Rainfall types and associated vertical stability-shear weather regimes during the 2002 West African monsoon season

Enhanced surface and upper-air observations from the field campaign of the IMPETUS project are used to partition rainfall amounts over the West African Soudanian zone during the 2002 summer monsoon season into several characteristic types and subtypes of precipitating systems. The most prominent rainfall subtype was fast moving, long-lived and extensive cloud clusters that often developed far upstream over the central Nigerian highlands in the afternoon hours and arrive at the Upper Ouémé Valley (UOV) after midnight. These organized convective systems (advective OCSs, Subtype Ia) accounted for 50% of the total rain amount in the UOV catchment in Benin during 2002. Subtypes Ia and IIa (i.e. locally-developing OCSs) were found to pass by or organize when a highly sheared environment with deep and dry mid-tropospheric layers was present over the UOV. These systems were most frequent outside the peak of the monsoon season. The second major type of organized convection, termed mesoscale convective system (Subtypes Ib, IIb, and IIIb) in the present study, contributed 26% to the annual UOV precipitation. They occurred in a less-sheared and moister tropospheric environment mainly around the height of the rainy season. A third distinct class of rainfall events occurred during an unusual synoptic situation in which a cyclonic vortex to the north of the UOV led to deep westerly flow. During these periods, the African Easterly Jet was lacking. The so-called vortex-type rainfalls (Subtypes IIIa, IIIb, and IIIc) contributed about 9% to the annual rainfall totals.