Some effects of rotation rate on planetary — scale wave flows

Summary A series of experiments was performed in a rotating annulus of fluid to study effects of rotation rate on planetary-scale baroclinic wave flows. The experiments reveal that change in rotation rate of fluid container causes variation in Rossby number and Taylor number in flows and leads to change in flow patterns and in phase and amplitude of quasi-stationary Waves. For instance, with increasing rotation rate, amplitude of quasi-stationary waves increases and phase shifts upstream. On the contrary, with decreasing rotation rate, amplitude of quasi-stationary waves decreases and phase shifts downstream. In the case of the earth's atmosphere, although magnitude of variation in earth's rotation rate is very small, yet it causes a very big change in zonal velocity component of wind in the atmosphere and of currents in the ocean, and therefore causes a remarkable change in Rossby number and Taylor number determining regimes in planetary-scale geophysical flows. The observation reveals that intensity and geographic location of subtropic anticyclones in both of the Northern and Southern Hemispheres change consistently with variation in earth's rotation rate. The results of fluid experiments are consistent, qualitatively, with observed phenomena in the atmospheric circulation.With 12 Figures     

Nonlinear waves in barotropic model

In this paper, from the system of equation describing a barotropic atmosphere using the method of Taylor expansion for the nonlinear terms, the periodic solutions of the nonlinear inertio-surface gravity waves and Rossby waves have been obtained.The finite-amplitude nonlinear inertio-surface gravity waves and Rossby waves with horizontal divergence satisfy all the KdV equation. The solutions are all the cnoidal function, i, e, the cnoidal waves which in-clude the linear waves and form the solitary waves under certain conditions. For the finite-amplitude Rossby waves with horizontal divergence, we find the new dispersive relation including both the wave number and the amplitude parameter. In case of small amplitude it is reduced to the Yeh formula. It is shown that the larger the amplitude and width, the faster the finite-amplitude inertio-surface gravity waves and the slower the finite-amplitude Rossby waves with horizontal divergence propagate. The blocking or cut-off system in which the amplitude and width are large may be considered as Rossby solitary waves.     

Generation of internal waves over a shelf

Experiments are performed in a 13-m cylindrical tank to study the generation of interfacial internal waves by barotropic sinusoidal waves passing over a slope. At each tidal cycle, there are two waves generated, one propagating onshore and the other propagating offshore. The amplitude of the waves increases with increasing forcing and evolves as nonlinear waves if the shelf width is smaller than the wavelength of the baroclinic tide. Rotation does not modify the generating mechanism but the amplitude of the generated waves decreases with increasing rotation rate; also no internal waves are generated when the forcing period is larger than the inertial period, and at high rotation rate, there are only dispersive waves propagating from the shelf break region. The experiments covered a large range of internal Froude number, Rossby number and temporal Rossby number and compare well with in situ observations.     

南极地区的大气环流流体物理实验

通过流体物理实验，研究南极地形及冷源对大气环流的影响。正压流体实验表明，由于地形动力强迫，在南极大陆沿岸及以北地区生成气旋性涡旋带，有３个低压中心。在地形上空生成巨大的反气旋。斜压流体实验表明，东移“行星波”的波数及波形有准周期低频振荡，其振荡周期相当于地球大气系统中的２１ｄ。东移的槽在１１０°Ｅ以东逐渐发展，在１６０°Ｗ的罗斯海附近发展得最深，再往东则逐渐减弱     

A fluid experiment of large-scale topography effect on baroclinic wave flows

The effects of topography on baroclinic wave flows are studied experimentally in a thermally driven rotating annulus of fluid.Fourier analysis and complex principal component (CPC) analysis of the experimental data show that, due to topographic forcing, the flow is bimodal rather than a single mode. Under suitable imposed experimental parameters, near thermal Rossby number ROT = 0.1 and Taylor number Ta = 2.2 × 107, the large-scale topography produces low-frequency oscillation in the flow and rather long-lived flow pattern resembling blocking in the atmospheric cir-culation. The ‘blocking’ phenomenon is caused by the resonance of travelling waves and the quasi-stationary waves forced by topography.The large-scale topography transforms wavenumber-homogeneous flows into wavenumber-dispersed flows, and the dispersed flows possess lower wavenumbers.     

