Search

The study investigates the rapid intensification (RI) of tropical cyclones (TCs) in the Northwestern Pacific. We found that rapid changes in the maximum wind speed (Vmax) and the minimum central pressure (Pmin) are not always concurrent. RI cases can be categorized into three types: (a) RIv, only Vmax strengthens rapidly; (b) RIp, only Pmin decreases rapidly; (c) RIpv, rapid changes in Vmax and Pmin occur concurrently. At the onset of RI, RIv‐type TCs exhibit the weakest intensity and the smallest size, with deep convection concentrated in the inner‐core region; RIp‐type TCs are characterized by the strongest cyclone intensity and the largest outer‐core size, with strong convection covering the inner‐ and outer‐core regions; RIpv‐type TCs show moderate intensity, size, and convection distribution. For RIpv, significant strengthening of wind profile is concentrated in the inner‐core region, while for RIp it is more prominent in the outer‐core. Plain Language Summary: The rapid intensification (RI) of tropical cyclones (TCs) continues to pose challenges in both operational forecasting and scientific research. The maximum wind speed (Vmax) and the minimum central pressure (Pmin) are two commonly used TC intensity indicators. In most of existing studies, one single indicator (usually Vmax) is used to investigate the RI process. Whether there is a difference between RI cases defined by 24‐hr strengthening of Vmax and 24‐hr deepening of Pmin is an issue deserving discussion. The study found that rapid changes in Vmax and Pmin are not always concurrent. In the cases where only Vmax strengthens rapidly, the outer‐core size of TC expands slowly. While in the cases where only Pmin deepens rapidly, the enhancement of outer circulation is distinctive. Since the Pmin, as an integral variable, reflects TC information of both the intensity and the size, RI cases classified via these two commonly used indicators (Vmax and Pmin) could help us further understand the relationship between TC intensity and size. Key Points: Rapid changes in the maximum wind speed (Vmax) and the minimum central pressure (Pmin) are not always concurrent in tropical cyclones (TCs)TCs that only experience a rapid increase in Vmax (RIv) tend to have a significantly weaker lifetime maximum intensityThe evident expansion of the outer circulation occurs in TCs of which only Pmin deepens rapidly (RIp) [ABSTRACT FROM AUTHOR]

This study aimed to explore the predictability of the most long‐lived tropical cyclone (TC) Freddy in 2023 while it traversed westward across the southern tropical Indian Ocean during the first 18 days of its existence. Global ensemble forecasts revealed southward track deflection and intensity underestimation of Freddy. We identified three key factors contributing to the limited predictability of Freddy, which are associated with the Mascarene High, Storm Dingani, and Freddy itself. The large track errors of Freddy can be attributed to the underestimated strength of the Mascarene High, the more northeastern position of Dingani, and the presence of excessively large or small sizes of Freddy. These findings were further validated through a high‐resolution regional model. Specifically, Freddy's track and intensity most closely matched the observations when these three factors were most closely represented. It underscores the pivotal role played by the interaction between TCs and multi‐scale systems in TC forecasts. Plain Language Summary: In 2023, Storm Freddy emerged as the most long‐lived tropical cyclone (TC) in record, lasting 35 days over the southern tropical Indian Ocean and spanning both weather and sub‐seasonal to seasonal time ranges. The primary objective of this study is to understand the factors contributing to the poor predictability of Freddy in forecasts spanning over 2 weeks. This holds significant importance as our understanding about the ability of the numerical models to predict long‐lived TCs remains limited. Using over 7,000 global ensemble forecasts from five global Numerical Weather Prediction (NWP) centers and a high‐resolution regional model, we identified three key factors contributing to the limited predictability of Freddy: the strength of the Mascarene High, the position of Storm Dingani (2023), and the size of Freddy. In large track‐error results of the global forecasts and regional simulations for Freddy, the strength of the Mascarene High was underestimated, Dingani was located further northeast, and Freddy was either too large or too small. Our investigation emphasizes the crucial role played by the interaction between the TC and multi‐scale systems in TC forecasts. This is meaningful for the improvement of Numerical Weather Prediction models to deal with extreme TC events in the future. Key Points: Due to the long lifespan of Storm Freddy, many forecasting challenges across several numerical models aroseThe limited predictability of Freddy was related to the underestimated Mascarene High and the more northeastern position of Storm DinganiThe inaccurately forecasted size of Freddy, whether excessively large or small, also contributed to the large track errors [ABSTRACT FROM AUTHOR]

