Effect of the Initial Vortex Structure on Intensity Change During Eyewall Replacement Cycle of Tropical Cyclones:A Numerical Study

This study investigates the effect of the initial tropical cyclone(TC)vortex structure on the intensity change during the eyewall replacement cycle(ERC)of TCs based on two idealized simulations using the Weather Research and Forecasting(WRF)model.Results show that an initially smaller TC with weaker outer winds experienced a much more drastic intensity change during the ERC than an initially larger TC with stronger outer winds.It is found that an initially larger TC vortex with stronger outer winds favored the development of more active spiral rainbands outside the outer eyewall,which slowed down the contraction and intensification of the outer eyewall and thus prolonged the duration of the concentric eyewall and slow intensity evolution.In contrast,the initially smaller TC with weaker outer winds corresponded to higher inertial stability in the inner core and weaker inertial stability but stronger filamentation outside the outer eyewall.These led to stronger boundary layer inflow,stronger updraft and convection in the outer eyewall,and suppressed convective activity outside the outer eyewall.These resulted in the rapid weakening during the formation of the outer eyewall,followed by a rapid re-intensification of the TC during the ERC.Our study demonstrates that accurate initialization of the TC structure in numerical models is crucial for predicting changes in TC intensity during the ERC.Additionally,monitoring the activity of spiral rainbands outside the outer eyewall can help to improve short-term intensity forecasts for TCs experiencing ERCs.

Roles of Upper-Level Descending Inflow in Moat Development in Simulated Tropical Cyclones with Secondary Eyewall Formation

This study investigated the effects of upper-level descending inflow(ULDI)associated with inner-eyewall convection on the formation of the moat in tropical cyclones(TCs)with secondary eyewall formation(SEF).In our numerical experiments,a clear moat with SEF occurred in TCs with a significant ULDI,while no SEF occurred in TCs without a significant ULDI.The eyewall convection developed more vigorously in the control run.A ULDI occurred outside the inner-eyewall convection,where it was symmetrically unstable.The ULDI was initially triggered by the diabatic warming released by the inner eyewall and later enhanced by the cooling below the anvil cloud.The ULDI penetrated the outer edge of the inner eyewall with relatively dry air and prevented excessive solid-phase hydrometeors from being advected further outward.It produced extensive sublimation cooling of falling hydrometeors between the eyewall and the outer convection.The sublimation cooling resulted in negative buoyancy and further induced strong subsidence between the eyewall and the outer convection.As a result,a clear moat was generated.Development of the moat in the ongoing SEF prevented the outer rainband from moving farther inward,helping the outer rainband to symmetrize into an outer eyewall.In the sensitivity experiment,no significant ULDI formed since the eyewall convection was weaker,and the eyewall anvil developed relatively lower,meaning the formation of a moat and thus an outer eyewall was less likely.This study suggests that a better-represented simulation of inner-eyewall convective structures and distribution of the solid-phase hydrometeors is important to the prediction of SEF.

Identification and Analysis of High-Frequency Oscillations in the Eyewalls of Tropical Cyclones

High-frequency oscillations, with periods of about 2 hours, are first identified by applying wavelet analysis to observed minutely wind speeds around the eye and eyewall of tropical cyclones（TCs）. Analysis of a model simulation of Typhoon Hagupit（2008） shows that the oscillations also occur in the TC intensity, vertical motion, convergence activity and air density around the eyewall. Sequences of oscillations in these variables follow a certain order.

