An Application of the Adjoint Method to a Statistical-Dynamical Tropical-Cyclone Prediction Model （SD-90） Ⅱ： Real Tropical Cyclone Cases

In the first paper in this series, a variational data assimilation of ideal tropical cyclone （TC） tracks was performed for the statistical-dynamical prediction model SD-90 by the adjoint method, and a prediction of TC tracks was made with good accuracy for tracks containing no sharp turns. In the present paper, the cases of real TC tracks are studied. Due to the complexity of TC motion, attention is paid to the diagnostic research of TC motion. First, five TC tracks are studied. Using the data of each entire TC track, by the adjoint method, five TC tracks are fitted well, and the forces acting on the TCs are retrieved. For a given TC, the distribution of the resultant of the retrieved force and Coriolis force well matches the corresponding TC track, i.e., when a TC turns, the resultant of the retrieved force and Coriolis force acts as a centripetal force, which means that the TC indeed moves like a particle; in particular, for TC 9911, the clockwise looping motion is also fitted well. And the distribution of the resultant appears to be periodic in some cases. Then, the present method is carried out for a portion of the track data for TC 9804, which indicates that when the amount of data for a TC track is sufficient, the algorithm is stable. And finally, the same algorithm is implemented for TCs with a double-eyewall structure, namely Bilis （2000） and Winnie （1997）, and the results prove the applicability of the algorithm to TCs with complicated mesoscale structures if the TC track data are obtained every three hours.     

DYNAMIC ANALYSIS OF INHERENT CAUSES FOR DEVIATION OF TROPICAL CYCLONE TRACKS

To have a clearer picture of mechanisms responsible for the deviation of tropical cyclone (to be simplified as TC hereafter) tracks, the current work assumes the TC as a circular vortex with a radius of R. A general motion equation of TC is then determined by averaging its horizontal motion equation over the sentire region of TC. In the meantime, with the moving track of TC assumed as a characteristic arc, the curvature equation is derived for the track of movement and patterns of its deviation due to TC structure and variation are discussed. The result shows that the scale, size, maximum wind speed and radius are factors causing the deviation of TC tracks. In addition, asymmetric structure of TC is also important for the deviation of tracks. The results, achieved with hypothesis, agree with facts in some cases but disagree with them in others, which are to be verified with more observations or numerical simulations.     

THE EFFECTS OF BOGUS TYPHOON AND OBSERVED OCEANIC DATA ON THE ABILITY OF T_(213)L_(31) TO PREDICT TC TRACK

The abilities of typhoon (TC) track prediction by a medium-range forecast model T213L31 at National Meteorological Center are analyzed and its ability to improve its TC forecasts is discussed. The results show that about 57% of the TCs could be predicted by T213L31 but the initial position errors are large.The 43% area without the prediction of TC tracks is concentrated between 13°N and 20°N and east of 120°E and lack of conventional observation data is the main reason for the absence of TC prediction in this area.The adding of bogus TC could improve the ability of TC track prediction when there is no TC vortex in the analysis field, but could only have positive effects on the short-range TC track prediction when there is TC vortex in the T213L31 analysis field.     

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

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.     

ANALYSIS OF THE CLIMATIC CHARACTERISTICS OF LANDING TROPICAL CYCLONES IN EAST CHINA

The intensity, landing time, track trend and intensity variation of tropical cyclones (TCs) after landfall are analyzed using the TCs data (of best track from the China Meteorological Administration) between 1949 and 2006 for the western North Pacific and South China Sea. The trend differences of track and intensity between the TCs that directly land in East China and those making the second landfall in East China after landing in Taiwan Island are categorically discussed. The results show that the first kind of landing TCs are more likely to go northward or turn while the second kind of TCs have a larger tendency to keep going northwest. The intensity of the first kind of TCs is more persistent than the second one. There is a higher percentage for the intensity to be weakened significantly if the TCs keep going west to northwest or southwest after landing.     