This study examines the changes in the outer size distribution of landfalling tropical cyclones (TCs) over the Chinese mainland from 1977 to 2020. The period was divided into two epochs: 1977–98 and 1999–2020. The results show that the size distribution of landfalling TCs over South China has no apparent change, while that of landfalling TCs over East China (LTCEC) is narrower in the second epoch, and the difference in the median sizes between East China and South China become more significant. Furthermore, it is found that LTCEC formed over the western part of the western North Pacific (W-WNP) shifted to a larger size range (300–500 km) at landfall, while those formed over the eastern part of the western North Pacific (E-WNP) rarely grew to extremely large size (>500 km). Further investigation revealed that over the W-WNP, the genesis position of LTCEC migrated equatorward during the second epoch, leading to a longer TC lifetime before landfall. Also, the increase of background relative vorticity and moisture associated with the southward migration is conducive to larger initial vortices. For TCs originating from the E-WNP, the change in the active area of TC passages reduced the frequency of TCs affecting the Chinese coast. Moreover, the growth of TC size during the intensification stage was significantly suppressed, lowering the occurrence probability of extremely large TCs. Changes in the large-scale thermodynamic environments between the two epochs were explored. Increased static stability and decreased convective available potential energy are possible factors limiting TC size increase. [ABSTRACT FROM AUTHOR]

A record-breaking precipitation event occurred in the Henan province of China in July 2021 (217HP). To identify the moisture source of the event, ensemble experiments with 120 members were conducted in this study. Results show that the precipitable water during this extreme event was primarily contributed by the low-level southeasterly (LLSE) water vapor transport. The LLSE was largely enhanced by the pressure gradient force maintained by the western Pacific subtropical high and further amplified by the latent heat release in the rainfall system over Henan. The positive moisture advection by the LLSE and evaporative water occurred below 950 hPa and was redistributed into higher levels by the LLSE jet-enhanced subgrid vertical turbulent transport. As a result, the combination of the enhanced LLSE centered around 950 hPa and the increase of moisture below 850 hPa were the main drivers for the continuous strengthening of LLSE moisture transport, with the former playing the dominant role. It is also found that not only the presence but also the intensity of the LLSE jet were important for reproducing the extreme rainfall. The impact of binary tropical cyclones In-Fa and Cempaka on the low-level moisture transport was also examined. We found that In-Fa (2021) presented an uncertain impact on 217HP, while Cempaka (2021) was found to be unfavorable for 217HP. Different from the LLSE water vapor transport, Cempaka mainly acted to weaken the southwesterly wind to the southwest of Henan by reducing the pressure gradient and impeding the water vapor transport toward Henan. [ABSTRACT FROM AUTHOR]

