Downscaling West African climate : uncertainties, sensitivity to the model physics and regional variability

We present here a set of regional climate simulations, complementary to the CORDEX-Africa modeling exercice, performed over West Africa during the 1989-2010 period using the non-hydrostatic model WRF. Lateral and SST forcings are provided by ERA-Interim reanalyses. The regional domain [45W 45E 10S 30N] encompasses West Africa and the nearby Atlantic Ocean and Gulf of Guinea. The grid resolution is moderate (80km, with 28 levels on the vertical) in order to reduce computational costs and multiply sensitivity experiments. A first set of fifteen 20-year long simulations addresses the influence of the model physics (convective, radiative and land surface schemes). A second set of simulations consists in perturbing the atmosphere initial conditions in order to obtain a fifteen-member ensemble experiment, allowing us to disentangle the reproducible (i.e. forced) and irreproducible (i.e. chaotic) components of the regional atmosphere variability. The respective weight of the model internal variability and the influence of its physical package will then be quantified. Eventually, two-way nested simulations during given years of interest will explicitly resolve atmospheric convection over the Niamey mesoscale site and its surroundings, enabling us to explore the simulation of meso-scale convective systems and their coupling with the surface. Although all sets of simulations are not completed yet, this presentation offers the opportunity to present the first results, rank uncertainties in the regional climate simulations, and document the capability of the WRF model in simulating the West African climate and its natural variability