Alignment of Track Oscillations during Tropical Cyclone Rapid Intensification

Recent studies on tropical cyclone(TC)intensity change indicate that the development of a vertically aligned TC circulation is a key feature of its rapid intensification(RI),however,understanding how vortex alignment occurs remains a challenging topic in TC intensity change research.Based on the simulation outputs of North Atlantic Hurricane Wilma(2005)and western North Pacific Typhoon Rammasun(2014),vortex track oscillations at different vertical levels and their associated role in vortex alignment are examined to improve our understanding of the vortex alignment during RI of TCs with initial hurricane intensity.It is found that vortex tracks at different vertical levels oscillate consistently in speed and direction during the RI of the two simulated TCs.While the consistent track oscillation reduces the oscillation tilt during RI,the reduction of vortex tilt results mainly from the mean track before RI.It is also found that the vortex tilt is primarily due to the mean vortex track before and after RI.The track oscillations are closely associated with wavenumber-1 vortex Rossby waves that are dominant wavenumber-1 circulations in the TC inner-core region.This study suggests that the dynamics of the wavenumber-1 vortex Rossby waves play an important role in the regulation of the physical processes associated with the track oscillation and vertical alignment of TCs.

Kinetic Energy Budgets during the Rapid Intensification of Typhoon Rammasun (2014)

In this study,Typhoon Rammasun(2014)was simulated using the Weather Research and Forecasting model to examine the kinetic energy during rapid intensification(RI).Budget analyses revealed that in the inner area of the typhoon,the conversion from symmetric divergent kinetic energy associated with the collocation of strong cyclonic circulation and inward flow led to an increase in the symmetric rotational kinetic energy in the lower troposphere.The increase in the symmetric rotational kinetic energy in the mid and upper troposphere resulted from the upward transport of symmetric rotational kinetic energy from the lower troposphere.In the outer area,both typhoon and Earth’s rotation played equally important roles in the conversion from symmetric divergent kinetic energy to symmetric rotational kinetic energy in the lower troposphere.The decrease in the symmetric rotational kinetic energy in the upper troposphere was caused by the conversion to asymmetric rotational kinetic energy through the collocation of symmetric tangential rotational winds and the radial advection of asymmetric tangential rotational winds by radial environmental winds.

Analysis of an Ensemble of High-Resolution WRF Simulations for the Rapid Intensification of Super Typhoon Rammasun(2014)

Diagnostics are presented from an ensemble of high-resolution forecasts that differed markedly in their predictions of the rapid intensification(RI)of Typhoon Rammasun.We show that the basic difference stems from subtle differences in initializations of(a)500-850-h Pa environmental winds,and(b)midlevel moisture and ventilation.We then describe how these differences impact on the evolving convective organization,storm structure,and the timing of RI.As expected,ascent,diabatic heating and the secondary circulation near the inner-core are much stronger in the member that best forecasts the RI.The evolution of vortex cloudiness from this member is similar to the actual imagery,with the development of an inner cloud band wrapping inwards to form the eyewall.We present evidence that this structure,and hence the enhanced diabatic heating,is related to the tilt and associated dynamics of the developing inner-core in shear.For the most accurate ensemble member:(a)inhibition of ascent and a reduction in convection over the up-shear sector allow moistening of the boundary-layer air,which is transported to the down-shear sector to feed a developing convective asymmetry;(b)with minimal ventilation,undiluted clouds and moisture from the down-shear left quadrant are then wrapped inwards to the up-shear left quadrant to form the eyewall cloud;and(c)this process seems related to a critical down-shear tilt of the vortex from midlevels,and the vertical phase-locking of the circulation over up-shear quadrants.For the member that forecasts a much-delayed RI,these processes are inhibited by stronger vertical wind shear,initially resulting in poor vertical coherence of the circulation,lesser moisture and larger ventilation.Our analysis suggests that ensemble prediction is needed to account for the sensitivity of forecasts to a relatively narrow range of environmental wind shear,moisture and vortex inner-structure.