SATELLITE-BASED ANALYSIS ON THE CONCENTRIC EYEWALL REPLACEMENT CYCLES OF SUPER TYPHOON MUIFA (1109)

Multisatellite data is used to analyze the characteristics of three eyewall replacement cycles(ERCs) during the lifetime of Typhoon Muifa(1109).Spiral rainbands evolutions,concentric eyewall(CE) structure modes,CE durations,and intensity changes are discussed in detail.In addition,an ERC evolution model of Typhoon Muifa is given.There are four main findings.(1) The outer spiral rainband joins end to end to form the outer eyewall after it disconnects from the original(inner) eyewall.The inner eyewall weakens as the outer eyewall becomes axisymmetric and is intensified.The contraction of the outer eyewall causes the inner eyewall to dissipate rapidly.Finally,the ERC ends with an annular eyewall or spiral rainbands.(2) Although the CE duration times of Typhoon Muifa's three ERCs covered a large range,the CE structures were all maintained for approximately 5 h from the formation of the axisymmetric outer eyewall to the end of the cycle.(3) There is no obvious precipitation reflectivity in the eye or moat region for the subsidence flow.The convection within the two eyewalls is organized as a radially outward slope with increasing height.(4) Typhoon intensity estimation results based on ADT may not explain the intensity variations associated with ERC correctly,while the typhoon's warm core data retrieved from AMSU-A works well.

Simulated Sensitivity of the Tropical Cyclone Eyewall Replacement Cycle to the Ambient Temperature Profile

In this study, the impacts of the environmental temperature profile on the tropical cyclone eyewall replacement cycle are examined using idealized numerical simulations. It is found that the environmental thermal condition can greatly affect the formation and structure of a secondary eyewall and the intensity change during the eyewall replacement cycle. Simulation with a warmer thermal profile produces a larger moat and a prolonged eyewall replacement cycle. It is revealed that the enhanced static stability greatly suppresses convection, and thus causes slow secondary eyewall formation. The possible processes influencing the decay of inner eyewall convection are investigated. It is revealed that the demise of the inner eyewall is related to a choking effect associated with outer eyewall convection, the radial distribution of moist entropy fluxes within the moat region, the enhanced static stability in the inner-core region, and the interaction between the inner and outer eyewalls due to the barotropic instability. This study motivates further research into how environmental conditions influence tropical cyclone dynamics and thermodynamics.

Unusual Evolution of the Multiple Eyewall Cycles in Super Typhoon Hinnamnor(2022)

Hinnamnor was the first super typhoon in the western North Pacific basin in 2022.It had several prominent characteristics,such as rapid intensification after its formation,three eyewall cycles,and a sudden recurvature of its track.Based on multi-source observational and reanalysis datasets,two secondary eyewall formation(SEF)cycles occurred during Super Typhoon Hinnamnor’s lifetime.The first SEF happened near the time when Hinnamnor achieved its maximum intensity,and it seems that its internal dynamics dominated the SEF process after the development of shear-induced asymmetric spiral rainbands.The merger of a tropical depression with Hinnamnor led to a continuous increase in both its inner-core size and outer-core circulation,causing generation of the second SEF.It is inferred that the external and internal dynamics worked together during the second SEF process.The concentric eyewall structure maintained for approximately 84 h under the moderate vertical wind shear.Also,unique changes in intensity accompanied the two structural changes.

Influences of Different PBL Schemes on Secondary Eyewall Formation and Eyewall Replacement Cycle in Simulated Typhoon Sinlaku (2008)

The effects of different planetary boundary layer （PBL） processes on the secondary eyewall formation （SEF） and eyewall replacement cycle （ERC） in Typhoon Sinlaku （2008） are investigated by using the Weather Research and Forecasting （WRF） model with six different PBL schemes. The SEF and ERC have been successfully simulated with all the six PBL schemes and the mechanism for the SEF and ERC proposed in our previous study has been reconfirmed. It is demonstrated that both the intensification of the storm and the inward-moving outer spiral rainband contribute to the SEF. After the SEF, the associated diabatic heating enhances the secondary eyewall further, and the transfer of moist air from outer region to the primary eyewall is cut off by the secondary eyewall. In such a way, the primary eyewall dies and an ERC completes. It is found that some simulated features of the SEF and ERC, such as the time and location of the SEF and duration of the ERC, do vary from one simulation to another. In order to describe the features of the SEF and ERC quantitatively, a concentric eyewall index （CEI） is defined and a threshold of the CEI is suggested to determine the onset of the secondary eyewall. The differences in the simulated SEF and ERC are discussed and some possible causes are suggested. In addition, based on the CEI threshold and the conservation law of angular momentum, a formula to predict the location of SEF is also proposed and applied to all the six simulations. The success and failure of the formula are then discussed.

