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.     

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 _T^U from positive to negative leads to the start of ET, and could be considered as a potential predictor in operationally forecasting an ET event.     

A CLIMATOLOGY OF EXTRATROPICAL TRANSITION OF TROPICAL CYCLONES IN THE WESTERN NORTH PACIFIC

Based on best-track data and JRA-25 reanalysis, a climatology of western North Pacific extratropical transition (ET) of tropical cyclone (TC) is presented in this paper. It was found that 35% (318 out of 912) of all TCs underwent ET during 1979–2008. The warm-season (June through September) ETs account for 64% of all ET events with the most occurrence in September. The area 120°E–150°E and 20°N–40°N is the most favorable region for ET onsets in western North Pacific. The TCs experience ET at latitudes 30°N–40°N and have the greatest intensity in contrast to other latitude bands. The distribution of ET onset locations shows obviously meridional migration in different seasons. A cyclone phase space (CPS) method was used to analyze the TC evolution during ET. Except for some cases of abnormal ET at relatively high latitudes, typical phase evolution paths—along which TC firstly showed thermal asymmetry and an upper-level cold core and then lost its low-level warm core—can be used to describe the main features of ET processes in western North Pacific. Some seasonal variations of ET evolution paths in CPS were also found at low latitudes south of 15°N, which suggests different ET onset mechanisms there. Further composite analysis concluded that warm-season ETs have generally two types of evolutions, but only one type in cold season (October through next May). The first type of warm-season ETs has less baroclinicity due to long distance between the TC and upper-level mid-latitude system. However, significant interactions between a mid-latitude upper -level trough and TC, of either approaching or being absorbed into the trough, and TC’s relations with downstream and upstream upper-level jets, are the fingerprints for both a second type of warm-season ETs and almost all the cold-season ETs. For each type of ETs, detailed structural characteristics as well as precipitation distribution are illustrated by latitude.     

西北太平洋变性台风时空分布特征

利用中国气象局上海台风研究所整编的1961-2000年共40 a热带气旋年鉴资料,对发生在西北太平洋上的变性台风的时空分布特征进行了诊断分析和研究.研究发现,发生于西北太平洋上的变性台风的年频数呈现出明显的年代际变化特征,主要特征是20世纪60年代偏多,70至80年代显著减少,90年代初又略有回升,至90年代下半期每年发生变性的台风个数均极少;年际变化趋势呈现出逐年减少的特征,与西北太平洋上生成的总台风频数的变化趋势一致;西北太平洋上的变性台风多发生于夏、秋两季,特别集中于夏季与秋季的转换时期(变性比例分别达到40%及46%);秋季较夏季台风发生变性的位置整体偏东;台风变性前移动路径主要集中于朝鲜半岛以南及日本海附近,变性后路径多北上偏东;西北太平洋各月变性台风在变性后6小时内平均强度均减弱,变性后12小时内平均强度仍继续减弱,变性后强度加强的气旋的最低平均气压仅在6、7月份较变性前最明显.进一步通过对NCEP/NCAR月平均再分析资料的500 hPa高度场的EOF分析,发现夏、秋两季,纬向环流指数与台风变性频数呈显著负相关;中高纬度500hPa距平高度场在夏半年为正距平区,对应着高压,宜于冷空气入侵向高纬地区北上的台风,促进台风发生变性.     

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.     

台风Faxai(1403)低纬变性的结构演变特征分析

利用气旋相空间法(cyclone phase space,CPS)对1403号台风Faxai变性前后的环境场及结构演变特征进行分析。结果表明:相空间法能够很好地指示低纬变性台风Faxai的变性起止时间。此次过程是由减弱的台风环流与TC西北侧的短波槽结合发展产生,分析台风Faxai的结构演变特征可知,变性阶段TC低层厚度场由均匀对称分布转为非均匀分布,增大了环境斜压性,变性后B值最大达30 m,为弱的斜压非对称结构。Faxai东侧的偏南风暖湿气流与偏北风气流相交汇,使得经向位温梯度增加从而在TC东北象限形成一带状锋区,锋区正好位于南北两大风圈之间的位置。整个变性阶段Faxai西侧几乎无明显冷锋锋生,只在环流东北侧有一定程度的暖锋锋生,这与典型的锋面气旋的发展过程有所不同。变性前,TC呈现对称分布的暖核结构;变性阶段,冷空气从热带低压西侧对流层中低层下沉入侵,TC呈现左侧冷、右侧暖的非对称斜压结构,中层增温可能与槽后强的下沉气流有关。对锋生函数各分量分析发现,散度场主导了气旋周围的标量锋生,倾斜项的贡献次之,涡度场是引起旋转锋生的主要因素,其余两项可忽略不计。      

