A Comprehensive Radiation Scheme for Numerical Weather Prediction Models with Potential Applications in Climate Simulations

Abstract

A comprehensive scheme for the parameterization of radiative transfer in numerical weather Prediction (NWP) models has been developed. The scheme is based on the solution of the δ-two-stream version of the radiative transfer equation incorporating the effects of scattering, absorption, and emission by cloud droplets, aerosols, and gases in each part of the spectrum. An extremely flexible treatment of clouds is obtained by allowing partial cloud cover in any model layer and relating the cloud optical properties to the cloud liquid water content. The latter quantity may either be a prognostic or diagnostic variable of the host model or specified a priori depending on cloud type, height, or similar criteria. The treatment of overlapping cloud layers is based on realistic assumptions, but any different approach requires only minor modifications of the code. The scheme has been tested extensively in the framework of the intercomparison of radiation codes in climate models (ICRCCM, WMO 1984, 1990). Radiative fluxes and heating rates, calculated in a few milliseconds of CPU time with our scheme, are in very good agreement with reference calculations, which may require several thousand CPU seconds for the same purpose. First experiments, using our parameterization scheme within the framework of a global weather forecast model, give promising results. Subject to the results of further experimentation, our code will be part of the parameterization schemes used in the operational weather prediction models of the DWD (Deutscher Wetterdienst). However, the generality of the scheme, particularly the flexibility of the code, extends its scope to other applications, such as climate simulations. In the long run, one of the decisive advantages of the method described here lies in the fact that the cost of computations varies only linearly with the number of atmospheric model levels, unlike the quadratic behavior of the so-called ?emissivity-type? methods.