Vortex Rossby Waves in a Numerically Simulated Tropical Cyclone. Part II: The Role in Tropical Cyclone Structure and Intensity Changes

Abstract

In Part I, the author analyzed the asymmetric structure in the inner core of a numerically simulated tropical cyclone and found that the asymmetry near the eyewall in the mid?lower troposphere is dominated by wavenumber-1 and -2 vortex Rossby waves. These waves are found to be well coupled with asymmetries in eyewall convection and thus may play an important role in the life cycle of a tropical cyclone. In this paper, analyses are extended to include the role of these vortex Rossby waves in tropical cyclone structure and intensity changes. The waves are found to transport angular momentum from the eyewall to the eye, accelerating tangential winds in the eye at the expense of decelerating the tangential wind in the eyewall, and thus they play an important role in the inner core dynamics of the tropical cyclone. Convection in the eyewall is enhanced between the downstream trough and upstream ridge in the vortex Rossby waves but suppressed between the downstream ridge and upstream trough. This close relationship stems from inflow (outflow) associated with the waves in the former (latter) region. Propagation of these waves around the eyewall can produce changes in eyewall shape and polygonal eyewalls with cyclonic rotation. The waves also propagate radially outward and stagnate at radii of 70?90 km, where the radial potential vorticity gradient disappears or reverses its sign. It is at these radii where strong outer spiral rainbands most frequently occur. These outer rainbands spiral cyclonically inward and occasionally perturb the eyewall. In many cases, outward-propagating inner spiral rainbands can be initiated and emanated from the eyewall, especially when the eyewall is perturbed by an outer spiral rainband. When such a perturbation is strong and in phase with strong vortex Rossby waves in the eyewall, the eyewall may experience a breakdown and then be recovered through the axisymmetrization process. The eyewall breakdown (recovery) is accompanied by a weakening (intensifying) cycle of the tropical cyclone.