Rapid Intensification of a Sheared Tropical Storm

Abstract

A weak tropical storm (Gabrielle in 2001) experienced a 22-hPa pressure fall in less than 3 h in the presence of 13 m s?1 ambient vertical wind shear. A convective cell developed downshear left of the center and moved cyclonically and inward to the 17-km radius during the period of rapid intensification. This cell had one of the most intense 85-GHz scattering signatures ever observed by the Tropical Rainfall Measuring Mission (TRMM). The cell developed at the downwind end of a band in the storm core. Maximum vorticity in the cell exceeded 2.5 ? 10?2 s?1. The cell structure broadly resembled that of a vortical hot tower rather than a supercell. At the time of minimum central pressure, the storm consisted of a strong vortex adjacent to the cell with a radius of maximum winds of about 10 km that exhibited almost no tilt in the vertical. This was surrounded by a broader vortex that tilted approximately left of the ambient shear vector, in a similar direction as the broad precipitation shield. This structure is consistent with the recent results of Riemer et al. The rapid deepening of the storm is attributed to the cell growth within a region of high efficiency of latent heating following the theories of Nolan and Vigh and Schubert. This view is supported by a rapid growth of wind speed and vorticity in the storm core during the 1-h lifetime of the cell, and by the creation of a narrow 7°C spike in 700-hPa temperature adjacent to the cell and coincident with the lowest pressure. The cell is not seen as the cause of rapid intensification. Rather, it is part of a multiscale process: (i) development of a new circulation center within the downshear precipitation shield, (ii) continued ambient shear creating a favored region for cell formation just downshear of the new center, and (iii) the development of the intense cell that enhanced diabatic heating close to the center in a region of high efficiency of kinetic energy production. This sheared, asymmetric rapid intensification of Tropical Storm Gabrielle is contrasted with the nearly symmetric composite given by Kaplan and DeMaria.