ON THE APPLICATION OF THE CLASSIC KESSLER AND BERRY SCHEMES IN LARGE-EDDY SIMULATION-MODELS WITH A PARTICULAR EMPHASIS ON CLOUD AUTOCONVERSION, THE ONSET TIME OF PRECIPITATION AND DROPLET EVAPORATION

Many Large Eddy Simulation (LES) models use the classic Kessler parame terisation either as it is or in a modified form to model the process of cloud water autoconversion into precipitation. The Kessler scheme, being linear, is particularly useful and is computationally straightfo rward to implement. However, a major limitation with this scheme lies in its inability to predict different autoconversion rates for maritim e and continental clouds. In contrast, the Berry formulation overcomes this difficulty, although it is cubic. Due to their different forms, it is difficult to match the two solutions to each other. In this pape r we single out the processes of cloud conversion and accretion operat ing in a deep model cloud and neglect the advection terms for simplici ty. This facilitates exact analytical integration and we are able to d erive new expressions for the time of onset of precipitation using bot h the Kessler and Berry formulations. We then discuss the conditions w hen the two schemes are equivalent. Finally, we also critically examin e the process of droplet evaporation within the framework of the class ic Kessler scheme. We improve the existing parameterisation with an ac curate estimation of the diffusional mass transport of water vapour. W e then demonstrate the overall robustness of our calculations by compa ring our results with the experimental observations of Beard and Prupp acher, and find excellent agreement.