The formation of African easterly waves (AEWs) and mesoscale convective systems (MCSs) in eastern North Africa and its impacts on the tropical cyclogenesis over the eastern Atlantic Ocean is studied. Based on numerical simulations using the Weather Research and Forecast (WRF) model, the AEWs during the hurricane season may be generated by the orography and shear zone established by the Somali jet and anticyclonic circulation associated with the Asian monsoon to the north of the Ethiopian Highlands (EH) and Red Sea. The vortices often form on the lee of major mountain ranges and propagate downstream by upper-level easterly flow. The MCSs are originated from the moist convection over the major mountain ranges, such as EH, Asir Mountains, and Darfar Mountains, triggered by diurnal sensible heating. An MCS may merge with a vortex in the shear zone and form a coupled AEW-MCS system. The WRF is employed to simulate the pre-hurricane AEW-MCS system of Tropical Storm Debby (2006) near the EH. Finer-resolution numerical simulations demonstrate that the vortex generated on the lee and MCS over the mountain eventually merge and become an AEW-MCS system which might serve as a precursor of tropical cyclone. The larger-scale environments conducive to the formation of the AEW-MCS system are also investigated, but using a much larger numerical model domain.

This work is supported by a grant from the National Oceanic and Atmospheric Administration, Educational Partnership Program under the cooperative agreement NA06OAR4810187.