A dual-Doppler analysis of Hurricane Guillermo (1997): Vortex resiliency in vertical shear flow

Hurricanes possess an intrinsic ability to remain vertically aligned when tilted by external vertical shear flow. The resiliency of the hurricane vortex has been explained in recent studies using dry dynamical models. The question then arises as to what the role of moist processes is when considering hurricane resiliency, and, furthermore, how moist processes impact the more general evolution of the vertically-sheared hurricane. While recent "full physics" numerical simulations have begun to offer insights into the behavior of the sheared hurricane, few observational case studies have been performed with sufficient spatial coverage and temporal resolution to address outstanding questions pertaining to the vortex dynamics. In this study, a unique airborne dual-Doppler radar data set is examined with the objective of documenting the evolving vertical tilt, axisymmetric structure, and asymmetric structure of a rapidly intensifying hurricane in an environment of non-negligible vertical shear.

A synthesis of vertical incidence and dual-Doppler measurements collected during repeated penetrations of Hurricane Guillermo (1997) on August 2 and 3 is used to 1) document the evolution of local vertical shear and the vertical tilt of the vortex axis, 2) identify and characterize the low azimuthal wavenumber convective asymmetry commonly observed in sheared hurricanes, 3) document the radial profile of azimuthal-mean vorticity, which is believed to play a key role in the intrinsic resiliency of the vortex, and 4) document the modulation of the asymmetric vorticity structure of the hurricane through its interaction with the external vertical shear flow.

Preliminary results indicate vigorous eyewall convection in the downshear to downshear-left quadrant of Guillermo. The relative vorticity within the eyewall and at the eye-eyewall interface is also observed to be highly asymmetric. Sources for the vorticity asymmetry, and implications for the sheared hurricane evolution, will be discussed.