A Modeling Study Of The Genesis Of Tropical Storm Debby (2006) From An African Easterly Wave

Tropical cyclogenesis continues to be one of the most challenge problems in tropical meteorology today. While a robust theoretical framework (marisupial pouch diagram) for tropical cyclogenesis within African Easterly Waves has recently been developed, little work has been focused on the mesoscale process and interactions with the African Easterly Waves during tropical cyclogenesis in north Atlantic basin. This study investigates the genesis of Tropical Storm Debby during the 2006 north Atlantic hurricane season using a series of high-resolution model simulation with the finest grid size of 1.33 km. In addition to a control simulation used to study the mesoscale processes during tropical cyclogenesis,20 members ensemble simulations are conducted to invest the dynamical processs taking place during tropical cyclogenesis.Results show that several midlevel vortices are generated in leading convective bands and then advected rearward into stratiform regions. The midlevel vortices merge into a mesovortex as they become closer. It is found that the genesis of Tropical Storm Debby is trigged by the co-location of the mid-level mesovortex and a convective generated low-level vortex with an intense low-level convergence zone. Results also show that the upper-level warming plays an important role in tropical storm genesis by hydrostatically maintaining the midlevel pouch and producing mesoscale surface pressure falls.Using simulation results of two cases under different large-scale conditions, we show that the upper-level warming could account for more than half of the minimum sea-level pressure falls in both cases. It is found that upper-level flow and vertical wind shear tend to suppress the formation and development of upper-level warming. Results also suggest that both upper- and low-level processes be considered in understanding tropical storm genesis.In addition, ensemble simulations are used to further investigate the formation mechanisms by showing some parameteric differences between the stronger and weaker members. The dominant pattern of mean sea-level pressure ensemble differences is associated with the intensity of the pre-tropical depression, explaining large part of the total variance at the time of tropical storm genesis, which confirms the importance of midlevel vortices and convections.