Direct/indirect effects of aerosols and their separate contributions to Typhoon Lupit(2009):Eyewall versus peripheral rainbands

As a typhoon approaches the continent,the position where anthropogenic aerosols penetrate,the convection competition between the eyewall and peripheral rainbands,and the separate contributions of direct aerosol-radiation interactions(ARI)and indirect aerosol-cloud interactions(ACI),yield uncertainties in the convection intensification area and hence the typhoon intensity.Typhoon Lupit(2009)was simulated using the Weather Research and Forecasting Model with Chemistry(WRF-Chem)to investigate and isolate the direct and indirect effects of aerosols on the intensity,convection,and precipitation of the typhoon.Three simulations(CTL,CLEAN,and CTLARIOFF)were designed,representing a polluted case(CTL,considering the ingestion of anthropogenic aerosols with ARI and ACI),a clean maritime case(CLEAN,mainly with sea salt aerosols),and a polluted case without aerosol radiative forcing(CTLARIOFF,as per CTL but without ARI).The results showed that anthropogenic aerosols could penetrate into both the peripheral rainbands and the eyewall when the typhoon was approaching the Asian continent.Owing to the representation of the real aerosol scenario,the simulated typhoon intensity weakened and was closer to observed values in the CTL experiment.The ARI dominated over ACI with the opposite effects.Specifically,the ACI mainly enhanced the formation of ice-phase hydrometeors within the upper level of the eyewall with more freezing latent heat releases,leading to an invigoration of eyewall convection.These excess ice-phase particles melted after they descended into the warm layer below the 0°C level,which accelerated the accretion of cloud droplets by raindrops(Pcacr)and hence the mixed phase precipitation process in the eyewall.The dynamic feedback induced by the ACI enhanced the boundary layer inflow and the upper layer outflow,supporting the maintenance of strong eyewall convection and intensification of the typhoon.Inversely,the ARI heated the distant periphery low-level atmosphere at an altitude of 1-2 km by the absorbing polluted aerosols.The heated air,driven by the radial inflow,firstly went through the periphery rainbands of the typhoon and invigorated convection there due to the low-level warming.Then,the enhanced periphery convection inhibited the further transport of warm moist air into the eyewall,resulting in weakening of the eyewall convection and hence typhoon intensity.In sum,for the polluted scenario,as the typhoon approached the continent,ARI played a dominant role over ACI.The WRF-Chem model with full consideration of aerosol-cloud-radiation interactions is advantageous in terms of reliably simulating typhoon intensity and precipitation distribution.

A Numerical Investigation of the Eyewall Evolution of a Tropical Cyclone

In this study, a high-resolution numerical simulation is conducted to investigate the eyewall evolution of Typhoon Imbudo （2003）. The eyewall contraction, breakdown, and reformation are successfully simulated by the model. The eyewall accordantly shrinks throughout the whole troposphere prior to landfall, while it exhibits different variations after landfall in the lower and upper troposphere, respectively. It is found that the dry air advected into the storm inner core through a low-θe channel, the reduced surface latent heat transfer, and the increased inflows in the coastal region are associated with the eyewM1 contraction. Accompanied with the high-to-low wavenumber change in the vortex Rossby waves, the initial polygonal eyewall transforms to an elliptical one. Such a wavenumber change is likely associated with the change of interaction between the rainbands and the eyewall. The corresponding features of the time-averaged and vertical dynamic and thermodynamic structures are also examined during the storm passage. A tangential wind budget analysis indicates that a strong acceleration due to the contributions of both the eddy and the mean circulation is located in the lower layer in the eyewall during pre-landfall, and the mean circulation contribution to the change in the tendency of the azimuthally averaged tangential wind counteracts the eddy contribution.

