Simulation and analysis of the moist vortex associated with the extreme rain event of Ouagadougou in 2009

Abstract

Abstract This study analyses the mechanisms involved in the formation of a moist meso‐vortex associated with an extreme rain event that occurred in Burkina Faso on 1 September 2009, on the basis of high‐resolution convection‐permitting simulations. After an evaluation of the 6‐day simulation skill to capture the main characteristics of this event, budgets of heat, moisture and relative vorticity are calculated. Results allow us to propose the following scenario for the occurrence of this extreme rain event, completing the large‐scale analysis of the same event performed in the companion article. Firstly, the arrival of a large‐scale wet spell is a key factor, creating a quite unusual wet environment over the Sahel, resulting in weak rain evaporation. As over oceanic regions, it favours intense heating at low levels and thus strong mean ascent and convergence. Secondly, the combination with the arrival of the trough of an African easterly wave provides a second key factor. The associated relative vorticity maximum on the southern flank of the African easterly jet (AEJ) core is boosted by the strong low‐level convergence through the stretching term. The vortex deepening is favoured by the tilting and by eddies at upper levels. Thirdly, in turn the quasi‐balanced vortex circulation brings warm and moist air from the north, favouring the convection triggering to the west of the vortex in the down‐shear direction, in agreement with Raymond and Jiang's theory for long‐lived mesoscale convective systems. This scenario exhibits a positive feedback that could explain its extreme character.