Dynamic instabilities and energy conversion processes in hurricane core regions.

摘要

A series of numerical simulations of axisymmetric hurricane-like vortices is performed to examine dynamic instabilities and energy conversion processes in a hurricane core area. The numerical experiments in this study consist of simulations of idealized dry vortices and moist vortices. The dry experiment is designed to show that the existence of baroclinic and barotropic instabilities is possible in realistic hurricane-like vortices. In order to generalize the dry simulation results, the simulations are extended to more realistic moist vortices. All numerical experiments are performed using the Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model version 5 (PSU/NCAR MM5) with a 6km horizontal grid.; Three steady-state vortices are designed for the dry experiments. A control vortex is built based on the results of a simulation of Hurricane Floyd (1999). Then, two axisymmetric vortices, EXP-1 and EXP-2, are constructed by modifying the wind and mass fields of the control vortex. The EXP-1 vortex is designed to satisfy the necessary condition of baroclinic instability, while the EXP-2 vortex satisfies the necessary condition of barotropic instability. These modified vortices are thought to lie within the natural range of the structural variability of hurricanes. In order to focus on internal effects on the stability of the dry vortices, all external forcings are eliminated. The dry vortices are constructed on an f-plane, and the experiments are performed without moist and boundary layer processes.; The stability of the dry vortices is examined by analyzing the behavior of small magnitude of an initial perturbations imposed on the vortices. When a vortex is found to be unstable, the type of instability is determined by the energy source of a growing perturbation. To identify the energy source of the perturbation, a linearized eddy energy equation is derived. The EXP-1 and EXP-2 vortices are found to be unstable with respect to small, imposed perturbations, while the control vortex is stable. Small perturbations added to the EXP-1 and EXP-2 vortices grow exponentially at the expense of available potential energy and kinetic energy of the primary vortex, respectively. By definition, therefore, the EXP-1 vortex is baroclinically unstable, while the EXP-2 vortex is barotropically unstable. (Abstract shortened by UMI.)