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1.
利用1958-1997年的NCEP/NCAR再分析资料,以南海季风爆发日为临界日期,计算了40年合成的季风爆发前月平均带状基流;在该基流上,计算了球面正压涡度方程中Rossby波的稳定性;并用谱函数展开法定义和计算了发展型波包的演变.结果显示:南海夏季风爆发前气候平均场上有球面Rossby波的正压不稳定,该不稳定主要由南半球的西风急流所激发,且不稳定扰动的最大振幅均出现在南半球西风急流以南.球面Rossby波发展型波包的最大振幅随时间会由两个半球的中高纬度向低纬扩展,虽然不能越过赤道,却激发了热带地区的积云对流,积云对流的爆发并向季风区传播,加速了大气环流的调整,其结果造成了南海夏季风的爆发.可见,南海夏季风的爆发虽是局地现象,但其爆发原因却是全球性的. 相似文献
2.
本文用转盘实验的方法,研究了均质正压流体运动中扰动与基本带状环流的非线性相互作用。着重考察了两类典型初始扰动场——即迭加在基流上的槽状扰动和多闭合涡旋中心扰动结构的演变过程。发现:非线性相互作用过程是以波动形式表现的。两类初始扰动场各自经历不同的演变过程,并趋向于不同的平衡态。槽状扰动结构有利于基流对扰动动能的吸收,流场最终趋向于轴对称带状环流状态,而且纬向波数越大,吸收过程越明显。多闭合涡心结构不利于基流对扰动动能的吸收。基流与扰动在动能随时间的变化上表现出准周期性振荡。实验结果与旋转适应过程理论有很好的一致性。 相似文献
3.
大尺度涡旋的正压不稳定 总被引：5，自引：4，他引：5
用极坐标中线性化的正压涡度方程，将大尺度圆形基流上扰动的稳定性问题化为广义特征值的问题，得到了扰动振幅增长率沿方位角方向波数的关系。然后以不稳定模态的扰动流函数场作为初始场，在强、弱两类圆形基流的条件下，用直角坐标中准地转正压模式，分别实施积分时间为６０ｈ的试验。结果在模式大气中，初始扰动均随时间而增幅；强圆形基流的扰动增幅更快，并形成与台风螺旋云系看似有些相近的结构。 相似文献
4.
最敏感扰动的演变与夏季乌拉尔地区的持续性异常环流 总被引：1，自引：1，他引：0
通过数值实验对初始扰动不稳定发展建立异常环流型的过程进行了分析。选择了东亚夏季风异常的１９９１年和１９８５年两个典型年份的平均环流作为基流,以共轭敏感性分析得到的最敏感扰动作为初始扰动,分析其在不同的环流背景下的发展情况。结果说明,１９９１年欧亚地区的基流不稳定性较强,且初始扰动结构有利扰动发展,因而通过能量频散在乌拉尔地区激发扰动并强烈发展,建立起阻塞形势。而１９８５年的扰动在乌拉尔地区形成正涡度距平,不利于阻高的建立。从而导致１９９１年和１９８５年夏季乌拉尔地区几乎相反的异常环流。一系列对比实验说明,异常环流型的建立,不仅依靠能量的频散,更依靠扰动通过正压不稳定过程从基流吸收能量而发展。它既有赖于基流自身的不稳定,又取决于初始扰动的结构以及相对于基流的位置。基流选择了具有特定结构的初始扰动型,只要在有利位置上给予扰动,就能够激发出扰动波列,建立强的持续异常环流型。不论初始波列的位相在一定范围内如何改变,扰动总是倾向于在基流的特定不稳定区域发展。 相似文献
5.
6.
亚洲夏季风结构和变动与大气运动流场的斜压和正压特征:正压模分析 总被引：8，自引：0，他引：8
使用ＥＣＭＷＦ１９８０—１９８６年７ａ格点资料对大气运动的正压模进行了分析。指出：对流层中大气正压运动流场显示的副热带高压带仅能反映出行星风带的季节性移动；与亚洲夏季风有关且反映季风局地性的则是副热带高压带南侧的东风带上的波状扰动；东风带上的经向风分量存在着纬向传播且传播方向和扰动幅度与印度、东亚季风子系统有关；随着北半球夏季的到来，亚洲季风区大气运动的斜压模有较大的增长且斜压运动动能占气柱内整层平均总动能的绝大部分，而在赤道附近的其他经度上则是正压运动动能的成分明显增长。 相似文献
7.
进一步了球面上正压准地转运动中不稳定波的Ｈｏｗａｒｄ半圆定理，发现在某些特定的基本流分布下，该定理可以发展成圆弧定理，由此得到不稳定扰动增长率的更为精确的估计式。 相似文献
8.
