Vortex Rossby Waves and Secondary Eyewall Formation in a High-resolution Simulation of Hurricane Katrina (2005)

The dynamics of secondary eyewall formation (SEF) is one of the most important research topics in the study of fully developed tropical cyclones (TCs). There is robust observational evidence that the phenomenon is a common feature of major hurricanes, which pose the most risk to our society. SEF is known to be followed by a series of events linked to sometimes rapid intensity changes, i.e., an eyewall replacement cycle, and has also been identified as one of the most significant processes involved in storm growth. Despite this importance, a unified theory of, and explanation for, SEF has remained elusive, and thus any application to TC intensity forecasting is precluded.

To investigate the physical processes responsible for SEF, a full physics, high-resolution (1.33 km) NCAR Advanced Hurricane Weather Research and Forecasting Model simulation of Hurricane Katrina (2005) is examined. The analysis reveals the cyclonic and outward propagation of convective rainbands, exhibiting the properties of wavenumber one vortex Rossby waves (VRWs), from the primary eyewall 4-18 hours before SEF. Initiated through a barotropic-convective instability, the waves stagnate at a radius of approximately three times the radius of maximum wind, their critical radius and also the radius of SEF.

A number of mechanisms have been hypothesized by which VRWs can lead to SEF, including wave-mean flow interaction and the redistribution of potential vorticity and moisture in the inner core (which modulate the structure of the secondary circulation and/or can establish an environment favorable to convection radially outside the primary eyewall). Each of these dynamical linkages between VRWs, increased convective activity outside the core and SEF will be explored in the presentation.