Simulations of the propagation and diurnal cycle of organized convection in Africa

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 (MCSs). For Sahelian Africa, MCSs account for most of the seasonal precipitation. Most of these systems are initiated in the lee of the Ethiopian Highlands and the Darfur Mountains, propagate westward, aid in easterly wave generation, and modulate the diurnal cycle of rainfall across northern, tropical Africa. However, existing observation networks are unable to resolve their structure and the interactions of the mesoscale and synoptic circulations. High resolution simulations over the US predicted the mode of mesoscale convection and their interaction with the synoptic-scale environment. The current study applies similar methods to predict the evolution and diurnal cycle of organized convection in Africa.

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 initiation of convection in the lee of the Ethiopian Highlands and generated several episodes of convection that propagated to West Africa. In general, long-lived episodes are associated with high values of lower-tropospheric equivalent potential temperature in the lee of the highlands and downstream. The zonal speeds of most long-lived episodes exceeded 10 m/s while short lived episodes had a mean zonal speed of less than 7 m/s. Two mechanisms were primarily responsible for limiting the duration of deep convection. Up-shear spreading of the cold pool from previous convection suppresses convection from the east and the advection of dry mid-tropospheric air from upstream was associated with weak or little convection. The model also produced potential vorticity maxima 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.

For northern, tropical Africa, a broad minimum in the frequency of deep convection occurs one or two hours before local noon, except for axes of high frequency that are due to propagating convective systems. Unlike the highlands where convection is most frequent during the afternoon to late evening, relatively flat regions to the west have nighttime to early morning maxima in convection due to propagating MCSs. In order to simulate a mean diurnal cycle, the model is run with a time-averaged lateral boundary condition that varies only with time of day. The simulated diurnal cycle is compared with diurnal cycles of organized convection observed by satellite.