共查询到20条相似文献，搜索用时 342 毫秒
1.
Several numerical experiments are conducted to examine the influence of mesoscale, bottom topography roughness on the inertial circulation of a wind-driven, mid-latitude ocean gyre. The ocean model is based on the quasi-geostrophic formulation, and is eddy-resolving as it features high vertical and horizontal resolutions (six layers and a 10 km grid). An antisymmetrical double-gyre wind stress curl forces the baroclinic modes and generates a strong surface jet. In the case of a flat bottom, inertia and inverse energy cascade force the barotropic mode, and the resulting circulation features strong, barotropic, inertial gyres. The sea-floor roughness inhibits the inertial circulation in the deep layers; the barotropic component of the flow is then forced by eddy-topography interactions, and its energy concentrates at the scales of the topography. As a result, the baroclinicity of the flow is intesified: the barotropic mode is reduced with regard to the baroclinic modes, and the bottom flow (constrained by the mesoscale sea-floor roughness) is decoupled from the surface flow (forced by the gyre-scale wind). Rectified, mesoscale bottom circulation induces an interfacial form stress at the thermocline, which enhances horizontal shear instability and opposes the eastward penetration of the jet. The mean jet is consequently shortened, but the instantaneous jet remains very turbulent, with meanders of large meridional extent. The sea-floor roughness modifies the energy pathways, and the eddies have an even more important role in the establishment of the mean circulation: below the thermocline, rectification processes are dominant, and eddies transfer energy toward permanent mesoscale circulations strongly correlated with topography, whereas above the thermocline mean flow and eddy generation are influenced by the mean bottom circulation through interfacial stress. The topography modifies the vorticity of the barotropic and highest baroclinic modes. Vorticity accumulates at the small topographic scales, and the vorticity content of the highest modes, which is very weak in the flat-bottom case, increases significantly. Few changes occur in surface-intensified modes. In the deep layers of the model, the inverse correlation between relative vorticity and topography at small scales ensures the homogenization of the potential vorticity, which mainly retains the largest scales of the bottom flow and the scale of β. 相似文献
2.
In this paper, the adaptation process in low latitude atmosphere is discussed by means of a two-layer baroclinic model on the equator β plane, showing that the adaptation process in low latitude is mainly dominated by the internal inertial gravity waves. The initial ageostrophic energy is dispersed by the internal inertial gravity waves, and as a result, the geostrophic motion is obtained in zonal direction while the ageostro-phic motion maintains in meridional direction, which can be called semi-geostrophic balance in barotropic model as well as semi-thermal-wind balance in baroclinic model. The vertical motion is determined both by the distribution of the initial vertical motion and that of the initial vertical motion tendency, but it is unrelated to the initial potential vorticity. Finally, the motion tends to be horizontal. The discussion of the physical mechanism of the semi-thermal-wind balance in low latitude atmosphere shows that the achievement of the semi-thermal-wind balance is due to the adjustment between the stream field and the temperature field through the horizontal convergence and divergence which is related to the vertical motion excited by the internal inertial gravity waves. The terminal adaptation state obtained shows that the adaptation direction between the mean temperature field and the shear flow field is determined by the ratio of the scale of the initial ageostrophic disturbance to the scale of one character scale related to the baroclinic Rossby radius of deformation. The shear stream field adapts to the mean temperature field when the ratio is greater than 1, and the mean temperature field adapts to the shear stream field when the ratio is smaller than 1. 相似文献
3.
Changes in the normal mode energetics of the general atmospheric circulation are assessed for the northern winter season (DJF) in a warmer climate, using the outputs of four climate models from the Coupled Model Intercomparison Project, Phase 3. The energetics changes are characterized by significant increases in both the zonal mean and eddy components for the barotropic and the deeper baroclinic modes, whereas for the shallower baroclinic modes both the zonal mean and eddy components decrease. Significant increases are predominant in the large-scale eddies, both barotropic and baroclinic, while the opposite is found in eddies of smaller scales. While the generation rate of zonal mean available potential energy has globally increased in the barotropic component, leading to an overall strengthening in the barotropic energetics terms, it has decreased in the baroclinic component, leading to a general weakening in the baroclinic energetics counterpart. These global changes, which indicate a strengthening of the energetics in the upper troposphere and lower stratosphere (UTLS), sustained by enhanced baroclinic eddies of large horizontal scales, and a weakening below, mostly driven by weaker baroclinic eddies of intermediate to small scales, appear together with an increased transfer rate of kinetic energy from the eddies to the zonal mean flow and a significant increase in the barotropic zonal mean kinetic energy. The conversion rates between available potential energy and kinetic energy, C, were further decomposed into the contributions by the rotational (Rossby) and divergent (gravity) components of the circulation field. The eddy component of C is due to the conversion of potential energy of the rotational adjusted mass field into kinetic energy by the work realized in the eddy divergent motion. The zonal mean component of C is accomplished by two terms which nearly cancel each other out. One is related to the Hadley cell and involves the divergent component of both wind and geopotential, while the other is associated to the Ferrel cell and incorporates the divergent wind with the rotationally adjusted mass field. Global magnitude increases were found in the zonal mean components of these two terms for the warmer climate, which could be the result of a strengthening and/or widening of both meridional cells. On the other hand, the results suggest a strengthening of these conversion rates in the UTLS and a weakening below, that is consistent with the rising of the tropopause in response to global warming. 相似文献
4.