Three-Dimensional Structure and Low-Level Features of the North Pacific Storm Track from 1999 to 2005

A storm track is a region in which synoptic eddy activities are statistically most prevalent and intense. At daily weather charts, it roughly corresponds to the mean trajectories of cyclones and anticyclones. In this paper, the recent QuikSCAT （Quick Scatterometer） satellite sea winds data with a 0.5°×0.5° horizontal resolution, and the NCEP （National Centers for Environmental Prediction） 10-m height Gaussian grid wind data and pressure-level reanalysis data, are employed to document the spatial structure of the North Pacific storm track in winter （January） and summer （July） from 1999 to 2005. The results show that in winter the North Pacific storm track is stronger, and is located in lower latitudes with a distinct zonal distribution. In summer, it is weaker, and is located in higher latitudes. Based on the horizontal distributions of geopotential height variance at various levels, three-dimensional schematic diagrams of the North Pacific storm track in winter and summer are extracted and presented. Analyses of the QuikSCAT wind data indicate that this dataset can depict the low-level storm track features in detail. The double storm tracks over the Southern Oceans found by Nakamura and Shimpo are confirmed. More significantly, two new pairs of low-level storm tracks over the North Pacific and the North Atlantic are identified by using this high-resolution dataset. The pair over the North Pacific is focused in this paper, and is named as the ＂subtropical storm track＂ and the ＂subpolar storm track＂, respectively. Moreover, statistical analyses of cyclone and anticyclone trajectories in the winters of 1999 to 2005 reveal as well the existence of the low-level double storm tracks over the North Pacific.     

FACTORS AFFECTING THE TRACK OF TROPICAL CYCLONES AFTER LANDFALL IN EASTERN CHINA

The correlation and composite analysis are carried out in this paper to study major factors affecting the track of tropical cyclones (TCs) after their landfall in the east of China. The mid-tropospheric environmental steering flow is found to dominate the movement of a TC even after landfall, with the inertia and Coriolis force two other subordinates. A key region is discovered covering the east of China and Yellow Sea, in which the environmental flow significantly affects the movement of TCs making landfall in this part of China. When the subtropical high in this region strengthens and extends westward, accompanied by northward shrink of the westerly trough, the TC tends to move westward after landfall and disappear inland. However, when the subtropical high in this region weakens and shrinks eastward, accompanied by southward push of the westerly trough, the TC tends to recurve after landfall and re-enter the sea at a location to the north of the site of landfall. The environment before the landfall of a TC has little impact on its post-landfall track, which is sensitive to the environmental change 12 – 24 hours after landfall. A 6-hour lag is found between the environmental change and the movement of a TC after landfall.     

ANALYSIS OF TROPICAL CYCLONE PRECIPITATION FOR DIFFERENT INTENSITY CLASS IN NORTHWEST PACIFIC WITH TRMM DATA

Combined with TRMM products and Tropical Cyclone (TC) best track data in Northwest Pacific from 1 January 2003 to 31 December 2009, a total of 118 TCs, including 336 instantaneous TC precipitation observations are established as the TRMM TC database, and the database is stratified into four intensity classes according to the standard of TC intensity adopted by China Meteorological Administration (CMA): Severe Tropical Storm (STS), Typhoon (TY), Severe Typhoon (STY) and Super Typhoon (SuperTY). For each TC snapshot, the mean rainfall distribution is computed using 10-km annuli from the TC center to a 300-km radius, then the axisymmetric component of TC rainfall is represented by the radial distribution of the azimuthal mean rain rate; the mean rain rates, rain types occurrence and contribution proportion are computed for each TC intensity class; and the mean quadrantal distribution of rain rates along TCs motion is analyzed. The result shows that: (1) TCs mean rain rates increase with their intensity classes, and their radial distributions show single-peak characteristic gradually, and furthermore, the characteristics of rain rates occurrence and contribution proportion change from dual-peak to single-peak distribution, with the peak rain rate at about 5.0 mm/h; (2) Stratiform rain dominate the rain type in the analysis zone, while convective rain mainly occurred in the eye-wall region; (3) The values of mean rain rate in each quadrant along TCs motion are close to each other, relatively, the value in the right-rear quadrant is the smallest one.     

