EFFECTS OF ALTERNATIVE CLOUD RADIATION PARAMETERIZATIONS IN A GENERAL-CIRCULATION MODEL

Using the National Center for Atmospheric Research (MCAR) general circ ulation model (CCM2), a suite of alternative cloud radiation parameter izations has been tested. Our methodology relies on perpetual July int egrations driven by +/-2 K sea surface temperature forcing. The tested parameterizations include relative humidity based clouds and versions of schemes involving a prognostic cloud water budget. We are especial ly interested in testing the effect of cloud optical thickness feedbac ks on global climate sensitivity. All schemes exhibit negative cloud r adiation feedbacks, i.e., cloud moderates the global warming. However, these negative net cloud radiation feedbacks consist of quite differe nt shortwave and longwave components between a scheme with interactive cloud radiative properties and several schemes with specified cloud w ater paths. An increase in cloud water content in the warmer climate l eads to optically thicker middle- and low-level clouds and in turn neg ative shortwave feedbacks for the interactive radiative scheme, while a decrease in cloud amount leads to a positive shortwave feedback for the other schemes. For the longwave feedbacks, a decrease in high effe ctive cloudiness for the schemes without interactive radiative propert ies leads to a negative feedback, while no distinct changes in effecti ve high cloudiness and the resulting feedback are exhibited for the sc heme with interactive radiative properties. The resulting magnitude of negative net cloud radiation feed-back is largest for the scheme with interactive radiative properties. Even though the simulated values of cloud radiative forcing for the present climate using this method dif fer most from the observational data, the approach shows great promise for the future.