The Effect of Warm Water and Its Weak Negative Feedback on the Rapid Intensification of Typhoon Hato(2017)

Typhoon Hato (2017) went through a rapid intensification (RI) process before making landfall in Zhuhai,Guangdong Province, as the observational data shows. Within 24 hours, its minimum sea level pressure deepened by35hPa and its maximum sustained wind speed increased by 20m s-1. According to satellite observations, Hato encountered a large area of warm water and two warm core rings before the RI process, and the average sea surface temperature cooling (SSTC) induced by Hato was only around 0.73℃. Air-sea coupled simulations were implemented to investigate the specific impact of the warm water on its RI process. The results showed that the warm water played an important role by facilitating the RI process by around 20%. Sea surface temperature budget analysis showed that the SSTC induced by mixing mechanism was not obvious due to the warm water. Besides, the cold advection hardly caused any SSTC, either. Therefore, the SSTC induced by Hato was much weaker compared with that in general cases. The negative feedback between ocean and Hato was restrained and abundant heat and moisture were sufficiently supplied to Hato. The warm water helped heat flux increase by around 20%, too. Therefore, the warm water influenced the structure and the intensity of Hato. Although there might be other factors that also participated in the RI process, this study focused on air-sea interaction in tropical cyclone forecast and discussed the impact of warm water on the intensity and structure of a tropical cyclone.

Role of ocean upper layer warm water in the rapid intensification of tropical cyclones: A case study of typhoon Rammasun(1409)

Rammasun intensified rapidly from tropical storm to super typhoon in the northern South China Sea（NSCS）before its landfall on Hainan Island. Analysis of observed data shows that the anomalous ocean upper layer warm water（WW） is important to the rapid intensification of Rammasun. During the period of Rammasun, sea surface temperature（SST） in the NSCS was much warmer than the climatological SST. The anomalous WW supplied more energy to Rammasun, resulting in its rapid intensification. Numerical simulations further confirm that the NSCS WW plays an important role in the rapid intensification of Rammasun. As the WW is removed, the intensification of Rammasun is only 25 h Pa, which is 58.1% of that in the original SST-forced run.

Upper-tropospheric potential vorticity anomaly before the rapid intensification of Typhoon Mujigae(2015)and its response to reduced SST

As the strongest typhoon in 2015,the rapid intensification(RI)process of Typhoon Mujigae is simulated at the cloud-resolving scale by utilizing the Weather Research and Forecasting model,coupled with observed sea surface temperature(SST)and reduced SST as the control and sensitivity experiments,respectively.The spatiotemporal distribution characteristics of uppertropospheric potential vorticity(PV)and its response to SST changes are analyzed simultaneously.The results show a significant upper-tropospheric PV anomaly 6 h before RI,which indicates the approaching RI of Typhoon Mujigae.Deep convection overlies the strong signals of upper-tropospheric PV anomalies.More insight into the PV budget analysis verifies that the diabatic heating effect and vertical advection terms associated with deep convection dominate the upper-tropospheric PV anomaly,inferring that deep convection plays a crucial role in the upper-tropospheric PV anomaly.As SST decreases,diabatic heating and vertical advection effects are weakened due to reduced deep convection;therefore,the PV anomaly in the model run is not stronger than that in the control run,and the RI process of Mujigae disappears.Therefore,from the viewpoint of upper and lower-level interactions,how SST impacts upper-tropospheric PV anomalies and their interactions with RI are clarified.Thus,in addition to SST,the upper-tropospheric PV anomaly might be a significant factor in differentiating between RI and non-RI tropical cyclones.

Characteristics of the convective bursts and their relationship with the rapid intensification of Super Typhoon Maria(2018)

