NUMERICAL EXPERIMENTS IN SYMMETRIC AND ASYMMETRIC MOTIONS IN THE BAROTROPIC PRIMITIVE EQUATION MODEL ATMOSPHERE

Three numerical experiments have been carried out by using a spectral barotropic primitive equation mo-del.It is found that the results obtained are quite similar to those with the barotropic filtered model.Themain results read as follows:(1)In the case with symmetric orography or without orography,if the motion is symmetric(with re-spect to the equator,the same is true hereafter)at the initial instant,then it would be symmetric afterwards.(2)The antisymmetric orography distribution could cause antisymmetric motion,and the original sym-metric motion might become asymmetric.In order to explain the above results,it has been proved that they are theoretically valid.And it is foundthat if the motion is antisymmetric at the initial instant,then it would become asymmetric.Therefore,nopure antisymmetric motion could be maintained.     

EFFECTS OF OROGRAPHY AND HORIZONTAL DIFFUSION ON THE GENERATION OF THE ASYMMETRIC MOTION IN THE BAROTROPIC FILTERED MODEL ATMOSPHERE

By utilizing the barotropic vorticity equation including effects of orography and horizontal diffusion,the linearized equations describing symmetric and antisymmetric motions and their analytic solutions are pre-sented.It can be found from the solutions that no matter what kind of motion may be,each solution consistsof three waves,namely,Rossby wave related to initial values,marching wave propagating at Rossby wavevelocity and stationary wave.The latter two are closely related to orography and horizontal diffusion.However,if the motion is symmetric at the initial instant,then the antisymmetric components of orographyand of horizontal diffusion are likely to lead to the generation of antisymmetric motion.In the steady state,the symmetric flow is connected with symmetric orography and horizontal diffusion and the antisymmetricflow with antisymmetric orography and horizontal diffusion.Further,in order to verify the above analysis,three numerical experiments have been made.The results show that antisymmetric orography can produceantisymmetric motion.Finally,the atmospheric interactions between Northern and Southern Hemispheresare discussed.     

THE SENSITIVITY OF THE NUMERICAL SIMULATION TO OROGRAPHY SPECIFICATION IN THE LOWRESOLUTION SPECTRAL MODEL-PART II: IMPACT OF THE SMOOTHED OROGRAPHY AND RIPPLES ON SIMULATIONS

In order to investigate the impact of the smoothed orography and the spurious orographic ripples on simulations in the low-resolution spectral model, three different numerical tests, that is, the unsmoothed orography scheme, the smoothed orography scheme and non-ripples scheme are performed. In this paper, the model used by us is the same as Part I except for orographic specification.The results from simulations indicate that, as far as the climatic simulation is concerned, some aspects of the simulated stationary disturbances, zonal and meridional wind, temperature and precipitation in the low-resolution spectral model with properly smoothed mountains are significantly improved, especially in winter hemisphere.The deep ripples in the model with the unsmoothed orography produce spurious high pressure regions at the surface with subsidence, and suppress rainfall, causing an unrealistic splitting of the precipitation area in northern winter and summer. Removal of the deep ripples by using the special proc     

在正压过滤模式大气中地形和水平扩散对非对称运动形成的作用

利用考虑地形和水平扩散作用的正压涡度方程,在线性情况下,给出了描写对称运动和反对称运动(都是对赤道来说的)的方程和它们各自的解析解。分析这些解可以看出,不论对于对称运动或反对称运动,解都由三部份组成:一是与初始流场有关的Rossby波;一是以Rossby波波速传播的,与地形和水平扩散有关的行进波;另一是与地形和水平扩散有关的定常波。但是,如初始时刻运动是对称的,则地面拔海高度和水平扩散系数的反对称分布可以引起反对称运动,从而,引起非对称的运动。在定常情况下,则对称的流场,和地面拔海高度、水平扩散系数的对称分布有关,反对称的流场,和地面拔海高度、水平扩散系数的反对称分布有关。最后,还用数值试验,证明反对称地形可以引起反对称运动;还讨论了南北半球的相互影响的问题。     

