Tropical cyclone track predictability

Summary In this study an attempt is made to estimate theinherent limits to tropical cyclone mean absolute track position errors out to 72 hours ahead and to compare these estimates with the position errors currently being obtained inpractice at weather centres around the world. A knowledge of the magnitude of the difference between the lower limit to predictability and that being achieved with state-of-the-art numerical weather prediction (NWP) models is of vital importance. A small difference would indicate that there is little further need for continued initiatives in the prediction of tropical cyclone tracks. On the other hand, a large difference would imply either that the problem requires continued emphasis or if there has been no significant trend towards reducing the forecast track errors, that present research and development techniques need to be extended or new procedures developed.It was found that the difference between the inherent and practical limits of tropical cyclone track position errors is presently about 35 to 40 per cent for advanced baroclinic NWP systems, a moderate to large difference, and one that is almost invariant between tropical cyclone basins. For simpler models, such as barotropic models, the difference is closer to 45 per cent but is again almost invariant. As far as the authors are aware, these are the first estimates of the lower bounds of tropical cyclone track predictability. Finally, very recent research studies with emerging range of high quality data, high density data sources, improved models and new data assimilation techniques suggest that the difference possibly is now down to about 30 to 35 per cent. This value is encouragingly small but still large enough to continue active research programs in improving tropical cyclone motion prediction. Much of the forecast track errors now come from major forecast errors associated with tropical cyclones that follow erratic tracks.With 3 Figures     

Effect of Baroclinicity on Vortex Axisymmetrization.Part Ⅱ：Baroclinic Basic Vortex

The effect of baroclinicity on vortex axisymmetrization is examined within a two-layer dynamical model.Three basic state vortices are constructed with varying degrees of baroclinicity：（i） barotropic,（ii） weak baroclinic,and （iii） strong baroclinic.The linear and nonlinear evolution of wavenumber-2 baroclinic disturbances are examined in each of the three basic state vortices.The results show that the radial propagating speed of the vortex Rossby wave at the lower level is larger with the stronger baroclinicity,resulting in a faster linear axisymmetrization process in the stronger baroclinic vortex.It is found that the nonlinear axisymmetrization process takes the longest time in the strongest baroclinic vortex among the three different basic vortices due to the weaker kinetic energy transfer from asymmetric to symmetric circulations at the lower level.A major finding in this study is that the same initial asymmetric perturbation can have different effects on symmetric vortices depending on the initial vortex baroclinicity.In numerical weather prediction models,this implies that there exists a sensitivity of the subsequent structural and intensity change solely due to the specification of the initial vertical shear of the tropical cyclone vortex.     

Impact of vortex-removal from environmental flow in cyclone track prediction using Lagrangian advection model

In the present study, a new approach is discussed to find out the residual steering flow from the high-resolution global Numerical Weather Prediction (NWP) model-forecasted wind fields, which have been used in the Lagrangian advection model to determine the track of tropical cyclones formed in the Indian Ocean. The Lagrangian advection model is newly developed model and conceptually closer to the dynamical models, which utilizes environmental steering flow and the effect due to earth’s rotation (the beta-effect) to determine the motion of cyclone. In this approach, the effect of environmental flow on the cyclone track is examined by removing the existing cyclone vortex from the steering flow which is determined by potential vorticity approach. A new approach based on vortex pattern matching has been used to identify the cyclone vortex and to remove it from the steering flow. The tracks of five tropical cyclones (viz., Nargis, Khai_Muk, Nisha, Aila and Phyan) which were formed in the North Indian basin during the period 2008–2009 have been generated by the Lagrangian advection model using the proposed scheme. The position errors were computed with respect to the Joint Typhoon Warning Center (JTWC) best track analysis positions and compared with that of without-vortex-removal scheme. The results show that the mean track errors for five cyclones are reduced by 6–35?% for 12–72?h forecast in case of vortex-removal scheme as compared to the without-vortex-removal scheme.     

