PC2: A prognostic cloud fraction and condensation scheme. II: Climate model simulations

A prognostic cloud fraction and prognostic condensate scheme (PC2) has been developed for the Met Office Unified Model. A companion paper discussed the motivation for a new scheme and described its formulation in detail. In this paper we describe the results of climate model simulations, concentrating on the mechanisms by which the cloud and condensate predicted by the model change between the Control and new scheme. We demonstrate that the detrainment of condensate from the convection scheme directly into the large scale. as parametrized in the PC2 scheme, produces improved simulations of deep tropical cloud. We also show that the unphysical strong link between cloud fraction and condensed water content that is present in the Control scheme has been broken by using PC2. but that it is still challenging to produce optically thin cloud in a large-scale model. Shallow convection proves to be a difficult cloud type to parametrize using a prognostic scheme. although the PC2 scheme performs well. The use of increased vertical resolution, in both the Control and PC2. improved the simulation of cloud when compared to observations. (C) Crown Copyright 2008. Reproduced with the permission of Her Majesty's Stationery Office. Published by John Wiley & Sons, Ltd.