Mechanism of Concentric Eyewall Replacement Cycles and Associated Intensity Change

Abstract

o understand the mechanisms responsible for the secondary eyewall replacement cycles and associated intensity changes in intense tropical cyclones (TCs), two numerical experiments are conducted in this study with the Weather Research and Forecasting (WRF) model. In the experiments, identical initial conditions and model parameters are utilized except that the concentration of ice particles is enhanced in the sensitivity run. With enhanced ice concentrations, it is found that the secondary eyewall forms at an increased radius, the time required for eyewall replacement is extended, and the intensity fluctuation is relatively large. The enhanced concentrations of ice particles at the upper tropospheric outflow layer produces a noticeable subsidence region (moat) surrounding the primary eyewall. The presence of the moat forces the secondary eyewall to form at a relatively large radius. The axisymmetric equivalent potential temperature budget analysis reveals that the demise of the inner eyewall is primarily due to the interception of the boundary layer inflow supply of entropy by the outer convective ring, whereas the advection of low entropy air from the middle levels to the boundary inflow layers in the moat is not essential. The interception process becomes inefficient when the secondary eyewall is at a large radius; hence, the corresponding eyewall replacement is slow. After the demise of the inner eyewall, the outer eyewall has to maintain a warm core not only in the previous eye, but also in the moat. The presence of low equivalent potential temperature air in the moat results in the significant weakening of storm intensity. The results found here suggest that monitoring the features of the moat and the outer eyewall region can provide a clue for the prediction of TC intensity change associated with eyewall replacement.