Track-Pattern-Based Characteristics of Extratropical Transitioning Tropical Cyclones in the Western North Pacific

Based on the Regional Specialized Meteorological Center(RSMC)Tokyo-Typhoon Center best-track data and the NCEP-NCAR reanalysis dataset,extratropical transitioning(ET)tropical cyclones(ETCs)over the western North Pacific(WNP)during 1951–2021 are classified into six clusters using the fuzzy c-means clustering method(FCM)according to their track patterns.The characteristics of the six hard-clustered ETCs with the highest membership coefficient are shown.Most tropical cyclones(TCs)that were assigned to clusters C2,C5,and C6 made landfall over eastern Asian countries,which severely threatened these regions.Among landfalling TCs,93.2%completed their ET after landfall,whereas 39.8%of ETCs completed their transition within one day.The frequency of ETCs over the WNP has decreased in the past four decades,wherein cluster C5 demonstrated a significant decrease on both interannual and interdecadal timescales with the expansion and intensification of the western Pacific subtropical high(WPSH).This large-scale circulation pattern is favorable for C2 and causes it to become the dominant track pattern,owning to it containing the largest number of intensifying ETCs among the six clusters,a number that has increased insignificantly over the past four decades.The surface roughness variation and three-dimensional background circulation led to C5 containing the maximum number of landfalling TCs and a minimum number of intensifying ETCs.Our results will facilitate a better understanding of the spatiotemporal distributions of ET events and associated environment background fields,which will benefit the effective monitoring of these events over the WNP.

COMPARATIVE ANALYSIS OF THE INTENSIFYING AND WEAKENING LANDFALL TROPICAL CYCLONES DURING EXTRATROPICAL TRANSITION OVER CHINA

Based on the Tropical Cyclone(TC) Yearbooks data and JRA-25 reanalysis data from the Japan Meteorological Agency(JMA) during 1979-2008, dynamic composite analysis and computation of kinetic energy budget are used to study the intensifying and weakening TCs during Extratropical Transition over China. The TCI shows strong upper-level divergence, strengthened low-level convergence and significantly enhanced upward motion under the influence of strong upper-level troughs and high-level jets. The TCI is correspondingly intensified after Extratropical Transition(ET); TCW exhibits strong upper-level divergence, subdued low-level convergence and slightly enhanced upward motion under the influence of weak upper-level troughs and high-level jets. It then weakens after ET. The increase(decrease) of the generation of kinetic energy by divergence wind in TCI(TCW) at low level is one of the major reasons for TCI's intensification(TCW's weakening) after transformation. The generation of kinetic energy by divergence wind is closely related to the development of a low-level baroclinic frontal zone. The growth of the generation of kinetic energy by rotational wind in TCI at upper level is favorable for TCI's maintenance, which is affected by strong upper-level troughs. The dissipation of the generation of kinetic energy by rotational wind in TCW at upper level is unfavorable for TCW's maintenance, which is affected by weak upper-level troughs.

A Comparison of Precipitation Distribution of Two Landfalling Tropical Cyclones during the Extratropical Transition

The precipitation distributions associated with two landfalling tropical cyclones（TCs） during extratropical transition（ET） were examined in this study.Their distinction is that the bulk of precipitation fell to the left of the TC track in one TC and to the right in the other.The analyses indicate that,for the TC Haima（2004） case,accompanied by the approach of a deep midlatitude trough throughout the depth of the troposphere,the warm and moist air advection by the southeasterly flow north of TC was favorable for warm advection and frontogenesis to the northwest of the TC.Due to the steepening of equivalent potential temperature（θ e）,the air-parcel uplift along the θ e surface,in collaboration with thermally direct circulation related to frontogenesis,led to enhanced precipitation northwest of the TC.In contrast,for TC Matsa（2005） embedded within a moister environment,a weak midlatitude trough was situated at the mid-upper level.The convection was triggered by the conditional instability at the lower level and then sustained by dynamic forcing at the mid-upper level so that the heavy precipitation occurred to the northeast of TC.For the two TC cases,the precipitation enhancement was also linked to the upper-level anomalous divergence associated with the jet-related forcing on the right side of the jet entrance.From the quasigeostrophic perspective,the advection of geostrophic absolute vorticity by the thermal wind most likely served as an indication reflecting the displacement of the vertical motion relative to the center of the TC.

