A satellite-based climatology of convective precipitation episodes over Africa

Convective precipitation in Africa is examined as part of a broader study to characterize the global character of warm season, continental precipitation. Warm season, continental rainfall is often produced from long-lived episodes of organized convection. Carbone et al. 2002 [JAS] used radar data to demonstrate that organized convection in North America is comprised of phase-coherent sequences. These episodes often occur in subtropical conditions and “weakly forced” in a mid-latitude dynamical sense. Similar patterns of warm-season rainfall have been reported for China, Australia, Tropical North Africa, and Europe.

A climatology of convective episodes is derived using digitized images from the European geostationary satellite (Meteosat). The infrared images have a spatial resolution of 5km at the satellite sub-point and are available at 30 minute intervals. The continent is divided into several domains for tracking precipitating convection. The northern domain encompasses 5ºS to 20ºN and 20ºW to 40ºE from May to August. The central domain covers 15ºS to 15ºN and 20ºW to 45ºE for the autumn and spring transition months. The mid-latitude domain covers 35ºS to 15ºS and 10ºE to 45ºE from November to February. Hovmoller strips are drawn through each domain and thresholds of 235K and 213K are used to identify the cloud systems that are most likely to be precipitating. The mean diurnal cycle was determined by computing the occurrence of precipitating convection at a particular longitude at the same time of day. Global Reanalysis data are used to analyze the large-scale environments associated with the development of deep convection. Reanalysis pressure level data have a 2.5-degree grid and are provided daily at 0000, 0600, 1200, and 1800UTC.

A disproportionately large fraction of episodes have their origin in the lee of major mountain ranges (e.g, the Ethiopian highlands in North Africa, the South African Escarpment, and the plateaus of central East Africa). While clearly insufficient in itself, a major generating factor is thermal forcing associated with large scale elevated heat sources. Episodes propagate across continents when steering winds are present in the mid- to upper troposphere. The highly organized systems sometimes propagate and regenerate across the entire continent. The propagation leads to a delayed-phase shift in the diurnal maximum of convective precipitation over some regions. Episodes occur in the presence of moderate vertical shear of the horizontal wind. In mid-latitude southern Africa, this is a common condition associated with the deep westerlies, while in Sahelian Africa, it is associated with the migration of the African Easterly Jet. The coherence of convection may be explained in part by the formation of gravity currents, trapped gravity waves and mesoscale convective vortices associated with [in] the dissipative stage of deep convection. The waves and vortices often greatly outlive the originating convective system and later induce new convection. Some vortices have been the precursors to tropical cyclones.

The phase speeds of the precipitating convection episodes in tropical north Africa were similar to those observed in the US and East Asia although the average zonal span and duration were longer. Long-lived propagating episodes were less common in mid-latitude southern Africa and the correlation between duration and span was significantly weaker than other regions. Precipitating convection in mid-latitude southern Africa appears similar to that of mid-latitude Australia.