Mechanisms of Secondary Eyewall Formation in Tropical Cyclones

Accurate predictions of hurricane intensity remain a challenging problem, because mechanisms leading to hurricane intensity variability are poorly understood. Secondary eyewall formation (SEF) is one of them, which is often associated with large and sudden variations of tropical cyclone intensities.

A variety of possible mechanisms for SEF have been proposed by previous studies, including the dynamical adjustment to latent heating due to outer rainband, and unbalanced boundary layer dynamics. To test these ideas, we built an axisymmetric dry model to calculate the secondary circulation induced by latent heating due to outer rainband before SEF. This provides background forcing for a limited domain cloud resolving model (CRM) that covers only the secondary eyewall region. The CRM is further coupled with the dry model to simulate the feedback between variations in the convective heating in the secondary eyewall region and the cyclone scale circulation. Explicitly dividing the system into a background forcing component and a component involving feedback between cyclone scale dry dynamics and convection provides us the flexibility to test the roles of different processes including enhanced surface evaporation, low level convergence, and unbalanced boundary layer dynamics. Results from this approach will be discussed.