中尺度对称不稳定和横波不稳定的波动性质 总被引：2，自引：0，他引：2
使用纬向线性以及非线性切变基流下中尺度扰动的Boussinesq近似方程组，讨论了两种典型的中尺度扰动发生对称不稳定或者横波不稳定时，其不稳定的一些特征以及扰动的波动性质。研究结果表明：（1）对于扰动的等位相面平行于基本气流方向的对称性扰动来说，对称不稳定的波动实质是沿着与基本气流方向相垂直的方向传播的重力惯性内波的不稳定。基流二次切变对于中尺度对称扰动来说是一个不稳定因子，并且驱动不稳定的中尺度对称扰动在南北方向传播；（2）对于扰动的等位相面垂直于基本气流方向的横波性扰动来说，在基本气流为常数或者只具有线性切变的情况下，此时根本不存在涡旋Rossby波，横波不稳定的波动实质则是沿着基本气流方向双向传播的重力惯性内波的不稳定。如果考虑基本流场的风速存在二次切变或者非线性切变时，此时就会产生一支新的波动（涡旋Rossby波），涡旋Rossby波相对于基本气流^-U0是单向传播的，涡旋Rossby波产生的物理根源是基本流场的风速^-U二次切变（β＊=^-Uzz≠0），此时横波型不稳定可能是混合的涡旋Rossby——重力波的不稳定。实际大气中，涡旋Rossby波对于中够尺度对流云核、暴雨团等天气系统的发生、发展和演变的物理机制具有极其重要的意义。 相似文献
9.
初步分析了两类大尺度园形基流上扰动的正压不稳定问题,对不稳定模态的扰动增长率和扰动振幅的径向分布等进行了讨论. 相似文献
10.
涡旋Rossby波传播的不同特征 总被引：3，自引：4，他引：3
用一个柱坐标系涡度正压方程的半谱模式，实施了4组试验，研究了不同初始扰动涡旋分布条件下，涡旋Rossby波的传播特征问题。结果指出：正值、负值相间的沿方向角二波分布的初始场，与只有正值涡旋的初始场相比，其后涡旋Rossby波的传播有明显的差别，即前者向内传播的涡量比后者衰减得快，而且前者是单峰传播，后者是双峰传播。 相似文献
11.
Study on the Dynamic Process of the Onset of South China Sea Summer Monsoon 总被引：1，自引：0，他引：1 
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下载免费PDF全文 By using the NCEP/NCAR reanalysis data from 1958 to 1997, we first looked into the atmospheric flow conditions in the one month immediately prior to the onset of the South China Sea summer monsoon （SCSSM） each year. A monthly-averaged zonal basic flow of 40-yr composite was then calculated. The stability of Rossby wave in the basic flow was studied based on the spherical barotropic vorticity equation. Furthermore, the spectral function expansion method was adopted to define and compute the evolvement of a developing wave packet. The results indicate that there exists barotropic instability of spherical Rossby wave in the climatically-averaged flow field before the SCSSM onset. The instability is triggered by the westerly jet stream in the Southern Hemisphere, and the strongest instable perturbation lies to the south of the westerly jet stream. The peak of the developing spherical Rossby wave packet propagates from mid and high latitudes to low latitudes, though not crossing the equator, spurring the cumulus convection in the tropical zones. The eruption of the cumulus convection and its spread to monsoon regions help to speed up the adjustment of the general circulation and the SCSSM onset. It is concluded that elements that contribute to the SCSSM onset are on global scale, albeit the onset itself looks like a local phenomenon. 相似文献
12.
With the aid of a global barotropic model, the role of the interaction of the synoptic-scale disturbance and the planetary flow in block onset is examined by a 4-dimensional variational approach. A cost function is defined to measure the squared errors of the forecasted stream functions during block onset period (day 4 and day 5 in this study) over a selected blocking domain. The sensitivity of block onset with respect to the initial synoptic-scale disturbance is studied by examining the gradient of the defined cost function with respect to the initial (during the first 24 hours) vorticity forcing, which is evaluated by the adjoint integration. Furthermore, the calculated cost function and gradient are connected with the limited-memory quasi-Newton optimization algorithm for solving the optimal initial vorticity forcing for block onset. For two studied cases of block onset (northern Atlantic and northern Pacific) introducing the optimal initial vorticity forcing, the nonlinear barotropic advection process mostly reconstructs these blocking onset processes. The results show that the formation of blocking can be correctly described by a barotropic nonlinear advection process, in which the wave- (synoptic-scale) flow (planetary-scale) interaction plays a very important role. On an appropriate planetary-scale flow, a certain synoptic-scale disturbance can cause the blocking onset by the interaction between the synoptic scale perturbations and the planetary scale basic flows. The extended forecasts show that the introduction of the optimal initial vorticity forcing can predict the blocking process up to the 7th or 8th day in this simple model case. The experimental results in this study show that the 4-dimensional variational approach has a good potential to be applied to study the dynamics of the medium-range weather processes. This simple model case study is only an initial trial. Applying the framework in this study to a complex model will further our understanding of the mechanism of the atmospheric/oceanic processes and improve their prediction. 相似文献
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14.
