The evolution of tropical storm Gabrielle (2001)

The kinematic structure of a developing asymmetric tropical cyclone is not very well understood by the scientific community. Some studies have shown that vertical shear of the horizontal wind can be beneficial in the early stages and results in a stronger vortex than a zero sheared environment (Molinari et al. 2006, Guinn and Schubert 1993, Enagonio and Montgomery 2001). This presentation will focus on the structure and evolution of Tropical Storm Gabrielle during the 15th and 16th of September and provide a brief comparison with asymmetric Hurricane Humberto (2001).

Gabrielle (2001) formed off the coast of Florida in the Gulf of Mexico on September 11. The influence of an upper level trough approaching from the north was believed to help intensify the storm prior to landfall on the 14th of September (Molinari et al. 2006). As the storm crossed land and emerged over the Atlantic on the 15th, the structure remained highly asymmetric. Gabrielle maintained tropical storm strength for the next 48 hours before intensifying into a hurricane on the 17th. Humberto (2001) formed under an upper level low in the western Atlantic in late September and was sampled over three consecutive days with 4 research aircraft.

The analyses of these two highly asymmetric storms were performed using Global Positioning System (GPS) sondes that were dropped during aircraft reconnaissance flights from NOAA's WP-3D's. Horizontal maps and vertical cross-sections are constructed by applying a cubic spline scheme to the GPS sonde data to generate the initial fields. In Tropical Storm Gabrielle, 60 sondes were deployed during the 15th and 16th of September. The analysis for Hurricane Humberto used 46 sondes from the 23rd of September.

Plan view images of Tropical Storm Gabrielle show strongest low-level inflow on the eastern flank of the circulation and maximum tangential velocities occurring just to the northwest of the center. The vertical structure of the tropical storm does not change drastically with height, while the horizontal structure is disorganized and dissimilar to that found in a mature tropical cyclone. Radial velocities were found to be the strongest in the lowest 60 meters and decreased slowly with height. The circulation center was elongated in a southwest to northeast oriented axis on the 16th. This is believed to be from the fast motion, about 10 m/s, off the east coast of the United States, which contributed to the asymmetric structure.

The few analyses of tropical storms available (e.g., Yanai 1961, Heymsfield et al. 2006) support the contention that a tropical storm is highly asymmetric with respect to cloudiness and rainfall. We will apply the GPS sondes to further define basic tropical storm kinematic structure in the lower troposphere. Ultimately we wish to learn why tropical storms are often asymmetric, and how they may remain so even as they achieve hurricane status. Future work will focus on the inflow structure and depth in tropical cyclones and the evolution of such structures with intensity.