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 the European Organization for the Exploitation of Meteorological Satellites (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 North Africa. Specifically, the Ethiopian Highlands (EH) play an essential role in the formation of the AEW which, in turn, was able to merge with the convective clouds over EH, Darfur Mountains (DF) and Cameroon Mountains (CM) organized into an AEW-MCS system, which is similar to that found in a previous study on Tropical Storm Alberto (2000), but differs in taking a longer merging process due to weaker pre-Helene AEW and MCS.

Based on WRF analysis of the pre-Helene AEW-MCS system, this study found that vertical moisture flux played a major role in the generation of AEW-MCS and maintenance of the convective cycles associated with the pre-Helene AEW-MCS system as it traveled westward across Northern Africa. The simulated propagation speed of the pre-Helene AEW-MCS is 8.3 ms-1, which is consistent with the observed speed. Initially, orographic forcing induced vortex leading to AEW on the lee of EH and MCSs over EH, DF, and CM. The AEW helped these MCSs organized into the pre-Helene AEW-MCS system. As the pre-Helene AEW-MCS system travels westward across the continent, the moisture flux fluctuated in reflection of the local environment. This study also identified several convective genesis periods and three lysis periods of the mesoscale convective system.

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.