RESEARCH PROGRESS ON THE STRUCTURE AND INTENSITY CHANGE FOR THE LANDFALLING TROPICAL CYCLONES

Landfalling tropical cyclones(LTCs)include those TCs approaching the land and moving across the coast.Structure and intensity change for LTCs include change of the eye wall,spiral rain band,mesoscale vortices,low-layer shear lines and tornadoes in the envelope region of TC,pre-TC squall lines,remote rain bands,core region intensity and extratropical transition(ET)processes,etc.Structure and intensity change of TC are mainly affected by three aspects,namely,environmental effects,inner core dynamics and underlying surface forcing.Structure and intensity change of coastal TCs will be especially affected by seaboard topography,oceanic stratification above the continental shelf and cold dry continental airflow,etc.Rapid changes of TC intensity,including rapid intensification and sudden weakening and dissipation,are the small probability events which are in lack of effective forecasting techniques up to now.Diagnostic analysis and mechanism study will help improve the understanding and prediction of the rapid change phenomena in TCs.     

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.     

两类登陆热带气旋的大尺度环流特征分析

采用动态合成分析方法,对1970-2006年登陆后北上类TC（tropicalcyclone）和西行类TC各7个样本做动态合成分析和诊断,结果表明：（1）北上类TC在背景场长波槽前北移靠近中纬度斜压锋区,通过吸附运动使TC低压并入西风槽,而西行类TC背景场没有长波槽,离中纬度斜压锋区较远;（2）北上类TC登陆时存在西南低空急流水汽输送带,当其强度减弱后,TC东南侧存在东南暖湿气流作为补充,而西行类TC减弱后逐渐与之分离,且不存在东南暖湿气流作为补充;（3）北上类TC高层辐散区与高空急流边界靠近,因此增强了其向东北方向的辐散,低层由于高层动量下传,加强了低空西风,从而使TC低压环流维持,而西行类TC离高空急流边界较远;（4）北上类TC从中纬度斜压锋区获取斜压能量,其环流垂直切变增强,相对涡度差负值增大,在高空TC中心散度由大变小后又由小变大的过程中,TC发生了变性,而西行类TC没有环境能量补给,逐渐填塞消亡。因此,当一个TC登陆后,其预报移动方向、水汽输送状况、与斜压锋区的关系以及高空辐散气流等特征,可以作为初步判定登陆TC将减弱消亡还是将变性加强的可能原因。     

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.     

中国大陆上变性加强热带气旋的诊断分析

利用1979-2007年日本气象厅热带气旋年鉴资料,对在中国大陆上发生变性的热带气旋进行了统计分析,结果表明:29 a间中国大陆上发生变性的热带气旋共有16个,占登陆中国热带气旋总数的8.56％,其中8个变性后加强.利用日本JRA-25再分析资料诊断分析了这8个变性加强热带气旋的湿位涡垂直分布特征以及影响热带气旋变性发...     

APPLICATION OF AN OBJECTIVE DISCRIMINATING METHOD IN THE EVOLUTION OF TROPICAL CYCLONE "HAIMA" DURING EXTRATROPICAL TRANSITION

An objective method for discriminating the process of extratropical transition (ET) in tropical cyclones is introduced. With this method, the gridpoint output data of NWP are used to calculate three parameters: storm-relative thickness symmetry (B), low-level (-VLT) and upper-level thermal wind (-VUT). This objective method is easy to calculate and convenient for operational use. To verify the method, this paper uses the NCEP reanalysis data to identify the evolution of ET for "Haima", a tropical storm (0421) that affected the eastern part of China in 2004. The result shows that the three parameters defined with the objective method are good indicators of the ET process.     

