The Interaction Between Cloud Regimes and Easterly Wave Activity over Africa: Convective Transitions and Mechanism

A growing body work using the International Satellite Cloud Project and reanalysis products suggest that African Easterly Waves (AEWs) are triggered in association with a “transition” process from a smaller and less well organized convective activity (identified as weather state type 3,WS3) into a larger and more well organized mesoscale convective activity (distinguished as weather state type 1, WS1). However, the transition process is unclear and the process how the mesoscale convection triggers AEWs is not well understood.

Composite analysis using satellite and reanalysis datasets show that increasing intradiurnal activity, increasing atmospheric instability (as determined by increasing equivalent potential temperature, Θ_E) and specific humidity (q) precedes the development of deep convection over East Africa. Large-scale and well-organized deep convection increases westwards from the Ethiopian highlands (EH). Atmospheric instability (increasing Θ_E) favours a high frequency of scattered, isolated type of convection to the east of EH at first, followed by large increase in instability and increasing humidity over the EH, which supports well organized and larger scale convection.

It is observed that Θ_E peaks when 700-hPa meridional wind anomalies (V700) are strong northerly at about the same time over EH, indicating the development of well-organized, larger scale convection. Maximum relative vorticity (ζ’) slightly precedes maximum convection over the EH. Thermodynamic processes suggest that the dominant process is WS3→WS1→AEW. Evidences show that high terrain associated with strong latent heating has an impact in transitioning of WS3 to WS1. The intradiurnal activity of deep convection peaks about half day before the peak in WS1 convection, suggesting the important influence of diurnal activity on deep convection. This conclusion is also supported by the dynamic results. Slightly before the WS1 peaks over the mountains and to the west of EH, low-level moist westerlies, low-level zonal wind shear, and positive relative vorticity increase between central Sudan and Ethiopia. Results in this study and past work provide evidence that AEW signatures over east Africa are weak during the time of convective transitioning, which is not surprising as East Africa is the region of initiation. Therefore, our study show that large scale and local environment enables the WS3 to merge and form bigger organized convective systems, WS1 types, as they propagate westwards and the WS1 type further develop over the region to the west of the EH. We note that the WS3 clouds also play an important role in moistening the midlevel atmosphere (by transporting positive q anomalies vertically upward). Also, increased vorticity in tandem with increased moderate wind shear, including increased upper level divergence, create an environment favorable for the development of deep and well-organized convection - WS1 type. This led us to conclude that the dominant pathway is WS3→WS1→ AEW.