Effects of orography and sea surface temperature on the cyclogenesis of tropical storm Debby (2006)

In mid-August 2006, an intense African easterly wave (AEW) propagated across the African continent. Coupled with a mesoscale convective system, this wave developed into the fourth named storm of the 2006 hurricane season, Tropical Storm (TS) Debby, on 23 August. Debby achieved maximum intensity of 45 knots (23.2 m/s) and minimum pressure of 999 mb. Although all conditions for tropical cyclogenesis and intensification existed, Debby was downgraded to a tropical depression on 26 August and dissipated on 28 August. In this study, we discuss cyclogenesis mechanisms related to TS Debby and present results from sensitivity experiments to study orographic and sea surface temperature (SST) 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. Several sensitivity experiments have been performed by varying the heights of Guinea Highlands (GH) or the SSTs. We found that local SST is positively correlated with tropical cyclogenesis, whereas the GH play a more crucial role in tropical cyclones initiated by AEWs. Following Berry and Thorncroft (2005), a lower-level (850-mb) vortex stalls and intensifies upon the arrival at the GH while higher-level (700-mb and higher) vorticity continues propagating over the GH and on to the Atlantic Ocean. One interesting finding is that both the removal and enhancement of the GH suppresses the intensity of the resulting TC. Relevant dynamical and physical processes associated with the above findings will be discussed.