西北太平洋热带气旋变性阶段强度变化的比较研究

选取西北太平洋上两个生命史中发生变性的热带气旋Yagi和Francisco,前者变性后有一个24小时的再增强过程,而后者则继续减弱直至消亡。利用日本气象厅提供的热带气旋资料和美国环境预报中心(NCEP)提供的FNL全球分析资料,对比分析两个TC在变性阶段的形势场,发现两者在高低层的环境场均具有明显的差异:Yagi在变性阶段其高空槽较强且在低层有一个与中纬度原先存在的温带气旋合并的过程;而Francisco在变性阶段其高空槽较弱,且变性后自行消亡。另外探讨了导致Yagi变性增强的原因,结果表明:(1)Yagi变性阶段与高空槽前的急流相互作用时,高空急流入口区左侧和出口区右侧的次级环流将产生高空辐散低空辐合的趋势,有利于低层TC低压的发展。同时,当Yagi在穿越急流的过程当中,垂直风切变的增加将导致斜压不稳定增强,低层锋区强烈发展,锋区内的斜压能量可能向TC动能转化,从而使得Yagi发展增强;(2)高空槽所对应的高层湿位涡下传可使得低层正涡度增长,从而在低层诱生出气旋性环流,有利于Yagi变性后重新发展;(3)Yagi与中纬度原先存在的温带气旋发生合并,温带气旋所带来的较高纬度冷空气的入侵增强了低层的水平温度梯度,使得低层锋区强烈发展,从Yagi以一个锋面气旋的形式而再度发展,促使其变性后进一步增强。而这些特征都是Francisco所不具备的。     

PRELIMINARY RESEARCH ON THE RELATIONSHIPS BETWEEN THE INNER CORE SIZE AND OUTER SIZE OF TROPICAL CYCLONES AND THEIR INTENSITY

Based on the Regional Spectral Model (RSM) re-analysis data from Japan Meteorological Agency (JMA) with a horizontal resolution of 20 km and a time interval of 6 h, this study works on the outer and inner core size of 2174 samples of tropical cyclones (TCs) occurring over the western North Pacific between 2001 and 2007. Some conclusions have been drawn on the basis of preliminary analysis of the TC inner core size and outer size and their relationship with TC intensity. First, the outer size increase (decrease) helps TCs intensify (weaken). Second, the enlargement (shrinking) of the inner core size helps TCs intensify (weaken) if TCs have a large inner core (with radius of maximum winds larger than 120 km). Contrarily, when TCs have small inner core (with radius of maximum winds smaller than 120 km), the enlargement (shrinking) of the inner core is good for weakening (intensifying) of TCs.     

IMPACTS OF TWO TROPICAL CYCLONES EXPERIENCING EXTRATROPICAL TRANSITION DURING NORTHWARD PROGRESSION ON THE RAINFALL OF LIAODONG PENINSULA

Both of Typhoon Winnie (9711) and Matsa (0509) underwent an extratropical transition (ET) process when they moved northward after landfall and affected Liaodong Peninsula. However, Matsa produced half as much rainfall as Winnie, although it struck Liaodong Peninsula directly while Winnie passed through the Bohai Sea. The relations between the ET processes and the precipitation over Liaodong Peninsula are examined. The result shows that the precipitation difference between Winnie and Matsa was closely related to the interactions between the westerly systems and typhoons during their ET processes. Winnie was captured by the upper westerly trough and then coupled with it when moving to the mid-latitudes, and the positive anomaly of moist potential vorticity (MPV) was transported downward from the upper troposphere over the remnant circulation of the tropical cyclone (TC). It was favorable to the interaction between tropical warm and wet air and westerly cold air, causing convective cloud clusters to form and develop. The rain belt composed of several meso-β cloud clusters over the Liaodong Peninsula, resulting in heavy rainfall. On the other hand, Matsa did not couple with any upper trough during its ET process and the positive anomaly of MPV in the upper troposphere and its downward transfer were weak. Only one meso-β cloud cluster occurred in Matsa’s rain belt during its ET process that tended to lessen rainfall over Liaodong Peninsula.     

黄渤海登陆热带气旋活动的统计分析

利用中国气象局整编的1949-2000年的热带气旋资料,分析了52年间在黄渤海沿岸登陆的热带气旋的年际和季节分布、强度、移向和移速、源地及变性等气候特征,揭示了此类气旋大部分强度较强并易发生陆上变性,变性后增强的气旋的雏持时间较长等事实,并发现了其变性的高频地区位于黄海北部至中朝边境。     

2017年冬季海洋天气评述

2017年冬季（2017年12月—2018年2月）大气环流特征为：北半球极涡呈偶极型分布,中高纬度呈4波型。12月,亚洲中东部中高纬度环流经向度较大,有利于冷空气南下。2018年1月,西伯利亚冷高压较12月更强,冷空气自北向南影响我国近海。2月,冷空气活动减弱,有温带气旋入海并发展。我国近海出现了19次8级以上大风过程,其中冷空气大风过程14次,冷空气和温带气旋共同影响的大风过程2次,冷空气与热带气旋共同影响的大风过程1次,热带气旋大风过程2次。2 m以上的海浪过程有19次,未出现2 m以上大浪的天数仅有10 d。我国近海出现6次比较明显的海雾过程,出雾区域在北部湾附近海域,出雾时间在夜间—早晨时段。西北太平洋和南海共生成4个台风。海面温度整体呈下降趋势。     