人造热带气旋垂直结构问题初探

在人造热带气旋中引入了倾斜垂直结构，并将其用于西折后9806号热带气旋登陆路径的数值模拟。结果表明：应用该方案所得到的模拟结果优于应用正压结构的人造热带气旋方案的模拟结果要准确。因此，此类热带气旋的预报中，其初始涡旋可采用倾斜垂直结构，从而提出改进热带气旋初始涡旋的一种新思路。若对其作进一步研究、推广，则具有广阔的业务应用前景。     

2002年西北太平洋和南海热带气旋路径主客观预报评价

利用目前中央气象台热带气旋路径实时业务预报中使用的各种主客观预报产品资料,对2002年西北太平洋和南海热带气旋路径实时业务预报中的主客观预报进行对比分析检验。结果显示：虽然在整体上主观预报要优于客观模式的结果,但客观模式的预报能力已接近主观预报,有时甚至还好于主观预报,特别是48小时以上时效的客观模式较主观预报具有一定的优势;而在客观模式中,全球模式优于台风模式;热带气旋路径数值模式产品的使用对提高热带气旋路径业务预报水平具有十分重要的作用。     

On the bogussing of tropical cyclones in numerical models: A comparison of vortex profiles

Summary At the resolutions currently in use, and with the sparse oceanic data coverage, numerical analyses cannot adequately represent tropical cyclone circulations for use in numerical weather prediction models. In many cases there is no circulation present at all. Most numerical weather prediction centers therefore employ a bogussing scheme to force a tropical cyclone vortex into the numerical analysis. The standard procedure is to define a synthetic data distribution based on an analytically prescribed vortex, which is passed to the analysis scheme as a set of high quality observations.In this study, four commonly used bogus vortices are examined by comparing resultant forecast tracks in an environment at rest, and in a background flow that simulates a typical monsoon trough-subtropical ridge structure. There are three main findings, each of which has significance for operational tropical cyclone track prediction. First, great care is needed in the choice of the characteristics of the bogus vortex, such as the radius and magnitude of the maximum wind. Second, the tropical cyclone trajectories can be very sensitive to their initial position in the idealised environment. Third, the bogus vortex can substantially influence the environment, especially over longer time periods and for vortices of larger size.With 9 Figures     

Evaluation of the ECMWF 32-day ensemble predictions during 2009 season of western North Pacific tropical cyclone events on intraseasonal timescales

The performance of the ECMWF 32-day ensemble predictions of western North Pacific tropical cyclone events (formation plus track) made once a week during the 2009 season is evaluated with the same procedures as for the 2008 season. Seventeen of the 23 tropical cyclones during the 2009 season occurred during multiple storm scenarios that are more difficult to predict, and many of the deficient track predictions involved unusual and rapidly changing tracks that typically involve interactions with adjacent synoptic circulations that are not predictable on intraseasonal timescales (10–30 days). Such incorrect predictions of the duration and tracks of these multiple cyclones were found to degrade the performance in predicting subsequent tropical cyclone formations and tracks during the 32-day integration. Predominantly northward tracks throughout the life cycle tended to be less predictable on intraseasonal timescales. Given these caveats, the overall performance of the ECMWF ensemble for the 12 typhoons was more successful than during the 2008 season. However, the performance for three tropical storms during the 2009 season was less successful due to the difficult track forecast scenarios. A surprisingly good performance was found in predicting the formation location and early track segments of eight minimal tropical storms or tropical depressions. The less satisfactory aspect for many of the late season tropical depressions was that the ECMWF ensemble continued to predict member vortices for extended periods after the system had actually dissipated.     

