VENTILATION FLOW IN A BAROCLINIC VORTEX RELATED TO TROPICAL CYCLONE MOTION

The tropical cyclone motion is numerically simulated with a quasi-geostrophic baroclinic model.The flow field of a tropical cyclone is decomposed into its axisymmetric and asymmetric components.The relation between the ventilation flow vector and the motion vector of the tropical cyclone is inves-tigated.The results of numerical experiments indicate:(1) There are both large-scale beta gyres andsmall-scale gyres in the asyrnmetric flow field.(2) The interaction between small-scale gyres andlarge-scale beta gyres leads to the oscillation of translation speed and translation direction for the tropi-cal cyclone.(3) There are the large deviations between the ventilation flow vector calculated bymeans of Fiorino and Elsberry's method and the motion vector of tropical cyclone.(4) The ventila-tion flow vector computed using the improved method closely correlates with the motion vector of thetropical cyclone.     

斜压涡旋中的通风气流与热带气旋移动的关系

应用准地转斜压模式数值模拟了热带气旋的移动。将热带气旋的流场分解为轴对称分量和非对称分量,研究非对称流场中的通风气流矢量与热带气旋移动矢量的关系。数值试验结果表明:(1)在热带气旋的非对称流场中,不但有大尺度β涡旋,而且还有小尺度涡旋。(2)小尺度涡旋与大尺度β涡旋之间的相互作用导致热带气旋移向的摆动和移速的振荡。(3)应用Fiorino和Elsberry的方法计算的通风气流矢量与热带气旋移动矢量有很大偏差。(4)应用改进的方法计算的通风气流矢量与热带气旋移动矢量相关密切。     

NUMERICAL STUDY ON FUNCTIONS AFFECTING TROPICAL CYCLONE STRUCTURE AND MOTION*

By using a beta-plane quasigeostrophic barotropic model,four numerical experiments with integration time more than five days are performed.Results show that the vorticity advection term and beta term are two basic factors significantly affecting tropical cyclone structure and motion.     

热带气旋逆时针打转物理机制的研究

应用一个无基本气流的准地转斜压模式数值模拟热带气旋逆时针打转运动。分析结果表明：对称气流对非对称涡度的平流引起非对称流场中的小尺度涡旋和通风气流逆时针旋转；旋转的通风气流引导热带气旋作逆时针打转运动。     

NUMERICAL SIMULATION OF TROPICAL CYCLONE MOVEMENT WITH THE RELATED CRITICAL POINT PHENOMENON*

A study is made of the paths of tropical cyclones under the joint action of environmental flow and heating fields in terms of a quasi-geostrophic barotropic model with the thermal forcing and dissipation terms involved,in which 16 experiments are designed and integrated for more than 7 model days.Results show that for different parameter values of the environmental flow and heating field,and the cyclone scale,such paths as counterclockwise looping,zigzag form and westward march with an abrupt turning towards the north are reproduced clearly in the model atmosphere,exhibiting their critical points of nonlinear character.     

INTERACTION BETWEEN TROPICAL CYCLONE AND MEIYU FRONT*

Generally speaking,the convection activities are inactive over western Pacific warm pool and tropical cyclone (TC) activity seldom occurs over the offshore of East Asia during the period of Meiyu rainfall.However,if a TC is active in this area,the Meiyu rainfall will often weaken or end up.Based on a statistical study with the data from 1980 to 1995,it is found that about 91% of 23 TC activities affected the intensity of Meiyu rainfall,and 50% of the end-up of Meiyu events were related to the active TCs and the change of subtropical high.The present paper simulates the effect of TC on Meiyu circulation by using MM4 model,and the results agree with the observations.From the point of view of vapor and energy transport,the landing of TC cuts not only the transport of the water vapor to Changjiang-Huaihe River basin from the Bay of Bengal but also the conversion of the mean flow energy to the Meiyu circulation because of the TC forcing to the zonal circulation.These two effects make the convection and perturbation existing in Meiyu region lack the supply of the vapor and energy for their maintenance and lead to the end of Meiyu rainfall.     

USE OF A NEW STEERING FLOW METHOD TO PREDICT TROPICAL CYCLONE MOTION

A tropical cyclone is a kind of violent weather system that takes place in warmer tropical oceans and spins rapidly around its center and at the same time moves along surrounding flows. It is generally recognized that the large-scale circulation plays a major role in determining the movement of tropical cyclones and the effects of steering flows are the highest priority in the forecasting of tropical cyclone motion and track. This article adopts a new method to derive the steering flow and select a typical swerving track case (typhoon Dan, coded 9914) to illustrate the validity of the method. The general approach is to modify the vorticity, geostropical vorticity and divergence, investigate the change in the non-divergent stream function, geoptential and velocity potential, respectively, and compute a modified velocity field to determine the steering flow. Unlike other methods in regular use such as weighted average of wind fields or geopoential height, this method has the least adverse effects on the environmental field and could derive a proper steering flow which fits well with storm motion. Combined with other internal and external forcings, this method could have wide application in the prediction of tropical cyclone track.     