This study investigates the evolution of tropopause layer cooling (TLC) in an idealized tropical cyclone (TC) as well as the physical mechanism for its formation throughout its lifetime. TLC development is closely related to the intensity change of the TC. The TLC within the inner core of a TC coincides with the occurrence of the TC warm core, whereas the enhancement of the TLC precedes the strengthening of the warm core and the intensification of the TC. This phenomenon suggests that the TLC could be a valuable signal for predicting TC intensity and intensity change. After the onset of rapid intensification, the TLC continues to enhance until it is replaced by the warm core in the inner core region, and it expands to the outer core area and forms a cold ring above the TC outflow layer. The budget analyses of the azimuthal mean potential temperature indicate that advection plays a leading role during all stages of the TLC evolution. Prior to and during the rapid intensification stages of a TC, advection (due to eddy motion) contributes to a cooling of the upper layer above the TC center. This upper‐level cooling is assumed to be related to the uplift caused by overshooting and convection within the inner core. However, the advection process caused by the azimuthal vertical motion dominated the broad cooling across the whole TLC area. Plain Language Summary: Observations and numerical simulations show that there is typically a cold anomaly above the TC warm core. This cooling can work with the warm core to reduce the upper‐tropospheric stability and affect the water vapor and momentum transport between the TC and the stratosphere. However, little is known about how the cold anomaly evolves during the life cycle of a TC and how it can be related to changes in TC intensity. This study examines the evolution of the cold anomaly throughout the lifetime of a TC with an idealized TC simulation. The cold anomaly emerges at the TC center before rapid intensification starts and continues to enhance during TC development. It moves radially and forms a cold ring around the TC until the mature stage. The advection, specifically the vertical mean motion, dominates the generation of the TLC, but the advection due to eddy motions is responsible for the cold anomaly near the center. Our findings demonstrate the importance of tropopause layer cooling on TC structure and intensity changes and indicate the necessity of additional observations of this cold anomaly to improve TC predictions. Key Points: The cooling tendency in the tropopause layer enhances continuously during the preintensification and rapid intensification stages of tropical cyclone (TC)The adiabatic uplift caused by overshooting convection can cool the upper layer in the TC center through azimuthal‐eddy advectionThe broad ring of the cold anomaly above the TC outflow layer is related to adiabatic cooling through the azimuthal mean vertical motion [ABSTRACT FROM AUTHOR]

The rainfall distribution in a tropical cyclone (TC) is affected by many factors/processes. Most of previous studies have focused on individual TCs. Little is known about the rainfall distribution in binary TCs. This study examines the rainfall distribution in binary TCs over the western North Pacific based on the observational data. When two TCs become nearby, the asymmetric component of rainfall shows an increasing trend with rainfall significantly suppressed in Quadrant IV of the TC located to the west when orienting the two TCs in the west‐east direction. The suppression becomes remarkable once the separation distance between the two TCs is within about 2,050 km. Furthermore, the convective activity in one TC is related to the deep‐layer vertical wind shear (VWS) from its companion. Rainfall is enhanced downshear‐left in a TC, consistent with a single TC embedded in an environmental VWS as found in previous studies. Plain Language Summary: The rainfall distribution in each tropical cyclone (TC) of binary‐TC systems may be more complicated than that in a single TC. Whether the rainfalls of binary TCs have some special features has not been studied before. This observational study examines the rainfall distribution in binary TCs over the western North Pacific during 2001–2020. The results show that the asymmetric component of TC rainfalls strengthened with the decreasing of the separation distance between the two TCs. If orient binary TCs in the west‐east direction, the rainfalls of the western TC were distinctly weakened in its Quadrant IV. The suppression effect cannot be neglected within 2,050‐km separation distance between binary TCs. Composite analyses indicate that the asymmetric rainfalls in one of the binary TCs were related to the deep‐layer vertical wind shear induced by its companion. The consistent pattern of rainfall asymmetry indicates a highly predictable component of rainfall distribution at vortex‐scale in binary TCs and can help improve the precipitation forecast of binary TCs. Key Points: The rainfall asymmetry increases with the decreasing separation distance between binary tropical cyclones (TCs)If the two TCs are oriented west‐east and within about 2,050 km, rainfall in the western TC is suppressed in its Quadrant IVThe asymmetric convective activity in one TC can be explained by the deep‐layer vertical wind shear from its companion [ABSTRACT FROM AUTHOR]

