Studies of Tropical Cyclone Vorticity Dynamics using airborne Doppler-derived wind fields

Recent numerical and theoretical work on the role of vorticity redistribution in tropical cyclone (TC) intensity change is presented using observed wind fields derived from airborne dual-Doppler radar measurements. The relatively broad spatial coverage of the TC core afforded by dual-Doppler sampling permits a reasonable decomposition of the TC vorticity field into the dominant azimuthally-symmetric component and low azimuthal wavenumber perturbations to that mean. The focus here will be on what insight into the TC vorticity evolution Doppler radar observations provide beyond that accessible using flight-level in-situ measurements alone.

Two principal cases will be discussed. The first is mature Hurricane Olivia (1994). At the beginning of the second day of observation of Olivia, the symmetric vorticity took the form of an annulus encircling the eye with near-uniform values extending from the lower to middle troposphere. During the observation period a strong wavenumber two asymmetry in vorticity (i.e., an elliptical eye) emerged at low levels followed by an apparent mixing of elevated vorticity from the annulus into Olivia's eye. Based on recent vortex dynamical theory, this kind of evolution is expected of a storm weakening from its peak intensity, as Olivia was. In the second case the vorticity evolution during the incipient stages of what eventually became Hurricane Guillermo (1991) is presented. How pre-existing vorticity in the lower to middle troposphere merged to form a vertically-coherent vortex structure and why it managed to hold together in the presence of environmental vertical shear are addressed using the wind fields constructed from airborne Doppler measurements over consecutive days during the TEXMEX experiment.