Tropical cyclone track and intensity prediction: The generation and assimilation of high-density, satellite-derived data

Summary The impact of recent scientific and technological advances in tropical cyclone track, intensity and structure modeling is discussed. Since the early 1990s, developments have occurred in remote sensing, data assimilation procedures, numerical models and high performance computing. In particular, there is now quasi-continuous high spatial and temporal resolution data coverage over the previously data-sparse oceans where tropical cyclones spend most of their life cycles. There has been a rapid development of data assimilation methodologies capable of using these data to initialize high-resolution prediction models. Model developments have reached a stage of maturity where the representation of many of the physical processes necessary for improved tropical cyclone track and intensity prediction are now included. Finally, available computer power has reached the teraflop range. Most operational centers have high performance computers capable of tropical cyclone modeling at resolutions necessary for skillful track and intensity simulations. This article focuses on combining all of the above developments in a tropical cyclone data analysis and prediction system. The system has produced statistically significant reductions in the mean forecast error statistics for tropical cyclone track predictions and resulted in far more realistic simulations of tropical cyclone intensity and structure. A large number of tropical cyclones have been modeled, with emphasis on those classified as being “difficult” storms to predict accurately. These difficult storms are most responsible for rapidly growing forecast errors. Our results are illustrated by case studies of such tropical cyclones. Received October 9, 2001 Revised December 28, 2001     

Improved tropical cyclone track predictions using error recycling

Summary Errors produced by a nonlinear predictive scheme contain information about both the observations and the prediction system. Therefore, its error history would be expected to contribute to increasing the skill of the predictions if it is included in the forecast. In this study an error recycling procedure is developed for tropical cyclone track prediction. Errors are defined here as differences between the model forecast and the best track position. Error histories are incorporated into a nonlinear analogue, or simplex, forecast scheme and applied to tropical cyclone track prediction, using the archives of observed position data associated with the forecast errors. Various forecast experiments of the cyclone tracks are performed: standard simplex predictions using observed positions only; simplex predictions improved by error forecasts based on libraries of both observations and the recycled forecast errors; and, finally, predictions that include NWP-model forecasts and their errors as predictors. The resulting gains in skill of predictions out to 72 hours ahead are found to be substantial. Received August 12, 1999 Revised November 5, 1999     

Critical time requirements for operational use of deterministic and ensemble tropical cyclone track forecasts

Tropical cyclone track forecasts have been improved, and forecast intervals have been extended to five days, owing to improved global and regional numerical model guidance. Critical time requirements that must be met for operational use of the deterministic model track forecasts are summarized for the U.S. and other selected non-U.S. tropical cyclone warning centers. One of the most accurate deterministic model forecasts from the European Center for Medium-range Weather Forecasts arrives too late to be used with other models at the + 6 h warning time, and thus is at least 12 h old before it can be operationally used. The time-critical nature of the tropical cyclone warning system is a major obstacle to operational use of single-model, or proposed multi-model, ensemble prediction system (EPS) mean and spread information, which is 12 h (or 18 h) delayed. This EPS mean and spread must also be superior to the mean and spread of the consensus of deterministic models that are available six hours earlier. These requirements must be met before the EPS tropical cyclone tracks will be operationally useful in specifying the uncertainty in the official track forecasts, which is the next challenge in tropical cyclone track warnings.     

Ensemble forecasting of tropical cyclone motion using a barotropic model. Part III: Combining perturbations of the environment and the vortex

Summary This is the third in a series of papers to investigate the applicability of the ensemble forecasting (EF) technique in the prediction of tropical cyclone (TC) motion. In the previous two papers, either the environment or the vortex was perturbed and the other unperturbed component was then merged onto the perturbed component at the initial time. In the present study, the separately-perturbed environment and vortex fields are combined at this time. The objective is to determine the extent to, and the synoptic pattern under which, such a combination can improve the TC motion forecast compared with perturbing only one component.The study makes use of the same barotropic model as the previous studies and the same dataset – 66 cases from the Tropical Cyclone Motion Experiment TCM90. Perturbations of the environment and those of the vortex are first generated separately using the breeding of growing modes (BGM) method, and then combined at the initial forecast time. The performance of this combined scheme, labeled as BGMC, is then compared with that of the scheme with only the environment or the vortex perturbations (termed BGME and BGMV, respectively).The BGMC distribution of ensemble forecast tracks are found to be basically similar to those in BGME but the spread is reduced. Some poor forecast members in BGME also become close to the best track in BGMC. The relative skill scores of the BGMC forecasts relative to the best track are almost all positive but those under the perfect model assumption are negative because the control forecast is better. While both BGMC and BGME schemes can improve TC forecast track under transition synoptic conditions, BGMC also achieve a higher success rate under complicated vortex and environment interactions. In general, the BGMC scheme is superior to the BGMV scheme.     