Experiments of DSAEF_LTP Model with Two Improved Parameters for Accumulated Precipitation of Landfalling Tropical Cyclones over Southeast China

The Dynamical-Statistical-Analog Ensemble Forecast model for landfalling tropical cyclones (TCs) precipitation (DSAEF_LTP) utilises an operational numerical weather prediction (NWP) model for the forecast track, while the precipitation forecast is obtained by finding analog cyclones, and making a precipitation forecast from an ensemble of the analogs. This study addresses TCs that occurred from 2004 to 2019 in Southeast China with 47 TCs as training samples and 18 TCs for independent forecast experiments. Experiments use four model versions. The control experiment DSAEF_LTP_1 includes three factors including TC track, landfall season, and TC intensity to determine analogs. Versions DSAEF_LTP_2, DSAEF_LTP_3, and DSAEF_LTP_4 respectively integrate improved similarity region, improved ensemble method, and improvements in both parameters. Results show that the DSAEF_LTP model with new values of similarity region and ensemble method (DSAEF_LTP_4) performs best in the simulation experiment, while the DSAEF_LTP model with new values only of ensemble method (DSAEF_LTP_3) performs best in the forecast experiment. The reason for the difference between simulation (training sample) and forecast (independent sample) may be that the proportion of TC with typical tracks (southeast to northwest movement or landfall over Southeast China) has changed significantly between samples. Forecast performance is compared with that of three global dynamical models (ECMWF, GRAPES, and GFS) and a regional dynamical model (SMS-WARMS). The DSAEF_LTP model performs better than the dynamical models and tends to produce more false alarms in accumulated forecast precipitation above 250 mm and 100 mm. Compared with TCs without heavy precipitation or typical tracks, TCs with these characteristics are better forecasted by the DSAEF_LTP model.     

NUMERICAL ANALYSIS ON THE ROLE OF TROPICAL STORM NAMTHEUN IN THE UNUSUAL TRACKS OF THREE TROPICAL CYCLONES IN 2010

Tropical cyclones (TCs) Lionrock, Kompasu, and Namtheun were formed successively within 40 hours in 2010. Over the next several days afterwards, these TCs exhibited unusual movements which made operational prediction difficult. Verifications are performed on the forecasts of the tracks of these TCs with six operational models, including three global and three regional models. Results showed that the trends of TC tracks could be reproduced by these models, whereas trajectory turning points and landfall locations were not simulated effectively. The special track of Lionrock should be associated with its direct interaction with Namtheun, according to a conceptual model of binary TC interaction. By contrast, the relation between Kompasu and Namtheun satisfied the criteria for a semi-direct interaction. Numerical experiments based on the Global and Regional Assimilation and Prediction System-Tropical Cyclone forecast Model (GRAPES-TCM) further confirmed the effect of Namtheun on the unusual tracks of Lionrock and Kompasu. Finally, the physical mechanism of binary TC interaction was preliminarily proposed.     

An Application of the Adjoint Method to a Statistical-Dynamical Tropical-Cyclone Prediction Model (SD-90) Ⅱ: Real Tropical Cyclone Cases

In the first paper in this series, a variational data assimilation of ideal tropical cyclone (TC) tracks was performed for the statistical-dynamical prediction model SD-90 by the adjoint method, and a prediction of TC tracks was made with good accuracy for tracks containing no sharp turns. In the present paper, the cases of real TC tracks are studied. Due to the complexity of TC motion, attention is paid to the diagnostic research of TC motion. First, five TC tracks are studied. Using the data of each entire TC track, by the adjoint method, five TC tracks are fitted well, and the forces acting on the TCs are retrieved. For a given TC, the distribution of the resultant of the retrieved force and Coriolis force well matches the corresponding TC track, i.e., when a TC turns, the resultant of the retrieved force and Coriolis force acts as a centripetal force, which means that the TC indeed moves like a particle; in particular, for TC 9911, the clockwise looping motion is also fitted well. And the distribution of the resultant appears to be periodic in some cases. Then, the present method is carried out for a portion of the track data for TC 9804, which indicates that when the amount of data for a TC track is sufficient, the algorithm is stable. And finally, the same algorithm is implemented for TCs with a double-eyewall structure, namely Bilis (2000) and Winnie (1997),and the results prove the applicability of the algorithm to TCs with complicated mesoscale structures if the TC track data are obtained every three hours.     

