Mesoscale Convective System Rainfall in the Sahel

Based on a full-resolution Meteosat dataset, an extensive climatological study of the mesoscale convective systems (MCSs) observed by satellite over the Sahel leads to the definition of a subpopulation of MCSs—called organized convective systems (OCSs)—that represents only 12% of the total number of MCSs observed during 9 yr over the central Sahel while accounting for almost 80% of the total convective cloud cover defined at the 233-K threshold. Using a high-resolution rainfall dataset, it is shown that these OCSs are also the main source of rain in this region, accounting for about 90% of the seasonal rainfall, with a mean areal rainfall of 14.7 mm per system. All of the OCSs are associated with a rain event, and more than 90% of the major rain events are associated with an OCS. These figures are compared with those obtained for mesoscale convective complexes (MCCs). Each MCC produces more rainfall on average (19 mm per system) but there are only a few of them (1.2% of the total number of MCSs), and they consequently produce only 19% of the seasonal rainfall. The interannual rainfall variability is first determined by the year-to-year fluctuation of the number of events defined from satellite rather than by the fluctuations of their mean rain efficiency. In fact, the total rain yield of an OCS appears to be linked primarily to its duration (which itself is largely determined by its spatial extension) rather than to its average rain rate. The diurnal cycle over the region is also studied, and it is shown that it is largely conditioned by the propagative nature of the OCSs associated with orography-driven generations located a few hundred kilometers to the east of the validation area.