Generation and propagation of MCCs and a mesovortex associated with an African easterly wave as a precursor of Hurricane Alberto (2000)

In this study, we found that mesoscale convective complexes (MCCs) and a mesovortex (MV) were embedded within an African easterly wave (AEW) that led to the genesis of Hurricane Alberto (2000). Analysis of satellite imagery also revealed that a significant number of other pre-cyclogenic AEWs originated from the Ethiopian Highlands (EH). The pre-Alberto AEW propagated westward at an average speed of 11.6 m s-1 and has an average wavelength of about 2200 km. According to EOM analyses, vertical moisture flux was crucial to generation and maintenance in the convective cycle associated with the pre-Alberto system as it traveled westward across North Africa. Based on the cloud top area and brightness values observed from satellite data, 4 genesis and 3 lysis stages are identified within a cycle of moist convection associated with the pre-Alberto disturbance. The availability of water vapor is the most essential factor controlling the convective cycle of the pre-Alberto disturbance over the African continent. The presence of significant topography affected the timing and placement of the genesis and lysis stages by regulating the water vapor supply.

A regional climate model was adopted to investigate the formation mechanisms of the MCC, MV, and AEW. The simulated fields indicated that convection was generated over the EH, and the MV and AEW was generated near the lee of the EH. These features comprised the pre-Alberto system and propagated westward. The EH is important for focusing and organizing AEW features by acting as a source point for a consistent stationary wave mode, lee side vorticity generation, and vertical moisture flux sufficient for initial convection. Planetary boundary layer (PBL) effects over the EH contribute to the initialization of convection through diurnal, diabatic heating. As the convection propagates downstream, an MCC develops at the lee of the EH and combines with orographically generated vorticity. PBL effects and moisture availability are important for maintaining AEWs. Surface moisture fluxes serve to affect these systems by aiding in convective development.