Genesis of Tropical Storm Debby (2006) from an African Easterly Wave

The transformation of an African easterly wave (AEW) into Tropical Storm Debby (2006) is examined using a 4-day, nested-grid simulation with the Weather Research and Forecasting (WRF) model at the finest grid length of 1.33 km. The model is initialized at 0000 UTC 20 August 2006 when the AEW was over land near the coast of Guinea. Tropical cyclogenesis (TCG) occurred as it moved into a warm ocean surface. The model-simulated results compare favorably to the best track and satellite observations. In this presentation, we will focus on the generation of different vortex structures in the vertical during the genesis stage. It is found that Debby is initiated from a midlevel mesoscale cyclonic vorticity zone associated with an African easterly jet (AEJ). As the disturbance moves off shore, a meso-β-scale mesolow begins to intensify, with the bottom-up growth of cyclonic vorticity in the presence of intense low-level convergence and localized deep convection close to the small-scale vortex center. Meanwhile, a vertically tilted, much larger-scale cyclonic circulation at the midlevel strengthens in the stratiform region, with a warm column above a cold layer. TCG occurs at 36 h into the integration, when both the midlevel circulation and lower-level lower pressures are collocated in the vertical, with a deep warm column at the vortex center. Because of the dry intrusion associated with the Sahara dust, only a relatively small-sized tropical storm develops. A time series of the vertical warming profiles near the vortex center shows a weak upper-level and a stronger lower-level warm core, respectively. A hydrostatic calculation reveals that the upper-level warm warming is more critical than the lower-level one in determining the surface pressure fall. An increase in the upper-level warming provides a trigger to the genesis of Debby. However, the lower-level warming core amplifies faster than the upper-level one during the intensification stage of Debby.