EFFECTS OF VERTICAL WIND SHEAR ON TROPICAL CYCLONE INTENSITY CHANGE

The effects of vertical wind shear on tropical cyclone(TC) intensity change are examined based on the TC data from the China Meteorological Administration and the NCEP reanalysis daily data from 2001 to 2006.First,the influence of wind shear between different vertical levels and averages in different horizontal areas are compared.The results indicate that the effect of wind shear between 200 and 850 hPa averaged within a 200-800 km annulus on TC intensity change is larger than any other calculated vertical wind shear.High-latitude and intense TCs tend to be less sensitive to the effects of VWS than low-latitude and weak TCs.TCs experience time lags between the imposition of the shear and the weakening in TC intensity.A vertical shear of 8-9 m/s(9-10 m/s) would weaken TC intensity within 60 h(48 h).A vertical shear greater than 10 m/s would weaken TC intensity within 6 h.Finally,a statistical TC intensity prediction scheme is developed by using partial least squares regression,which produces skillful intensity forecasts when potential predictors include factors related to the vertical wind shear.Analysis of the standardized regression coefficients further confirms the obtained statistical results.

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.

THE INTENSITY VARIATION OF THE SUMMER INTERTROPICAL CONVERGENCE ZONE IN WESTERN NORTH PACIFIC AND ITS IMPACT ON TROPICAL CYCLONES

Based on the satellite data from the National Oceanic and Atmospheric Administration and the NCEP/NCAR reanalysis data, the variation of the intensity of convection over the Intertropical Convergence Zone （ITCZ） in summer and its impacts on tropical cyclones are studied. In this paper, an intensity index of the ITCZ is proposed according to Outgoing Longwave Radiation （OLR） in the region of （5°-20°N, 120°-150°E） in the western North Pacific （WNP）. Then strong and weak ITCZ years are classified and different variables during the strong/weak ITCZ years are analyzed. The composite results show that the ITCZ anomaly is connected to the general atmospheric circulation and SST distribution. In the strong ITCZ years, the subtropical anticyclone weakens and shifts northward. Besides, there is salient cyclonic anomaly at the low level and anticyclonic anomaly at the high level. SST patterns in the preceding winter resemble to those of La Nifia. It could persist into the succeeding summer. However, it is opposite in the weak ITCZ years. The impact of the ITCZ anomaly on the tropical cyclone （TC） formation and track is also discussed. There are more TCs over the WNP （5°-20°N, 120°-150°E） in the strong ITCZ years and there is a significant increase in the northward recurving TCs. In the weak ITCZ years, fewer TCs occur and the frequency of the northwestward track is higher.

Reexamination of the Relationship between Tropical Cyclone Size and Intensity over the Western North Pacific

This study reexamines the correlation between the size and intensity of tropical cyclones(TCs) over the western North Pacific from the perspective of individual TCs, rather than the previous large-sample framework mixing up all TC records.Statistics show that the positive size-intensity correlation based on individual TCs is relatively high. However, this correlation is obscured by mixing large samples. The weakened correlation based on all TC records is primarily due to the diversity in the size change relative to the same intensity change among TCs, which can be quantitatively measured by the linear regression coefficient(RC) of size against intensity. To further explore the factors that cause the variability in RCs that weakens the size-intensity correlation when considering all TC records, the TCs from 2001 to 2020 are classified into two groups according to their RC magnitudes, within which the high-RC TCs have a larger size expansion than the low-RC TCs given the same intensity change. Two key mechanisms responsible for the RC differences are proposed. First, the highRC TCs are generally located at higher latitudes than the low-RC TCs, resulting in higher planetary vorticity and thus higher planetary angular momentum import at low levels. Second, the high-RC TCs are susceptible to stronger environmental vertical wind shear, leading to more prolific outer convection than the low-RC TCs. The positive feedback between outer diabatic heating and boundary layer inflow favors the inward import of absolute angular momentum in the outer region, thereby contributing to a larger size expansion in the high-RC TCs.

