The Interaction between Convective Systems and Easterly Wave Actvity over Africa: Convective Transitions and Mechanisms"

The initiation of African Easterly Waves (AEWs) and their variability, including their role on convection is still an active area of research. Early studies viewed AEW-genesis as related to the presence of an unstable mid-tropospheric easterly jet, while recent work suggested that the mid-level jet is only marginally unstable and, therefore, AEWs are triggered in association with pre-existing large scale convective activity. Still others argue that new AEWs are triggered by energy dispersion from upstream wavepackets associated with old AEWs. This, however, does not answer the question as to how waves are initiated when energy dispersion is non-existent.

More recent work using the International Satellite Cloud Project and reanalysis products suggested that AEWs are triggered in association with a “transition” process from a smaller and less well organized convective activity into a larger and more well organized mesoscale convective activity. However, the transition process is unclear. Also, the process how the mesoscale convection triggers AEWs is unclear. Further, the large-scale environment and mechanisms that determine AEW-initiation, including the vertical wind shear, requires investigation. In particular, the wave-triggering region in East Africa is composed of complicated high terrain and the effect of topography and diurnal cycle has not been explored in previous studies.

This work revisited the AEW-genesis and their interaction with different types of convection. A suit of multiple satellite observed and reanalysis datasets are used to investigate the AEW-genesis and the direct connection between different types of deep convection and the large-scale dynamics. We will present results that show how the smaller-scale, isolated type of deep convection transition to MCS-type deep convection across Africa. The role of dynamical parameters, including vertical wind shear is investigated. We will present results that show the influence of diurnal variance on convective transitions. Additionally, the atmospheric state during the transitions is investigated using a more direct measure of atmospheric stability, the equivalent potential temperature, and highlights of this result will be presented.