Numerical simulations of the evolution of long-lived episodes of organized convection in Africa

The prediction of convectively-generated precipitation remains a challenge especially in the tropics where observation networks are inadequate for resolving mesoscale circulations. Observational studies have documented the common occurrence of deep convection and heavy rainfall as episodes that exhibit temporal and spatial coherence across continents during the warm season. These episodes are comprised of mesoscale convective systems, which account for most of the seasonal precipitation in the Sahel. These orographically-driven systems propagate westward, aid in easterly wave generation, and modulate the diurnal cycle of rainfall across the Sahel. Studies over the US have examined the role of planetary boundary layer circulations in the initiation of convection near high terrain. The current work examines the initiation and evolution of episodes within several hundred km west of the Ethiopian Highlands and the Darfur Mountains.

The Advanced Research Weather Research and Forecasting (WRF) modeling system is used to simulate a convectively-active 14-day period during August 2006. The model is integrated over a large sub-continental domain over the eastern and central Sahel and uses a deep convection-permitting horizontal grid spacing of 4 km. The simulation reproduces the daily triggering of convection in the lee of the Ethiopian Highlands and the generation of long-lived episodes of convection. The model also produced waves with periodicity of about 2.5 days, similar to the period of African Easterly Waves. These periodic potential vorticity maxima were associated with convection in the lee of the mountains.