Tropical Cyclone Evolution via Potential Vorticity Anomalies in a Three-Dimensional Balance Model

Abstract

A new mechanism of vortex intensification by convectively forced vortex Rossby waves was proposed by Montgomery and Kallenbach. As demonstrated by them, the axisymmetrization process is described by vortex Rossby waves that eventually propagate outward before their symmetrization. Montgomery and Kallenbach were able to relate these waves to intensity changes in barotropic hurricane-like vortices. In the present work these ideas are applied to better understand structure change and intensification of hurricanes in a baroclinic setting. The work of Möller and Montgomery, who examined the wave kinematics and wave?mean flow interaction of vortex Rossby waves in a barotropic model, is extended here to three dimensions. The model is based on the asymmetric balance theory of Shapiro and Montgomery. A nonlinear prognostic model is used to examine the effect of convectively generated potential vorticity (PV) disturbances on the evolution of a hurricane-like vortex on an f plane. This investigation generalizes that of Montgomery and Enagonio, who studied tropical cyclogenesis using a quasigeostrophic balance model, to a larger Rossby number. Convection is represented to the extent that the prescribed initial PV anomalies could be convectively forced. As in this formulation gravity waves are excluded, the dynamics of vortex Rossby waves and their interaction with the mean vortex and each other can be focused upon. Simple relaxation (?axisymmetrization?) experiments with monochromatic azimuthal-wavenumber disturbances show that vortex Rossby waves propagate both radially and vertically. The higher the wavenumber the weaker the vertical propagation of the PV asymmetries and corresponding response of the basic state. Experiments where double-cluster PV anomalies are superimposed complement the cyclogenesis results of Montgomery and Enagonio. The lower-level cyclonic PV anomaly intensifies the vortex while symmetrizing for a wide range of anomaly amplitudes. Depending on the strength of the cluster, however, the upper-level anticyclonic PV anomaly is expelled outward (stronger anomaly), as in Montgomery and Enagonio, or is symmetrized (weaker anomaly) similar to the lower-level positive PV anomaly. When the ongoing process of convection is simulated by adding double-cluster PV anomalies to the PV fields (so-called pulsing), the tropical storm intensifies to hurricane strength whose intensity depends on the location and extent of the anomaly. These results confirm that there exists an alternative means of tropical cyclone intensification to the symmetric mode.