The Generation, Maintenance and Propagation of the Pre-Helene African Easterly Wave and Mesoscale Convective System over Africa: A Numerical Study and Analysis of the Environment

Based on EUMETSAT infrared (IR) satellite imagery and controlled simulations using the Weather Research and Forecasting (WRF) model, the pre-Hurricane Helene (2006) African Easterly Wave and mesoscale convective system (AEW-MCS) were found to be initially formed over eastern Africa. Specifically, Ethiopian Highlands (EH) plays an essential role in the formation of the AEW which, in turn, was able to get the convective clouds over EH, Darfur Mountains (DF) and Cameroon Mountains organized into an AEW-MCS system, in a way similar to that found in a previous study on Tropical Storm Alberto (2000).

Based on WRF analysis of the pre-Helene AEW-MCS system, we found that vertical moisture flux played a major role in the generation and maintenance of the convective cycles associated with the pre-Helene AEW-MCS system as it traveled westward across Northern Africa. Initially, orographic forcing induced vertical motion and the redistribution of moisture to provide upward moisture flux. As the pre-Helene AEW-MCS system travels westward across the continent, the moisture flux fluctuated in reflection of the local environment. We identified several convective genesis periods and three lysis periods of the mesoscale convective system, which was embedded in the AEW, traveled across the African continent.

In addition to the supply of upward vertical flux from the environment, AEW also played an important role in keeping the MCS coherent. It was also found that the pre-Helene AEW-MCS is slightly modulated by orography downstream from the EH and DF regions.