The Effect of Asymmetric Wind Structures of Tropical Cyclones on Their Tracks

1. IntroductionStudies on the tracks of tropical cyclones (TCs)have been one of the core problems in TC dynamics.In earlier researches, TCs are often viewed aspoint vortices, Rankine vortices. For example, Yeh(1950) and Kuo (1969), adopting dynamical methods,studied the motions of TCs and the effects of environ-ments. Their findings indicated that the motion tracksof TCs are characterized by cyclonic trochoidal oscil-lations.Syono (1955), by solving the barotropic vorticityequation, obta…     

西北太平洋TC移动速度异常及预报误差特征的分析

利用国家气象局和上海台风研究所(CMA-STI)整编的西北太平洋1970—2009年热带气旋(TC,Tropical Cyclone)及TC最佳路径数据集和2005—2010年的TC路径预报误差资料,应用百分位法,确定TC移动速度异常指标,分析了40 a来西北太平洋TC移动速度及其变化异常发生的时空分布特征,研究了TC速度预报误差对路径预报误差的影响及其与大尺度引导气流之间的关系。结果显示:1)西北太平洋TC移速及移速变化累积概率达95%(5%)分位数的阈值分别为10.8 m·s~(-1)(1.43 m·s~(-1))和2.42m·s~(-1)(-1.72 m·s~(-1))。2)快速移动及加速的TC大都出现在日本海地区,而缓慢的和减速移动TC主要出现在南海区域。3)TC移动速度异常的季节变化表现为,快速移动的TC在5月出现的频率达到最高,缓慢移动的TC在10月频率达到最高,加速移动的TC在6月频率达到最高。4)近6 a的TC移速预报误差对TC路径预报误差的贡献平均约为41.6%。5)对TC路径预报误差偏大,且移速预报误差贡献大的个例分析显示,该个例大尺度环境引导气流偏弱使TC移动速度偏慢。而如果预报的大尺度环境引导气流偏强,使预报的TC移速偏快,那么就容易导致大的路径预报误差。     

The relationships between tropical cyclone tracks and local SST over the western North Pacific

Tropical Cyclone (TC) tracks over the western North Pacific (WNP) during 1949–2007, obtained from China Meteorological Administration/Shanghai Typhoon institute, are classified into three track types. These types are the main pathways by which TCs influence the coast of East Asia. The relationships between local sea surface temperature (SST) in WNP and TC tracks are revealed. Results show that the local SST plays an important role in TC tracks, though the relationships between local SST and the frequencies ...     

Statistical distributions of singular vectors for tropical cyclones affecting Korea over a 10-year period

To investigate the statistical sensitivity distributions of tropical cyclone (TC) forecasts over the Korean Peninsula, total energy (TE) singular vectors (SVs) were calculated and evaluated over a 10-year period. TESVs were calculated using the fifth-generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model (MM5) and its tangent linear and adjoint models with a Lanczos algorithm over a 48-h period. Chosen cases were 21 TCs that affected the Korean Peninsula among 230 TCs that were generated in the western North Pacific from 2001 to 2010. Sensitive regions indicated by TESVs were mainly located near mid-latitude troughs and TC centers but varied depending on TC track and environmental conditions such as subtropical high and mid-latitude trough. The cases were classified into three groups by clustering TC tracks based on the finite mixture model. The two groups that passed through the western and southern sea of the Korean Peninsula had maximally sensitive regions in the mid-latitude trough and largely sensitive regions around the TC center, while the other group that passed straight through the eastern sea of the Korean Peninsula had maximally sensitive regions near the northeastern region of the TC center. Vertically, the former two clustered groups had the westerly tilted TESVs and potential vorticity structures under the mid-latitude troughs at the initial time, indicating the TCs were in a baroclinic environment. Conversely, the straight-moving TCs were not in a baroclinic environment. Based on the results in the present study, the TCs moving toward a fixed verification region over the Korean Peninsula have different sensitivity regions and structures according to their moving tracks and characteristic environmental conditions, which may provide guidance for targeted observations of TCs affecting the Korean Peninsula.     

