Vortical structures accompanying secondary eyewall formation in Hurricane Rita (2005)

Uniquely detailed observations obtained by the ELDORA Doppler radar during the RAINEX project shed new light on cloud-scale processes internal to several intense storms encountered during the notable 2005 Atlantic hurricane season. Here we explore measurements obtained within Hurricane Rita on 22 September 2007, as this Category-5 storm was in the midst of an eyewall replacement cycle. Multiple circumnavigations of Rita's inner core by the NRL P-3, which carried ELDORA into the storm under guidance provided by surveillance radars aboard two NOAA P-3 aircraft, afforded simultaneous high-resolution views of Rita's inner (primary) and outer (secondary) eyewalls during this critical evolutionary phase.

In conjunction with appearance of a shallow but distinct low-level wind maximum within Rita's nascent outer eyewall, volumetric ELDORA velocity data reveal coherent curvilinear vorticity structures that arced radially outward and downshear with height. With an observed separation in the range of 5-10 km, these prominent modulations of the velocity field are distinct from more tightly spaced (sub-kilometer scale) boundary-layer rolls previously observed during the landfall phase of some storms. Although they appear to extend to appreciable heights (> 10 km MSL), these alternating cyclonic/anticyclonic vorticity bands are distinct from more vertically erect (and locally more intense) cyclonic maxima observed at this time in the mixing zone along the inner edge of Rita's primary eyewall. On the whole, however, the outer eyewall exhibited far less smooth and less circularly-symmetric velocity structure than did the inner eyewall. The outer eyewall was characterized by multiple convective-scale velocity maxima (corresponding to the aforementioned banded vorticity structures) These observations suggest that an active axisymmetrization process was spinning up the secondary wind maximum (e.g., Moeller and Montgomery, 1999). The potential relationship of these newly observed secondary eyewall structures to simulated vortex Rossby wave dynamics unique to the “beta skirt” region (Terwey and Montgomery, 2007) will also be explored.

Moeller, J.D., and M.T. Montgomery, 1999: Vortex Rossby waves and hurricane intensification in a barotropic model. J. Atmos. Sci., 56, 1674-1687.

Terwey, W.D., and M.T. Montgomery, 2007:Secondary eyewall formation in two idealized, full-physics modeled hurricanes. J. Geoph. Res. (submitted).