A 16-year Climatology of the West African Offshore Rainfall Maximum

Monsoon rainfall is a critical part of the West African climate and understanding its variability is crucial to the livelihood of its population. Satellite rainfall estimates reveal a consistent rainfall maximum just off the coasts of Sierra-Leone and Liberia during the rainy season of May through October. A 16-year rainfall climatology is conducted in this study to determine the cause of such copious amounts of rainfall off the southwestern coast of West Africa. The Tropical Rainfall Measuring Mission Multi-satellite Precipitation Analysis (3B42) and Precipitation radar (2A25) data were used in the rainfall climatology, and the European Center for Medium-Range Weather Forecasts Reanalysis Interim data were used for the analysis of atmospheric variables. A 7°×10° region surrounding the offshore maximum in the averaged seasonal rainfall was designated as the study region. Total kinematic moisture fluxes for August indicate low-level moisture convergence and mid-level moisture divergence in the study region. Composites of daily rainfall and mid-level meridional winds centered on the days with maximum rainfall in August show that the heaviest rainfall day follows the strongest mid-level northerlies. Reflectivity and rain type composites show that stratiform rain dominates the region. The principal attributes and rainfall characteristics of mesoscale convective systems change as they propagate off the continent. The composites suggest that the dominant contribution to the August offshore rainfall maximum derives from the trailing stratiform portion of mesoscale convective systems; the rainfall is most prolific in the area of convergence located in the northerly phase of African Easterly Waves propagating off the continent.