Charles A. Lin 《Dynamics of Atmospheres and Oceans》1985,9(1):1-16
Baroclinic instability of a zonal flow with latitudinal structure is examined using a nonlinear quasi-geostrophic, two-level β-plane model. An initially small perturbation with the structure of the linearly most unstable mode is allowed to grow to finite amplitude through nonlinear interaction. Because of latitudinal asymmetries of the basic zonal flow, a spectrum of meridional modes is generated in the perturbation. The time evolution of zonal wind and perturbation meridional structures, and their Fourier meridional mode spectra are examined. The radius of deformation is an important meridional scale in both the zonal flow and perturbation. This is especially true during the barotropic decay phase of the baroclinic wave. Time series of energy conversion terms show there is no energy accumulation. 相似文献
5.
作为气象研究中的一个难点问题,暖区暴雨的动力学一直为学界所关注。基于多尺度子空间变换(MWT)以及基于MWT的局地多尺度能量学分析和正则传输理论,对2018年5月7日的一次闽南暖区暴雨进行研究以了解其多尺度动力过程。首先将原始物理量场重建到三个尺度子空间:背景流子空间、天气尺度子空间和暴雨子空间。重构场上可以很好地看出背景环流尺度的高低空急流,以及暴雨尺度上的垂直环流。以往的研究普遍认为暖区暴雨的动力过程具有弱斜压性这一特征,而就此次事件而言,正压失稳和斜压失稳都起着很关键的作用,暴雨主要落区内既发生了正压失稳,也发生了斜压失稳。研究表明,对流层不同高度上的动力学存在差异,低层主要表现为正压失稳,天气尺度子空间与背景流子空间向暴雨子空间传输的动能相当; 中层主要是混合失稳,除正压失稳外,斜压正则传输也将有效位能从背景流子空间传输到了暴雨子空间,再通过浮力转换将有效位能转为动能,从而维持暴雨在中层的动力过程; 高层则与低层相似,但只存在背景流子空间向暴雨子空间的能量传输。 相似文献
6.
Summary The problem of along-stream ageostrophic frontogenesis is studied by employing a numerical model at meso-alpha and meso-beta scales in simulations of the downstream circulations over the Front Range of the Rocky Mountains. Three-dimensional real data simulations at these two scales of motion are used to diagnose the transition from semigeostrophic cross-stream frontogenesis accompanying a propagating baroclinic upper-level jet streak to midtropospheric along-stream ageostrophic frontogenesis. This along-stream ageostrophic frontogenesis results from the perturbation of the jet streak by the Rocky Mountain range. The case study represents an example of internal wave dynamics which are forced by the drag of the Rocky Mountains on a strong jet streak in the presence of a low-level inversion.The simulation results indicate that, unlike semi-geostrophic frontogenesis, a front (which is alligned perpendicular to the axis of the jet stream) may form when significant adiabatic heating occurs within a stratified shear flow over horizontal length scales shorter than the Rossby radius of deformation. The mechanism responsible for the frontogenesis is the growth of the divergent along-stream wind velocity component which becomes coupled to the front's along-stream pressure gradient force. This nonlinear interaction produces hydrostatic mesoscale frontogenesis as follows: 1) vertical wind shear in the along-stream plane strengthens resulting in the increasingly nonuniform vertical variation of horizontal temperature advection as the ageostrophic wind component grows in magnitude downstream of the meso-scale terrain-induced adiabatic heating, 2) increasing along-stream differential vertical motions (i.e., along-stream thermally indirect circulation with warm air sinking to the west and cold air rising to the east) tilt the vertical gradient of isentropes into the horizontal as the vertical temperature gradient increases due to the previous process in proximity to horizontal gradients in the along-stream component of the ageostrophic wind, 3) as tilting motions act to increase the along-stream horizontal temperature gradient, the along-stream confluence acts to nonuniformly increase the along-stream frontal temperature gradient which increases the along-stream pressure gradient force resulting in further accelerations, ageostrophy, and frontal steepening as part of a scale contraction process.The evolution of the aforementioned processes results in the three-dimensional hydrostatic frontogenesis accompanying the overturning of isentropic surfaces. These adjustments act to turn air parcels to the right of the southwesterly geostrophic wind vector at successively lower atmospheric levels as the scale contraction continues. This simulated along-stream front is verified from diagnostic analysis of the profiler-derived temperature and wind fields.With 17 Figures 相似文献
7.
