Orographic influence on the cyclogenesis of tropical storm Debby (2006)

In this study, we investigate the sensitivity of the cyclogenesis of tropical storm (TS) Debby (2006) to local orography. It is known that several necessary conditions exist for tropical cyclogenesis and among them are sufficiently large sea surface temperature (SST) utilized in maximum potential intensity (MPI) theory, mid-level moisture, low vertical wind shear, and lower-level vorticity, coming from a preexisting cyclonic disturbance. Although a theoretical framework is possible for discussing the genesis potential for tropical cyclones in terms of these quantities, we wish to investigate the effect of the passage of the African easterly wave (AEW)/mesoscale convective system (MCS) precursor to TS Debby over the Guinea highlands (GH) and more specifically the Fouta Djallon highlands (FJH), a mountainous region of Guinea, west Africa. We discuss cyclogenesis mechanisms related to TS Debby and present results from sensitivity experiments to study orographic effects on the cyclogenesis of Debby by performing numerical experiments utilizing the Weather Research & Forecasting - Advanced Research WRF (WRF-ARW) model with nested grids of resolution 27, 9, and 3 km, respectively. Several sensitivity experiments have been performed by varying the heights of GH. From our experiments, we find that passage of an AEW/MCS disturbance plays a crucial role in the genesis of TS Debby. Upon passage of the precursor over the GH, part of the precursor continues propagating over the east Atlantic, whereas the lower level components of the precursor are directly influenced by the terrain height. One interesting finding is that both the removal and enhancement of the GH suppresses the intensity of the resulting TC. In addition to affecting the size and location of the cyclonic vorticity of the AEW precursor, local orography plays a role in moisture processes associated with the MCS precursor to Debby, and has an overall effect on tropical cyclogenesis. Relevant dynamical and physical processes associated with the above findings will be discussed.