How Does Tropical Cyclone Size Affect the Onset Timing of Secondary Eyewall Formation？

By using idealized numerical simulations, the impact of tropical cyclone size on secondary eyewall formation（SEF） is examined. Both unbalanced boundary layer and balanced processes are examined to reveal the underlying mechanism. The results show that a tropical cyclone（TC） with a larger initial size favors a quicker SEF and a larger outer eyewall. For a TC with a larger initial size, it will lead to a stronger surface entropy flux, and thus more active outer convection. Meanwhile, a greater inertial stability helps the conversion from diabatic heating to kinetic energy.Furthermore, the progressively broadening of the tangential wind field will induce significant boundary layer imbalances. This unbalanced boundary layer process results in a supergradient wind zone that acts as an important mechanism for triggering and maintaining deep convection. In short, different behaviors of balanced and unbalanced processes associated with the initial wind profile lead to different development rates of the secondary eyewall.

THE FORMATION MECHANISM OF CONCENTRIC DOUBLE EYEWALL TYPHOON-PART Ⅰ:DYNAMICAL ANALYSIS

The strong vortex will mutually adjust the thermodynamic field and dynamic field to a state of gradient balance whilst forced by an external cold source,namely,the gradient adjustment process,in which a linearized two-layer model is dealt with in this paper.The analyses show that on account of the heterogeneous radial distribution of the cold source,the adjustment of the thermodynamic and dynamic fields results in the two-peak tangential wind feature,which is analogous to the character of concentric double eyewalls in the strong typhoon.Consequently,the gradient adjustment may be one possible mechanism for the formation of a concentric double-eye typhoon.

OUTER VORTEX WIND STRUCTURE CHANGES DURING AND FOLLOWING TROPICAL CYCLONE SECONDARY EYEWALL FORMATION

Some recent studies have utilized flight-level(700 mb) winds to document the maximum wind speeds(Vmax)and radius of Vmax(Rmax) of the original and secondary eyewalls during 24 Atlantic hurricane eyewall replacement cycles(ERC).In this study,Hurricane Wind(H*Wind) analyses of Atlantic hurricanes during 2003-2005 are utilized to document changes in the outer vortex surface wind profile beyond the secondary eyewall,with a focus on the radii of gale-force winds(R34) that are often defined operationally as size changes.In Mode 1,complete and partial ERCs in which the pre-,during-,and post-ERC outer wind profiles have approximately the same shape,the outward displacements of Rmax leads to size(R34) increases as much as 100 km.Mode 2 ERCs are characterized by sharpened wind profiles outside the secondary eyewall that offset the larger Rmax radii to produce only small R34 increases.While statistically significant results are not obtained,the differences in size changes for Mode 1 and Mode 2 SEF cases suggest practical significance for forecasts and warnings.

THE FORMATION MECHANISM OF CONCENTRIC DOUBLE EYEWALL TYPHOON——PART Ⅱ:NUMERICAL SIMULATIONS

In the context of a non-hydrostatic axisymmetric compressible numerical model,the concentric double eyewall structure of the typhoon was replicated well to discuss the formation mechanism of concentric eyewall with the aid of gradient wind adjustment.Evidence suggests that in the weakening phase of the typhoon,the heterogeneous radial distribution of the sensible and convective latent heating over the sea surface will induce the gradient wind unbalance accompanied by mutual gradient wind adjustment between the thermodynamic field and dynamic fields,which results in a two-peak tangential wind and concentric double eyewall structure.

