Application of a Global Mesoscale Model to Study Hierarchical Multiscale Interactions during the Tropical Cyclogenesis associated with African Easterly Waves in 2006

In this study, we conduct numerical experiments with the NASA global mesoscale model to examine the multiscale interactions of tropical cyclogenesis for Cape Verde-type hurricanes, aimed at improving simulations of regional hurricane climate near the Cape Verde islands, off the west coast of Africa. Previous studies (e.g., Landsea, 1993) have shown that nearly %85 of intense hurricanes have their origins as African easterly waves (AEWs), and these AEWs could form via the release of barotropic and/or baroclinic instability by an African Easterly Jet (AEJ). Therefore, to understand regional hurricane climate near the Cape Verde islands, it is important to accurately simulate the multiscale interactions among hurricanes, AEWs, and AEJ, and surface mechanic and thermal processes. As recent advance in high-resolution global modeling has shown a potential for improving multiscale interactions between mesoscale hurricanes and synoptic flows, extended-range (30day) simulations with the NASA global mesoscale model are conducted to examine model's ability to simulate AEWs in 2006 and its modulation on the formation of Hurricane Helene (2006). By comparing simulations with NCEP Reanalysis and Observations collected during the NAMMA (NASA African Monsoon Multidisciplinary Analyses), we will show that formation and propagation of six AEWs are realistically simulated as well as the formation of Hurricane Helene with a lead time of 20 days. However,the simulated Helene has an overestimated growth rate. In addition, remarkable simulations of an AEJ averaged over the entire 30 days are also obtained. As the global mesoscale model allows to study the two-way interactions, we then discuss the impacts of terrains (e.g., Guinea Highlands) on the simulations of the AEJ, AEWs and hurricanes.