The Parameterization of the Precipitation Process in the LMD General Circulation Model

The LMD atmospheric General Circulation Model includes a cloud water budget equation in which the condensation of water vapour and the precipitation of cloud water are treated as separate processes. Such a distinction has become a standard feature of many models in the recent years. We show here that it has little impact on the simulated monthly mean precipitation. It does however affect strongly the water vapour mixing ratio, the cloud water content and the cloud distribution. If we submit the model to an external perturbation, such as an increase in greenhouse effect, these parameters cause powerful feedback processes, which may affect the mean precipitation: the parameterization of the precipitation process then appears as a key process determining the amplitude of the climate response.