The development of intense convective systems in West Africa in wet and dry years, 1998-2005

The 8-year (1998-2005) history of the TRMM satellite encompasses both wet and dry wet seasons in West Africa. Starting with 1998, we identified convective systems from ice scattering signatures in the 85GHz TRMM Microwave Imager data. Each convective system's minimum brightness temperatures were ranked, so that intense convective systems were defined as those in the 10th percentile or lower. We examined how the characteristics of the moist monsoon layer and the coherence and intensity of the jet affected the location and frequency of intense convective systems in dry vs. wet years. For this period, we used high-resolution re-analysis data to diagnose the position and strength of the African easterly jet and the inland penetration of the moist monsoon layer. We observed a distinct separation between the peak activity of weak convective systems (south of 10N and east of 10E) and the peak activity of intense convective systems (north of 15N in wet years, farther south in dry years). The peak activity of intense convective systems underwent poleward migration to remain near the edge of the monsoon layer, in the high CAPE/high shear environment poleward of the African easterly jet. In wet years, most of the intense convective systems originated in the vicinity of orography north of 12N. Since intense convective systems supply more than 70% of the precipitation north of 10N, we associate dry years with the failure of the monsoon layer to overlap regularly with the few crucial areas where intense convective systems most frequently originate.