Simulation of a squall line associated with an african easterly wave: sensitivity to the parameterization of convection

The general circulation models used to make seasonal and climate predictions generally have a poor representation of the rain-producing synoptic weather systems. Therefore it is difficult to adequately assess their rainfall prediction; hence the effect of climate change on rainfall. It is then important to improve the parameterization of convection associated with these systems. In this work which is part of the West African Monsoon Project (EU Project), we focus on a squall line associated with an African easterly wave that occurred on 21-08-1992 during the Hapex-Sahel experiment. This squall line has been chosen because of the availability of a large data set (high resolution observation surface data, radar, satellite and ECMWF analyses and reanalyses data) useful for validation. Simulations have been carried out for 24 hours using the Meteo-France spectral limited area model Aladin at different horizontal resolution (of the order of 10 to 80 km) and ECMWF analyses or reanalyses as lateral boundary conditions. The convection scheme, used in these simulations, has been modified with regard to the closure assumption (CAPE versus moisture convergence), the cloud profile, the inclusion of downdrafts and the convective momentum transport. Runs have been carried out using the last two versions of the model. Their main difference is on the formulation of the relationship between soil, vegetation and atmophere; the first one being rather crude and the last one (ISBA scheme) more realistic. Results showing the sensitivity of the simulations to all these schemes are presented.