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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 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. 

Abstract This study revisits the issue of why tropical cyclones (TCs) develop more rapidly at lower latitudes, using ensemble axisymmetric numerical simulations and energy diagnostics based on the isentropic analysis, with the focus on the relative importance of the outflow-layer and boundary layer inertial stabilities to TC intensification and energy cycle. Results show that although lowering the outflow-layer Coriolis parameter and thus inertial stability can slightly strengthen the outflow, it does not affect the simulated TC development, whereas lowering the boundary layer Coriolis parameter largely enhances the secondary circulation and TC intensification as in the experiment with a reduced Coriolis parameter throughout the model atmosphere. This suggests that TC outflow is more likely a passive result of the convergent inflow in the boundary layer and convective updraft in the eyewall. The boundary layer inertial stability is found to control the convergent inflow in the boundary layer and depth of convection in the eyewall and thus the temperature of the energy sink in the TC heat engine, which determines the efficiency and overall mechanical output of the heat engine and thus TC intensification. It is also shown that the hypothesized isothermal and adiabatic compression legs at the downstream end of the outflow in the classical Carnot cycle are not supported in the thermodynamic cycle of the simulated TCs, implying that the hypothesized classical TC Carnot cycle is not closed. It is the theoretical maximum work of the heat engine, not the energy expenditure following the outflow downstream, that determines the mechanical work used to intensify a TC.