IMPACT OF SEA SPRAY ON TROPICAL CYCLONE STRUCTURE AND INTENSITY CHANGE

In this paper,the effects of sea spray on tropical cyclone(TC)structure and intensity variation are evaluated through numerical simulations using an advanced sea-spray parameterization from the National Oceanic and Atmospheric Administration/Earth System Research Laboratory(NOAA/ESRL),which is incorporated in the idealized Advanced Research version of the Weather Research and Forecast (WRF-ARW)model.The effect of sea spray on TC boundary-layer structure is also analyzed.The results show that there is a significant increase in TC intensity when its boundary-layer wind includes the radial and tangential winds,their structure change,and the total surface wind speed change.Diagnosis of the vorticity budget shows that an increase of convergence in TC boundary layer enhances TC vorticity due to the dynamic effect of sea spay.The main kinematic effect of the friction velocity reduction by sea spray produces an increment of large-scale convergence in the TC boundary layer,while the radial and tangential winds significantly increase with an increment of the horizontal gradient maximum of the radial wind, resulting in a final increase in the simulated TC intensity.The surface enthalpy flux enlarges TC intensity and reduces storm structure change to some degree,which results in a secondary thermodynamic impact on TC intensification.Implications of the new interpretation of sea-spray effects on TC intensification are also discussed.

A SIMULATION STUDY OF THE INFLUENCE OF LAND FRICTION ON LANDFALL TROPICAL CYCLONE TRACK AND INTENSITY

A quasi-geostrophic barotropic vorticity equation model is used to simulate the influences of topographic forcing and land friction on landfall tropical cyclone track and intensity. The simulation results show that tropical cyclone track may have sudden deflection when the action of topographic friction dissipation is considered, and sudden deflection of the track is easy to happen and sudden change of tropical cyclone intensity is not clear when the intensity of tropical cyclone is weak and the land friction is strong. The land friction may be an important factor that causes sudden deflection of tropical cyclone track around landfall.

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.

Decadal Variations of Intense Tropical Cyclones over the Western North Pacific during 1948–2010

Using Joint Warning Typhoon Center （JTWC） best track data during the period 1948-2010, decadal and interdecadal changes of annual category 4 and 5 tropical cyclone （TC） frequency in the western North Pacific basin were examined. By allowing all of the observed TCs in the JTWC dataset to move along the observed TC tracks in a TC intensity model, the annual category 4 and 5 TC frequency was simulated. The results agreed well with observations when the TC intensity prior to 1973 was adjusted based on time-dependent biases due to changes in measurement and reporting practices. The simulated and adjusted time series showed significant decadal （12-18 years） variability, while the interdecadal （18-32 years） variability was found to be statistically insignificant. Numerical simulations indicated that changes in TC tracks are the most important factor for the decadal variability in the category 4 and 5 TC frequency in the western North Pacific basin, while a combined effect of changes in SST and vertical wind shear also contributes to the decadal variability. Further analysis suggested that the active phase of category 4 and 5 TCs is closely associated with an eastward shift in the TC formation locations, which allows more TCs to follow a longer journey, favoring the development of category 4 and 5 TCs. The active phase corresponds with the SST warming over the tropical central and eastern Pacific and the eastward extension of the monsoon trough, thus leading to the eastward shift in TC formation locations.

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.

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.

NUMERICAL SIMULATION ON RE-INTENSIFICATION OF TROPICAL REMNANT RE-ENTERING THE SEA:A CASE STUDY

When Typhoon Toraji(2001)reached the Bohai Gulf during 1-2 August 2001,a heavy rainfall event occurred over Shandong province in China along the gulf.The Advanced Research version of the Weather Research and Forecast(WRF-ARW)model was used to explore possible effects of environmental factors,including effects of moisture transportation,upper-level trough interaction with potential vorticity anomalies,tropical cyclone(TC)remnant circulation,and TC boundary-layer process on the re-intensification of Typhoon Toraji,which re-entered the Bohai Gulf after having made a landfall.The National Centers for Environmental Prediction(NCEP)global final(FNL)analysis provided both the initial and lateral boundary conditions for the WRF-ARW model.The model was initialized at 1200 UTC on 31 July and integrated until 1200 UTC on 3 August 2001,during which Toraji remnant experienced the extratropical transition and re-intensification.Five numerical experiments were performed in this study,including one control and four sensitivity experiments.In the control experiment,the simulated typhoon had a track and intensity change similar to those observed.The results from three sensitivity experiments showed that the moisture transfer by a southwesterly lower-level jet,a low vortex to the northeast of China,and the presence of Typhoon Toraji all played important roles in simulated heavy rainfall over Shandong and remnant re-intensification over the sea,which are consistent with the observation.One of the tests illustrated that the local boundary layer forcing played a secondary role in the TC intensity change over the sea.

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.

NUMERICAL SIMULATION STUDY ON THE RAPID INTENSIFICATION OF TYPHOON HAIKUI(1211) OFF THE SHORE OF CHINA

Forecasting the rapid intensification of tropical cyclones over offshore areas remains difficult. In this article,the Weather Research and Forecast(WRF) model was used to study the rapid intensification of Typhoon Haikui(1211)off the shore of China. After successful simulation of the intensity change and track of the typhoon, the model output was further analyzed to determine the mechanism of the rapid change in intensity. The results indicated that a remarkable increase in low-level moisture transportation toward the inner core, favorable large-scale background field with low-level convergence, and high-level divergence played key roles in the rapid intensification of Typhoon Haikui in which high-level divergence could be used as an indicator for the rapid intensity change of Typhoon Haikui approximately 6 h in advance. An analysis of the typhoon structure revealed that Typhoon Haikui was structurally symmetric during the rapid intensification and the range of the eyewall was small in the low level but extended outward in the high level. In addition, the vertically ascending motion, the radial and tangential along wind speeds increased with increasing typhoon intensity, especially during the process of rapid intensification. Furthermore, the intensity of the warm core of the typhoon increased during the intensification process with the warm core extending outward and toward the lower layer. All of the above structural changes contributed to the maintenance and development of typhoon intensity.

