African Easterly Wave Dynamics in Convection Permitting Simulations: Rotational Stratiform Instability as a Conceptual Model

We examine the upscale effect of moist convection on African easterly waves (AEWs) by limiting condensational heating and initial ambient moisture in convection-permitting simulations. Moist convection is fundamental in maintaining and destabilizing AEWs. The contribution from barotropic-baroclinic instability, albeit important, is relatively smaller. Mesoscale convective systems (MCSs) are initiated downstream of the AEW troughs and are associated with extensive trailing stratiform cloud regions. Using a potential vorticity (PV) budget, we show that the attendant diabatic heating profile reinforces the AEW. A model for destabilization is proposed that relies on the phasing of stratiform heating and the PV anomaly of the AEW. It qualitatively resembles stratiform instability and stretched building blocks hypotheses introduced in previous studies. The generation of PV by deep moist convection in the vicinity of the trough counters the shearing effect of the background flow. This helps maintain an upright PV column, which is conducive for formation of tropical cyclones. AEW propagation is dominated by advective processes and intermittently modified by moist convection when large MCSs move ahead of the AEW. Plain Language Summary African easterly waves (AEWs) are weather systems that impact a broad region of Africa and the tropical Atlantic. The interaction between AEWs and cloud systems is not well understood though. We investigate this problem by simulating two AEWs and then controlling the extent of clouds within these simulations. We find that cloud systems are essential for the growth and maintenance of the AEWs. Multiple cloud systems combine together to produce regions of high and expansive clouds coinciding with the trough of the AEWs. Precipitation formation within those cloud regions leads to a release of heat which in turn leads to new regions of rotation. Because these new regions of rotation coincide with the AEW trough, they maintain and enhance the AEW. Further, new regions of rotation generated in the same manner, but close to the surface by regions of deep cloud at the head of the cloud systems, leads to the deepening of the AEW circulation. The latter point, has implications for the downwind initiation of tropical cyclones in association with AEWs over the tropical Atlantic.