Estimating the subgrid variance of saturation, and its parametrization foruse in a GCM cloud scheme

The prediction of cloud fraction and condensate in the unified weather fore casting/climate prediction model of the UK Meteorological Office, and many other general-circulation models, depends upon a probability distribution o f a saturation variable in a grid box. A method of estimating the width of the probability distribution of the saturation variable on the scales of on e grid box of the climate configuration (2.5 degrees latitude x 3.75 degree s longitude) has been created, using analyses from the global weather forec asting configuration (0.55 degrees latitude x 0.833 degrees longitude). Thi s width was found to be potentially very variable in time at many locations , therefore the use of climatological values was inappropriate, A parametri zation was created to predict the width of the probability distribution of the saturation variable for use in the climate model. The concept of this e mpirical parametrization was based upon a scaling relationship often observ ed, and which is present in the global analyses. This new statistically bas ed parametrization replaced the standard approximation used in the Hadley C entre climate model, and significantly reduced the model's biases concernin g water vapour, temperature and cloud amount in the upper troposphere.