Book Description

Tropical cyclogenesis remains a challenging research topic, in part due to the limited understanding of the interactions between the mesoscale processes and the TC-seedling synoptic-scale vortex. The interactions between African easterly waves (AEWs) and mesoscale convective systems (MCSs) over Africa and east Atlantic and how they pertain to the formation of tropical cyclone (TCs) are documented. First, an algorithm that objectively tracks MCSs over Africa is developed to analyze the morphology and climatology of these systems over Africa. It is shown that realistic MCS propagation over Africa is attained when the tracker accounts for the African Easterly Jet (AEJ) mean background flow in which the MCSs propagate on. With an MCS dataset in place, a climatology AEW-MCS dataset is developed to perform a wave-relative analysis of AEW-MCS systems over Africa and over the Main Development Region. It is found that the MCSs of developing AEW-MCS systems (those that develop into TCs) are more likely to be in phase with and propagate at the same speed as the AEW trough. In contrast, the MCSs of non-developing AEW-MCS systems are more likely to be positioned south of the AEW trough and do not necessarily propagate at the same speed. In addition, it is found that developing AEWs over West Africa are associated with a larger number of Convective Cloud Clusters (CCCs; squall line-type systems) than non-developing AEWs. Over West Africa, the fraction of extremely large MCS areas associated with developing AEWs is larger than for non-developing AEWs. These findings support the notion that both the position of moisture and latent heating relative to the AEW trough are essential for cyclogenesis. At the synoptic-scale, significant differences between developing and non-developing AEWs are identified over eastern Africa (region of AEW onset) that can inform tropical cyclogenesis prediction. TC precursors when compared to non-developing AEWs experience: anomalously active West African Monsoon, stronger northerly flow, more intense zonal Somali jet, anomalous convergence over the Marrah Mountains (region of AEW forcing), and a more intense and elongated African easterly jet (AEJ). These more favorable large scale condition for developers reflect in their initial 850-hPa dynamic and thermodynamic structure over eastern Africa. It is found that AEWs that initiate over eastern Africa and cross the west coast of Africa are more likely to undergo tropical cyclogenesis than those initiating over central or west Africa. Developing AEWs are also more likely to be southern-track AEWs than non-developers.