Secondary Eyewall Formation in Tropical Cyclones - Unbalanced Dynamics in the Boundary Layer

• Main Authors: Yi-Hsuan Huang, 黃怡瑄
• Other Authors: 吳俊傑
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/79472347014227906494

博士 === 國立臺灣大學 === 大氣科學研究所 === 100 === Typhoon Sinlaku (2008) is a case in point under T-PARC (THORPEX – Pacific Asian Regional Campaign) with the most abundant flight observations taken and with great potentials to address major scientific issues of tropical cyclones. In a recent study, a new method for vortex initialization based on EnKF data assimilation and the WRF model was adopted to simulate the life cycle of Sinlaku. A high-spatial/temporal-resolution and model/observation-consistent dataset was constructed by continuously assimilating (with an update cycle every 30 minutes) tracks, the mean surface structure of Sinlaku and all available measurement data for Sinlaku during a 4-day period. This dataset provides a unique opportunity to study the dynamical processes of concentric eyewall formation in Sinlaku. Using this previously constructed dataset, this study investigates the evolutionary of the concentric eyewall structure and aim to explore the key dynamical mechanisms and conditions for the secondary eyewall formation (SEF) in a tropical cyclone. Precursors to SEF found in our study suggest a possible application of a newly proposed spin-up paradigm to the SEF problem. We herein present a new model for SEF based on an axisymmetric view of the problem, including precursor characteristics and the associated evolution of the boundary layer flow and a dynamical interpretation. The findings point to a sequence of structure changes that occur in the outer-core region of a mature tropical cyclone, culminating in the formation of a secondary eyewall. The sequence begins with a broadening of the tangential wind field, followed by an increase of the corresponding boundary layer inflow underneath the broadened swirling wind, and an enhancement of a zone of organized convergence in the boundary layer where the secondary eyewall forms. The narrow region of strong boundary layer convergence is associated with the generation of supergradient winds in and just above the boundary layer that acts to rapidly decelerate inflow there. The progressive strengthening of the boundary layer inflow and the generation of an effective adverse radial force therein leads to an eruption of air from the boundary layer to support deep convection outside the primary eyewall in a favorable thermodynamic and kinematic environment. This presented paradigm for SEF is attractive on physical grounds because its simplicity and consistency with a set of 3-D numerical simulations. It is herein suggested that the unbalanced response in the boundary layer to an expanding swirling flow serves as an important mechanism for initiating and sustaining an approximate ring of deep convection in a narrow supergradient-wind zone in the vortex’s outer-core region. This progressive boundary layer control on SEF, involving in a positive feedback loop among the evolving primary and secondary circulation, implies that the boundary layer scheme and its coupling to the interior flow need to be adequately represented in numerical models to improve the understanding of SEF, as well as the accuracy of SEF forecasts, including the timing and preferred radial intervals.