The structure of West African Squall Lines and their environmental moisture budget

Summary The Squall Lines are the most important convective systems of the West African Sahel region yielding precipitation during the summer months mainly from June to September.The composite method is used to show the average structure of the continental Squall Lines observed during the WA-MEX period July 1979. Composite fields of flow and moisture are produced of the areas within the Squall Lines as well as of their environment to show the coupling between both the Squall Lines and the larger scales features such as the African Easterly Jet, the Easterly Waves and the Monsoon Front. The Squall Lines are preferably situated west of the Easterly Wave trough whereas the area east of the trough mostly is Squall Line free. Here the northward directed water vapour flux into the Sahel region is enhanced. Furthermore, north-ward moisture transport occurs north of the Squall Lines. The water vapour of both fluxes from the south as well as the water vapours available from intensive local evaporation in the Squall Line free space are then incorporated in the subsequent Squall Lines and thus provide the Sahel région with the essential rains.With 10 Figures