Tropical transition of tropical cyclones Gabrielle and Humberto during the 2007 North Atlantic season

The 2007 North Atlantic tropical cyclone (TC) season featured 15 named tropical disturbances, including eight tropical storms, six hurricanes, and one subtropical storm. A distinguishing aspect of the 2007 North Atlantic TC season was that nine of the 15 (60%) TCs formed via the tropical transition (TT) genesis pathway, a higher percentage than expected from the long-term climatology (50%) for this genesis pathway. The motivation for focusing on TCs that form via the TT genesis pathway is twofold: 1) these storms can be difficult to predict because they can form on relatively short timescales (< 24 h) and in close proximity (< 1000 km) to U.S. coastal zones, and 2) these storms provide an opportunity to study the physical processes that govern an important TC genesis pathway. This goal of this presentation will be to document the ambient environment and physical processes associated with the TT genesis pathway. This task will be accomplished by means of two linked illustrative case studies that occurred during the 2007 North Atlantic season: TCs Gabrielle and Humberto.

The genesis of TCs Gabrielle and Humberto occurred during 30 August–7 September and 11–12 September 2007, respectively, over the western North Atlantic and the Gulf of Mexico. Distinguishing features of both TCs were that: 1) the pre-genesis low-level disturbances formed and intensified in conjunction with a common long-lived upper-level potential vorticity (PV) streamer that drifted slowly southwestward from the western North Atlantic to the Gulf of Mexico, and 2) low-level vorticity growth occurred in conjunction with episodic outbreaks of deep convection in the vicinity of a weak baroclinic zone that persisted along the Gulf of Mexico coast. Accordingly, we will take full advantage of the abundant available radar and surface observations to document some of the mesoscale aspects of the pre-genesis disturbances that ultimately evolved into TCs Gabrielle and Humberto. Available satellite and radiosonde data, as well as high-resolution Rapid Update Cycle analyses, will also be used to help document the vertical structure evolution and life cycles of a number of convectively driven mesoscale vorticity centers that contributed to the eventual organization of the incipient tropical disturbances that became TCs Gabrielle and Humberto, respectively.