简单热带海气耦合模型中不同扰动形式间的耦合作用

在局地热平衡情况下研究了简单热带海气耦合模式中不同扰动形式间的耦合，依次讨论了由大气准定常Kelvin波与海洋R0ssby波、大气准定常Rossby波与海洋Kelvin波、大气准定常Kelvin波与海洋KelVin波、大气准定常Rossby波与海洋Rossby波组成的耦合系统的性质，并研究了存在于其中的耦合扰动的特征。     

正压大气中的代数Rossby孤立波

本文研究了基本气流具有线性弱切变的非线性Rossby波,得到了一个非线性发展方程为Kubota方程,在一定的条件下,可变为Benjamin-Ono方程,并指出当代数Rossby孤立波的振幅越大时,代数Rossby孤立波的传播速度越小,基流切变越强,代数Rossby孤立波越慢,同时我们还指出在代数Rossby孤立波的振幅满足一定的条件下,代数Rossby孤立波才随纬度的增高(β减小)而减慢。并且代数Rossby孤立波的结构与大气中的偶极子阻塞是一致的。     

从Rossby波能量频散理论到准定常行星波动力学研究的发展

本文是为纪念叶笃正院士诞辰百周年和他对大气动力学发展的重大贡献而撰写的.叶先生在20世纪40年代所提出的罗斯贝波能量频散理论不仅至今仍广泛应用于天气预报,而且开创了准定常行星波动力学的研究.在罗斯贝波频散理论的启迪和引领下,行星波动力学和大气环流异常遥相关的研究取得重要进展.特别是关于准定常行星波在二维和三维球面大气的传播特征以及北半球夏季大气环流异常的EAP(East Asia-Pacific)型和"丝绸之路(Silk Road)"型遥相关及其机理已做出系统的研究,本文简要地回顾这些研究.并且,本文还回顾了在叶先生所提出罗斯贝波能量频散理论的引领下,近年来我们关于东亚冬、夏季风和我国气候灾害的年际和年代际变化的内动力学机理研究所取得的进展.     

A numerical method for solving the evolution equation of solitary Rossby waves on a weak shear

In this paper an evolution equation in integral-differential form for finite amplitude Rossby waves on a weak shear is presented and an efficient method for its numerical solution is set up. It is shown that a propa-gation of solitary wave is possible whenever a proper weak shear in basic flows acts with the nonlinear effects and dispersion of the media, both in the atmosphere and in the ocean. To test the numerical method for solving the evolution equation, a series of experiments are carried out. The results indicate that the solitary solutions do exist and interact with each other in quite a succinct, manner. Therefore the method is successful and efficient for solving initial value problems of the above equation. The time decoupling problem arising in the numerical scheme and the related filtering technique are discussed. A variety of interesting phenomena such as the interaction of solitary Rossby waves, damping, dispersion and the development of nonlinear wave train are numerically studied.     

Rossby wave propagation in a barotropic atmosphere

The ideas of ray tracing from geometrical optics and wave propagation in a slowly varying medium are applied to Rossby waves propagating in a barotropic atmosphere.The propagation of low-frequency Rossby waves in a zonally symmetric basic state is compared with that for stationary waves presented by Hoskins and Karoly (1981). These ideas are then used to study the propagation of Rossby waves in a basic state with zonally varying middle latitude or low latitude jets. Conditions which allow cross-equatorial wave propagation are presented. For a zonally varying middle latitude jet, there is weak wave convergence in regions of decreasing jet speed, However, this is not sufficient to explain the enhanced wave amplitude found in numerical-model experiments using a zonally varying basic state.     

非线性正压Rossby波的精确解

本文从半地转近似下的正压大气运动方程组出发,导出了非线性正压Rossby波所精确满足的KdV方程,求得了椭圆余弦波解和孤立波解,并讨论了非线性波动的波速公式及其振幅与其它波参数间的关系。同时指出,在对大气模式进行数学简化时必须尽可能保证原模式所具有的一些守恒关系,否则有可能使原问题的解受到歪曲。     

低纬大气Kelvin波和Rossby波的非线性相互作用

利用多重尺度摄动法,对低纬大气Kelvin波和Rossby波的波包演变进行了分析,得到两波的非线性相互作用方程为耦合的非线性复系数Landau方程组。数值计算表明两波相遇将使波振幅变化,波宽变窄;两波非线性相互作用可能是低纬强烈天气突然爆发的原因之一。     

地球旋转向量水平分量对赤道β平面上波动的影响

文中利用合理的赤道 β平面近似方程组研究 β效应和地球旋转向量水平分量 (即Coriolis力分量 f*=2 Ω cosφ)共同作用下低纬度大气波动的特征。定性结果分析表明 ,若扰动与纬度有关 (混合 Rossby-重力波和 Rossby波 ) ,f*虽然不影响波动传播的频率特征 ,但修改其决定波动经向结构的振幅。如果扰动与纬度无关 (Kelvin波 ) ,则 f*的影响消失 ,这与 f*- f平面上波动的情况显然不同。     

Large scale perturbations in extratropical atmosphere—Part I: On rossby waves

This study reexamines the propagation mechanism and geostrophic property of the classical two-dimensional Rossby waves in a non-divergent barotropic atmosphere. It will be found that propagation of large scale atmospheric waves depends crucially on horizontal divergence. A small Rossby number in Rossby waves is not sufficient for the waves to have a small ageostrophic component, because the two-dimensional classical Rossby waves do not manifest the geostrophic balance as good as observed in the atmosphere.     

