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Abstract Hurricane Patricia (2015), the most powerful tropical cyclone (TC) on record, formed its secondary eyewall when its center was about 113 km offshore before its landfall at the southwestern coast of Mexico at around 2300 UTC 23 October. The ARW-WRF Model reproduced well the main features, allowing for a detailed investigation of the secondary eyewall formation (SEF). Our results show that the secondary eyewall developed from a stationary banding complex (SBC), originating from the intersection of two outer rainbands (OR1 and OR2) on the western side of the TC. This process was largely regulated and enhanced by the coastal terrain through the orographic channel effect. The results from sensitivity experiments show that increasing terrain height amplified the channel effect, accelerating airflow between the TC vortex and the terrain, strengthening convergence into OR1, and promoting midlevel descending inflow conducive to convective enhancement downstream in the SBC. While the terrain weakened low-level moisture transport, it also positioned OR2 closer to OR1, facilitating the formation of the SBC and accelerating the moat development. Backward trajectory analysis revealed that the inflows below the upper-level outflow layers of both the primary and secondary eyewalls contributed to moat development. With increasing terrain height, dry air transported into the moat region by the upper-level inflows from the secondary eyewall significantly increased, further suppressing convection in the moat. These findings offer novel insights into the understanding of SEF processes and underscore the importance of the topographic effects in shaping outer rainband organization, contributing to the moat and SEF. 

Abstract Hurricane Patricia (2015) formed over the eastern North Pacific and is the most intense tropical cyclone (TC) on record with a maximum sustained wind speed of 95 m s⁻¹, which presented a great forecasting challenge due to its unprecedented rapid intensification, record-breaking lifetime maximum intensity, and subsequent rapid weakening. The intensity and structure changes in Patricia were successfully simulated in a control experiment using a two-way interactive, quadruply nested version of the Weather Research and Forecasting Model with both initial and lateral boundary conditions from the Global Forecast System Final Analysis data. The successful simulation resulted from the inclusion of dissipative heating, realistic horizontal mixing length, and sea-spray-mediated heat flux. The relative contributions of these processes were assessed based on a series of ensemble-based sensitivity experiments and energetic diagnostics. Results show that dissipative heating and reduced horizontal mixing length had the most significant impacts on the intensification rate of Patricia after it reached an intensity of category 3, contributing 25.8% and 28.9% to the intensification rate and 11.7% and 14.1% to the lifetime maximum intensity, respectively. The contribution by spray-mediated heat flux increased significantly with wind speed, contributing up to 20.1% to the intensification rate and 20% to the surface energy flux under the eyewall at the wind speed of 90 m s⁻¹. An alternative surface drag coefficient scheme and a constant surface roughness for moisture and heat were also tested and discussed via sensitivity experiments. The study provides insights into the physical processes key to successful simulations and forecasts of extremely strong TCs. 

Abstract Tropical cyclone (TC) lifetime maximum intensity exhibits a distinct bimodal distribution, with peaks at tropical storm and major hurricane strength. Using a best‐track‐based algorithm to identify eyewall replacement cycle (ERC) storms, we show that ERC storms overwhelmingly populate the high‐intensity peak. Both reintensifying and non‐reintensifying ERC storms contribute, but those unable to reintensify cluster near 120–140 kt, defining the secondary peak. In contrast, reintensifying ERC storms can achieve higher intensities when moving over warmer seas with greater ocean heat content and reduced vertical wind shear. The scarcity of storms at intermediate intensities (85–105 kt) arises from rapid intensification (RI), which drives systems quickly through this range. These results clarify that while RI explains the trough at mid‐intensities, ERCs, by halting or enabling further strengthening, shape the high‐intensity peak and its upper tail. Incorporating ERC dynamics into intensity statistics may improve understanding and prediction of TC extremes. 

Abstract Secondary eyewall formation (SEF) in tropical cyclones (TCs) emerges from a complex interplay of internal dynamics and environmental influences. Motivated by observations linking low inertial stability in the TC outflow layer to eyewall replacement cycles, we investigate how variations in outflow‐layer inertial stability control both the initiation and radial position of SEF. Idealized simulations reveal that reduced outflow‐layer inertial stability enhances upper‐level divergence and updraft in the TC outer core, fostering the growth of stratiform rainbands. By averaging secondary circulation over the domain grids featuring stratiform precipitation, it is explicitly shown that the strength of the mesoscale descending inflow (MDI) is greater within the widespread and more developed stratiform clouds. Such stratiform‐induced MDI can dynamically and thermodynamically broaden the tangential wind field in the lower altitudes. As a result, the ensuing increase in boundary‐layer inertial stability and inflow supplies greater absolute vorticity influx in the outer‐core region, making the tangential wind tendency peaks and the secondary eyewall intensifies at a larger radius. This study highlights the role of MDI in the coupling between the upper‐ and lower‐tropospheric dynamics. 

Abstract The timescale of eyewall replacement cycle (ERC) is critical for the prediction of intensity and structure changes of tropical cyclones (TCs) with concentric eyewall (CE) structures. Previous studies have indicated that the moat width can regulate the interaction between the inner and outer eyewalls and has a salient relationship with the ERC timescale. In this study, a series of sensitivity experiments are carried out to investigate the essential mechanisms resulting in the diversity of the duration of CEs using both simple and full‐physics models. Results reveal that a larger moat can induce stronger inflow under the same inner eyewall intensity by providing a longer distance for air parcels to accelerate in the boundary layer. Thus, there is greater inward absolute vorticity flux to sustain the inner eyewall. Besides, the equivalent potential temperature (θ_e) budget indicates that the vertical advection and surface flux of moist entropy can overbalance the negative contribution from the horizontal advection and lead to an increasing trend ofθ_ein the inner eyewall. This suggests that the thermodynamic process in the boundary layer is not indispensable to the inner eyewall weakening. It is also found that the contraction rate of the secondary eyewall, which directly influences the moat width, is subject to the activity of outer spiral rainbands. By directly introducing positive wind tendency outside the eyewall and indirectly promoting a vertically tilted eyewall structure, active convection in the outer region will impede or even suspend the contraction of the outer eyewall and hence extend the ERC timescale.