Mesovortices in Hurricane Isabel

n 12 September 2003, satellite images of Atlantic Hurricane Isabel astounded the tropical cyclone science community as a myriad of elaborate patterns revealed themselves within the clouds in Isabel’s eye. Patterns of convoluted clouds in hurricane eyes have been previously documented (Fletcher et al. 1961; Kossin et al. 2002; Montgomery et al. 2002), but Isabel showed us something new. As the first visible satellite images became available after local sunrise (~1100 UTC), a remarkable pentagonal pattern resembling a starfish was clearly evident in Isabel’s eye. The pattern was due to the presence of six distinct mesovortices—one in the center and five others arranged fairly symmetrically around the center—and remained fairly steady for a few hours while rotating cyclonically within the eye (Fig. 1). One of the exciting aspects of the appearance of the starfish pattern in Isabel’s eye is that this pattern was previously found (Kossin and Schubert 2001) within a theoretical framework of maximum simplicity— unforced two-dimensional barotropic flow. Using aircraft flight-level data, the authors found that the tangential flow in the eyewalls of intense hurricanes can resemble a vortex sheet (their Fig. 1), and that these flows can support barotropic instability at high azimuthal wavenumbers and fast growth rates. To study how such instability might evolve within the hurricane inner core, a fully nonlinear, two-dimensional barotropic model was initialized with flow analogous to the observed flows. One outcome of such numerical integrations is shown in Fig. 2 (their Fig. 9). The maximum growth of the instabilities of the initial flow MESOVORTICES IN HURRICANE ISABEL