For Super Typhoon Maria(2018),the multi-intensity change stages are identified and reproduced by numerical simulation.It is rather difficult to perform a perfect simulation for such a repeatedly reinforced typhoon during its long life-cycle and remote path.In this study,the rapid intensification(RI)episode is focused on to investigate the convective burst(CB)characteristics and the relationship between the CBs and the RI of Maria.For Typhoon Maria,1)the spatial pattern of the inner-core CBs in distinct shear-relative quadrants,instead of the overall inner-core CBs frequently used in previous studies,presents cyclonic rotation from downshear to upshear quadrants during RI,producing a higher efficiency for tropical cyclone(TC)spinup,which accelerates the RI process.2)Dual meanings/relationships exist between CBs and RI for Maria,in contrast to the previous argument that CBs might be an indicator or a precursor to RI.The sudden growth of CBs prior to RI provides a precursor for the upcoming RI.Additionally,the appearance of the CB peak soon after RI indicates RI could lead to more intensive deep convections.The overlap of CBs with high inertial stability inside the radius of maximum wind plays a significant role in the RI of TCs.3)The synoptic attributions to CBs are also explored for the entire troposphere,fitting in the bottom-up thinking of convection growth.The CBs might be associated with high convective available potential energy in the boundary layer,a strengthening of the deep-layer secondary circulation,and an enhanced upper-level eddy momentum flux convergence.

Analysis of the Rapid Intensification of Typhoon “Mekkhala” (2006) over the Offshore Area Based on Satellite Data

Analysis of the cloud macro characteristics of typhoon “Mekkhala” is based on FY-4A stationary meteorological satellite data. Aiming at the precipitation process during the “Mekkhala” tropical storm and typhoon, the precipitation structure characteristics were analyzed using the precipitation data retrieved from polar orbiting satellites. The results show that: in the life process of “Mekkhala”, its cloud system always presents an asymmetric structure, and the cloud area and cloud top height on the north and south sides also change constantly. When the intensity of “Mekkhala” reaches the maximum, its minimum brightness temperature range is also the largest, and the spiral structure is also the most obvious;during the precipitation process of the “Mekkhala” tropical storm and typhoon, the near-surface precipitation rate is roughly distributed in a ring shape, from the precipitation rate of the FY3-D polar-orbiting satellite and the GCOM-W1 satellite. In terms of product comparison, the precipitation rate product of the GCOM-W1 satellite responds better to low-level precipitation.

Cloud Microphysical Characteristics of Typhoon Meranti(2016)during Its Rapid Intensification:Model Validation and SST Sensitivity Experiments

Cloud microphysics plays an important role in determining the intensity and precipitation of tropical cyclones(TCs).In this study,a high-resolution numerical simulation by WRF(version 4.2)of Typhoon Meranti(2016)during its rapid intensification(RI)period was conducted and validated by multi-source observations including Cloud-Sat and Global Precipitation Mission satellite data.The snow and ice particles content were found to increase most rapidly compared with other hydrometeors during the RI process.Not all hydrometeors continued to increase.The graupel content only increased in the initial RI stage,and then decreased afterwards due to precipitation during the RI process.In addition,sea surface temperature(SST)sensitivity experiments showed that,although the intensity of the TC increased with a higher SST,not all hydrometeors increased.The graupel content continued to increase with the increase in SST,mainly due to the accumulation of more lower-temperature supercooled water vapor at the corresponding height.The content of snow decreased with the increase in SST because stronger vertical motion at the corresponding height affected the aggregation of ice crystals.

Predictability and dynamics of the rapid intensification of Super Typhoon Lekima (2019)

This study explores the effect of the initial axisymmetric wind structure and moisture on the predictability of the peak intensity of Typhoon Lekima(2019)through a 20-member ensemble forecast using the WRF model.The ensemble members are separated into Strong and Weak groups according to the maximum 10-m wind speed at 48 h.In our study of Lekima(2019),the initial intensity defined by maximum 10-m wind speed is not a good predictor of the intensity forecast.The peak intensity uncertainty is sensitive to the initial primary circulation outside the radius of maximum wind(RMW)and the initial secondary circulation.With greater absolute angular momentum(AAM)beyond the RMW directly related to stronger primary circulation,and stronger radial inflow,Strong group is found to have larger AAM import in lowlevel,helping to spin up the TC.Initial moisture in innercore is also critical to the intensity predictability through the development of inner-core convection.The aggregation and merger of convection,leading to the TC intensification,is influenced by both radial advection and gradient of system-scale vortex vorticity.Three sensitivity experiments are conducted to study the effect of model uncertainty in terms of model horizontal grid resolution on intensity forecast.The horizontal grid resolution greatly impacts the predictability of Lekima’s intensity,and the finer resolution is helpful to simulate the intensification and capture the observed peak value.