Thermodynamic Characteristics of Tropical Cyclones with Rapid Intensity Change over the Coastal Waters of China

In order to investigate the different thermodynamic mechanisms between rapid intensifying （RI） and rapid weakening （RW） tropical cyclones （TCs）, the thermodynamic structures of two sets of composite TCs are analyzed based on the complete-form vertical vorticity tendency equation and the NCEP/NCAR reanalysis data. Each composite is composed of five TCs, whose intensities change rapidly over the coastal waters of China. The results show that the maximum apparent heating source Q1 exists in both the upper and lower troposphere near the RI TC center, and Q1 gets stronger at the lower level during the TC intensification period. But for the RW TC, the maximum Q1 exists at the middle level near the TC center, and Q1 gets weaker while the TC weakens. The maximum apparent moisture sink Q2 lies in the mid troposphere. Q2 becomes stronger and its peak-value height rises while TC intensifies, and vice versa. The increase of diabatic heating with height near the TC center in the mid-upper troposphere and the increase of vertical inhomogeneous heating near the TC center in the lower troposphere are both favorable to the TCs＇ rapid intensification; otherwise, the intensity of the TC decreases rapidly.

Large-scale characteristics of landfalling tropical cyclones with abrupt intensity change

Data from the China Meteorological Administration and ERA-Interim are used to examine the environmental characteristics of landfalling tropical cyclones(TCs)with abrupt intensity change.The results show that,of all 657 landfalling TCs during 1979-2017,71%,70%and 65%of all landfalling TDs,TSs and TYs,respectively,intensify.Of all the 16595 samples,4.0%and 0.2%of typhoons and tropical storms,respectively,experience over-water rapid intensification(RI)process during their life cycle.Meanwhile,4.5%and 0.6%of typhoons and tropial storms,respectively,undergo overwater rapid decay(RD).These two kinds of cases,i.e.,RI and RD,are used to analyze their associated large-scale conditions.Comparisons show that the RI cases are generally on the south side of the strong western Pacific subtropical high(WPSH);warm sea surface temperatures(SSTs)and sufficient water vapor fluxes existing in RI samples is a dominant feature that is conducive to the development of TCs.Also,the moderate low-level relative vorticity is favorable for TC intensification.On the contrary,the RD TCs are located on the west side of the WPSH;significant decreasing SSTs and low-level water vapor transport may synergistically contribute to RD.Simultaneously,low-level relative vorticity seems to be unfavorable for the development of TCs.

OPERATIONAL PERSPECTIVES ON TROPICAL CYCLONE INTENSITY CHANGE PART 2:FORECASTS BY OPERATIONAL AGENCIES

This review summarizes experiences at operational centers to forecast tropical cyclone(TC) intensity change as presented to the International Workshop on Tropical Cyclones(IWTC-9) in Hawaii in 2018. Some operational forecast centers have been able to leverage advances in intensity guidance to increase forecast skill, albeit incrementally, while others have struggled to make any significant improvements. Rapid intensity changes continue to present major challenges to operational centers and individual difficult cases illustrate the forecasting challenges.It is noteworthy that the realization of a recommendation from IWTC-8 in 2014, to adapt guidance initially developed for the North Atlantic and North-East Pacific to other basins, has led to improved forecast skill of some agencies. Recent worldwide difficult cases are presented so that the research community can further investigate, potentially leading to improved intensity forecasts when similar cases are observed in the future.

OPERATIONAL PERSPECTIVES ON TROPICAL CYCLONE INTENSITY CHANGE PART 1:RECENT ADVANCES IN INTENSITY GUIDANCE

This review summarizes techniques used by operational centers to forecast tropical cyclone intensity change as presented to the International Workshop on Tropical Cyclones(IWTC-9)in Hawaii in 2018.Recent advances and major changes over the past four years are presented,with a special focus on forecasting rapid intensity changes.Although intensity change remains one of the most difficult aspects of tropical cyclone forecasting,objective guidance has shown some improvement.The greatest improvements are realized when consensus methods are utilized,especially those that blend statistical-dynamical based guidance with dynamical ocean-coupled regional models.These models become even more skillful when initialized with inner core observational data.Continued improvement and availability of intensity guidance along with associated forecaster training are expected to deliver forecasting improvements in the future.

