Concentric eyewall replacement cycle and intensity change

The primary purpose of this study is to understand secondary eyewall formation and associated intensity change in tropical cyclones (TC). Idealized numerical simulations were conducted by using the high-resolution Weather Research and Forecasting (WRF) model. The model successfully simulated the formation of an annular hurricane (AH) through a concentric eyewall replacement cycle. The result suggests that concentric eyewall replacement is an efficient route to the formation of an AH. The bottom-up mixing of elevated potential vorticity (PV) in the concentric eyewalls associated with barotropic instability is the primary mechanism for the transition to the AH.;The formation of the secondary eyewall is investigated from a PV perspective. Large positive PV anomalies are initially maximized at the middle troposphere in the outer region of the TC, and then develop at the lower levels prior to the establishment of the outer convective ring. The changing of PV distribution is similar to that in tropical cyclogenesis scenario. The development of low-level convective vortices in a stratiform precipitation region changes PV vertical distribution. After the low-level circulation is enhanced, an outer convective ring develops through a positive feedback between the surface winds and upward latent and sensible heat fluxes from the underlying ocean.;TC intensity changes associated with eyewall replacement cycles vary from case to case. To understand this issue, two parallel experiments were conducted with identical initial conditions and model parameters, except a slight modification in microphysics scheme. The modification of the microphysics scheme results in systematic enhancement of freezing hydrometer concentration. The storm in the experiments experiences a secondary eyewall replacement cycle, but the replacement lasts longer and the intensity change is more significant in the experiment with enhanced ice concentration. The high concentration of ice particles tends to enhance downdrafts in the outer region, resulting to a weak outer eyewall and a clear moat. The weak outer eyewall has a low ability to maintain a warm core in the original eye when the inner eyewall dissipates. The low entropy air in the moat adds additional load. High ice concentration leads to a notable weakening of storm intensity.