Three numerical experiments are designed to investigate internal instabilities of hurricane-like vortices and their effects on hurricane core structure. The goal is to examine the roles played by barotropic and baroclinic energy conversions in the early growth stages of eddies in the eyewall and to document the nature of these eddies. The PSU/NCAR Mesoscale Model version 5.3 (MM5) is used for this study. To investigate the internal mechanisms, all external forcings, such as environmental wind, beta effect, are removed, and both moist processes and boundary layer processes are eliminated from the simulations. Three axisymmetric, quasi-steady-state vortices are designed based on the result of a full-physics simulation of hurricane Floyd (1999). The control vortex is an axisymmetric version of Floyd. The two experimental vortices were created by altering the balanced temperature and wind fields to produce one vortex with maximum potential vorticity in the eyewall and one with increased radial gradients of temperature.

To examine the stability of these vortices, they are slightly perturbed with very weak initial vorticity anomalies in the eyewall, and the time evolutions of the perturbations are analyzed. The results indicate that there are baroclinic and barotropic instabilities in the two experimental vortices. As the perturbations evolve, they cause fluctuations of intensity through the energy conversions between eddy and mean kinetic energy. While the baroclinic energy conversion process intensifies the hurricane-like vortex, the barotropic energy conversion process weakens the vortex. The results suggest that both barotropic and baroclinic eddies can play roles in the observed structural oscillations in hurricane cores.