Fronts are ubiquitous dynamic processes in the ocean, which play a significant role in the ocean dynamical and ecological environments. In this paper strong temperature fronts are investigated on the shelf of the Northern South China Sea using high resolution satellite data. These fronts have large horizontal gradients exceeding 1 °C km−1 with spatial scales around several kilometers. The fronts generate meanders and eddies due to baroclinic instability, since these instabilities have spatial scales around the local first baroclinic mode deformation radius. The estimated Rossby number of the fronts is O(0.4), suggesting that the fronts tend to be ageostrophic and show submesoscale features. The Finite Size Lyapunov Exponent analysis of the generation mechanism indicates that the fronts are tightly related to the combined flow straining of geostrophic and Ekman currents. 相似文献
8.
管兆勇 《南京气象学院学报》2000,23(3):313-322
使用ＮＣＥＰ／ＮＣＡＲ４０年（１９５８～１９９７年）月平均再分析资料,通过动力学论断研究了大气斜压／正压运动动能的变化及其相互转换,分析了亚洲季风变动与这两种动能变化的联系。指出：季风区大气运动动能的组成和变化具有独特的特征。冬季风时期大气斜压运动动能与正压运动动能具有正相关线性关系,斜压运动能向正压运动动能转换;春、秋季无论是东亚还是印度季风区斜压运动动能与正压运动动能之间转换都处于极小值,只是 相似文献
9.
西北太平洋纬向扰动海温经验正交函数（EOF）分解第一和第三模态、第二和第四模态分别代表同期黑潮延伸体和亲潮强弱的配置关系,将两者的典型位相合成,可以分别得到延伸体收缩和扩张状态时的典型模态海温,本文以此及气候态海温作为初始海温强迫场,利用CESM1.2.0模式,讨论了延伸体的系统变异对北太平洋风暴轴的影响及其在不同能量转换过程的主要影响机制,结果表明,延伸体收缩状态下,北太平洋风暴轴强度整体加强,而扩张模态下强度减弱。空间分布上,收缩模态下,风暴轴主要体现为经向方向的变化,中心及其以北强度加强,中心以南减弱;扩张状态下,则主要表现为纬向方向的差异,中心及以西强度减弱明显,中心以东有所增强。对能量转换的诊断分析表明,正压能量转换过程对涡动动能的变化贡献很小,且在风暴轴中心附近,其作用主要为消耗涡动动能,延伸体收缩状态下其消耗作用增强,而扩张状态下消耗作用减弱,这一差异主要是由于不同海温异常强迫下瞬变涡旋的形变不同造成;斜压有效位能释放比正压能量转换大一个量级以上,该过程几乎全部通过基流的经向温度梯度和经向涡动热量输送的相互作用完成,在这一过程中大气斜压性（经向温度梯度）起了关键性作用,大气斜压性异常、基流经向温度梯度异常、斜压有效位能释放异常与风暴轴异常的空间分布均具有较好的对应关系,该过程可能也是延伸体海温异常影响北太平洋风暴轴的主要物理过程;涡动有效位能需要进一步转换为涡动动能才能产生瞬变涡旋运动,涡动有效位能释放的量级与斜压有效位能的释放相当,但数值要小,这一过程通过冷暖空气的上升下沉运动完成,延伸体异常模态下,扰动垂直速度和扰动温度的负相关性的变化与涡动有效位能向涡动动能转换的变化也有较好的对应关系。 相似文献
10.
根据绝热无耗散大气总动能和总位涡拟能守恒的特性，本文讨论了二维平面上动能谱分布随时间变化的规律。结果表明，大气谱动能变化必然涉及至少三种不同的空间尺度；或遵从双向转化原则，或遵从单向转化原则。在讨论能量的双向转化和单向转化条件的基础上，本文还分别研究斜压大气和正压大气中能量转化的一般状况。 相似文献
11.