RECENT ADVANCES IN RESEARCH AND FORECASTING OF TROPICAL CYCLONE TRACK, INTENSITY, AND STRUCTURE AT LANDFALL

This review prepared for the fourth International Workshop on Tropical Cyclone Landfall Processes(IWTCLP-4) summarizes the most recent(2015-2017) theoretical and practical knowledge in the field of tropical cyclone(TC) track, intensity, and structure rapid changes at or near landfall. Although the focus of IWTCLPIV was on landfall, this summary necessarily embraces the characteristics of storms during their course over the ocean prior to and leading up to landfall. In the past few years, extremely valuable observational datasets have been collected for TC forecasting guidance and research studies using both aircraft reconnaissance and new geostationary or low-earth orbiting satellites at high temporal and spatial resolution. Track deflections for systems near complex topography such as that of Taiwan and La Réunion have been further investigated, and advanced numerical models with high spatial resolution necessary to predict the interaction of the TC circulation with steep island topography have been developed. An analog technique has been designed to meet the need for longer range landfall intensity forecast guidance that will provide more time for emergency preparedness. Probabilistic track and intensity forecasts have also been developed to better communicate on forecast uncertainty. Operational practices of several TC forecast centers are described herein and some challenges regarding forecasts and warnings for TCs making landfall are identified. This review concludes with insights from both researchers and forecasters regarding future directions to improve predictions of TC track, intensity, and structure at landfall.

华南地区热带气旋登陆前强度突变的大尺度环境诊断分析

利用每6小时一次的NCEP再分析资料,对华南地区登陆前突然减弱和突然增强的两类热带气旋(TC)进行大尺度诊断分析,结果表明:(1)突然增强的TC位于副高的西南侧或南侧,低空有明显的西南气流卷入TC内部,而突然减弱的TC基本在副高西侧或西北侧;(2)突然增强TC的低空辐合、高空辐散均较强;(3)充足的水汽输送是TC登陆前突然增强的另一重要原因。

近海热带气旋强度突变的垂直结构特征分析

应用1949-2003年共55年的《台风年鉴》和《热带气旋年鉴》资料以及NCEP／NCAR再分析资料，给出热带气旋强度突变标准，对中国近海突然增强和突然减弱的两组热带气旋进行合成分析和对比分析。结果表明，近海热带气旋强度变化与南亚高压、副热带高压的强度变化呈反相变化关系；环境风垂直切变小于5m／s是南海近海热带气旋突然增强的必要条件，热带气旋强度突变对环境风垂直切变变化的响应时间为18-36h；热带气旋中心附近存在数值在-6～6m／s之间纬向分布的环境风垂直切变密集带，在热带气旋突然增强时刻，中心附近环境风垂直切变经向梯度最大；风垂直切变在热带气旋突然增强过程中逐渐减弱，而在热带气旋突然减弱过程中逐渐增强；热带气旋中心附近是高低层相对涡度垂直切变的强负值区，在热带气旋突然增强过程中相对涡度垂直切变逐渐减小，在突然增强时刻最小。

中国近海突然增强台风统计分析

利用NCEP/NCAR再分析资料、中国气象局热带气旋年鉴资料,对1949—2013年中国近海突然增强台风进行了统计分析,表明近海台风突然增强是一个小概率事件,占近海台风总数的10.1 9%。近海突增台风存在年代际变化,在20世纪50—70年代出现一个高峰,60年代达最高值,2000年以后明显减少;近海突增台风存在明显的时空分布特征,发生在4-10月,以8、9月最多;多数出现在南海,在黄海也有发生。对近海突增台风的大尺度环流特征作动态合成分析表明,突增台风通常处于副热带高压脊西南部;温度场上被暖脊包围;水汽输送上有持续的水汽输入台风环流;海温场上处高海温海域,同时,处于弱风垂直切变环境中,有利于台风突然增强。统计结果进一步表明,高海温、弱风垂直切变环境下,台风强度突增明显,其比例占所有突增台风总数的71.4%;当仅处于低海温环境下或仅处于强风垂直切变环境下,突然增强台风现象极少,比例都是所有突增台风总数的2.9%;但当高海温、强风垂直切变环境下,台风仍会出现突然增强现象,占所有突增台风总数的2.9%,在低海温、弱风垂直切变极端环境下,不出现突然增强台风。

超强台风“桑美”(2006)近海急剧增强过程数值模拟试验

应用PSU/NCAR非静力平衡中尺度模式MM5(V3.5)设计试验方案,对超强台风"桑美"(2006)在我国近海的急剧增强和减弱过程进行数值模拟研究,模式较好地再现了台风的路径和强度变化;通过地形敏感性试验,着重研究了地形对近海台风强度变化的影响。结果表明:(1)"桑美"强度变化与南亚高压、副热带高压的强度变化呈反相变化关系,当南亚高压和副热带高压减弱时,台风急剧增强;台风中心附近对流层高层辐散的增强导致"桑美"急剧增强,对流层中低层辐散的增强以及中层辐合的增大与"桑美"的减弱密切相关;来自海洋的暖湿气流是"桑美"发展的关键条件;低层气旋性涡旋并入台风环流是"桑美"近海急剧增强的重要原因。(2)凝结加热过程对"桑美"的近海维持和发展增强非常重要,尤其是对流层中上层凝结潜热的突然增强有利于台风在近海的急剧增强。(3)小范围地形对"桑美"在近海的强度和路径有一定影响,但作用相对较小,而且主要表现在台风登陆前后;大范围地形导致水平风场的非对称分布和台风中心附近垂直运动的异常,最终影响到台风的强度变化。

