Genesis of Tropical Storm Debby (2006) within an African Easterly Wave: Roles of the Bottom-Up and Midlevel Pouch Processes

Abstract

Abstract The “bottom up” generation of low-level vortices (LVs) and midlevel vortices (MVs) during the genesis of Tropical Storm Debby (2006) and the roles of a midlevel “marsupial pouch” associated with an African easterly wave (AEW) are examined using an 84-h simulation with the finest grid size of 1.33 km. Results show that several MVs are generated in leading convective bands and then advected rearward into stratiform regions by front-to-rear ascending flows. Because of different Lagrangian storm-scale circulations, MVs and LVs are displaced along different paths during the early genesis stages. MVs propagate cyclonically inward within the AEW pouch while experiencing slow intensification and merging under the influence of converging flows. The MVs’ merging into a mesovortex is accelerated as they come closer to each other in the core region. In contrast, the low-level Lagrangian circulation is opened as a wave trough prior to tropical depression (TD) stage, so the LVs tend to “escape” from the pouch region. Only after the low-level flows become closed do some LVs congregate and contribute directly to Debby’s genesis. The TD stage is reached when the midlevel mesovortex and an LV are collocated with a convective zone having intense low-level convergence. Results also show the roles of upper-level warming in hydrostatically maintaining the midlevel pouch and producing mesoscale surface pressure falls. It is found that the vertically tilted AEW with a cold dome below is transformed to a deep warm-core TD vortex by subsiding motion. A conceptual model describing the key elements in the genesis of Debby is also provided.