Vortex Rossby Waves and vorticity mixing in a MM5 simulation of Hurricane Bob (1991)

It has been suggested that convectively forced Vortex Rossby Waves play a pivotal role throughout the growth, development and mature phases of Tropical Cyclones (TCs) (Montgomery and Kallenbach 1997). The existence and importance of these features, however, has not yet been demonstrated in a full physics model. Only recently have 3-D models been capable of accurately representing explicit convection and Vortex Rossby Wave features near the TC eye wall.

Using data from a PSU/NCAR MM5 simulation of Hurricane Bob(1991) on a 4km grid (Braun and Tao 1999), we examine the evolution of vorticity and establish the existence of Vortex Rossby Waves in a full physics context. The vorticity dynamics is examined from a variety of perspectives, including: radius-time Hovmoller diagrams, eddy momentum and eddy vorticity flux analysis. In addition, using the high resolution data obtained with a 1.3 km inner nested grid, we present evidence for vorticity mixing in and around the eywall, similar to the conceptual model proposed by Schubert, et al. (1999).

This work represents a first step towards a more complete understanding of the mesoscale wind and vorticity structure of TCs.