A STATISTICAL ANALYSIS ON THE DIFFERENT FEATURES OF TC EXTRATROPICAL TRANSITION OVER LAND AND OCEAN

The extratropical transitions(ETs)of tropical cyclones(TCs)over China and the ocean east to 150°E are investigated by the use of best-track data and JRA-25 reanalysis spanning 1979-2008.The ET events occurring north of 25°N and in the warm season(from May to October)are extracted from the reanalysis to emphasize the interaction between TC and midlatitude circulation.Statistical analysis shows that 18.5%of the warm-season TCs go through land ETs north of 25°N in the western North Pacific.And 20.5%of the ET events occur over the ocean east of 150°E.Most(62.2%)ET TCs over China gradually die out after ET,but more(70.7%)ocean ET cases have post-ET reintensification.The evolutions in cyclone phase space and the composite fields for land and ocean ETs,as well as the ET cases with and without post-ET reintensification,are further analyzed.It is found that most TCs with ET over China and those without post-ET reintensification evolve along the typical ET phase path as follows:emergence of thermal asymmetry→losing upper-level warm core→losing lower-level cold core→evolving as extratropical cyclone.The TCs undergoing ETs over ocean and those with post-ET reintensification form a high-level cold core before the ET onset.The TCs with land ET have long distance between the landing TC and a high-level trough.That makes the TC maintain more tropical features and isolates the TC flow from the upstream and downstream jets of the midlatitude trough.The structure of circulation leads to weak development of baroclinicity in land ET.On the contrary,shorter distance between ocean TC and high-level trough makes the high-level trough absorb the TC absolutely.Under that baroclinicity-favorable environment,strong cold advection makes the TC lose its high-level warm core before ET onset.The composite fields confirm that the TC with ocean ET has stronger baroclinic features.Generally,the TC at land ET onset is located to the south of the ridge of the subtropical high,which tends to prevent the TCs from interacting with midlatitude circulation.But for the ocean ET,the situation is just the opposite.Similar analyses are also carried out for the TCs with and without post-ET reintensification over both land and ocean east of 150°E.The results further prove that the TC with stronger baroclinic characteristics,especially in the circumstance favorable to its interaction with high-level midlatitude systems,has more opportunity to reintensify as an extratropical cyclone after ET.

COMPARISONS OF THE CHARACTERISTICS OF TROPICAL CYCLONES EXPERIENCING EXTRATROPICAL TRANSITION IN THE WESTERN NORTH PACIFIC BASED ON DIFFERENT DATASETS

The differences in the climatology of extratropical transition(ET) of western North Pacific tropical cyclones(TCs) were investigated in this study using the TCs best-track datasets of China Meteorological Administration(CMA),Japan Meteorological Agency(JMA) and the Joint Typhoon Warning Center(JTWC). The results show that the ET identification, ET completion time, and post-ET duration reported in the JTWC dataset are greatly different from those in CMA and JMA datasets during 2004-2010. However, the key differences between the CMA and JMA datasets from 1951 to 2010 are the ET identification and the post-ET duration, because of inconsistent objective ET criteria used in the centers. Further analysis indicates that annual ET percentage of CMA was lower than that of JMA, and exhibited an interannual decreasing trend, while that of JMA was an unchanged trend. The western North Pacific ET events occurred mainly during the period June to November. The latitude of ET occurrence shifted northward from February to August,followed by a southward shift. Most of ET events were observed between 35°N and 45°N. From a regional perspective,TCs tended to undergo ET in Japan and the ocean east to it. It is found that TCs which experienced the ET process at higher latitudes were generally more intense at the ET completion time. TCs completing the ET overland or offshore were weaker than those finishing the ET over the ocean. Most of the TCs weakened 24 h before the completion of ET.In contrast, 21%(27%) of the TCs showed an intensification process based on the CMA(JMA) dataset during the post-ET period. The results presented in this study indicate that consistent ET determination criteria are needed to reduce the uncertainty involved in ET identification among the centers.

ANALYSIS OF STRUCTURE EVOLUTION AND ENVIRONMENTAL CONDITIONS OF TROPICAL CYCLONES OVER THE WESTERN NORTH PACIFIC DURING EXTRATROPICAL TRANSITION

Fifty-eight extratropical transition(ET) cases in the years 2000-2008, including 2,021 observations(at 6-hour intervals), over the western North Pacific are analyzed using the cyclone phase space(CPS) method, in an effort to get the characteristics of the structure evolution and environmental conditions of tropical cyclones(TCs) during ET over this area. Cluster analysis of the CPS dataset shows that strong TCs are more likely to undergo ET. ET begins with the increment of thermal asymmetry in TCs, along with the generation and intensification of an upper-level cold core, and ends with the occurrence of a lower-level cold core. ET lasts an average duration of about 28 hours. Dynamic composite analysis of the environmental field of different clusters shows that, in general, when TCs move northward,they are gradually embedded in the westerlies and gradually transform into extratropical cyclones under the influence of the mid-and higher-latitude baroclinic systems. As for those TCs which complete ET, there is always much greater potential vorticity gradient in the northwest of them and obvious water vapor transport channels in the environment.

