Origin and a formation mechanism of pre-TC African easterly wave-mesoscale convective systems in eastern Africa

In this study, the origins and formation mechanism of the pre-Debby (2006) mesoscale convective system (MCS) and African easterly wave (AEW) were investigated by using satellite imagery and numerical weather prediction modeling results. Based on the Meteosat-8 infrared imagery, we were able to trace the pre-Debby MCS back to Ethiopian Highlands (EH) and the Asir Mountains (AS) as early as 8/11/18Z. Numerical experiments were performed using a mesoscale model (ARW) to help identify the pre-Debby MCS and AEW. The formation of the pre-Debby AEW-MCS system was explained by the splitting-advection-merging mechanism in eastern North Africa: (1) originated from the stationary AS MCS and then split from it and advected downstream, (2) merged with the EH MCS and then split from it and advected downstream, and (3) gained its strength by merging with the Darfur Mountains (DM) MCS and then split from it and advected further downstream. The mechanism is also applicable to the formation and evolution of the pre-Alberto (2000) MCS. Analysis of the relative vorticity fields indicated that there existed two mesoscale vorticity maxima (MV), one (EH MV) associated with the EH-MCS and the other (AP MV) associated with the cyclonic shear near the southern Arabia Peninsula, produced by the high pressure systems over Indian Ocean and North Africa. The EH-MCS and EH MV moved downstream and then merged with the EH lee vortex to form a coupled AEW-MCS system. This AEW-MCS system was strengthened by the AP MV which moved westward at a faster speed. We also found that the strong cyclonic shear along the southern flank of the Arabian Peninsula was near the Inter-Tropical Convergence Zone (ITCZ). The proposed mechanism appears to be applicable to some other pre-TC AEW-MCS systems. This work is supported by a grant from the NOAA EPP under the cooperative agreement NA06OAR4810187.