Formation and maintenance of an African easterly wave-mesoscale convective system in 2004 over East Africa and the Arabian Peninsula

The precursors of an intense coupled system (A04) of an African easterly wave (AEW) and a mesoscale convective system (MCS) over the lee of Darfur Mountains around 1800 UTC, August 12, 2004 were traced back to the southern Arabian Peninsula, Asir Mountains, and Ethiopian Highlands using gridded satellite (GridSat) data, European Centre Medium-Range Weather Forecast Intermediate (ERA-I) data, and the Advanced Research Weather Research and Forecasting (WRF) model simulated data. A vorticity budget analysis was performed to investigate the dynamics and mechanisms that contributed to the enhancement of A04‘s vorticity perturbation. It was found that convergence of southeasterly and northwesterly wind from anticyclones over Arabian and Indian Ocean generates cyclonic vorticity over the southern Arabian Peninsula that was advected by an easterly wind maximum embedded within the African easterly jet. Convergence and vorticity advection were the most important factors to the formation of A04‘s vorticity perturbation, while cloud clusters were generated by diurnal heating over the Asir Mountains, Ethiopian Highlands and Darfur Mountains. Both vorticity perturbation and cloud clusters went through their formation and propagation phases, intensified by lee vortices, and then eventually merged into an AEW-MCS system on the lee of the Darfur Mountains, consistent with the conceptual model proposed in Lin et al. (2013).