Measurements in a turbulent patch in a rotating,linearly-stratified fluid

A series of experiments is described in which a turbulent patch is generated locally by an oscillating grid positioned at one end and mid-depth of a rotating channel filled initially with a linearly-stratified fluid. Measurements have been made of vertical density profiles through the patch both during sustained oscillations and following cessation of grid forcing. Temporal variations in patch size and structure, Thorpe scales, mixedness parameter and available potential energy are deduced from these measurements, and the effects thereon of varying the background rotation rate, initial buoyancy frequency and grid action are investigated. For the growth phase of the patch, previous results obtained by other workers are confirmed and extended. Because the rapid turbulent motions implied a large Rossby number, rotation was not important during this phase. During the decay phase, rotational effects are shown to become important, and the presence of rotation is found to retard the decay of both the mixedness and the Thorpe scales of the density overturns within the patch. The work is novel in that measurements of the patch parameters listed above have not previously been carried out in the presence of rotation. The results are relevant to studies of such patches that have been observed in the ocean and atmosphere.     

Activities of low-frequency waves in the tropical atmosphere and enso

ENSO, particularly the occurrence of ENSO is still an important research object in climatic variation. Using the ECMWF data, the relationship between ENSO and the activities of low-frequency waves in the tropical atmosphere is analyzed in this paper. It is shown that the occurrence of ENSO is closely related to the intraseasonal oscillation and the quasi-stationary waves (the period > 90 days) in the tropical atmosphere. Associated with the occurrence of El Nino event, the kinetic energy of low-frequency waves has obvious variation: the kinetic energy of atmospheric intraseasonal (30–60 days) oscillation (ISO) decreases abruptly and the kinetic energy of quasi-stationary waves increases abruptly. Moreover, the ISO and quasi-stationary waves propagate eastward clearly corresponding to El Nino; but they clearly propagate westward in La Nina cases.     

Activities of Low-Frequency Waves in the Tropical Atmosphere and ENSO

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Mesoscale vortex shedding from large islands: A comparison with laboratory experiments of rotating stratified flows

Summary Mesoscale vortex shedding in the wake of large islands has been analysed with respect to the influence of stratification and background rotation. Comparison with results from laboratory experiments on rotating-stratified flows past isolated obstacles based on Rossby and Froude numbers have shown that forFr<0.4 vortex shedding can be expected for moderate rotation rates. Rossby numbers for cases of atmospheric vortex streets were found to be much larger than in laboratory experiments. Depending on the Froude number different flow pattern in the wake of three-dimensional obstacles have been identified in both, laboratory experiments and atmospheric observations.With 5 Figures     

地形强迫对大气长波调整的可能影响

地形作为大气的外部强迫,其动力和热力作用对波动结构演变及极端天气出现都有不能忽视的作用。本文通过数值求解考虑地形强迫的β平面正压准地转位势涡度方程,探讨了地形强迫作用对大气长波调整的可能影响,结果表明:同非线性作用和纬向非均匀基流作用一样,无基流情形下具有纬向差异的地形分布影响了大气长波结构的演变,也能强迫出大气长波调整现象。大气长波调整依赖于地形的高度和地形分布,地形越高,长波越容易出现波数的调整;地形波数越大,即地形结构复杂,越不易出现波数变化。大气长波调整还与纬度有关,纬度越高,β越小,地形强迫作用越突出,长波调整容易出现;反之,低纬度以β效应为主的线性波动不易出现波数调整。大气长波调整对波动初始波动的振幅不敏感,但依赖于波动的初始结构。此外,有基流作用时,地形强迫还是诱发定常波的重要因素,且定常波流场结构依赖于地形高度分布,与波动初始结构无关。     

夏季东亚高空副热带西风急流季节内异常的环流特征及前兆信号

位于东亚中纬度上空的东亚高空副热带西风急流是东亚季风环流系统中的重要成员,我国夏季降水雨带的季节内变化受东亚高空副热带西风急流位置季节内异常变化影响。根据1979~2008年中国降水资料、NCEP/NCAR再分析资料以及NOAA ERSST V3月平均海表温度资料,利用统计分析和物理量诊断方法对夏季东亚高空副热带西风急流位置季节内异常的东亚大气环流特征及外强迫信号的物理过程进行了探讨。研究指出:6月东亚高空副热带西风急流位置异常主要受欧亚大陆中高纬东传的Rossby波列位相变化影响,春季北大西洋海温异常是欧亚大陆中高纬度Rossby波列位相变化的最显著的外强迫信号;7月东亚高空副热带西风急流位置异常主要受西太平洋热带向副热带传播的Rossby波列位相变化影响,春季西太平洋热带海温异常是西太平洋热带向副热带传播的Rossby波列位相变化的最显著的外强迫信号;8月东亚高空副热带西风急流位置异常主要受南亚大陆向东亚大陆热带、副热带传播的Rossby波列位相变化影响,春季印度洋海温异常是南亚大陆向东亚大陆热带、副热带传播的Rossby波列位相变化的最显著的外强迫信号。