Song-Lak Kang 《Boundary-Layer Meteorology》2009,132(1):59-81
A theoretical approach suggests that the surface heterogeneity on a scale of tens of kilometres can generate mesoscale motions
that are not in a quasi-stationary state. The starting point of the theoretical approach is the equations of horizontal velocity
and potential temperature that are low-pass filtered with a mesoscale cut-off wavelength. The transition of the generated
mesoscale motions from a quasi-stationary state to a non-stationary state occurs when horizontal advection is strong enough
to level out the potential temperature gradient on the surface heterogeneity scale. Large-eddy simulations (LES) suggest that
the convective boundary layer (CBL) changes to a non-stationary state when forced by a surface heat-flux variation of amplitude
of 100W m−2 or higher and a wavelength of the order of 10 km. Spectral analysis of the LES reveals that when the mesoscale motions are
in a quasi-stationary state, the energy provided by the surface heat-flux variation remains in organized mesoscale motions
on the scale of the surface variation itself. However, in a non-stationary state, the energy cascades to smaller scales, with
the cascade extending down into the turbulence scale when the wavelength of the surface heat-flux variation is on a scale
smaller than 100 times the CBL height. The energy transfer from the generated mesoscale motions to the CBL turbulence results
in the absence of a spectral gap between the two scales. The absence of an obvious spectral gap between the generated mesoscale
motions and the turbulence raises questions about the applicability of mesoscale models for studies on the effect of high-amplitude
surface heterogeneity on a scale of tens of kilometres.
The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
12.
本文利用中国自动站与CMORPH（Climate Prediction Center Morphing technique for the production of global precipitation estimates）融合的逐时降水量0.1°网格数据集资料挑选出一次典型的梅雨锋暴雨个例,运用WRF中小尺度模式进行模拟,对模拟得到的高分辨率结果进行Barnes滤波,最后将滤波结果代入动能和位能方程中,目的是定量地分析各个尺度能量的变化以及它们之间的相互作用对暴雨强度的影响。研究发现:模式模拟的降水过程和强度与实况较为吻合,推导的能量方程适用于这次暴雨过程。三种尺度能量之间的相互作用包含了各种跨尺度能量的相互作用。在整个暴雨过程中,跨尺度之间的斜压能量转换包括位能向动能的能量转换和动能向位能的能量转换。同尺度之间的斜压能量转换总是单向的,且量值较大,动能的强度主要靠位能向动能的能量转换来维持。斜压能量转换的多少影响着暴雨的强弱。大尺度斜压能量转换在中高层比较强,中尺度斜压能量转换在低层较强,尤以β中小尺度系统变化最为显著,β中小尺度系统扰动是影响暴雨强度的关键系统。风切变的大小影响各尺度动能之间的能量转换。温度或位温梯度的大小影响各尺度位能之间的能量转换。位能与动能之间的能量转换主要与各尺度垂直速度和温度的垂直分布有关,暖空气上升冷空气下沉是各个尺度位能向动能转换的主要过程。 相似文献
13.
斜压气流的中尺度稳定性 Ⅱ.横波型不稳定 总被引：18，自引：1，他引：18
本文是“斜压气流的中尺度稳定性”的第二部份,讨论基本流对横波型中尺度扰动的稳定性。 将f-平面的Eady模型推广到非地转情况,得到斜压基流的双模态不稳定谱:在天气尺度和次天气尺度上出现Eady模态,在几十至几百公里的惯性尺度上出现非地转斜压中尺度模态。在垂直剖面上中尺度模态呈非对称“猫眼”流型,在水平方向上散度与涡度交替分布。对同一线性风速廓线,中尺度模态的发展率约为Eady模态的4倍,两种扰动发展的主要能源都是旋转基流中储存的斜压有效位能。 相似文献
14.
Zhang Kesu 《Acta Meteorologica Sinica》1988,2(3):313-322
This is the second part of "Mesoscale Instability of a Baroclinic Basic Flow" which discusses the instability of a basic flow against mesoscale perturbations of transversal type.A bi-mode instability spectrum is obtained by generalizing the Eady model to ageostrophic regime in an f-plane:Eady modes present at the synoptic and subsynoptic scales,while the ageostrophic baroclinic mesoscale modes present at the inertial scales of a few tens to hundreds kilometers.The mesoscale mode is featured by an asymmetric "eat eyes" pattern in the vertical cross section and by an alternative distribution of divergence and vorticity in the horizontal direction.The growth rates of the mesoscale modes are about four times larger than those of Eady modes in magnitudes for the same wind profile.The major energy source for development both Eady mode and mesoscale mode is the baroclinic available energy stored in the rotational basic flow. 相似文献
15.