Diagnostic Study of an Extreme Explosive Cyclone over the Kuroshio/Kuroshio Extension Region

Explosive cyclones(ECs)occur frequently over the Kuroshio/Kuroshio Extension region.The most rapidly intensified EC over the Kuroshio/Kuroshio Extension region during the 42 years(1979-2020)of cold seasons(October-April)was studied to reveal the variations of the key factors at different explosive-developing stages.This EC had weak low-level baroclinicity,mid-level cyclonic-vorticity advection,and strong low-level water vapor convergence at the initial explosive-developing stage.The low-level baroclinicity and mid-level cyclonic-vorticity advection increased substantially during the maximum-deepening-rate stage.The diagnostic analyses using the Zwack-Okossi equation showed that diabatic heating was the main contributor to the initial rapid intensification of this EC.The cyclonic-vorticity advection and warm-air advection enhanced rapidly in the middle and upper troposphere and contributed to the maximum rapid intensification,whereas the diabatic heating weakened slightly in the mid-low troposphere.The relative contribution of the diabatic heating decreased from the initial explosive-developing stage to the maximum-deepening-rate stage due to the enhancement of other factors(the cyclonic-vorticity advection and warm-air advection).Furthermore,the physical factors contributing to this EC varied with the explosive-developing stage.The non-key factors at the initial explosive-developing stage need attention to forecast the rapid intensification.

Growing Threat of Rapidly-Intensifying Tropical Cyclones in East Asia

This study examines the long-term change in the threat of landfalling tropical cyclones(TCs) in East Asia over the period 1975–2020 with a focus on rapidly intensifying(RI) TCs. The increase in the annual number of RI-TCs over the western North Pacific and the northwestward shift of their genesis location lead to an increasing trend in the annual number of landfalling RI-TCs along the coast of East Asia. The annual power dissipation index(PDI), a measure of the destructive potential of RI-TCs at landfall, also shows a significant increasing trend due to increases in the annual frequency and mean landfall intensity of landfalling RI-TCs. The increase in mean landfall intensity is related to a higher lifetime maximum intensity(LMI) and the LMI location of the landfalling RI-TCs being closer to the coast. The increase in the annual PDI of East Asia is mainly associated with landfalling TCs in the southern(the Philippines, South China, and Vietnam) and northern parts(Japan and the Korean Peninsula) of East Asia due to long-term changes in vertical wind shear and TC heat potential. The former leads to a northwestward shift of favorable environments for TC genesis and intensification, resulting in the northwestward shift in the genesis, RI, and LMI locations of RI-TCs. The latter provides more heat energy from the ocean for TC intensification, increasing its chances to undergo RI.

A comparison study of atmospheric circulations and potential vorticity anomaly between the two rapid-intensified typhoons

For two rapid-intensification typhoons-Mujigae(2015)and Vicente(2012)-the atmospheric circulation conditions and potential vorticity(PV)anomaly are compared.Although similar in their rapid-intensification(RI)rate,their atmospheric circulation conditions differ considerably,with the absence or presence of an upper-tropospheric inverted trough(IT)being their main difference.The IT provides useful clues for the onset of RI,by estimating the interaction between the environmental upper-tropospheric IT and the typhoon based on eddy momentum flux convergence calculation.The trough–typhoon interaction is examined by comparing the PV transport process for the two cases.An isolated positive PV column develops vertically near Mujigae’s onset of RI,which is not influenced by synoptic-scale PV advection.However,for Vicente,another source-advection from a high-latitude PV reservoir along the upper-tropospheric IT-joins the built-up high-PV anomaly in favor of RI.