一个分析大气谱模式的合理性的定性方法

利用对称运动和非对称运动的理论结果,如‘对称运动的不变性’,可以推得:如初始时刻运动是对称的,对于绝热无摩擦的谱模式,在时间积分过程中,总有 1.和反对称运动有关的谱系数等于零; 2.全球预报和半球预报在同一半球内处处相同; 3.和赤道对称的任意两个区域,其各自的运动彼此对称。 数值试验也证实了这些结果。 在有外力强迫的情形,如地形(或加热),如其分布是对称的,则在初始运动是对称的情况下,‘对称运动的不变性’仍然成立。但如果地形(或加热)分布是非对称的,则运动的对称性会减小,而变成非对称运动,并出现越赤道气流。这些也经过数值试验加以证实。 利用上述理论结果作为指导,可以用所设计的模式进行试验、分析,从而,确定模式设计是否合理,有关软件是否有误等。     

正压过滤模式大气中的对称运动和非对称运动

把流函数分成对赤道对称和对赤道反对称的两部分,可以把正压涡度方程分离为描写大气对称运动和反对称运动的两个方程。根据它们,讨论了在全球范围内几种基本物理量,如涡度和绝对角动量等在对称和反对称情况的守恒性。还给出了这两类运动的能量收支方程和能量转换表达式。可以看出,在大气中不仅有纬圈平均动能和扰动动能之间的转换,也有对称运动产生的和反对称运动产生的动能之间的转换。 还在涡度方程中考虑地形和水平扩散的情形下,提出了大气非对称性质产生的机制。结果表明:非对称的地形分布和水平扩散系数的分布可能是导致非对称运动的原因。 最后,还对全球预报和半球预报,从物理观点作了理论比较。     

ABRUPT CHANGE OF QUASI-GEOSTROPHIC FLOW FORCED BY OROGRAPHY AND DIABATIC HEATING

A low-order spectral model is used to study abrupt change of quasi-geostrophic current forced byboth orography and diabatic heating. In certain parametric combinations, along with the gradualchange of external thermal forcing, it is found that abrupt changes of both the spectral coefficient andthe locations of the westerly jet stream can be stimulated. Introducing the orography into the model results in a change of the parametric range in whichthe multiple equilibrium states in flow patterns exist and the shift of the parametric critical points atwhich the abrupt change of the locations of westerly jet streams is created, as well as increase of thewave components of the equilibrium flow patterns. The aspects of simulated "South Branch"jet streams are more similar to the observational fact in the presence of both the orography and thediabatic heating than those in the case of the heating forcing.     

A BAROTROPIC QUASI-GEOSTROPHIC MODEL WITH LARGE-SCALE TOPOGRAPHY, FRICTION AND HEATING

Based on the barotropic equations including large-scale topography, friction and heat factor, a barotropic quasi-geostrophic model with large-scale topography, friction and heating is obtained by means of scale analysis and small parameter method. It is shown that this equation is a basic one, which is used to study the influence of the Tibetan Plateau on the large-scale flow in the atmosphere. If the friction and heating effect of large-scale topography are neglected, this model will degenerate to the general barotropic quasi-geostrophic one.     

EFFECTS OF THE ENVELOPE DEGREE OF OROGRAPHY ON THE SIMULATED CLIMATE PROPERTIES*

A primitive equation model with a zonal domain and a p-σ incorporated vertical coordinate system is used to study the effects of the envelope degree of orography on the simulated properties of both winter and summer climates.Results show that the orography with a larger envelope degree can improve the simulations to certain extent,especially in winter.It is found that the simulated properties of climate are mainly determined by the co-effects of the land-sea and the orographic distributions,and the envelope degree of orography has a secondary effect.However it should be taken properly in order to improve simulations:otherwise,it would be harmful.Different envelope degrees of orography can be adopted in different areas.     

ACTUAL ANALYSIS AND THEORETICAL SIMULATION ON POSITION OF SUBTROPICAL HIGHS OVER WESTERN PACIFIC

In this paper,using a nonlinear low-order barotropic system with orography and momentum forcing aswell as dissipation,we have theoretically calculated the two kinds of curves for the seasonal northward movementof subtropical highs.It has been shown that the theoretical results resemble basically with the analysed resultsof wave spectrum data.     

