The Role of Boundary Layer Dynamics in Secondary Eyewall Formation

The possible impacts of the boundary layer processes on secondary eyewall formation (SEF) have been presented and discussed during the past few years. Aside from the question whether the boundary layer dynamics plays a dominant or a minor role in SEF, this study attempts to understand, with a given planetary boundary layer (PBL) scheme, how flow in the boundary layer and lower troposphere responses to aloft atmosphere (mostly in gradient wind balance), and the associated uncertainties and variabilities.

The ensemble simulations of Typhoon Sinlaku (2008) presented in our two previously-published works in 2012 provide the prescribed vortex structure in the free atmosphere, and a nonlinear boundary layer model presented by Kepert and Wang in 2001 is adopted to investigate the problem. Responsible mechanism(s) for the obtained structure and the role of boundary layer dynamics in SEF are discussed. The variabilities of the tropical cyclone structure in related to boundary layer dynamics prior to SEF are assessed by using the vortices in the aforementioned ensemble simulations, and by applying different PBL parameterizations with different settings of parameters.