2018年春季海洋天气评述

2018年春季（3—5月）大气环流特征为：北半球极涡呈偶极型分布,中高纬度呈4波型。3月,亚洲中东部中高纬度环流呈经向型,利于冷空气南下。4月,冷空气势力减弱。5月,温带气旋活动增多。我国近海出现了15次8级以上大风过程,其中冷空气大风过程有8次,冷空气和温带气旋共同影响的大风过程有3次,入海温带气旋大风过程有2次,强对流导致雷暴大风过程2次。我国近海浪高在2 m以上的海浪过程有14次。春季共有11次比较明显的海雾过程,分别为：3月3次,4月3次,5月5次。西北太平洋和南海共生成1个台风,全球其他各大洋共有热带气旋 14个,分别为北大西洋1个、南太平洋6个、南印度洋 5个、北印度洋2个。海面温度整体呈上升趋势。     

AN OBSERVATIONAL STUDY ON DISTRIBUTION OF PRECIPITATION ASSOCIATED WITH LANDFALLING TROPICAL CYCLONES AFFECTING CHINA

In order to provide an operational reference for tropical cyclone precipitation forecast,this study investigates the spatial distributions of precipitation associated with landfalling tropical cyclones(TCs) affecting China using Geostationary Meteorological Satellite 5(GMS5)-TBB dataset.All named TCs formed over the western North Pacific that made direct landfall over China during the period 2001-2009 are included in this study.Based on the GMS5-TBB data,this paper reveals that in general there are four types of distribution of precipitation related to landfalling TCs affecting China.(a) the South-West Type in which there is a precipitation maximum to the southwestern quadrant of TC;(b) the Symmetrical South Type in which the rainfall is more pronounced to the south side of TC in the inner core while there is a symmetrical rainfall distribution in the outer band region;(c) the South Type,in which the rainfall maxima is more pronounced to the south of TC;and(d) the North Type,in which the rainfall maxima is more pronounced to the north of TC.Analyses of the relationship between precipitation distributions and intensity of landfalling TCs show that for intensifying TCs,both the maximum and the coverage area of the precipitation in TCs increase with the increase of TC intensity over northern Jiangsu province and southern Taiwan Strait,while decreasing over Beibu Gulf and the sea area of Changjiang River estuary.For all TCs,the center of the torrential rain in TC shifts toward the TC center as the intensity of TC increases.This finding is consistent with many previous studies.The possible influences of storm motion and vertical wind shear on the observed precipitation asymmetries are also examined.Results show that the environmental vertical wind shear is an important factor contributing to the large downshear rainfall asymmetry,especially when a TC makes landfall on the south and east China coasts.These results are also consistent with previous observational and numerical studies.     

西北太平洋热带气旋路径异常偏折的分类特征

基于中国台风网CMA-STI热带气旋（TC）最佳路径资料,对1949—2016年西北太平洋TC路径发生异常偏折的地理位置进行K-means聚类分析,并将其分为五个区域。对各区TC路径异常偏折的频数、方向变化、周期及时间变率等特征进行分析。结果表明:(1)不同分区TC异常偏折高频月份不同,纬度较高区域主要发生在夏季,纬度较低区域则主要发生在秋季。(2)异常右折TC在发生偏折前移向主要为西北向,偏折后为北向;异常左折TC偏折前主要为北向,偏折后主要转为西北向。(3)西北太平洋TC异常偏折总频数存在准2~4年、准3~6年的年际变化周期,其长期变化趋势表现为20世纪80年代中期之前呈增加趋势,其后呈减少趋势, 低纬区域年变化与之最为相似,中高纬区域变化趋势不明显。（4）将研究区域按5 °×5 °进一步栅格化统计TC异常偏折频数的时间变率,发现其地理分布表现为中国沿海为正、台湾岛以东海域为负的变化特征。其中沿海的增加趋势主要由异常右折增加引起,台湾岛以东洋面的减弱趋势主要由异常左折的减少引起。(5)异常右折TC强度增强的高频中心主要位于菲律宾半岛以东洋面,次中心位于中国南海中部,而强度减弱位于台湾岛西南区域;异常左折TC强度增强的高频中心位于南海中部,强度减弱中心位于我国东南沿海。