A dual-Doppler analysis of Hurricane Guillermo (1997): Interactions between the eye and eyewall during rapid intensification

Recent observations and numerical simulations indicate that the azimuthal distribution of deep hurricane convection can be significantly influenced by environmental vertical wind shear and internal dynamical processes. In such instances, the convection can acquire a persistent low-azimuthal wavenumber structure composed of multiple transient convective cells. The roles played by such convective asymmetries in hurricane evolution are not well understood. In particular, the extent to which evolving low-wavenumber flow across the eye-eyewall interface influences the generation, organization, and evolution of asymmetric convection remains an open question. The objective of this study is to elucidate the organization and evolution of observed asymmetric eyewall convection within the context of the low-wavenumber flow along the eye-eyewall boundary during a period of rapid intensification.

This study utilizes an extensive observational dataset collected during repeated dual-aircraft penetrations of Hurricane Guillermo on 2 August 1997 during a 6 h period. Estimates of the environmental vertical wind shear and azimuthal low-wavenumber flow are derived from three-dimensional dual-Doppler (3DDD) wind fields within 60 km of the storm center. Evolution of the flow is determined from 10 unique 3DDD winds fields. The azimuthal distribution and evolution of transient convective cells is determined from animated radar imagery. Dynamic interactions across the the eye-eyewall interface are determined from forward and backward air parcel trajectories derived from the 3DDD wind fields. Flight-level, GPS dropwindsonde, and vertical incident radar data is also used to elucidate various aspects of the eyewall convection.

Preliminary results suggest that moderate northerly vertical wind shear was impinging on the inner-core vortex. The eyewall convection exhibits a persistent wavenumber-one structure with enhanced convection in the southern and eastern quadrants. Transient convective cells episodically develop in the southern quadrant, and then advect cyclonically through the eastern quadrant before dissipating in the northern quadrant. The 3DDD wind fields depict an evolving asymmetric wind field along the eye-eyewall interface. Currently, a census of air parcel trajectories which cross the interface is being performed to document their frequency and spatial relationship to any episodic convective bursts.