南海台风模式对台风利奇马快速增强预报能力研究

针对台风利奇马(1909),分析中国气象局南海台风模式(CMA-TRAMS)和欧洲中期天气预报中心高分辨率模式(HRES)对台风快速增强的业务预报情况,并基于CMA-TRAMS,从水平分辨率、初始场和边界条件、物理参数化方案等角度设计并开展数值敏感性试验。CMA-TRAMS和HRES对“利奇马”增强具有一定预报能力,但对快速增强的速度预报明显低于实况,均不能满足24 h和12 h快速增强标准,可达到6 h快速增强标准。CMA-TRAMS采用3 km分辨率对“利奇马”移动路径和强度变化的预报效果优于9 km分辨率,但未改进快速增强预报效果;采用3 km嵌套9 km的方案,模式对台风快速增强的预报效果明显提升。采用MRF边界层参数化方案对台风路径、强度、快速增强的预报效果总体优于YSU方案。海温参数化结合32层垂直分辨率的初始场和边界条件的方案明显提高了快速增强预报效果,预报快速增强的频次、增强的最大速度更接近实况。分析表明,海温参数化方案使海气温差增大,在短时间内对大气、海洋之间的热量输送和交换有明显影响,海洋向大气输送的感热通量和台风内核区的潜热通量加强使内核更暖湿、气压负倾向增大,是预报效果改进的主要原因。

Evaluation of Two Initialization Schemes for Simulating the Rapid Intensification of Typhoon Lekima (2019)

Two different initialization schemes for tropical cyclone(TC)prediction in numerical models are evaluated based on a case study of Typhoon Lekima(2019).The first is a dynamical initialization(DI)scheme where the axisymmetric TC vortex in the initial conditions is spun up through the 6-h cycle runs before the initial forecast time.The second scheme is a bogussing scheme where the analysis TC vortex is replaced by a synthetic Rankine vortex.Results show that although both initialization schemes can help improve the simulated rapid intensification(RI)of Lekima,the simulation employing the DI scheme(DIS)reproduces better the RI onset and intensification rate than that employing the bogussing scheme(BOG).Further analyses show the cycle runs of DI help establish a realistic TC structure with stronger secondary circulation than those in the control run and BOG,leading to fast vortex spinup and contraction of the radius of maximum wind(RMW).The resultant strong inner-core primary circulation favors precession of the midlevel vortex under the moderate vertical wind shear(VWS)and thus helps vortex alignment,contributing to an earlier RI onset.Afterwards,the decreased vertical shear and the stronger convection inside the RMW support the persistent RI of Lekima in DIS.In contrast,the reduced VWS is not well captured and the inner-core convection is weaker and resides farther away from the TC center in BOG,leading to slower intensification.The results imply that the DI effectively improves the prediction of the inner-core process,which is crucial to the RI forecast.

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。

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。

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。

陆地摩擦对登陆热带气旋路径和强度影响的模拟研究

采用准地转正压涡度方程模式,对地形强迫和陆地摩擦作用下登陆热带气旋的路径和强度进行了模拟研究。模拟结果表明:在考虑地形摩擦耗散作用时,热带气旋可能出现路径的突然偏折;在热带气旋强度比较弱而陆地摩擦作用比较强的情况下,热带气旋路径容易发生突然偏折,且其强度的突然变化不明显;陆地摩擦作用可能是造成登陆前后热带气旋路径突然偏折的一个重要因素。

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。

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。

环境风垂直切变对热带气旋“碧利斯”的影响

用WRF中尺度数值模式,对0604号强热带风暴"碧利斯"进行了数值模拟。通过对模拟结果与实况的对比分析,表明WRF模式对强热带风暴"碧利斯"有较好的模拟能力。在整个模拟过程中可以分为两个阶段:第一为发展阶段,其强度不断增强,从热带风暴发展成强热带风暴,环境垂直切变方向从北北东风(NNE)转变为北北西风(NNW),对应涡旋内区的上升运动和降水有一个气旋性旋转,上升运动和降水较分散地分布在切变矢量的两侧;第二为维持阶段,该阶段的强度变化不大,上升运动和降水趋于集中并逐渐向切变矢量左侧偏移,这与早期的一些研究结果相一致,原因是当切变方向变化时,低层的相对流入发生了变化,低层相对流入流出的分布决定了700 hPa上热带气旋内区的上升运动和降水的分布。

台风Danas(1324)对台风Fitow(1323)影响的诊断分析及数值试验

利用FNL资料和WRF模式进行数值试验,并在此基础上研究了1324号台风Danas对1323号台风Fitow的影响特征。分析结果表明,台风Fitow登陆前、后的强度变化及其东边界与台风Danas相连的水汽输送通道的变化直接相关。台风Fitow登陆前,其水汽输入主要是由副热带高压西南侧环境场的偏东气流带来,而上游台风Danas截取了环境场输送给台风Fitow的水汽,抑制了Fitow的发展。但在台风Fitow登陆后,台风Danas环境场的水汽反过来起到转运作用,增加了Fitow的水汽收入,加剧其北侧的强降水。台风Danas以水汽通道为纽带影响台风Fitow登陆前的强度和在陆上非对称降水的分布。此外,台风Danas的北上影响了副热带高压的强度,使其东退且改变台风Fitow的引导气流,影响了Fitow的登陆位置;并在Fitow登陆后施加了偏南引导气流,令其逆时针打转。