Mechanisms governing the Eyewall Replacement Cycle in Numerical Simulations of Tropical Cyclones

A series of numerical simulations by the Weather Research and Forecasting Model (WRF) are used to investigate eyewall replacement cycles (ERCs) in tropical cyclone (TC)-like vortices on a f-plane. The simulated ERCs possess the key features similar to those observed in real TCs including the formation and development of a secondary tangential wind maximum associated with the outer concentric eyewall. The Sawyer-Eliassen equation and tangential momentum budget analyses are performed to diagnose the mechanisms underlying the secondary eyewall formation (SEF) and ERC. Our diagnoses show that the azimuthal-mean transverse circulation obtained directly from WRF simulations can be well represented by the diagnosed circulation from the Sawyer-Eliassen equation and many aspects of SEF and ERC can be well explained in the symmetric balanced framework. The study reveals the crucial role of outer rainband heating in governing the formation and development of the secondary tangential wind maximum and demonstrates that the strength of outer rainband heating must reach a certain level relative to the eyewall heating before SEF and the subsequent ERC can occur. Based on the analyses, a positive feedback among shallow convection, acceleration of tangential winds, and convergence of radial inflow in the boundary layer that leads to SEF and a mechanism for the demise of inner eyewall that involves the interaction between the transverse circulations induced by eyewall and outer rainband convection are proposed. The decomposition of tangential momentum budget indicates that the net tendency of tangential wind is a small residual resultant from a large cancellation between the tendencies induced by the resolved and sub-grid scale (SGS) processes. The large SGS contribution to the tangential wind budget may explain why the SEF and ERC in previous numerical studies show different characteristics and poses a great challenge for a timely correct forecast of this phenomenon.