Super Typhoon Hinnamnor(2022)with a Record-Breaking Lifespan over the Western North Pacific

Super Typhoon Hinnamnor(2022)was a rare and unique western North Pacific typhoon,and throughout its lifespan,it exhibited all of the major features that pose current challenges in typhoon research.Specifically,during different stages of its lifespan,it experienced a sudden change of track,underwent rapid intensification,interacted and merged with another vortex,expanded in size,underwent rapid weakening,produced a strong cold wake,exhibited eyewall replacement,and underwent extratropical transition.Therefore,a timely identification and review of these features of Hinnamnor(2022),as reported in this article,will help update and enrich the case sets for each of these scientific issues and provide a background for more in-depth mechanistic studies of typhoon track,intensity,and structural changes in the future.We also believe that Hinnamnor(2022)can serve as an excellent benchmark to quickly evaluate the overall performance of different numerical models in predicting typhoon’s track,intensity,and structural changes.

INTENSITY CHANGE CHARACTERISTICS OF TROPICAL CYCLONES IN THE WESTERN NORTH PACIFIC AS REVEALED BY THREE DIFFERENT DATASETS

Analyzed in this paper are the 20-yr(1991-2010)tropical cyclone(TC)intensity from three forecast centers in the Western North Pacific,i.e.China Meteorological Administration(CMA),Japan Meteorological Agency(JMA),and Joint Typhoon Warning Center(JTWC)of the United States.Results show that there is more or less discrepancy in the intensity change of a TC among different datasets.The maximum discrepancy reaches 22 hPa/6h(42 hPa/6h,33 hPa/6h)between CMA and JMA(CMA and JTWC,JMA and JTWC).Special attention is paid to the records for abrupt intensity change,which is currently a difficult issue for forecasters globally.It is found that an abrupt intensity change process recorded by one dataset can have,in some extreme cases,intensity change in another dataset varying from 0 to≥10 hPa/6h with the same sign or the opposite sign.In a total of 2511 cases experiencing rapid intensity change,only 14%have consensus among all the three datasets and 25%have agreement between two of the three datasets.In spite of such a significant uncertainty,the three datasets agree on the general statistical characteristics of abrupt intensity change,including regional and seasonal distribution,the relationship with initial intensity and TC moving speed,and persistence features.Notable disagreement is on very strong systems(SuperTY)and TCs moving very fast.

A High-resolution Simulation of Supertyphoon Rammasun(2014)--Part Ⅰ:Model Verification and Surface Energetics Analysis

A 72-h high-resolution simulation of Supertyphoon Rammasun （2014） is performed using the Advanced Research Weather Research and Forecasting model. The model covers an initial 18-h spin-up, the 36-h rapid intensification （RI） period in the northern South China Sea, and the 18-h period of weakening after landfall. The results show that the model reproduces the track, intensity, structure of the storm, and environmental circulations reasonably well. Analysis of the surface energetics under the storm indicates that the storm＇s intensification is closely related to the net energy gain rate （eg）, defined as the difference between the energy production （PD） due to surface entropy flux and the energy dissipation （Ds） due to surface friction near the radius of maximum wind （RMW）. Before and during the RI stage, the ~：g is high, indicating sufficient energy supply for the storm to intensify. However, the Sg decreases rapidly as the storm quickly intensifies, because the Ds increases more rapidly than the PD near the RMW. By the time the storm reaches its peak intensity, the Ds is about 20% larger than the PD near the RMW, leading to a local energetics deficit under the eyewall. During the mature stage, the PD and Ds can reach a balance within a radius of 86 km from the storm center （about 2.3 times the RMW）. This implies that the local PD under the eyewall is not large enough to balance the Ds, and the radially inward energy transport from outside the eyewall must play an important role in maintaining the storm＇s intensity, as well as its intensification.

EVOLUTION OF THERMODYNAMIC STRUCTURES DURING RAPID GROWTH AND DECAY OF EXTREMELY SEVERE CYCLONIC STORM CHAPALA(2015)

The structure and evolution of inner-core convective bursts and their differences associated with rapid intensification(RI) and rapid decay(RD) of tropical cyclone CHAPALA are examined. The inception of RI was associated with substantial increase of convective heating and its vertical extent in the inner core. Increase in diabatic heating was of the order of 12-21 oC, particularly in the middle and upper troposphere. Latent heat release produced a diabatically generated potential vorticity(PV) in vertical column. The immediate cause of RI was a significant increase of moisture flux from surface to 500 h Pa. This was accomplished primarily by updrafts of the order of 6-12 Pa s-1, representing the strong vertical motion distribution inside the warm core convective zone. The episode of deep convective bursts transpired during the period of RI. The evolving flow became highly symmetric and dominated by deep convective axisymmetric vortex structures. The RD coincided with the significant weakening in updraft of moisture flux consequently decrease of diabatic heating in the middle and upper troposphere and dissipation of upper and lower PV.

