The Structure and Evolution of Hurricane Elena (1985). Part II: Convective Asymmetries and Evidence for Vortex Rossby Waves

Abstract

A portable data recorder attached to the Weather Surveillance Radar-1957 (WSR-57) in Apalachicola, Florida, collected 313 radar scans of the reflectivity structure within 150 km of the center of Hurricane Elena (in 1985) between 1310 and 2130 UTC 1 September. This high temporal and spatial (750 m) resolution dataset was used to examine the evolution of the symmetric and asymmetric precipitation structure in Elena as the storm rapidly strengthened and attained maximum intensity. Fourier decomposition of the reflectivity data into azimuthal wavenumbers revealed that the power in the symmetric (wavenumber 0) component dominated the reflectivity pattern at all times and all radii by at least a factor of 2. The wavenumber 1 asymmetry accounted for less than 20% of the power in the reflectivity field on average and was found to be forced by the environmental vertical wind shear. The small-amplitude wavenumber 2 asymmetry in the core was associated with the appearance and rotation of an elliptical eyewall. This structure was visible for nearly 2 h and was noted to rotate cyclonically at a speed equal to half of the local tangential wind. Outside of the eyewall, individual peaks in the power in wavenumber 2 were associated with repeated instances of cyclonically rotating, outward-propagating inner spiral rainbands. Four separate convective bands were identified with an average azimuthal velocity of 25 m s−1, or ∼68% of the local tangential wind speed, and an outward radial velocity of 5.2 m s−1. The azimuthal propagation speeds of the elliptical eyewall and inner spiral rainbands were consistent with vortex Rossby wave theory. The elliptical eyewall and inner spiral rainbands were seen only in the 6-h period prior to peak intensity, when rapid spinup of the vortex had produced an annular vorticity profile, similar to those that have been shown to support barotropic instability. The appearance of an elliptical eyewall was consistent with the breakdown of eyewall vorticity into mesovortices, asymmetric mixing between the eye and eyewall, and a slowing of the intensification rate. The inner spiral rainbands might have arisen from high eyewall vorticity ejected from the core during the mixing process. Alternatively, because the bands were noted to emanate from the vertical shear-forced deep convection in the northern eyewall, they could have formed through the axisymmetrization of the asymmetric diabatically generated eyewall vorticity.

A Long-Lasting Vortex Rossby Wave–Induced Rainband of Typhoon Longwang (2005)

“…6b), which is inconsistent with the cyclonically propagating speed noted for VRWs (e.g., Wang 2002;Corbosiero et al 2006). A careful wave decomposition analysis following Corbosiero et al (2006) using the CWB radar images indicated that the convective segment used for the previous tracking is actually only a part of a longer rainband, more specifically, the tail of a long spiral rainband (Fig. 7).…”

“…6b). Relatively small radial propagation speeds of VRWs have been found in both numerical modeling [e.g., 4-5 m s -1 for wavenumber-1 VRWs in Wang (2002)] and observational studies [e.g., 5.2 m s -1 in Corbosiero et al (2006)]. The small radial propagation speed of this band is thus consistent with VRWs rather than the high-frequency gravity waves [e.g., greater than 15 m s -1 in Diercks and Anthes (1976)].…”

Life Cycles of Hurricane-Like Vorticity Rings

“…This was most likely due to either an exponential instability (allowed by the model's continuous vorticity profile) or nonlinear vorticity mixing. Elliptically shaped vorticity structures have been observed in hurricanes (Kuo et al 1999;Reasor et al 2000;Corbosiero et al 2006) and simulated as a nonlinear interaction between a monopole and a secondary ring of enhanced vorticity (Kossin et al 2000), but their formation dynamics are not clear in the evolution of unforced PV rings.…”

Paradigms for tropical cyclone intensification

“…The simulated vortex exhibits many realistic features of a mature tropical cyclone (e.g., Willoughby 1995, Kossin et al 2002, Corbosiero et al 2006, Marks et al 2008) with spiral bands of convection surrounding an approximately symmetric eyewall and a central eye that is free of deep convection (Fig. 9, upper panels).…”