3A.2
Cycles of deep convection over central and southern Africa
A. G. Laing, NCAR, Boulder, CO; and R. E. Carbone and V. Levizzani
Knowledge of the evolution of deep convection and precipitation over Africa serve a variety of vital purposes. 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 hence 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. Significant attention has been given to West Africa, especially the Sahel, where severe drought and extreme floods have had tremendous social impact. Few studies have focused on central and southern Africa and the variability of precipitation at the diurnal scale.
This study examines the occurrences of organized convection in central and southern Africa using four years (2000-2003) of digital infrared imagery. Reduced-dimension techniques are used to document the properties of cold clouds, which serve as proxies for deep convection and precipitation. The central Africa domain covers 15šS to 10šN and 20šW to 50šE for the two rainy seasons, March to May and September to November. The southern African domain covers 35šS to 15šS and 10šE to 45šE 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 ms-1, which is similar to warm-season organized convection over northern, tropical Africa, Australia, East Asia, Europe, and the United States. A large fraction of the convective episodes initiate in the lee of high terrain (e.g., the mountains of Cameroon, the mountains of East Africa, and the South African Escarpment). A major generating factor is thermal forcing associated with large scale elevated heat sources. Over central Africa, episodes initiate on an almost daily basis, propagate, and regenerate on regional and continental scales. Daily propagation is sometimes interrupted by large-scale circulation changes which have a periodicity of one-to-two weeks. 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 and mid-latitude southern Africa, the mean diurnal cycle shows less evidence of propagation. For that domain, deep convective development and propagation occur when low-level, northerly winds bring warm, moist air from the equator. 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 mid-latitude, southern Africa, this is a common condition associated with the deep westerlies; while in tropical Africa, vertical shear is associated with the migration of 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.
Recorded presentationSession 3A, African Climate: III. Convection and Synoptic Systems
Monday, 21 January 2008, 1:30 PM-2:30 PM, 215-216
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