Modeling studies of African Easterly Waves in relation to Tropical Cyclogenesis along the West African Coast

This study performed model simulations of three separate events of tropical cyclogenesis (TC-genesis) off the West African coast between the years of 2006 and 2008. In previous studies, main aspects of concentration have been on the environmental conditions within and surrounding a tropical cyclone in its developmental stages after it has been classified as a tropical depression. The purpose of this study was to investigate the processes that take place during the transition of an African Easterly Wave (AEW) and any associated Mesoscale Convective Systems (MCSs) or squall lines as they progress from continental West Africa into the maritime environment of the eastern Atlantic Ocean. This was accomplished through model simulations beginning 72-hours prior to each system in relation to an AEW being classified as a tropical depression by the National Hurricane Center (NHC). Aspects of concentration during model simulations were the microphysical environment regarding the proximity of Saharan Air Layer (SAL) dust to areas of TC-genesis and the impacts of topography (e.g., Guinea Highlands) in relation to accurately modeling TC-genesis.

The Weather Forecasting and Research (WRF) model version 3 was utilized to conduct numerical model simulations. Three tropical cyclones that were associated with AEWs and related MCSs or squall lines over continental West Africa that progressed off the coast, later achieving at least tropical storm strength, were selected to be investigated. These three tropical cyclones were: Tropical Storm Debby (2006), Hurricane Helene (2006), and Hurricane Josephine (2008).

Results demonstrated that the nested (i.e., 30 Km and 10 Km grid-spacing domains) WRF model simulation was able to recapture the evolution of each MCS or squall line in association with AEWs during all three events. Sensitivity experiments suggested that reducing the elevation of the Guinea Highlands produces an outcome with significant consequences in relation to TC-genesis. Although the highest peaks of the Guinea Highlands are only approximately 1300 m, simulation results suggested that topographic blocking may play an important role which enhances low level cyclonic flow. Another sensitivity case mimicking dust effects from the Saharan Air Layer (SAL) in microphysical processes demonstrated that more “dirty” air could potentially enhance cyclonic circulations as well as amounts of precipitation during the early stage of the cyclogenesis. On a larger scale, the impact of possible dry-air intrusion in association with SAL dust outbreaks that transport high concentrations of dust by meridional winds into favorable areas of TC-genesis just off the West African coast was evident.