An Application of the Adjoint Method to a Statistical-Dynamical Tropical-Cyclone Prediction Model （SD-90）Ⅱ：Real Tropical Cyclone Cases

In the first paper in this series, a variational data assimilation of ideal tropical cyclone （TC） tracks was performed for the statistical-dynamical prediction model SD-90 by the adjoint method, and a prediction of TC tracks was made with good accuracy for tracks containing no sharp turns. In the present paper, the cases of real TC tracks are studied. Due to the complexity of TC motion, attention is paid to the diagnostic research of TC motion. First, five TC tracks are studied. Using the data of each entire TC track, by the adjoint method, five TC tracks are fitted well, and the forces acting on the TCs are retrieved. For a given TC, the distribution of the resultant of the retrieved force and Coriolis force well matches the corresponding TC track, i.e., when a TC turns, the resultant of the retrieved force and Coriolis force acts as a centripetal force, which means that the TC indeed moves like a particle; in particular, for TC 9911, the clockwise looping motion is also fitted well. And the distribution of the resultant appears to be periodic in some cases. Then, the present method is carried out for a portion of the track data for TC 9804, which indicates that when the amount of data for a TC track is sufficient, the algorithm is stable. And finally, the same algorithm is implemented for TCs with a double-eyewall structure, namely Bilis （2000） and Winnie （1997）, and the results prove the applicability of the algorithm to TCs with complicated mesoscale structures if the TC track data are obtained every three hours.

EXPLICIT SIMULATION ON THE TRACK AND INTENSITY OF TROPICAL CYCLONE WINNIE (1997)

An explicit simulation of the tropical cyclone Winnie （1997） was conducted by use of a triply-nested 3-D nonhydrostatic mesoscale model MM5, before and after its landfall. The simulated results show that the model can well reproduce the track of Winnie （1997） with a BOGUS storm-like vortex inserted in the large-scale first-guess field at the initial model time. The mean track error in a 6 h interval is 54.5 km, which is much less than that obtained with the operational prediction. Comparisons between simulation and observation near the landfall place show that the main temporal variation features of sea level pressure and surface wind speed can also be successfully captured. The characteristics of the eyewall, the spiral cloud band and the corresponding precipitation were disclosed, and the applications of the model outputs were also discussed.

Lightning Activity and Its Relationship with Typhoon Intensity and Vertical Wind Shear for Super Typhoon Haiyan (1330)

Super Typhoon Halyan （1330）, which occurred in 2013, is the most powerful typhoon during landfall in the meteorological record. In this study, the temporal and spatial distributions of lightning activity of Haiyan were analyzed by using the lightning data from the World Wide Lightning Location Network, typhoon intensity and position data from the China Meteorological Administration, and horizontal wind data from the ECMWF. Three distinct regions were identified in the spatial distribution of daily average lightning density, with the maxima in the inner core and the minima in the inner rainband. The lightning density in the intensifying stage of Haiyan was greater than that in its weakening stage. During the time when the typhoon intensity measured with maximum sustained wind speed was between 32.7 and 41.4 m s-1, the storm had the largest lightning density in the inner core, compared with other intensity stages. In contrast to earlier typhoon studies, the eyewall lightning burst out three times. The first two eyewall lightning outbreaks occurred during the period of rapid intensification and before the maximum intensity of the storm, suggesting that the eyewall lightning activity could be used to identify the change in tropical cyclone intensity. The flashes frequently occurred in the inner core, and in the outer rainbands with the black body temperature below 220 K. Combined with the ECMWF wind data, the influences of vertical wind shear （VWS） on the azimuthal distribution of flashes were also analyzed, showing that strong VWS produced downshear left asymmetry of lightning activity in the inner core and downshear right asymmetry in the ralnbands.

Hurricane eye morphology extraction from SAR images by texture analysis

Tropical hurricanes are among the most devastating hazards on Earth.Knowledge about its intense inner-core structure and dynamics will improve hurricane forecasts and advisories.The precise morphological parameters extracted from high-resolution spaceborne Synthetic Aperture Radar(SAR)images,can play an essential role in further exploring and monitoring hurricane dynamics,especially when hurricanes undergo amplification,shearing,eyewall replacements and so forth.Moreover,these parameters can help to build guidelines for wind calibration of the more abundant,but lower resolution scatterometer wind data,thus better linking scatterometer wind fields to hurricane categories.In this paper,we develop a new method for automatically extracting the hurricane eyes from C-band SAR data by constructing Gray Level-Gradient Co-occurrence Matrices(GLGCMs).The hurricane eyewall is determined with a two-dimensional vector,generated by maximizing the class entropy of the hurricane eye region in GLGCM.The results indicate that when the hurricane is weak,or the eyewall is not closed,the hurricane eye extracted with this automatic method still agrees with what is observed visually,and it preserves the texture characteristics of the original image.As compared to Du’s wavelet analysis method and other morphological analysis methods,the approach developed here has reduced artefacts due to factors like hurricane size and has lower programming complexity.In summary,the proposed method provides a new and elegant choice for hurricane eye morphology extraction.

