@conference {bnh-3877, title = {Secondary eyewall formation in tropical cyclones}, booktitle = {AFAC17}, year = {2017}, month = {09/2017}, publisher = {Bushfire and Natural Hazards CRC}, organization = {Bushfire and Natural Hazards CRC}, address = {Sydney}, abstract = {

Roughly half of all intense tropical cyclones experience an eyewall replacement cycle. In these events, a new eyewall forms concentrically around the original one. This secondary eyewall develops its own wind maximum, and both the secondary eye and the wind maximum typically intensify and contract, whilst the original eyewall and wind maximum weaken and eventually dissipate. While the evolution of a storm with concentric eyewalls is reasonably well understood, the mechanism or mechanisms by which the outer eyewall forms remain elusive. Understanding secondary eyewall formation is an important problem, for the subsequent eyewall replacement cycle can significantly affect the intensity of the storm, and the formation process and replacement cycle are usually associated with a major expansion of the outer wind field. Both these factors significantly affect the cyclone{\textquoteright}s impact.

We investigate a high resolution simulation of an eyewall replacement cycle. Boundary layer convergence due to friction substantially influences the evolution of the convection, and we present evidence for a positive feedback involving convection, vorticity and frictional convergence that governs the subsequent evolution of the system. In this feedback, frictional convergence strengthens the convection, stretching of vortex tubes in the buoyant updrafts increases the vorticity, and the vorticity structure of the storm determines the strength and location of the frictional updraft.\ 

Changes in the structure and intensity of tropical cyclones cause difficulties for their \ management, especially if these changes occur in the last day or two before landfall. Our improved knowledge of these processes will lead to better forecasts and mitigation.

}, author = {Jeffrey Kepert} }