The response of the ECMWF model to changes in the cloud overlap assumption

The role of the cloud overlap assumption (COA) in organizing the cloud distribution through its impact on the vertical heating/cooling rate profile by radiative and precipitative/evaporative processes is studied in a series of experiments with a recent version of the ECMWF general circulation model, which includes a prognostic cloud scheme. First, the radiative forcing initially obtained for different COAs (maximum, MAX; maximum-random, MRN; and random, RAN overlap) is discussed from results of one-dimensional radiation-only computations. Ensembles of T_L95 L31 simulations for the winter 1987/88 (November-December-January-February) are then used, with the three different overlap assumptions applied on radiation only (RAD), evaporation/precipitation only (EP). or both (EPR). In RAD and EPR simulations, the main effect of a change in COA is felt by the model through the change in radiative heating profile, which affects in turn most aspects of the energy and hydrological budget. However, the role of the COA on the precipitation/evaporation, albeit smaller, is not negligible. In terms of radiative fluxes at the top and surface in the RAD and EPR simulations, RAN differs much more from MRN than MAX does, showing that for this vertical resolution, the majority of the clouds appear more in contiguous layers than as independent layers. Given the large sensitivity of both the model total cloud cover and surface and top-of-the-atmosphere radiation fields to the cloud overlap assumption used in the radiation and cloud scheme, it is very important that these quantities are not validated independently of each other, and of the radiative cloud overlap assumption. The cloud overlap assumption for precipitation processes should be made consistent with that for radiation.