The prediction of tropical cyclone (TC) intensity remains a major scientific challenge. Recent studies indicate that cloud‐radiation feedback (CRF) plays a positive role in the intensification of TCs during their genesis. However, little attention has been given to how CRF affects TC intensity after genesis. This study shows that CRF may prevents TCs from attaining higher maximum intensities. The ascending motion induced by the anomalous radiative heating of TC promotes more latent heating on the outer side of the upper eyewall, resulting in a more tilted eyewall. A more tilted eyewall leads to a larger inner‐core size and less inward flux of absolute vertical vorticity within the inner core, thus preventing the TC from reaching higher intensity. This work highlights that CRF may affect TC intensity by modulating the structure of the inner‐core convection, and further advances our understanding of the interaction between radiation effect and TC dynamics. Plain Language Summary: Cloud‐radiation feedback plays an important role throughout the life cycle of a tropical cyclone (TC). While many previous studies have focused on the effects of cloud‐radiation feedback (CRF) during TC genesis, little work has examined how CRF modulates TC intensity after TC genesis. With idealized simulations, this study shows that radiation‐induced secondary circulation leads to a more tilted eyewall, which results in a larger inner‐core size and less inward flux of absolute vertical vorticity, finally preventing TCs from reaching higher intensities. These results demonstrate that after TC genesis, CRF may affect storm intensity by modulating the structure of the inner‐core convection, a pathway previously overlooked. This study also reveals, counterintuitively, that CRF may have opposite effects on TC intensities at different stages of their lifetimes. Key Points: Cloud‐radiation feedback (CRF) results in a larger but weaker tropical cyclone (TC)Anomalous radiative heating induces a more tilted eyewall, leading to a larger radius of the maximum wind and less inward flux of vorticityThe effect of CRF on TC inner‐core processes is critical [ABSTRACT FROM AUTHOR]

The short‐range impacts of volcanic aerosols from the eruption of the Kilauea volcano on the genesis of tropical cyclone (TC) Wukong in 2018 are investigated by numerical simulations, as distinct from the studies on the aerosol climate effects. The aerosol−radiation and aerosol−cloud effects are isolated through sensitivity experiments. For the aerosol−radiation effect, the radiative cooling at the low levels and heating above by aerosols southeast of Wukong's periphery induce stable sinking and decrease latent heating in the environment far from the storm center. Therefore, the sea‐level pressure increases there, enhancing low‐level radial inflow to the storm, inner‐core convection and the release of latent heat, which is conducive to TC genesis. For the aerosol−cloud effect, volcanic aerosols in storm circulation can be converted into cloud condensation nuclei, invigorating inner‐core convection, and subsequently favoring TC genesis. However, the nonlinear effect resulting from the interaction between aerosol−radiation and aerosol−cloud effects is not conducive to TC genesis, because it weakens the aerosol−cloud effect through the deposition of aerosols and also limits the aerosol−radiation effect through the increasing of peripheral convection. Plain Language Summary: The role of sulfate aerosols released by volcanic eruptions in climatological tropical cyclone (TC) activities over different basins or hemispheres remains uncertain. These aerosols, composed of either solid or liquid particles, not only change the heating or cooling of atmosphere by scattering and absorbing sunlight, but also participate in cloud formation and evolution by conversion into nuclei of cloud droplets. Impacts of aerosol−radiation and aerosol−cloud interaction on a TC formation process during a volcanic eruption still lacks of detailed study for real cases. We investigate whether these effects of aerosols from the Kilauea volcanic eruption were beneficial or not to the formation of TC Wukong in 2018. The aerosol−radiation effect enhances cooling at the low levels of Wukong's environment and increases the sea‐level pressure there, which drives air to flow inward and is beneficial to TC genesis. On the other hand, lots of aerosols ingested into the storm invigorates the convection near the storm center, is also helpful. Instead, the interaction between the above two effects offset partly their own positive contributions to TC genesis. Our findings suggest that considering the interaction of volcanic aerosol−cloud−radiation is necessary to improve our understanding and prediction of TC activities. Key Points: Enhanced radiative cooling at low layer of Wukong's periphery induced by aerosols from Kilauea volcanic eruption advanced its genesisMicrophysical effect of the aerosols partly entering into Wukong's inner core invigorated convection so as to favor its genesisThe nonlinear interaction between the aerosol−radiation and the aerosol−cloud effects contributed negatively to Wukong's genesis [ABSTRACT FROM AUTHOR]

