCAR) community climate model. The techniques employed to treat the contribution by water vapor absorption to the cloud radiative properties, the overlaps between cloud layers, and the spectral dependence of the properties are described, and comparisons are made with a more detailed radiation code. Only minor adjustments were necessary to implement the cloud prediction scheme. Results from a suite of integrations are used to examine the effect on model simulations. There are improvements in the model's temperature structure and in the radiation budget and cloud radiative forcing when compared with results from the Earth Radiation Budget Experiment. It is shown that the changes in the cloud distribution over the subtropical oceans induce changes not only in the surface radiative fluxes but also in the surface turbulent fluxes, through modifications to the boundary layer stability. The turbulent fluxes change in such a way as to maintain the cloud. While the cloud distributions in these areas appear realistic, it is argued that important physical processes are ignored by the model and that a more comprehensive formulation of the cloudy boundary layer is required.