GRAPES-TCM业务试验结果分析

应用数值模式GRAPES-TCM2.1（GT2.1）的升级版GRAPES-TCM2.6（GT2.6）对2005年西北太平洋及南海热带气旋路径进行了后报试验和性能检验。在检验预报路径时考察了平均距离误差、系统偏差、距离误差的地理分布和平均移速误差,并在总体检验的基础上,根据与热带气旋路径相关的重要因素（路径类型、强度、有无登陆过程以及有无移向或移速突变）进行了分类检验。结果表明,GT2.6表现出良好的预报性能,其对所有样本的24、48和72小时平均距离误差分别为135.8、230.7和336.0km,但前12小时距离误差较大（近100km）。用系统偏差订正GT2.6对转向类样本的预报路径可获得明显的改进。GT2.6对130°E以西区域特别是靠近我国东南沿海的热带气旋路径预报表现较好。GT2.6的总体平均预报移速在48小时内都比最佳路径移速偏快,初始的12小时内偏快最多（近1m·s^-1）,在48小时后转为偏慢。GT2.6和GT2.1的前48小时路径预报性能的稳定性相当,总体误差特征相似,GT2.6对所有样本的48小时预报有显著改进,平均距离误差减小17km。初步分析表明,对GT2.6的改进应主要集中在数值预报初始场的改进方面,比如在初始化过程中采用较准确的涡旋分离方案、加入引导向量改进初始移动以及适当考虑加入非对称Bogus涡旋;扩大预报区域,并适当选取预报区域起始位置等。     

Estimating the Predictability Limit of Tropical Cyclone Tracks over the Western North Pacific Using Observational Data

In this study, the nonlinear local Lyapunov exponent (NLLE) approach was used to quantitatively determine the predictability limit of tropical cyclone (TC) tracks based on observed TC track data obtained from the Joint Typhoon Warning Center. The results show that the predictability limit of all TC tracks over the whole western North Pacific (WNP) basin is about 102 h, and the average lifetime of all TC tracks is about 174 h. The predictability limits of the TC tracks for short-, medium-, and long-lived TCs are approximately 72 h, 120 h, and 132 h, respectively. The predictability limit of the TC tracks depends on the TC genesis location, lifetime, and intensity, and further analysis indicated that these three metrics are closely related. The more intense and longer-lived TCs tend to be generated on the eastern side of the WNP (EWNP), whereas the weaker and shorter-lived TCs tend to form in the west of the WNP (WWNP) and the South China Sea (SCS). The relatively stronger and longer-lived TCs, which are generated mainly in the EWNP, have a longer travel time before they curve northeastwards and hence tend to be more predictable than the relatively weaker and shorter-lived TCs that form in the WWNP region and SCS. Furthermore, the results show that the predictability limit of the TC tracks obtained from the best-track data may be underestimated due to the relatively short observational records currently available. Further work is needed, employing a numerical model to assess the predictability of TC tracks.     

T213与T639模式热带气旋预报误差对比

应用国家气象中心全球谱模式T213L31(简称T213) 及其升级版本T639L60(简称T639) 对2009—2010年西北太平洋热带气旋数值预报的结果进行对比。结果表明:T213与T639模式24～120 h预报平均距离误差基本相近,但由于T639模式分辨率较高,T639模式的热带气旋强度预报明显好于T213模式。从分类误差来看,T639模式对于西北行登陆及转向热带气旋的路径预报好于T213模式,但对西行及北上热带气旋预报误差偏大。对于异常路径热带气旋预报,T639模式能较好预报环流形势的突然调整,对路径突变的热带气旋预报比T213模式有明显优势;从登陆类热带气旋预报的移向误差来看,T213模式存在东北偏北向系统性偏差,T639模式存在东北偏东向系统性偏差。     

Sensitivity Experiments on the Poleward Shift of Tropical Cyclones over the Western North Pacific under Warming Ocean Conditions

Recent studies found that in the context of global warming, the observed tropical cyclones (TCs) exhibit significant poleward migration trend in terms of the mean latitude where TCs reach their lifetime-maximum intensity in the western North Pacific (WNP). This poleward migration of TC tracks can be attributed to not only anthropogenic forcing (e.g., continuous increase of sea surface temperature (SST)), but also impacts of other factors (e.g., natural variability). In the present study, to eliminate the impacts of other factors and thus focus on the impact of unvaried SST on climatological WNP TC tracks, the mesoscale Weather Research and Forecasting (WRF) model is used to conduct a suite of idealized sensitivity experiments with increased SST. Comparisons among the results of these experiments show the possible changes in climatological TC track, TC track density, and types of TC track in the context of SST increase. The results demonstrate that under the warmer SST conditions, the climatological mean TC track systematically shifts poleward significantly in the WNP, which is consistent with the previous studies. Meanwhile, the ocean warming also leads to the decreased (increased) destructive potential of TCs in low (middle) latitudes, and thus northward migration of the region where TCs have the largest impact. Further results imply the possibility that under the ocean warming, the percentage of TCs with westward/northwestward tracks decreases/increases distinctly.