No tropical cyclone (TC) formed over the western North Pacific (WNP) in July 2020, which is record‐breaking. Some recent studies suggested that this extreme event was mainly caused by the strong WNP subtropical high (WNPSH) in July 2020, which might not be the only reason. In this study, results from statistical and composite analyses indicate that the strong WNPSH, the strong South Asian high, the westward extended tropical upper‐tropospheric trough, and the weak South Asian summer monsoon are all detrimental to TC genesis over the WNP and might contribute to the absence of TCs in July 2020. Results from sensitivity experiments using a regional atmospheric model that can resolve TCs further demonstrate that it was the collective effect, not any of the individual large‐scale circulation anomalies, that induced the extremely unfavorable conditions for TC genesis and thus led to the record‐breaking inactive TC month. Plain Language Summary: Tropical cyclones (TCs) become more active during the typhoon season (from July to September) over the western North Pacific (WNP). However, it is an exception in the records that no TC formed over the WNP in July 2020. Some recent studies suggest that this extreme event was mainly caused by the strong WNP subtropical high (WNPSH) in July 2020. This study identified four large‐scale circulation anomalies that are detrimental to TC genesis over the WNP and might contribute to this extremely inactive TC event, including the strong WNPSH, the strong South Asian high (SAH), the westward extended tropical upper‐tropospheric trough, and the weak South Asian summer monsoon (SASM). The strong WNPSH reduced the mid‐level moisture and the low‐level vorticity; the strong SAH and the westward shift of the tropical upper‐tropospheric trough increased the deep‐level vertical wind shear; and the weak SASM could not provide sufficient low‐level relative vorticity. It is the combined effect, not any of the individual large‐scale circulation anomalies, that induced the extremely unfavorable conditions for TC genesis and led to the absence of TCs over the WNP in July 2020. Key Points: No tropical cyclone (TC) formed over the western North Pacific (WNP) in July 2020, an extreme month that has never occurred in historyIt is due to the combination of anomalies in the WNP subtropical high, the South Asian high, the South Asian summer monsoon, and the tropical upper‐tropospheric troughTC‐resolving numerical experiments demonstrate the detrimental contributions of all the four large‐scale atmospheric circulations [ABSTRACT FROM AUTHOR]

In this study, the characteristics of the tropical cyclone (TC) warm cores in high resolution reanalysis dataset (ERA5) and a 25-km-mesh regional climate simulation over CORDEX East Asia domain during 1988–2009 are analyzed. The Kain–Fritsch scheme with new convection trigger function (Ma and Tan, Atmos Res 92:190–211, 2009; KFMT), the renewed Kain-Fritsch convective parameterization (Kain, J Appl Meteorol 43:170–181, 2004; KF) and the simplified Arakawa-Schubert scheme (Arakawa and Schubert, J Atmos Sci 31:674–701, 1974; SAS) are employed to illustrate the impact of cumulus parameterization schemes (CPSs) on the representation of warm core in the regional climate simulation. The TC intensity and warm core strength in ERA5 reanalysis is largely weaker than those in the regional climate simulations. In ERA5 reanalysis and three regional climate simulations, the warm core strength shows a significant positive correlation with TC intensity, but the warm core height has no correlation with TC intensity, especially during the intensifying stage of TCs. The results also show that CPS has a great impact on the warm core structures of simulated TCs. The TC warm core strength is strongest in KFMT experiment and weakest in SAS experiment, which is consistent with the differences in the TC intensities. The TC warm core heights in the KF and KFMT experiments are significantly higher than that in SAS experiment. These differences of TC warm core strength and height are mainly due to the discrepancy in the simulation of convective activities with different CPSs. The results of this study point out that the ability of climate model to simulate TC intensity is strongly related to the model capability to simulate the TC thermal structure, which is influenced by convection representations in the model. [ABSTRACT FROM AUTHOR]