夏季西北太平洋热带辐合带的强度变化特征及其对热带气旋的影响

利用卫星资料和再分析资料讨论夏季6—8月平均热带辐合带(Intertropical Convergence Zone,简称ITCZ)的强度变化特征及其对热带气旋的影响。首先利用120～150°E,5～20°N区域平均的对外长波辐射(Outgoing Long-wave Radiation,简称OLR)定义一个夏季ITCZ强度指数,再根据这个指数划分ITCZ强弱年并进行合成分析和相关分析。结果表明:ITCZ强弱的变化与高中低层大气环流和前冬海温场有显著的关系,ITCZ强年往往伴随有副热带高压的偏北、偏弱,对流层低层出现气旋性距平风场,高层出现反气旋性距平风场,同时整个太平洋前冬的SST的分布呈现类似La Ni a的海温分布,而弱年正好相反。另一方面,ITCZ的对流强弱对热带气旋(Tropical Cyclone,简称TC)的发生和路径均有重要影响,在ITCZ强年,相应区域上的TC个数增多,同时TC路径更易于打转;而ITCZ弱年,TC个数减少,TC路径更易于向西北行。

西北太平洋不同路径下热带气旋快速加强统计特征及影响因子分析

利用2002—2011年JTWC最佳路径资料和NCEP的1°×1°全球最终分析资料以及热带气旋年鉴,分析了西北太平洋不同路径下热带气旋(TC)快速加强(RI)的时空分布特征,并对不同路径下快速加强(RITC)和缓慢加强(Non-RITC)两组TC进行合成分析和对比分析。结果表明:转向路径发生RI频率最大,且转向路径中西转向的TC最易发生RI过程;其次是东北和西北行路径。在时间分布上,各个路径下RI的月际和日变化具有不同的位相分布特征;在空间分布上,大多数RI过程发生在菲律宾和台湾岛以东洋面,西行路径在南海北部也出现较多RI过程,转向路径RI过程多发生在转向处。各个路径下RITC与Non-RITC环境场存在较明显差异,RITC对流层上层的南亚高压相对较弱,中低层副高相对较强,对流层低层存在较大的相对湿度,且湿度大值区域位于TC移动方向前侧。不同路径下的快速加强的环境影响因素也有所不同。

热带气旋经过台湾岛强度变化特征

用中国气象局整编的1949—2006年共58年的《台风年鉴》或《热带气旋年鉴》资料,将资料线性插值到1小时,挑选出经过台湾岛的热带气旋(TC),用统计分析的方法,揭示TC经过台湾岛时的强度变化特征。结果表明,TC从东侧登陆台湾岛损失的强度为西侧登陆损失强度的2倍以上;TC登陆时的路径方向与台湾中央山脉长轴的交角越接近垂直,其过岛损失的强度越小,在岛逗留的时间越短;TC登陆台湾岛东侧时强度损耗与TC登陆前其自身的强度呈正相关,而登陆台湾岛西侧则没有明显的统计规律;TC从台湾西侧登陆时不但出现强度不变或者增强的几率更大,而且强度增强也更多。

登陆华南热带气旋强度变化与大尺度环流的关系

应用UK再分析资料,采用合成分析方法,对比分析了登陆华南的登陆前迅速增强的TC(Rapid Intensifying TC,简称RITC)和迅速减弱的TC(Rapid Filling TC,简称RFTC)登陆前24 h的大尺度环流背景特征。结果表明:从外流、入流强度和范围上看,RITC的低空入流和高空外流均明显强于RFTC,两类TC高空外流强度的差异比低空入流明显,RITC的次级环流径向范围大;从外流垂直伸展高度上看,RITC的平均外流主要集中在500 hPa以上,而RFTC的平均外流比较分散,向下伸展到850 hPa;从高空流场配置看,RITC上空除西北象限外均有较强外流,而RFTC仅在东北象限有较强外流,相应的RITC和RFTC的高空辐散在范围和强度上均有明显的差异,其中RITC的高空辐散明显强于RFTC;强烈的西南季风水汽输送是登陆华南的TC登陆前突然加强的先兆条件,RITC的对流活动明显比RFTC活跃;RITC的纬向风垂直切变比RFTC小,有利于RITC的强度增强。

