Interactions between desertic dust and deep convection in West Africa : Obserations and convective scale modeling

Abstract

West Africa shows the greatest uncertainties about the evolution of precipitations. Moreover, in a warming climate, this region is very suceptible to droughts which can be devastating for the local populations. This region is also the main source of desert dust in the world where production is increasing due to over-exploitation of soils. Besides the fact that dust is associated with outbreaks of meningitis, it also has a direct impact on the atmosphere since it absorbs and scatters solar radiation causing a change in atmospheric heating. By altering the atmospheric radiation budget, the dust can influence the fine-scale turbulence up to large-scale atmospheric circulations. Also, the processes involved in the interactions between desert dust and atmosphere are very diverse, complex, and constitute a major source of uncertainty in numerical prediction. In order to understand these processes, a modeling framework was developed which englobes fine to regional scales. It explicitly resolves the deep convection, the dust life cycle and its radiative impacts, but also considers their interactions over all of West Africa at a monthly scale. Under dust storm conditions during the dry season, the high amount of dust raised by the fronts, leads to a near doubling of the storm intensity. By this positive feedback, explicit modeling of dust improves the predictability of the storm. During the establishment of the monsoon in June, the frequent occurrence of dust is actively involved in the initiation of the monsoon onset : On the one hand, by weakening the Saharan heat low, and on the other hand, by strengthening the northward movement of the heart of the inter-tropical convergence and jets. At finer scales, the presence of dust also impacts the location and the diurnal cycle of convective systems. Also, dust can change the cloud cover which leads to stronger radiative impacts than the direct effect of dust. This thesis suggests that accounting for the coupling be