The Interaction between Deep Convection and Easterly Wave Activity over Africa: Convective Transitions and Mechanisms

Recent work using observational data from the International Satellite Cloud Climatology Project (ISCCP) and reanalysis products suggests that African easterly waves (AEWs) are formed in association with a “transition” process from a smaller and scattered convection into a larger and more well organized mesoscale convective activity. However, the transition process is unclear and how the mesoscale convection initiates AEWs is not well understood.

Analysis based on 25 years of ISCCP and reanalysis datasets show that increasing intradiurnal activity, atmospheric instability and specific humidity precede the development of well-organized convection over the Ethiopian highlands (EH). Atmospheric instability favors a high frequency of scattered, isolated convection to the east of EH, first, followed by a continuing and large increase in instability and increasing humidity, which supports well organized and larger scale convection. The timing of the changes of thermodynamic variables shows that the dominant transition process is scattered, weakly organized convection transitioning into the well-organized mesoscale type of convection and this initiates the AEWs.

Slightly before the mesoscale type of well-organized convection peaks over EH, low-level moist westerlies, low to mid-level wind shear, and positive relative vorticity increase over the region. Evidence shows that the large scale and local environment enables the scattered and less well-organized convection to merge and form larger and the well-organized convection type. The dynamic processes suggest that the dominant pathway for the AEW initiation is scattered convection transitioning to large and well-organized convection over EH and this initiates AEWs westward of EH.