Numerical simulation of episodes of organized convection in tropical northern Africa

A large-domain convection-permitting numerical model is used to simulate episodes of deep convection, which are generated during the day over the Ethiopian Highlands and then propagate westward over the eastern and central Sahel region (5°-20°N) of northern Africa. The simulation comprises 12.5 days within the African Monsoon Multidisciplinary Analysis (AMMA) field campaign in 2006. During this period, long-lived precipitation episodes that survived beyond a single diurnal cycle occurred in the lee of the Ethiopian Highlands only every 2-3 days in both the simulation and observations. This contrasts with some other latitudinal corridors in the lee of major topography, such as the central United States, where long-lived heavy precipitation episodes frequently occur on successive nights. The intermittency of long-lived events for the current case occurs despite regular daily triggering of convection along the upstream orography, and is linked to strong lower-tropospheric stabilization and reduction of daytime surface sensible heat flux due to residual cloudiness in the wake of long-lived precipitation events during the previous diurnal cycle. The vertical shear that helps organize deep convection is also weakened in the wake of the long-lived events by temporary disruptions of the midtropospheric African easterly jet. The environments of mesoscale convection are presented for the eastern Sahel, a region where most Sahelian convection originates, but about which little is known at the mesoscale. The study describes the potential for early identification of long-lived convection episodes that are likely to have high impact on the central Sahel and West Africa.