Characteristics of rapidly intensifying tropical cyclones in the South China Sea,1980–2016

The differences in the characteristics of the rapid intensification(RI)during the TCs that form in the SCS(referred as local TCs)and that enter the SCS from the western North Pacific(WNP;referred as entering TCs)have not been well studied,which could contribute the inaccuracy of current TC intensity forecast in the SCS.In this study,we used TC observations,reanalysis data and model experiments to analyze the RI occurrences during local TCs and entering TCs in 1980-2016.We found that the significant interannual and interdecadal variations in RI occurrences during local eastward-moving TCs were related to the strong intraseasonal oscillation(ISO)over the SCS and the WNP under La Niña conditions.RI during local westward-moving TCs showed insignificant variations as a result of the complex interactions among the monsoon trough,ISO and the large-scale circulation.RI during entering TCs showed strong interdecadal variations,with increased RI after 1997,even though the total number of entering TCs has decreased since 1997,which is a result of a higher number of entering TCs in the northwestern quadrant of the WNP,a stronger ISO and weak vertical windshear over the SCS and east of the Philippines under negative phase of Pacific Decadal Oscillation.The different variations and related mechanisms of RI indicates that distinct forecasting factors should be considered for intensity prediction during local eastward-and westward-moving TCs and entering TCs.

Physical processes associated with movement of maximum wind of Typhoon Rammasun(2014)

In this study,the movement of the maximum wind of Typhoon Rammasun(2014)was measured by the radial movement of the maximum symmetric rotational kinetic energy.The weather research and forecasting(WRF)model was used to simulate Typhoon Rammasun,and validated simulation data for the lower troposphere were analyzed to examine the physical processes responsible for the radial movement of the maximum wind.The radii,where maximum symmetric rotational kinetic energy and its maximum tendency were located,were compared to explain radial movement.The tendency in the lower troposphere is controlled by the flux convergence of symmetric rotational kinetic energy and the conversion from symmetric divergent kinetic energy to symmetric rotational kinetic energy,as well as frictional dissipation in the symmetric rotational kinetic energy budget.The inward movement before rapid intensification(RI)resulted from radial flux convergence;cyclonic circulation develops while moving inward.Stationary maximum symmetric rotational kinetic energy and RI were caused by the conversion,which was observed to be proportional to the symmetric rotational kinetic energy.Landfall increased terrain-induced friction dissipation,which led to outward movement and ended the RI.

An Empirical Model of Tropical Cyclone Intensity Forecast in the Western North Pacific

The relative impact of environmental parameters on tropical cyclone(TC) intensification rate(IR) was investigated through a box difference index(BDI) method, using TC best track data from Joint Typhoon Warning Center and environmental fields from the NCEP final analysis data over the western North Pacific(WNP) during 2000–2018.There are total 6307 TC samples with a 6-h interval, of which about 14% belong to rapid intensification(RI) category. The analysis shows that RI occurs more frequently with higher environmental sea surface temperature, higher oceanic heat content, and lower upper-tropospheric temperature. A moderate easterly shear is more favorable for TC intensification. TC intensification happens mostly equatorward of 20°N while TC weakening happens mostly when TCs are located in the northwest of the basin. Mid-tropospheric relative humidity and vertical velocity are good indicators separating the intensification and non-intensification groups. A statistical model for TC intensity prediction was constructed based on six environmental predictors, with or without initial TC intensity. Both models are skillful based on Brier skill score(BSS) relative to climatology and in comparison with other statistical models, for both a training period(2000–2018) and an independent forecast period(2019–2020).