NUMERICAL STUDY ON NONLINEAR INTERACTION BETWEEN TROPICAL CYCLONE AND SEA*

Given time-independent monthly mean sea temperature,numerical simulation was done of the path of the tropicalcy clone Forrest and its related physical quantity field in terms of the improved version of the σ-coordinate 10-level primitive equation model for tropical cyclones developed by the Guangzhou Institute of Marine Meteorology.Result ssuggest the similarity between the simulated and observed physical quantity fields except that the simulated path was to the right of the observed,with the deviation increasing with time.Sea and tropical cyclone,in effect,interact mutually.In an attempt to compare the results from coupling and uncoupling experiments,an oceanic mixed layer model was formulated based on experiments with sea response to cyclone and integrated in synchronous coupling with the aforementioned version of tropical cyclones,the computational domain covering the western North Pacific with the coastal configuration involved.The coupling experiment shows that the negative feedback of the interaction between sea and mature cyclone leads to weaker sea response as compared to the uncoupling case.Except for the location of the divergence core of oceanic current,the mixed layer depth and inhomogeneous distribution of sea surface temperature (SST) caused by the cyclone-core position and intensity,the values of the maximum oceanic current,departure of the mixed layer depth and SST drop were smaller in the coupling than in the uncoupling case,and so was the maximum wind near the cyclone core.The path from the coupling run is to the left of the course in the uncoupling experiment but closer to the observed one though remaining somewhat leftward.Even on a scale of less than 3 days,the effects of sea and sea-cyclone interaction on the cyclone were by no means negli-gible.     

β涡旋的垂直结构及其对热带气旋移动的影响

应用准地转斜压模式数值模拟热带气旋的移动，分析大尺度β涡旋的水平和垂直结构及其对热带气旋移动的。研究结果表明，两β涡旋之间的准均匀流（通风气流）仍然与热带气旋的运动相关。     

EFFECT OF THE LINEAR BETA TERM ON MOVEMENT AND DEVELOPMENT OF TROPICAL CYCLONE*

The dynamics of tropical cyclone is investigated in a nondivergent,barotropic model with no basic flow.The effect of linear beta term on the movement and development of tropical cyclone is emphatically demonstrated.The streamfunction tendency due to the symmetric component of linear beta term appears in a dipole-like pattern with an east-west symmetry,which maintains and intensifies the large-scale beta gyres and causes the tropical cyclone to have a westerly moving component.The streamfunction tendency due to the asymmetric component of linear beta term arises in an ellipse pattern with a north-south major axis,which weakens the tropical cyclone.The streamfunction tendency due to the asymmetric component of linear beta term and the intensity of large-scale cyclonic beta gyre synchronously vary in a fluctuating manner with time.     

环境气流及非绝热加热对热带气旋结构和移动的影响

设计一个三层的准地转斜压模式,研究了不同结构环境气流和环境非绝热加热场中热带气旋移动路径和结构的特征.数值试验的结果表明:(1)无外界影响下热带气旋向西北方向移动,垂直速度场有明显的螺旋结构;(2)热带气旋主要受基本环境气流引导,非对称环流中的小扰动干扰了热带气旋的移动;(3)非绝热加热场对热带气旋影响较大,热带气旋有向加热中心移动的倾向,同时热带气旋的水平、垂直结构都有较大的变化.     

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.     

DYNAMIC FACTORS RELATED TO DECAY PROCESS OF OFFSHORE TROPICAL CYCLONES IN NORTHWEST PACIFIC

Intensity variation of tropical cyclones(TCs),especially that of coastal or landfalling TCs,is of great concern in current research.Most of the research papers,however,focus on intensification processes of TCs;only a few discuss decay processes in the lifetime of a TC.In the daily weather operation related to TCs,it is challenging when a TC weakens and/or disappears suddenly,because it brings more difficulties than the forecast of intensifying TCs does.Overestimation of a decaying landfalling TC would lead to over-preparation of defensive measures and result in"crying wolf"mentality with adverse effects.This study summarized physical mechanisms that dominate the decaying process of TCs and listed several possible dynamical factors:reduced level of air temperature,too large or too small speed,contraction of TC size amplification of TC’s core,and lightning number in a TC.     

