High Resolution Dual-Doppler Observations of Tropical Cyclone Vortex Rossby Waves

Vortex Rossby waves (VRWs) have been hypothesized to be an initiation mechanism of the inner spiral bands observed in the inner core of tropical cyclones. VRWs are thought to propagate along the radial gradient of storm vorticity, suggesting a Rossby-type wave rather than a gravity-inertia wave. Although numerous numerical modeling studies have hypothesized their existence, very few observational studies have documented their structure. Since the early 2000s, the Shared Mobile Atmospheric Research and Teaching Radars (SMART-Rs) have sampled multiple landfalling tropical cyclones in which three-dimensional wind retrievals are possible in the inner core regions. Herein two of those cases are examined: Hurricane Isabel (September 2003) and Hurricane Irene (August 2011). Three-dimensional variational data assimilation dual-Doppler analyses (DDAs) are available every two to three minutes in Hurricane Isabel and every ten minutes in Hurricane Irene. In this study we examine a 20 minute period (1040 – 1100 UTC) from Hurricane Isabel and a one hour period (1000 – 1100 UTC) from Hurricane Irene.

VRWs are apparent below 2-5 km above radar level (ARL) as elongated bands of vorticity that appear to be sheared by the radial shear of the horizontal wind. Examining the three dimensional structure of vertical vorticity, it appears that the convection and rising motion associated with inner spiral bands lead vertical vorticity maxima in both Hurricane Irene and Hurricane Isabel. Conceptually, low-level convergence is expected on the leading edge of the vorticity maxima, initiating the updrafts sustaining spiral rainbands. Updrafts appear to be tropospherically deep in many cases with the strongest rising motion associated with the low-level vorticity maxima. However, the vorticity maxima also appear to extend through the depth of the troposphere and coupled with the updraft maxima aloft. As the waves propagate outward from the eyewall, they appear to converge at a radius at which point the waves dissipate (thought to be the radius at which the radial gradient of storm vorticity is small). Utilizing the formalized theory of VRWs, we show that the radial propagation speed of the observed waves is consistent with the dispersion relation presented by the literature. In addition to the existence of VRWs themselves, we suggest that the inner core environment rainfall is strongly influenced by the rainbands initiated by VRWs. Although a more robust analysis of the entire inner core environment rainfall distribution is needed, results from the DDAs in Hurricane Isabel suggest that an extensive portion of rainfall seen in the DDA domain is a result of VRW induced spiral rainbands.