Comparison between Two Case Studies of Developing and Nondeveloping African Easterly Waves during NAMMA and AMMA/SOP-3: Absolute Vertical Vorticity Budget

Abstract

Two West African disturbances observed in August and September 2006 during the National Aeronautics and Space Administration African Monsoon Multidisciplinary Analysis (NAMMA) and the Special Observing Period 3 (AMMA/SOP-3) have been simulated using the Méso-NH numerical model with explicit convection. The first disturbance spawned Hurricane Helene (2006) off the West African coast, and the second one, referred to as perturbation D, though relatively intense, failed to develop. Over the continent, each case was associated with a well-defined African easterly wave (AEW) trough with embedded growing and decaying convective activity of various size, duration, and intensity. The aim of this work is to investigate the contribution of these convective systems in the generation and maintenance of cyclonic vorticity associated with the AEW trough, with respect to the synoptic-scale processes. The absolute vorticity budgets are analyzed during the ?continental? and ?oceanic transition? stages of these AEW troughs in order to highlight the similarities and differences between the developing pre-Helene disturbance and the nondeveloping perturbation D. For the developing case, low- to midlevel cyclonic vorticity was produced by convective processes through tilting and stretching. Cyclonic vorticity was then transported upward through vertical advection associated with convection and outward through horizontal advection mostly induced by the large-scale midlevel diverging circulation related to the downstream AEW ridge. For the nondeveloping case, low- to midlevel cyclonic vorticity production through stretching and tilting, and its vertical transport were relatively similar over the continent but smaller over the oceanic transition because of weaker convective activity. The outward transport through horizontal advection was also weaker as there was little midlevel divergence induced by the downstream AEW ridge in this case.