Convective scale evolution during rapid intensification of a tropical cyclone

Dr. John (Jack) Beven of the National Hurricane Center has argued on the Tropical Storms list for two types of rapid intensification (RI) of tropical cyclones. He described them as the “closed eyewall type” and the “supercell type”. In the first, vertical wind shear is small and the precipitation field during RI is nearly symmetric, following the conditions for RI described by Kaplan and DeMaria (2003). In the latter, ambient vertical wind shear is moderate-to-large, the cloud and precipitation fields are highly asymmetric, and a single cell or cell cluster erupts. We will call the latter “intense cell type” rather than supercell-type for reasons that will become apparent in the talk. Beven argued that the latter type is capped in intensity, typically as a strong tropical storm, whereas the closed eyewall RI can lead to major hurricanes. The case study of interest here is Tropical Storm (TS) Gabrielle of 2001, and it represents the intense cell type of RI. This storm experienced a 20 hPa fall in minimum central pressure in less than 3 hours, during a time that ambient vertical wind shear exceeded 13 m/s. Our presentation will focus on the evolution of individual cells during a four-hour period before and during the rapid intensification. The National Lightning Detection Network, U.S. Air Force reconnaissance, and coastal radar will provide the key data. The cell of greatest interest developed within 30 km of the center and moved cyclonically and inward over the next hour. A TRMM overpass captured this cell near the time of its maximum intensity. The cell fell within the top 0.2% of all TRMM-measured convective cells (land or ocean) in terms of 85 GHz and 37 GHz scattering (Dan Cecil, personal communication, 2009), and fell within the top 0.007% of all cells in tropical cyclones (Haiyan Jiang, personal communication, 2009). The talk will address the following questions: (i) Was this a supercell? (ii) Why did the storm deepen so rapidly? (iii) What came first? TS Gabrielle will be compared with some other storms in the literature, including Guillermo (2001), Ophelia (2005), Claudette (2003), and Chantal (2001).