A high-resolution simulation of a West African rainy season using a regional climate model

The regional climate model Modele Atmospherique Regional (MAR) is applied to West Africa and the year 1992 is simulated. MAR reproduces the observed intraseasonal variations of rainfall. It is suggested that such a phenomenon is associated with oscillations between a weak and a strong regime of the Hadley cell. The later is correlated with a stronger meridional gradient of moist static energy in the planetary boundary layer and is responsible for an enhanced convergence of this quantity and a subsequent increase of convection and rain. An enhanced consumption of moist static energy and finally a weakening of the meridional circulation result. The meridional gradient of the moist static energy is restored by surface processes. The model also simulates the observed abrupt northward shift of the rainband in the first half of July. The spatial variability of the simulated monthly mean rainfall is in good agreement with the observations, although the model overestimates rainfall in some places from the beginning of August. Time series of daily mean rainfall are averaged over two 2.5degrees x 2.5degrees grid meshes in the Niamey region and in the Oueme high valley. Maxima reaching up to 40 mm/day are found in both areas, as in the observations. Atmospheric variables such as temperature and wind are briefly compared with the European Center for Medium-Range Weather Forecasting reanalyses. The main (cold) biases are located where the hydrological cycle simulated by MAR is too strong.

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