This study investigates the intensity change of binary tropical cyclones (TCs) in idealized cloud-resolving simulations. Four simulations of binary interaction between two initially identical mature TCs of about 70 m s−1 with initial separation distance varying from 480 to 840 km are conducted in a quiescent f-plane environment. Results show that two identical TCs finally merge if their initial separation distance is within 600 km. The binary TCs presents two weakening stages (stages 1 and 3) with a quasi-steady evolution (stage 2) in between. Such intensity change of one TC is correlated with the upper-layer vertical wind shear (VWS) associated with the upper-level anticyclone (ULA) of the other TC. The potential temperature budget shows that eddy radial advection of potential temperature induced by large upper-layer VWS contributes to the weakening of the upper-level warm core and thereby the weakening of binary TCs in stage 1. In stage 2, the upper-layer VWS first weakens and then restrengthens with relatively weak magnitude, leading to a quasi-steady intensity evolution. In stage 3, due to the increasing upper-layer VWS, the nonmerging binary TCs weaken again until their separation distance exceeds the local Rossby radius of deformation of the ULA (about 1600 km), which can serve as a dynamical critical distance within which direct interaction can occur between two TCs. In the merging cases, the binary TCs weaken prior to merging because highly asymmetric structure develops as a result of strong horizontal deformation of the inner core. However, the merged system intensifies shortly after merging. [ABSTRACT FROM AUTHOR]

The coupling of vortex tilt and convection, and their effects on the intensification variability of tropical cyclones (TCs) in directional shear flows, is investigated in this study. The height-dependent vortex tilt controls TC structural differences in clockwise (CW) and counterclockwise (CC) hodographs during their initial stage of development. Moist convection may enhance the coupling between displaced vortices at different levels and thus reduce the vortex tilt amplitude and enhance precession of the overall vortex tilt during the early stage of development. However, differences in the overall vortex tilt between CW and CC hodographs are further amplified by a feedback from convective heating and therefore result in much higher intensification rates for TCs in CW hodographs than those in CC hodographs. In CW hodographs, convection organization in the left-of-shear region is favored because the low-level vortex tilt is ahead of the overall vortex tilt and the TC moves to the left side of the deep-layer shear. This results in a more humid midtroposphere and stronger surface heat flux on the left side (azimuthally downwind) of the overall vortex tilt, thus providing a positive feedback and supporting continuous precession of the vortex tilt into the upshear-left region. In CC hodographs, convection tends to organize on the right side (azimuthally upwind) of the overall vortex tilt because the low-level vortex tilt is behind the overall vortex tilt and the TC moves to the right side of the deep-layer shear. In addition, convection organizes radially outward near the downshear-right region, which weakens convection within the inner region. These configurations lead to a drier midtroposphere and weaker surface heat flux in the downwind region of the overall vortex tilt and also a broader potential vorticity skirt. As a result, a negative feedback is established that prevents continuous precession of the overall vortex tilt. [ABSTRACT FROM AUTHOR]

Recent studies have demonstrated the importance of moist dynamics on the intensification variability of tropical cyclones (TCs) in directional shear flows. Here, we propose that dry dynamics can account for many aspects of the structure change of TCs in moist simulations. The change of vortex tilt with height and time essentially determines the kinematic and thermodynamic structure of TCs experiencing directional shear flows, depending on how the environmental flow rotates with height, that is, in a clockwise (CW) or counterclockwise (CC) fashion. The vortex tilt precesses faster and is closer to the left-of-shear (with respect to the deep-layer shear) region, with a smaller magnitude at equilibrium in CW hodographs than in CC hodographs. The low-level vortex tilt and accordingly more low-level upward motions are ahead of the overall vortex tilt in CW hodographs but are behind the overall vortex tilt in CC hodographs. Such a configuration of vortex tilt in CW hodographs is potentially favorable for the continuous precession of convection into the upshear region but in CC hodographs it is unfavorable. Most of the upward motions within a TC undergoing CW shear are concentrated in the downshear-left region, whereas those in the CC shear are located in the downshear-right region. Moreover, the upward (downward) motions are in phase with positive (negative) local helicity in both CW and CC hodographs. Here, we present an alternative mechanism that is associated with balanced dynamics in response to vortex tilt to explain the coincidence and also the distribution variability of vertical motions, as well as local helicity in directional shear flows. The balanced dynamics could explain the overlap of positive helicity and convection in both moist simulations and observations. [ABSTRACT FROM AUTHOR]