环境风速垂直切变对西北太平洋热带气旋强度变化的影响

利用2000---2006年中国气象局《热带气旋年鉴》和NCEP再分析日资料，对环境风垂直切变对西北太平洋热带气旋（TC）强度变化的影响进行统计分析。首先比较了不同高度层之间、不同水平区域平均的全风速垂直切变和纬向风速垂直切变对TC强度变化的影响，结果表明，全风速切变对TC强度变化的抑制作用显著大于纬向风速垂直切变；以200～800km的圆环区域平均计算的风速垂直切变与TC强度变化的负相关最显著；中高层的风速垂直切变与TC强度变化的相关优于中低层。其次，全风速切变大于8m／s后抑制TC增强，且这种抑制作用存在6．60h的滞后。全风速垂直切变大时，滞后时间较短：当全风速切变为8～9m／s（9—10m／s）时，TC强度在未来60（48）h开始减弱；当全风速切变大于10m／s时，TC在6h内开始减弱。最后，利用偏最小二乘回归建立TC强度变化的预报模型PLS—STIPSV。结果表明，加入风速垂直切变因子后对TC强度预报有所改进，并通过分析标准化回归系数进一步证实了上述的统计结果。

登陆台湾岛热带气旋强度和结构变化的统计分析

利用1949—2008年共60年的《台风年鉴》、《热带气旋年鉴》资料及CMA-STI热带气旋最佳路径数据集,2001—2008年美国联合台风警报中心(JTWC)热带气旋尺度相关资料及日本气象厅(JMA)的TBB资料,统计分析西北太平洋(包括南海)热带气旋(TC)在登陆台湾过程中强度和结构变化的基本特征,主要结论有:(1)TC登陆台湾时强度为台风及以上级别的样本数占总样本数约60%,主要出现在6—9月,东部登陆TC的强度一般比在西部登陆的强;(2)大部分TC在岛上维持6 h左右,登陆时最大风速≤5级和强度为超强台风的TC穿越台湾岛时移动比较缓慢;(3)126个登陆台湾的TC样本过岛后近中心海平面气压平均增加5.61 hPa,近中心最大风速平均减小3.58 m/s,在台湾东部地区登陆TC的衰减率比在西部登陆的大3倍左右;(4)TC在登陆台湾前6 h至离岛后6 h期间其8级和10级风圈半径均明显减小,TC形状略呈长轴为NE-SW向的椭圆状,而其最大风速的半径却逐渐增大;(5)TBB分析结果显示,TC登陆台湾前,其外围对流主要出现在南侧和西侧,结构不对称,登陆以后,TC北部及东部的对流显著发展,外围结构区域对称;但中心附近的强对流则从登陆前6 h开始逐渐减弱消失。表明TC穿越台湾过程中内核结构松散、强度减弱。

中国近海热带气旋强度突变的热力特征

应用2000-2006年的NCEP/NCAR再分析资料,通过合成分析和对比分析,利用全型垂直涡度倾向方程,研究中国近海热带气旋强度突变的热力特征。结果表明:(1)突然增强热带气旋在其中心附近对流层高低层均存在视热源Q_1的极大值中心,低层Q_1在突然增强过程中越来越强;而突然减弱热带气旋在中心附近对流层中层存在Q_1的极大值中心,而且在突然减弱过程中Q_1越来越弱。视水汽汇Q_2的极值中心在热带气旋强度变化过程中位于对流层中层,在突然增强过程中有所增大,而在突然减弱过程中有所减小。(2)Q_1的峰值高度在热带气旋突然增强和突然减弱过程中分别位于对流层高层和中层。Q_2的峰值高度在热带气旋突然增强过程中不断抬升,而在突然减弱过程中不断降低,这说明积云对流的垂直输送在热带气旋突然增强过程中起到一定作用。(3)热带气旋中心附近对流层中上层非绝热加热随着高度增加、对流层低层垂直非均匀加热的增大有利于热带气旋的突然增强,反之导致热带气旋突然减弱。