A cloud water variable has been included in the Meteorological Office atmospheric general circulation model. the cloud amount and water content are generated by a scheme which assumes a distribution of thermodynamic and water content variables about their grid-box-mean values. A parametrization of the rate of depletion of cloud water by precipitation is included. the model's boundary layer mixing is reformulated in terms of variables conserved during changes of state of water, allowing a more realistic calculation of atmospheric stability in cloudy regions.
The scheme produces realistic cloud distributions and the predicted cloud water content verifies well against microwave radiometer data in all regions except the mid-latitudes of the summer hemisphere. the parametrization of cloud optical properties in terms of the cloud water content will be an important future development of the model.