Modelling of the atmospheric column over the central sites during HAPEX-Sahel

The boundary layer development over the Central sites of HAPEX-Sahel has been simulated for 4 individual days. The study is divided into two parts. In the first part a surface scheme is calibrated for the dominant land cover of the area, the fallow savanna. This calibration is performed for a period of 54 days encompassing the transition from wet to dry. A comparison is then made with another more detailed surface scheme. The model, in spite of its drastic simplification of reality, is able to reproduce the overall water balance for the period. In terms of relative contribution of soil evaporation and vegetation transpiration it does not depart significantly from the more complex scheme. In the second part, the surface scheme is coupled to a one-dimensional version of the ARPEGE model (operational forecast model at Meteo-France). The importance of the direct heating of the boundary layer by aerosols is stressed. With minor modifications of calibration parameters from the first phase, boundary layer profiles of temperature and also mixing ratio are reproduced with a good accuracy.