SYMMETRIC AND ASYMMETRIC MOTIONS IN THE BAROTROPIC FILTERED MODEL ATMOSPHERE

By expressing the stream function in terms of the symmetric and the antisymmetric components with respect to the equator,the barotropic vorticity equation can be separated into two equations,one describing the behaviours of the atmospheric symmetric motion,and the other those of the atmospheric antisymmetric motion.From them,for the entire global surface at the equivalent barotropic level,the conservations of several basic quantities,such as vorticity,angular momentum etc.in the symmetric and antisymmetric cases,have been discussed.In addition,the energy budget equations and the energy conservation expressions for the two kinds of motion are given.It can be seen from them that there are not only the conversion between the zonal mean kinetic energy and the disturbed kinetic energy in the atmosphere,but also the conversion between the kinetic energy generated by the symmetric and antisymmetric motions.In the case of including orography and horizontal diffusion into the vorticity equation,a mechanism of the generation of asymmetric behaviours of the atmosphere is proposed.The results show that the asymmetric distribution of orography and that of horizontal diffusion coefficients are likely the causes leading to the asymmetric motion.Finally,a theoretical comparison between the global model and the hemispheric one from the physical point of view is made.     

对多层原始方程模式的合理性分析

本文分理论基础和数值分析两部份。在第一部份中,利用绝热无耗散多层原始方程模式从理论上证明:如在初始时刻t_0时运动是对称的(包括地形是对称的),则在t>t_0的任一时刻,运动将保持是对称的。还在考虑对称地形情况下用模式进行时间数值积分,证明了理沦结果的正确性,因而,理论结果可以作为分析模式合理性的基础。 但是,如考虑实际地形,则由于其对称部份和反对称部份数量相当,运动的对称性会大大降低,受影响的地区主要是北半球高山地区和南半球与之对称的低地。而且,这种影响是从低层逐渐传播到高层的。     

温度和纬圈平均西风等气象要素对赤道的对称分布和反对称分布

把任一气象要素分为对赤道对称和对赤道反对称的两部分，利用Oort等的气候资料进行分析，发现在全年中，许多气象要素，如500和1000百帕纬圈平均的高度、温度和西风等，主要呈对称分布。对称部分的绝对值和其沿经圈分布曲线的起伏，都远远大于其反对称部分的对应值。而且，对称部分的曲线起伏，除1000百帕的西风和高度不太明显外，在1—7月普遍呈减小的趋势。至于反对称部分，其变化趋势则与对称部分相反。为了解释上述现象，还对年平均的非绝热加热、摩擦和地形等进行了分析。发现地面拔海高度的反对称部分最明显。在12—2月和6—8月，净辐射、地面雷诺应力、地面（包括海洋）和大气之间的热交换（包括辐射、感热和潜热）的反对称部分也很明显，有时和对称部分相当，甚至超过；但它们在这两个季节的沿经圈分布的位相是相反的。这看来和上述气象要素中反对称部分的产生，以及对称运动维持及其随季节变化有密切关系。     

大气对称运动和反对称运动的物理性质以及它们之间的能量转换

采用p坐标系,利用控制原始方程模式大气中对称运动和反对称运动的方程组,对其物理性质,如对称的和反对称的能量、角动量等的守恒性和它们之间的相互转换,纬圈平均的和扰动的对称和反对称运动之间的能量的相互转换进行了讨论。结果指出:(1)对全球总的绝对角动量的变化有贡献的,只能是因地形而引起的位势高度的对称部分在纬圈方向的差值和耗失力的纬向分量;(2)不同形式能量之间的相互转换除和经典的情况类似外,还有对称能量和反对称能量之间的相互转换,可以制约对称运动和反对称运动的相互影响。     

Recent advancements in dynamical tropical cyclone track predictions

Summary Considerable advancements have recently been made in understanding tropical cyclone motion. Based on these new understandings, the requirements for accurate tropical cyclone motion prediction with dynamical models can be specified. Four issues related to dynamical track prediction are the initial specifications of the environmental wind field, the symmetric vortex and the asymmetric vortex structure, as well as the adequacy of the models to predict the time evolution of each of these three components of the total wind field. Recently developed barotropic and limited-region or global baroclinic models are examined in terms of these issues. The capability of the Hurricane Research Division barotropic model to provide skillful track forecasts to 48 h retrospectively substantiates that tropical cyclones motion is governed by barotropic dynamics to first order. Limited-region baroclinic models are demonstrated to have many of the numerical characteristics, physical process representations, and initial condition specifications that will be required to properly predict tropical cyclone tracks. In particular, the semioperational Geophysical Fluid Dynamics Laboratory model most closely addresses all of the above issues, and has demonstrated potential for markedly improved tracks for a small sample of cases. Finally, the inclusion of some aspects of tropical cyclone structure in the initial conditions of global baroclinic models has improved their track predictions. Thus, the outlook is for a significant improvement in dynamical track predictions.With 6 Figures