The diurnal cycle and propagation of deep convective clouds in Africa

The prediction of convective precipitation remains a challenge especially where observation networks are unable to resolve mesoscale circulations. In recent decades, analysis of satellite and radar observations has increased our understanding of convective precipitation. This study examines the propagation and evolution of precipitating convection in Africa as part of a broader study of warm-season precipitation across continents.

Five years (1999-2003) of Meteosat infrared imagery are analyzed. The continent is divided into several domains for tracking precipitating convection. The northern domain encompasses 5S to 20N and 20W to 40E from May to August. The central domain covers 15S to 15N and 20W to 45E for the autumn and spring transition months. The mid-latitude domain covers 35S to 15S and 10E to 45E from November to February. Reduced-dimension techniques are used to document the propagation of cold clouds, proxies for deep convection and precipitation. Their large-scale environments are diagnosed from global reanalyses.

Organized convection in Africa consists of coherent sequences or episodes. The phase speeds are similar to those in the US, East Asia and Australia although the average zonal span and duration of episodes in tropical N. Africa are greater than other regions.

A large fraction of the episodes initiate in the lee of high terrain, which is consistent with the principles of thermal forcing from elevated heat sources. Episodes propagate with steering winds in the mid-troposphere and occur with moderate vertical shear of the zonal wind, which itself varies with the migration of the African Easterly Jet in tropical N. Africa. In mid-latitude southern Africa, mid-upper level shear in the westerlies may be contributing to propagation but, since that region has fewer episodes, those findings are considered preliminary.

Propagation leads to a delayed-phase shift in the diurnal maximum of convective precipitation. Concerning the coherence of convection, the imagery shows evidence of gravity currents, trapped gravity waves and mesoscale convective vortices during the dissipative stage of deep convection. Waves and vortices often outlive the originating convective system and induce new convection later. Some vortices were precursors to tropical cyclones close to the west coast of Africa.