Tuesday, 25 October 2005
Alvarado F and Atria (Hotel Albuquerque at Old Town)
Arlene Laing, NCAR, Boulder, CO; and R. Carbone and V. Levizzani

Our ability to predict warm season precipitation is encumbered on our understanding of the development and environment of organized convective weather systems. Studies of the continental United States and East Asia have found that a large fraction of the warm season precipitation is produced by organized convection which occurs as long-lived episodes. Carbone et al 2002 {JAS} demonstrated that these episodes of convection propagate in a coherent manner, a characteristic which could be exploited to improve the prediction of warm season precipitation. This study examines convection in Sahelian, Soudian, and mid-latitude southern Africa and compares the characteristics of organized, deep convection over Africa with that of other continents.

Satellite observations are analyzed in a distance-time framework to document the lifecycles of convective precipitating systems. A threshold technique is applied to infrared data to identify the cold-cloud that is most likely to be precipitating. Global reanalyses are used to explore the large-scale environment associated with these episodes of deep convection.

Like other continents, convection is most often initiated in the lee of elevated terrain. In Africa, the Ethiopian Highlands and the South African escarpment serve as the primary sources of thermal heating. Clusters of precipitating convection propagate and regenerate across the continent and display similar coherence to that observed in North America and East Asia. The propagation leads to a delayed-phase shift in the diurnal maximum of convective precipitation over some regions.

Episodes of deep convection occur in the presence of moderate vertical shear of the horizontal wind. For Sahelian Africa, this is commonly associated with the African Easterly Jet, while in mid-latitude, southern Africa, it is associated with the westerlies. Some coherence may be explained by the formation of mesoscale convective vortices associated with the dissipative stage of deep convection. The vortices often greatly outlive the originating convective system and later induce new convection. Some vortices were the precursors to tropical cyclogenesis off the west coast of Africa.

The phase speeds of the precipitating convection episodes were similar to those observed in North America and East Asia although the average zonal span and duration were longer over Sahelian Africa. The vast longitudinal domain of northern African allows for cloud streaks to be tracked over great distances and through multiple diurnal cycles. Finally, these results support the notion that that continental precipitation cycles may be predictable beyond one or two days.

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