INNER AND OUTER STRUCTURES OF TROPICAL CYCLONE──Ⅰ.BAROTROPIC MODEL

A tropical cyclone is divided into inner and outer regions in which temporal and spacial scales and physical quantities are all different. Making use of the scale analysis and perturbation methods the governing equations of barotropic and baroclinic models are obtained for the two regions, respectively. In the two models the stream and pressure fields are solved analytically and the obtained structrues are approximately consistent with observations. It is shown that there are different governing equations for the inner and outer regions of the tropical cyclone. The inner region is governed by the cyclostrophic wind and an evolution equation, the outer region by the gradient wind and another evolution equation.     

INNER AND OUTER STRUCTURES OF TROPICAL CYCLONE Ⅱ. BAROCLINIC MODEL

A tropical cyclone is divided into inner and outer regions in which temporal and spacial scales and physical quantities are all different.Making use of the scale analysis and perturbation methods the gov-erning equations of barotropic and baroclinic models are obtained for the two regions, respectively.In the two models the stream and presure fields are solved analytically and the obtained structures are ap- proximately consistent with observations.It is shown that there are different governing equations for the inner and outer regions of the tropical cyclone.The inner region is governed by the cyclostrophic wind and an evolution equation, the outer region by the gradient wind and another evolution equation.     

Further experiments on cyclone track prediction with a quasi-Lagrangian limited area model

Summary Results of an earlier study of cyclone track prediction using a quasi-Lagrangian model (QLM) to generate track forecasts of up to 36 hours were reported by Prasad and Rama Rao (2003). Further experiments to produce track forecasts of up to 72 hours with an updated version of the same model have been carried out in the present study. In this case, the ability of the model to predict recent historical cyclones in the Bay of Bengal and Arabian Sea has been assessed. Analysis of some of the structural features of analyzed and predicted fields has been carried out. Such fields include wind distribution and vertical motion around the cyclone centre. In addition, the merging of an idealized vortex with the large scale initial fields provided by a global model, has been carried out for a particular case study of a May 1997 storm, which hit the Bangladesh coast. This current study has demonstrated that the model generates a realistic structure of a tropical cyclone with an idealized vortex. Performance evaluation has been carried out by computing the direct position errors (DPE). The results of which show that the mean error for a 24 h forecast is about 122 km, which increases to about 256 km for a 48 h forecast and 286 km for a 72 h forecast. These figures are comparable to similar errors in respect of tropical cyclone forecasts produced by an advanced NWP centre, viz., the UKMO global model during the corresponding period, 1997–2000 (obtained from UKMO web site). The average forecast errors of the UKMO model are 160 km for 24 h, 265 km for 48 h, 415 km for 72 h forecast ranges.     

NUMERICAL EXPERIMENT OF THE EFFECT OF ENVIRONMENTAL FLOWS ON TROPICAL CYCLONE MOTION

A barotropic prirnitive equation model is used to simulate the tropical cyclone motion.Tropical cyclonemovements under different environmental flow backgrounds were examined and sensitivity of tropical cyclonetracks were discussed.Conclusions of practical significance have been obtained in this paper.     