DIAGNOSIS OF THE DEVELOPMENTAL MECHANISM OF THE EXTRATROPICAL TRANSITION OF A TROPICAL CYCLONE USING POTENTIAL VORTICITY INVERSION OF FRONTOGENESIS

This paper diagnoses and analyses the developmental mechanism of a process of extratropical transition of a tropical cyclone which occurred over West Pacific Ocean based on a diagnosis method of potential vorticity inversion of frontogenesis.The study diagnoses quantitatively the role and effect of dynamic influence of westerly cold troughs,middle-latitude baroclinic frontal zones,cyclone cycles and unbalanced wind fields during the different stages of the extratropical transition of a tropical cyclone,and also discusses the interaction between them and the developmental mechanism.The results show that there are different developmental mechanisms during each stage of the extratropical transition and the processes are also unbalanced.

Cyclone Phase Space Characteristics of the Extratropical Transitioning Tropical Cyclones over the Western North Pacific

The cyclone phase space （CPS） method has been utilized to evaluate the extratropical transition （ET） of tropical cyclones （TCs） in many recent publications. However, these studies mainly focused over the North Atlantic basin. In this paper, the CPS characteristics of all the cyclones over the western North Pacific are investigated and discussed, with three parameters calculated from the best-track data of the Regional Specialized Meteorological Center in Tokyo and the Japanese 25-yr reanalysis data. It is concluded that most TCs over the western North Pacific possess the non-frontal and warm-core structure, while a larger number of cyclones that have undergone ET hold the frontal and cold-core structure. The spatial pattern of the CPS parameters indicates that the areas of tropical and extratropical cyclone activities could be demarcated by 30 - N. The composite and individual series of three parameters of the CPS indicate that the transformation of –V U T from positive to negative leads to the start of ET, and could be considered as a potential predictor in operationally forecasting an ET event.

Impact of the Subtropical High on the Extratropical Transition of Tropical Cyclones over the Western North Pacific

Recent publications have investigated the interactions between the extratropical transitions （ETs） of tropical cyclones （TCs） and midlatitude circulations; however, studies of ET events have rarely considered the relationship between the storm and the nearby subtropical high. The TC best-track data provided by the Regional Specialized Meteorological Center-Tokyo Typhoon Center of the Japan Meteorology Agency are used in conjunction with the NCEP/NCAR reanalysis data to discuss the potential effects of the subtropical high on ETs over the western North Pacific basin. When the western Pacific subtropical high （WPSH） is weakened and withdrawn toward the east, more TCs follow recurving paths and the midlatitude trough activity is intensified. These changes lead to enhanced ET activity. By contrast, when the WPSH strengthens and extends westward, the number of TCs that follow direct westward paths increases and the midlatitude trough is relatively inactive. These conditions lead to reduced occurrences of ET cases. Abnormal activity of the WPSH should be considered as an important factor in determining ET activity.

Re-Examination of the Potential Vorticity Metrics for Determining Extratropical Transition Onset and Completion Times Using High-Resolution Data

Based on a high-resolution dataset, this note re-examines the recently developed potential vorticity （PV） metrics for determining extratropical transition （ET） onset and completion times. The PV metrics use average 330-K isentropic potential vorticity （IPV） to determine the ET onset time, defined as the 330-K IPV minimum time. However, the suggested 330-K IPV threshold fails to determine the ET completion time using the 20-km resolution data, and this IPV method cannot resolve reintensifying and weakening tropical cyclone cases due to the absence of differentiation of lower-level IPV tendencies after ET onset between these two groups of cases.

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.

Phase transitions between tropical,subtropical,and extratropical cyclones:A review from IWTC-10

This review,which was adapted from a Tenth International Workshop on Tropical Cyclones(IWTC-10)report,discusses research findings and operational practices relevant to cyclone types and phase transitions(extratropical,subtropical,and tropical).The cyclone phase space(CPS)method is widely used in both historical investigations and real-time evaluation of cyclone type and transition;however,CPS parameter values depend on input data resolution,and universal thresholds do not currently exist to delineate when a cyclone transitions from one type to another.Assessments of phase transitions in a changing climate highlight potential latitude shifts in extratropical transition and increased potential for tropical transition,but realistic projections of future trends likely require high-resolution simulations that can capture the cyclone warm core.Operational meteorological centers apply varied approaches to cyclone classification via CPS parameters and other criteria,some of which depend on the tropical basin,yet these approaches cannot fully address challenges in operational classification and subsequently in communicating risks associated with these phase transitions.We recommend a multivariate historical assessment of tropical and subtropical cyclones across all basins in which they occur,including the South Atlantic Ocean and the Mediterranean Sea,to identify the potential for a more universal cyclone classification approach that meets operational needs.