EFFECT OF ENERGY DISPERSION ON THE STRUCTURE AND MOTION OF TROPICAL CYCLONE*

The energy dispersion of a typhoon vortex and its effect on the typhoon motion are studied using an analytical method of double-Fourier expansion as well as a numerical model in a β-plane nondivergent barotropic framework.The analytic model and the linear version of the numerical model give essentially the same result:the energy dispersion from a tropical cyclone can creat an L-H-L wave train to the east of the tropical cyclone.Three numerical experiments,integrated for 7 model days by the nonlinear model,indicate that the closed high in the wave train produces obvious influence on the structure and movement of the tropical cyclone.     

热带气旋及相关天气系统对上海强梅雨的影响分析

通过对1996年和1999年上海强梅雨的分析,发现在强梅雨年的梅雨期内140°E以西的西北太平洋上无热带气旋(以下简称TC)活动,使副高脊线和梅雨锋得以持久稳定,造成长江中下游地区的长时间强(多)梅雨。研究还表明,强梅雨年的副高脊线比常年偏北1～4个纬距,梅雨量的多少与副高、西风带系统的强弱、系统稳定控制的时间长短及其系统相互作用的位置有关。     

NUMERICAL STUDY ON THE EFFECTS OF SEA SURFACE TEMPERATURE ON TROPICAL CYCLONE INTENSITY-PART I:NUMERICAL EXPERIMENT OF THE TROPICAL CYCLONE INTENSITY RELATED TO SST *

In the context of a model of tropical cyclone intensity based on an improved meso-scale atmospheric model, numerical simulation is performed of the track and intensity variation of tropical cyclones(TC) arising from sea surface temperature(SST) variation over a specified sea region. Evidence suggests that the model is capable of modeling quite welt the track and intensity of TC:SST variation leads to an abrupt change in the cyclone intensity:the response of the cyclone to the abrupt SST change lasts 8-12 h.     

热带气旋路径集合预报方法研究Ⅰ——正压模式结果的初步分析

选择1979～1993年间的热带气旋为试验个例,通过扰动热带气旋初始位置和初始结构,构造集合成员, 用正压原始方程模式,进行路径集合预报试验, 并初步探讨预报成员的集合方法。试验结果表明:热带气旋定位误差影响路径预报,但扰动初始位置的集合平均预报与控制试验的预报水平相接近。扰动热带气旋初始结构的集合预报试验表明,约有60 %～70 %个例的集合路径预报得到改进。此外,试验结果还表明,当环境引导气流较弱时,进行扰动热带气旋初始结构的集合预报,预报结果的改善较明显。     

THE ENSEMBLE FORECASTING OF TROPICAL CYCLONE MOTION I：USING A PRIMITIVE EQUATION BAROTROPIC MODEL

1INTRODUCTIONErrorsofnumericalforecastingcomprisethoseintheinitialvalue,modelandcomputationandthefirstkindoferrorbecomesmoreandmoreoutstandingwiththeon-goingdevelopmentofcomputersandnumericalmodels.Subjecttoavailableobservationmeans,therealatmosphereisanythingbutapproximatelymeasured.Meanwhile,oneshouldpayattentiontothepossibility,assuggestedbyLorenz[1],thatanyslightdifferencesduringtheinitialstatemayleadtoresultsthatturnouttobetotallydissimilarfromwhatwouldbeexpectedotherwise,asfarasdefi…     

DEVELOPMENT OF A TROPICAL CYCLONE IN BAROTROPIC ENVIRONMENTAL FLOWS

Development of a tropical cyclone in barotropic environmental flows is investigated with a shallow water model. It is found that the tropical cyclone develops only when it is embedded in an environmental now with a southward relative vorticity gradient. The related physical mechanism is explained by analyzing the kinetic energy conversion between the environmental flow and tropical cyclone circulation.     

一种初始涡旋重定位方法及热带气旋路径数值模拟试验

采用平滑滤波和柱形滤波技术,从背景场中分离出初始涡旋环流和大尺度环境场,然后把分离出的涡旋环流进行平移和再植入到观测位置,最终得到一种初始涡旋重定位方法。针对2006—2007年7个热带气旋共23个时次,采用背景场直接加入热带气旋bogus模型和背景场进行初始涡旋重定位后加入热带气旋bogus模型2种方案,分别进行了对比模拟试验。结果表明:对背景场进行初始涡旋重定位后加入热带气旋bogus模型的方案模拟的24和48小时热带气旋路径平均误差都相对较小,初始涡旋重定位方法可以减小热带气旋模式因热带气旋初始位置偏差而引起的误差,有助于提高热带气旋模式的路径预报水平,有较好的业务应用前景。