Unbalanced Dynamics of Secondary Eyewall Formation in Tropical Cyclones - Part I: Momentum Budget

A new dynamical pathway to secondary eyewall formation (SEF) in tropical cyclones has been advanced from the axisymmetric perspective in our two companion works published in 2012, based on a dataset with model simulations and ensemble Kalman filter (EnKF) data assimilation of the abundant data taken during Typhoon Sinlaku (2008) in T-PARC (THORPEX - Pacific Asian Regional Campaign) field program. It was demonstrated that the unbalanced dynamics within and just above the boundary layer on account of the storm's expanding swirling flow serves as an important mechanism for initiating and sustaining a ring-like deep convection in a narrow supergradient-wind zone outside the primary eyewall.

This follow-up study, comprising of two-part works, provides further dynamical analysis to examine the aforementioned pathway to SEF. Based on momentum budget analyses, this Part I presentation investigates 1) how the tangential winds broaden prior to SEF; 2) how the secondary tangential wind maximum forms; and 3) how unbalanced processes impact the radial distribution of the boundary layer inflow. This study presents new evidence supporting the dynamically unbalanced pathway to SEF.

Analyses from higher-resolution simulations of Sinlaku will be presented in the part II paper.