Luo Zhexian 《Acta Meteorologica Sinica》1996,10(1):13-21
By using the linearized barotropic vorticity equation in polar coordinates the stability of perturbations on a large scale circular basic flow is transformed into a generalized eigenvalue problem,yielding the relationship between the growth rate of the amplitude of perturbations and the azimuthal wave number. Then, numerical experiments whose integration time is 60 model hours are performed in terms of a quasi-geostrophic barotropic model in Cartesian coordinates using the perturbation stream function field of unstable mode superimposed on a strong and weak circular basic flows as the initial fields. The experimental results reveal that the amplitudes of the initial perturbations in the model atmosphere grow with time. The amplitude of the perturbations superimposedon the strong circular basic flow grows quicker and forms a spiral-band-like structure. 相似文献
15.
罗哲贤 《Acta Meteorologica Sinica》1996,(1)
By using the linearized barotropic vorticity equation in polar coordinates the stability of pertur-bations on a large scale circular basic flow is transformed into a generalized eigenvalue problem,yielding the relationship between the growth rate of the amplitude of perturbations and the az-imuthal wave number. Then, numerical experiments whose integration time is 60 model hours areperformed in terms of a quasi-geostrophic barotropic model in Cartesian coordinates using the per-turbation stream function field of unstable mode superimposed on a strong and weak circular basicflows as the initial fields. The experimental results reveal that the amplitudes of the initial pertur-bations in the model atmosphere grow with time. The amplitude of the perturbations superimposedon the strong circular basic flow grows quicker and forms a spiral-band-like structure. 相似文献
16.
Based on a barotropic vortex model, generalized energy-conserving equation was derived and two
necessary conditions of basic flow destabilization are gained. These conditions correspond to generalized
barotropic instability and super speed instability. They are instabilities of vortex and gravity inertial wave
respectively. In order to relate to practical situation, a barotropic vortex was analyzed, the basic flow of which is similar to lower level basic wind field of tropical cyclones and the maximum wind radius of which is 500 km.
The results show that generalized barotropic instability depending upon the radial gradient of relative vorticity
can appear in this vortex. It can be concluded that unstable vortex Rossby wave may appear in barotropic vortex. 相似文献
17.
Lu Weisong 《Acta Meteorologica Sinica》1989,3(1):90-97
Based on a non-frictional and non-divergent nonlinear barotropic vorticity equation and its solutions oftravelling waves,the criteria for linear and nonlinear barotropic instability are gained respectively at an equilibriumpoint of the equation on a phase plane.The linear and nonlinear analytical solutions to instability waves arealso found.The computational results show that if their amplitudes are equal at the initial time,the amplitudeincrements of nonlinear instable barotropic wave are always less than those of linear instable barotropic wave.The nonlinear effects can slow down the exponential growth of linear instability.The time needed for makingthe amplitude double that of initial time by instabilities,is about 6h for linear instability and about 18h fornonlinear instability,the latter is in agreement with the observations in the real atmosphere. 相似文献
18.
Barotropic Process Contributing to the Formation and Growth of Tropical Cyclone Nargis 总被引：1，自引：0，他引：1
This study reveals the barotropic dynamics associated with the formation and growth of tropical cyclone Nargis in 2008,during its formation stage.Strong equatorial westerlies occurred over the southern Bay of Bengal in association with the arrival of an intraseasonal westerly event during the period 22-24 April 2008. The westerlies,together with strong tropical-subtropical easterlies,constituted a large-scale horizontal shear flow,creating cyclonic vorticity and thereby promoting the incipient disturbance that eventually evolved into Nargis.This basic zonal flow in the lower troposphere was barotropically unstable,with the amplified disturbance gaining more kinetic energy from the easterly jet than from the westerly jet during 25-26 April. This finding suggests that more attention should be paid to the unstable easterly jet when monitoring and predicting the development of tropical cyclones.Energetics analyses reveal that barotropic energy conversion by the meridional gradient of the basic zonal flow was indeed an important energy source for the growth of Nargis. 相似文献
19.
使用地形强迫正压涡度方程模式的输出资料，讨论了流函数异常EOF展开时间系数的特征时间尺度。由此研究了异常持续类型的确定及其对可预报性的影响问题。结果表明:异常持续类型与环流的演变有十分密切的关系，初始状态环流的结构对可预报性有明显的影响。最后探讨了对中期和延伸期数值预报的可能应用。 相似文献
20.
线性和非线性正压不稳定 总被引：4，自引：0，他引：4
本文从无摩擦的、无辐散的非线性正压涡度方程出发,利用行波解,在相平面上对方程的平衡点分别导得线性和非线性正压不稳定判据,与传统的不稳定判据作了比较分析。还求得了线性和非线性不稳定波的解析解,计算结果表明,若初始时刻两者波幅相等,则以后非线性不稳定波幅的增长值总小于线性不稳定波幅的增长值。非线性效应有使线性不稳定指数型增长减缓的作用。不稳定使得振幅比初始时刻增大一倍所需要的时间,对于线性不稳定大约是6h,对于非线性不稳定大约是18h,后者与实际大气中的观测事实是一致的。 相似文献