Modulation of eyewall convection by eye-eyewall mesovortices during the rapid intensification of Hurricane Guillermo (1997)

Observations and numerical simulations indicate that the azimuthal distribution of deep eyewall convection can be significantly influenced by the 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 within the inner core 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 flows in and near the eyewall of a rapidly intensifying hurricane.

This study utilizes an extensive Doppler radar dataset collected during multiple dual-aircraft penetrations of Hurricane Guillermo during a 6 hour period on 2 August 1997. A total of 10 unique dual-Doppler analyses are used to document the evolving convective and kinematic structures within and near the eyewall. During this time, the inner core exhibited a vorticity structure supportive of barotropic/baroclinic instabilities. The eyewall contained a persistent azimuthal wavenumber-one convective pattern consistent with the observed vertical wind shear. The eyewall also contained a rich spectrum of evolving higher-wavenumber mesovortices and deep convective cells. A census of the mesovortices was performed and two distinct types were observed. The first variety was a prominent azimuthal wavenumber-two structure within the low and mid-level eyewall. The second variety consisted of higher-wavenumber (three-five) features confined to the low levels and located along the eye-eyewall interface. The two types rotated at different speeds that, combined with the shear-induced asymmetries, had a profound impact on the convective structure and evolution of Guillermo's eyewall. A synopsis of our results and their comparison with previous numerical studies will be presented at the conference.