Cycles of deep convection over central and southern Africa

Africa is a major source of heating for the general circulation; primarily through latent heating from precipitation. The timing, sequence, and duration of precipitation affect soil moisture, runoff, and surface feedback processes. Improvements in weather and climate prediction depend on the ability of numerical models to reproduce the statistics of precipitation, such as timing and duration, at a variety of scales. Furthermore, knowledge of the spatial and temporal variability of precipitation is needed for African societies to manage agriculture, water resources, public health, renewable energy, and hazard mitigation. This study focuses on the variability of deep convection in central and southern Africa, with particular emphasis on the diurnal cycle.

Organized convection in central and southern Africa is examined using four years (2000-2003) of digital infrared imagery. Reduced-dimension techniques are used to document the propagation of cold clouds, which serve as proxies for deep convection and precipitation. The central Africa domain covers 15ºS to 10ºN and 20ºW to 50ºE for the two rainy seasons, March to May and September to November. The southern African domain covers 35ºS to 15ºS and 10ºE to 45ºE from November to February. Global analyses are used to diagnose the large-scale environments associated with the evolution of deep convection. The analyses are on a 1-degree grid and are provided daily at 0000, 0600, 1200, and 1800UTC.

Deep convection in central and southern Africa is frequently organized as long-lived “episodes” or coherent sequences. The phase speeds for most convective cloud streaks are 10-20 m s-1, similar to warm-season, organized convection over other continental regions. A large fraction of the convective episodes initiate in the lee of high terrain (e.g., the mountains of Cameroon, the East Africa mountains, and the South African Escarpment). Over central Africa, episodes initiate on an almost daily basis, propagate, and regenerate on regional to continental scales. Daily westward propagation is sometimes interrupted by large-scale circulation changes such as the passage of Kelvin waves. Fewer convective episodes occur over subtropical southern Africa.

The zonal propagation of organized convection shifts the diurnal maximum of precipitation westward over central Africa and eastward over the southern, subtropical domain. Over subtropical southern Africa, the mean diurnal cycle shows less evidence of propagation. For that domain, deep convective development and propagation occur with low-level northerly winds and westerly wind shear. Tropical cyclones are also major sources of precipitation for southeastern Africa.

Episodes of organized convection occur in the presence of moderate vertical shear of the horizontal wind. In the subtropical domain, shear is associated with the deep westerlies; in central Africa, it is associated with the African Easterly Jet and the Tropical Easterly Jet. Results of this study infer the potential for increased skill in sub-seasonal weather prediction and in projections of intra-seasonal to inter-annual variability.