The influence of air-sea interaction on the development and motion of a tropical cyclone: Numerical experiments with a triply nested model

Summary Tropical cyclone (TC)—ocena feedbacks are studied using a coupled tropical cyclone-ocean model consisting of an eightlayer triply-nested movable grid model of a TC and a three-layer primitive equation ocean model. The numerical results indicate that the TC-ocean interaction influences intensities, structures, and the trajectories of tropical cyclones. Two possible mechanisms, barotropic and baroclinic, influencing TC tracks under TC-ocean interaction are suggested. The barotropic mechanism is related to the changes of the vertically averaged TC structure, induced by the TC-ocean coupling. The baroclinic mechanism is related to the asymmetry of the condensation heating within the TC caused by the asymmetry of heat and moisture fluxes at the sea surface. This asymmetry arises due to the asymmetry in sea surface cooling relative to the storm center. The experiments indicate that the influence of TC-ocean interaction on the TC tracks is the greatest for the case of a zero background flow. In the case of a non-zero background flow the sensitivity of storm tracks to the coupling with the ocean decreases. It is found that the influence of the ocean coupling on the TC track is quite sensitive to the method of convective heating parameterization in the TC model. The TC-ocean interaction also results in a change of the amount and spatial distribution of precipitation.     

Simulations of the motion of tropical cyclone-like vortices in the presence of synoptic and mesoscale circulations

Initial mesoscale vortex effects on the tropical cyclone(TC) motion in a system where three components coexist(i.e.,an environmental vortex(EV),a TC,and mesoscale vortices) were examined using a barotropic vorticity equation model with initial fields where mesoscale vortices were generated stochastically.Results of these simulations indicate that the deflection of the TC track derived from the initial mesoscale vortices was clearly smaller than that from the beta effect in 60% of the cases.However,they may have a more significant impact on the TC track under the following circumstances.First,the interaction between an adjacent mesoscale vortex and the TC causes the emergence of a complicated structure with two centers in the TC inner region.This configuration may last for 8 h,and the two centers undergo a cyclonic rotation to make the change in direction of the TC motion.Second,two mesoscale vortices located in the EV circulation may merge,and the merged vortex shifts into the EV inner region,intensifying both the EV and steering flow for the TC,increasing speed of the TC.     

热带气旋路径趋势数值预报的试验研究

给出了一个嵌套于欧洲中期天气预报中心全球谱模式中的套网格模式，用于预报热带气旋路径趋势，提出了一种以欧洲中期天气预报中心提供的时距为24小时的预报场为基础的“接力预报”方法；设计了五种不同的“接力预报”方案；并对8507号和8509号两个热带气旋就各种不同预报方案进行了数值预报试验。结果表明，本模式对热带气旋路径趋势具有较强的预报能力，对疑难路径也具有一定的预报能力，为开展热带气旋的路径趋势预报提供了一个新的途径。文中还对影响热带气旋路径趋势的一些主要因子结合各预报结果进行了分析，为模式尽早投入业务提供了一定的依据。     

全球数值模式中的台风初始化Ⅰ:方案设计

由于缺少大量有效的观测资料,台风初始化对数值天气预报业务模式而言,仍然是一个悬而末决的难题.中国国家气象中心自从1996年将台风数值预报系统投入业务运行以来,一直使用经验的人造bogus涡旋台风初始化技术.实际上,不同时期的台风有着不同的环流结构,即使同一个台风在不同的生命期也具有不同的结构特征,而这些结构特征的差异并不能依靠现有的bogus涡旋技术体现出来,这种主观方法的统一性与台风在时空上的差异性形成了强烈的反差.最近,基于国家气象中心全球资料分析同化-预报循环系统,设计和发展了一套新的台风初始化业务方案,它主要由初始涡旋形成、涡旋重定位和涡旋调整3部分过程组成.相比于业务中使用的人造bogus涡旋台风初始化方案,新方案在很大程度上减少了人为因素对台风涡旋结构的影响,而更多地是依靠数值模式自身的动力和物理过程来协调约束产生三维空间的涡旋结构.应用新方案,文中对生成于西北太平洋的2006年0605号台风格美(Kaemi)进行了数值试验,初步分析表明,新方案在实现台风涡旋环流结构的初始化方面效果较好,同时,对台风格美多个时次的预报结果也显示,相比于业务使用的bogus方案而言,新方案对台风路径平均预报误差有了大幅度的降低.     

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.