Inner core convective asymmetries and vortex Rossby waves in Atlantic basin tropical cyclones

Recent observational and numerical modeling work has documented the importance of understanding the dynamics of the asymmetries within the core, i.e. the eyewall and inner rainband region, of tropical cyclones. These asymmetries have been found to influence not only storm structure, but storm intensity and intensity change through the interaction of the asymmetries with the parent vortex. This interaction has been well documented using potential vorticity (PV) and vortex Rossby wave (VRW) dynamics in numerical models. In nature, however, the three-dimensional, high resolution data sets necessary to diagnose PV are rare, and thus only a handful of observational studies have documented the existence of VRWs and speculated on their formation mechanisms and feedback to the parent vortex.

Because numerical simulations have shown VRWs to be well-coupled to convection, the continuous observations of radar reflectivity by ground-based and airborne platforms have allowed for the examination of inner core hurricane structure for the existence of VRWs. Fourier decomposition of the reflectivity data from Hurricane Elena (1985) into azimuthal wavenumbers elucidated the contribution of low wavenumbers (n < 3) to the inner core reflectivity structure. The reflectivity pattern was dominated by the symmetric, wavenumber 0 structure, but a strong wavenumber 1 signal in the eyewall was forced by environmental vertical wind shear and an elliptical eyewall produced significant power in wavenumber 2. Outside the eyewall, peaks in the power in wavenumber 2 were found to correspond to the repeated appearance of inner spiral rainbands, which propagated radially outward and cyclonically around the storm from the asymmetric eyewall convection north of the center.

A similar exploration of the inner core reflectivity structures of Atlantic basin Hurricanes Hugo (1989) and Andrew (1992) is currently underway. Key aspects of the research include cataloging the occurrence and propagation characteristics of inner spiral rainbands, diagnosing the contributions from different azimuthal wavenumbers to the reflectivity structure, and trying to document the formation mechanism(s) of the rainbands. As data becomes available from the 2005 season, it is anticipated that more storms will be added to the study and the results presented in due course.