A SHORT NOTE ON THE INTENSIFICATION AND EXTREME RAINFALL ASSOCIATED WITH HURRICANE LANE

Hurricanes Lane intensified in the Central Pacific with data from reconnaissance aircraft indicating it reached category 5 intensity on the Saffir-Simpson Hurricane Wind Scale while moving closer to Hawaii. The cyclone weakened as it moved closer to the Hawaiian Islands with its closest approach at 1500UTC 25 August 2018 when 175 km south southwest of Honolulu with a central pressure of 995 hPa. The impact on Hawaii was mainly record rainfall and the structure of this weather system is examined here to show how the winds turning anticyclonically with height in the lower to middle level troposphere play a crucial role in the intensification of Hurricane Lane to category 5 intensity and the generation of extreme rainfall.

Typhoon Winnie (1997) with a Very Large Eye:High Resolution Numerical Simulation

Typhoon Winnin （1997） was one of the hurricanes that had extremely large eyewall ever revorded with a diameter of eyewall reaching 370 kin. Using the Penn State Uhiversity/National Center for Atmospheric Researeh mesoscale model MM5 with 3-kan grid horizontal spacing on the finest nested mesh, Winnie was successfully simulated in terms of track, intensity, eye and concentric eyewalls. The dynamic and thermal structures of concentric eyewalls were studied based on the model output. It was found that the concentric eyewails and their surrounding wind fields were asymmetric in observation as well as in simulation. Winale＇s outer eyewall was associated with a maximum wind ring, a warm moist ring, and a high vorticity ring. The inner eyewall was associated with a secondary maximum wind ring and a warm moist ring. Upward motion dominated the whole layer of inner eyewall and the area above 2-km altitude of the outer eyew031. Downward motion was found inside the eye and the moat. Radial inflow happened in the boundary layer of the outer eyewall and the moat, but radial outflow dominated the middle and upper levels of the outer eyewall.

Characteristics of the Asymmetric Flow of Tropical Cyclone Shanshan(2006) During Its Turning and Intensification Period

In this paper,characteristics of the asymmetric flow of Tropical Cyclone（TC） Shanshan（2006） during its turning and intensification period over the oceanic area east of Taiwan are investigated,based on the simulation results from the nonhydrostatic mesoscale model WRF（Weather Research and Forecasting）.It is found that the symmetric flow strengthened as the TC intensified,whereas the amplitude of the asymmetric flow of wavenumber 2 increased more significantly,which was strong enough to be comparable with or even exceed that of wavenumber 1,becoming the main part of the asymmetric flow sometimes.The asymmetric waves rotated around the TC center mainly counterclockwise.The closer to the center,the faster the asymmetric waves rotated.Moreover,the asymmetric flow rotated rapidly（slowly） during the slow（rapid） intensification of the TC,and the radial wavenumber showed an increase during the TC intensification.Furthermore,because of the superposition of intensified symmetric flow with the positive perturbation of the asymmetric flow,the maximum wind speed of TC Shanshan became larger.During the merger of the double eyewalls of Shanshan,the symmetric flow showed less increase in strength and the intensification of maximum wind speed was mainly related to the energy accumulation caused by the phase change of the asymmetric waves.The energy accumulation was realized when the asymmetric waves altered the strength and distribution of the inner and outer maximum wind cores,leading to the combination of the inner and outer eyewalls and eventually resulting in the intensification of the TC.