The role of outer rainband convection in governing the eyewall replacement cycle in numerical simulations of tropical cyclones

Eyewall replacement cycles (ERCs) are frequently observed during the evolution of intensifying tropical cyclones (TCs). Although intensely studied in recent years, the underlying mechanisms of ERCs are still not well understood, and a timely accurate forecast of ERCs remains to be a challenge. To advance our understanding of ERCs and provide insights into the improvement of numerical forecast of ERCs, a series of three-dimensional full physics simulations is performed using the Weather Research and Forecasting (WRF) model to investigate ERCs in TC-like vortices on an f plane. The simulated ERC in the control experiment possesses key features similar to those observed in real TCs including the formation and development of a secondary tangential wind maximum associated with the outer eyewall. The Sawyer-Eliassen diagnoses and tangential momentum budget analyses are performed to investigate the mechanisms underlying the secondary eyewall formation (SEF) and ERC. The simulations show the crucial roles of outer rainband heating in governing the formation and development of the secondary tangential wind maximum and demonstrate that the outer rainband convection must reach a critical strength relative to the eyewall convection before SEF and ERC can occur. The diagnoses reveal a positive feedback among low-level convection, acceleration of tangential winds and convergence of radial flow in the upper boundary layer, and surface evaporation that leads to the development of outer rainband convection and formation of secondary eyewall, and a mechanism for the demise of inner eyewall that involves the interaction between the transverse circulations induced by eyewall and outer rainband convection.