Convective and orographic aspects in the formation of a pre–cyclogenic African easterly wave near the Ethiopian Highlands

The African easterly wave (AEW) that would form the basis of Hurricane Alberto (2000) originated in the vicinity of the Ethiopian Highlands (EH) 5 days prior to cyclogenesis over the eastern Atlantic Ocean. A non-hydrostatic mesoscale model is used to simulate the development of the pre-“Alberto” AEW, as well as to conduct sensitivity tests that highlight key features in the mesoscale environment that contribute to the development of the initial mesoscale convective complex (MCC) and the associated mesovortex (MV) of this AEW. Specifically, the effects of orography, sensible heating, and latent heating on the convective and kinematic development of this AEW are examined.

The elevated heat source of the EH surface and the surrounding moisture sources of the Red Sea and the Congolese rain forests are shown to be crucial to the initiation of the scattered afternoon convection, and the eventual maturity of the convection into an MCC, with the no-sensible-heat-flux (NSH) and no-latent-heat-flux (NLH) experiments. The reduced-terrain (RT) experiment indicates that convection alone produces a weak wave signature, and that the local orography contributes significantly to the development of the convection and of the pre-“Alberto” AEW. Additional sensitivity experiments excluding the effects of the planetary boundary layer (NPBL) and the presence of the Turkana Channel (NTCH) will be performed to investigate more precisely the formation mechanisms for the pre-“Alberto” AEW.

Comparing simulation results with satellite data, the development of this AEW appears to take place in four stages: 1) cellular convection develops in the local afternoon over higher mountain peaks of the EH, 2) convective cells conglomerate into an MCC during the local nighttime at the lee slopes of the EH, 3) an MV develops from the remnants of the MCC, and 4) the convectively-generated MV combines with a pre-existing lee trough to form the AEW.