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