This study investigates the quadrant-by-quadrant evolution of the low-level tangential wind near the eyewall of an idealized simulated mature tropical cyclone embedded in a unidirectional shear flow. It is found that the quadrant-averaged tangential wind in the right-of-shear quadrants weakens continuously, while that in the left-of-shear quadrants experiences a two-stage evolution: a quasi-steady stage followed by a weakening stage after the imposing of vertical wind shear. This leads to a larger weakening rate in the right-of-shear and a stronger jet in the left-of-shear quadrants. The budget analysis shows that the quadrant-dependent evolution of tangential wind is controlled through the balance between the generalized Coriolis force (GCF; i.e., the radial advection of absolute angular momentum) and the advection terms. The steady decreasing of the GCF is primarily responsible for the continuous weakening of jet strength in the right-of-shear quadrants. For the left-of-shear quadrants, the quasi-steady stage is due to the opposite contributions by the enhanced GCF and negative tendency of advections cancelling out each other. The later weakening stage is the result of both the decreased GCF and the negative tangential advection. The combination of storm-relative flows at vortex scale and the convection strength both within and outside the eyewall determines the evolution of boundary layer inflow asymmetries, which in turn results in the change of GCF, leading to the quadrant-dependent evolution of low-level jet strength and thus the overall storm intensity change. [ABSTRACT FROM AUTHOR]

A suite of idealized simulations of tropical cyclones (TCs) with weak to strong vertical wind shear (VWS) imposed during the mature stage was employed to examine the effects of VWS on the inner-core thermodynamics and intensity change of TCs using a three-dimensional full-physics numerical model as well as a budget analysis of moist entropy. For sheared TCs with shear-induced convective asymmetries, VWS tends to reduce moist entropy within the midlevel eyewall and the boundary layer (BL) but supply moist entropy outside the eyewall above the BL. Such changes in moist entropy reduce the radial gradient of moist entropy across the eyewall, resulting in weakening of the TC. Budget analysis showed that the intense eddy fluxes are mainly responsible for the reduction and/or increase in entropy in the sheared TCs. The entropy reduction within the midlevel eyewall is a result of both the radial eddy flux and the vertical eddy flux. These eddy fluxes are effective at introducing low-entropy air into the midlevel eyewall. Accompanying the flushing of midlevel low-entropy air into the BL, there is an increase in moist entropy outside the eyewall above the BL due to the upward transport of moisture from the BL by shear-induced convection. This represents a new potential pathway to further restrain the radial gradient of moist entropy across the eyewall and hence TC intensity in the sheared environmental flow. [ABSTRACT FROM AUTHOR]

More than half of major tropical cyclones (TCs) form concentric eyewalls (CEs) at least once in their life cycle. CE formation and the ensuing eyewall replacement can result in a rapid change in TC intensity and wind field structure. Over the past decade, the dynamics of secondary eyewall formation (SEF) have been the subject of intense research. The separation distance between the inner and outer eyewalls, however, can vary dramatically from case to case. The mechanisms determining the radial location where the secondary eyewall forms are much less understood.This study investigates the relationship between secondary eyewall size and the TC vortex-scale structural features prior to the formation of CE, utilizing satellite data, high-resolution reanalysis data and best-track dataset. The results show that there is a significant positive correlation between the secondary eyewall size and the strength of outer-core circulation and TC size. The stronger the outer-core circulation is and the larger the TC size is, the larger the secondary eyewall size is. The correlation between secondary eyewall size and the mid-level outer TC circulation features appears two days before CE formation, suggesting that stratiform processes occurring in the middle tropospheric levels playing important roles prior to SEF.Furthermore, the differences in vortex-scale structure between the major TCs that do not form obvious CE structure and the TCs that form CE structure are compared and analyzed. The strength of outer-core circulation in TCs without obvious CE structure is much weaker and the corresponding TC size is smaller. The results of this study demonstrate that TC needs to reach certain size as well as a certain intensity to form CE structure. Intense TCs with too small size are not prone to form obvious CE structure. [ABSTRACT FROM AUTHOR]