The Interaction Between Convection and African Easterly Waves

African Easterly Waves (AEWs) and convection have been associated with each other since the seminal studies on AEWs by Frank, Burpee, and Reed, among others, during the 1960s and 1970s. More recently, studies such as those by Berry and Thorncroft (2005, 2012) have shown that convection is integral to maintaining an AEW during its passage across West Africa. However, details of the exact mechanism(s) through which they interact need to be better understood. In this presentation we examine the findings of two complementary studies which examine composite average AEWs in reanalyses and case studies in convection-permitting WRF simulations. The evolution of potential vorticity (PV) in AEWs is a focus with the results of PV budgets presented.

We first note that the progression of cloud regimes within our simulations, and as shown prior by Janiga and Thorncroft (2016), resembles the stretched building blocks hypothesis whereby clouds progress from shallow convection to deep convection to stratiform cloud within the AEW. We then find that four processes are important to the AEW PV anomaly; two advective sources of PV and two diabatic sources of PV. Diabatic processes associated with coupled deep-convection generate PV in the lower levels counteracting the effects of time-mean shear and therefore maintaining a deep PV column. Meanwhile, diabatic processes associated with both coupled and time-mean mid-upper level stratiform regions enhance the AEWs and are typically larger than barotropic and baroclinic processes. Through experiments with WRF case studies that control for convective processes, we confirm both of these processes. Finally, given an as yet undefined convective coupling mechanism, we propose that AEWs are enhanced and maintained via a process akin to stratiform instability.