Secondary eyewall formation in high-resolution hurricane simulations

Secondary eyewall formation (SEF) is one of the most important research topics in the dynamics of fully developed tropical cyclones. There is documented observational evidence that the phenomena is a common feature of major tropical cyclones (those that pose the most risk to our societies) and SEF is known to be followed by a series of events linked to sometimes rapid intensity changes, i.e. an eyewall replacement cycle. Despite its importance, the dynamics of the formation of secondary eyewalls remain elusive and thus any application to tropical cyclone intensity forecasting is precluded.

Idealized numerical simulations suggest that both internal dynamics and external environmental factors may play important roles in SEF. Axisymmetric, barotropic and full-physics, but highly idealized, numerical frameworks have been used to explore the role of external forcing, vortex Rossby waves and the axisymmetrization of vorticity features in secondary eyewall formation. However, a more unified theory of, and explanation for, SEF requires the systematic study of the process in full physics, high resolution, realistic simulations.

Mesoscale integrations of the NCAR Advanced Hurricane WRF model at 1.33 km resolution, both in real-time and after season re-runs with a variety of parameterizations, are shown to undergo SEF. Preliminary examination of the azimuthal averages of the time evolution of precipitable water, tangential and radial winds at 850 hPa, for two cases (Katrina and Rita (2005)), suggest that SEF might be due to fundamentally different processes in each storm with one showing a radially outward evolution and the other a radially inward one. We analyze the simulations to elucidate the relative role of internal dynamics in the formation of secondary eyewalls with particular attention on vortex Rossby wave activity.