Cloud-Resolving Modeling of Mesoscale Convective Systems over West Africa during a Dusty Outbreak

Over West Africa, most of the precipitation arises from Mesoscale Convective Systems (MCSs). MCSs result from the scale interaction between microphysical and dynamical processes with many impacts and nonlinear feedbacks. This complex chain of interaction is poorly resolved by global models, which makes MCSs difficult to forecast. During springtime, the heavy load of dust renders this interaction scale even more complex. Cloud-resolving models (CRMs) can now perform simulations over large domains where processes from convective to regional scales are explicitly represented. From 9 to 14 June 2006, an intense dust outbreak occurred in Chad and Sudan. Its transport to the southwest by the African Easterly Jet (AEJ) and its interaction with an MCS passing over Benin has been well documented. A set of high- and low-resolution simulations (grid spacing of 2.5 and 20 km) of this 6-day long episode was run using a prognostic dust scheme over a large domain covering the northern part of Africa, from the Equator to the Mediterranean. Both the simulations capture the emission of dust and its transport by the AEJ as well as the rain band over South Western Africa. The high-resolution simulation produces a rain band further south in better agreement with actual rain retrievals. It also better captures the observed diurnal cycle of convection. The radiative impact of aerosols is analyzed by comparing the simulations with a set of dust-free simulations. The additional heating due to the heavy dust loading greatly weakened the AEJ, reduced it to the east and shifted the Sahelian convection to the south. This study was supported by the DACCIWA project.