Peter D. Killworth 《Dynamics of Atmospheres and Oceans》1980,4(3):143-184
The linear normal mode instabilities of a parallel shear flow which varies both vertically (z) and meridionally (y) in a quasigeostrophic, rotating, stratified fluid are considered. The β effect (variation of Coriolis parameter with y) is included. Both two-layer and continuous fluids are treated. Attention is concentrated on the types of instability possible for a given shear flow. It is found that the instability can be described adequately by three nondimensional parameters: Λ, the ratio of the horizontal length scale of the shear to the internal deformation radius: δ, which is either the ratio of layer depths in the two-layer fluid or the fractional depth of variation of the stratification in the continuous fluid; and β, suitably nondimensionalized.Asymptotic analyses, confirmed by direct numerical solutions, are performed for conditions in which various parameters become large or small. The β effect is essentially quantitative, whereas Λ and δ define the type of instability as barotropic (if the kinetic energy of the mean flow feeds the growing perturbations), baroclinic (if the available potential energy of the mean flow feeds the perturbations) or mixed (a combination of the two).The case of large Λ (the most relevant for oceanographic applications) is treated in detail. It is shown that y-independent problems have only limited relevance. For a fixed deformation radius and the y scale of the mean flow increasing without limit, the asymptote is not the case of no y variation in the mean flow. 相似文献
16.
17.
斜压模,正压模和异常海温时间变化的位相关系 总被引：5，自引：2，他引：3
利用１９５８－１９９７共４０年ＮＥＣＰ／ＮＣＡＲ再分析大气资料和ＧＩＳＳＴ海表温度资料集,研究了大气斜压模、正压模与海表温度变化这三者之间位相关系,并从大气动力学方程组解释了这些位相关系。指出：大气了斜压西风异常时间变化的位相落略微后于ＳＳＴＡ的变化位相,而正压西风异常的变化位相超前于ＳＳ－ＴＡ变化位相。斜压纬向风异常的变化又超前于正压异常纬向风的变化。斜压纬向风异常与海表温度异常之间存在正相关关 相似文献
18.
Characteristics of Submeso Winds in the Stable Boundary Layer 总被引：2，自引：2，他引：0
Larry Mahrt 《Boundary-Layer Meteorology》2009,130(1):1-14
The characteristics of submeso motions in the stable boundary layer are examined using observations from networks of sonic
anemometers with network sizes ranging from a few hundred metres to 100 km. This study examines variations on time scales
between 1 min and 1 h. The analysis focuses on the behaviour of the spectra of the horizontal kinetic energy, the ratios of
the three velocity variances, their kurtosis, the dependence of horizontal variability on time scale, and the inter-relationship
between vertical vorticity, horizontal divergence and deformation. Motions on larger time and space scales in the stable boundary
layer are found to be nearly two-dimensional horizontal modes although the ratio of the vorticity to the divergence is generally
on the order of one and independent of scale. One exception is a small network where stronger horizontal divergence is forced
by a decrease in surface roughness. The horizontal variability, averaged over 1 h, appears to be strongly influenced by surface
heterogeneity and increases with wind speed. In contrast, the time dependence of the horizontal structure on time scales less
than one hour tends to be independent of wind speed for the present datasets. The spectra of the horizontal kinetic energy
and the ratio of the crosswind velocity variance to the along-wind variance vary substantially between networks. This study
was unable to isolate the cause of such differences. As a result, the basic behaviour of the submeso motions in the stable
boundary layer cannot be generalized into a universal theory, at least not from existing data. 相似文献
19.
The effect of barotropic shear on baroclinic instability has been investigated using both a linear quasi-geostrophic β-plane channel model and a multilevel primitive equation model on the sphere when a nonmodal disturbance is used as the initial perturbation condition. The analysis of the initial value problem has demonstrated the existence of a rapid transient growth phase of the most unstable mode. The inclusion of a linear barotropic shear reduces initial rapid transient growth, although at intermediate times the transient growth rates of the sheared cases can be larger than in the unsheared case owing to downgradient eddy momentum fluxes. Certain disturbances can amplify by factors of 4.5–60 times (for the L2 norm), or 3–30 times (for the perturbation amplitude maximum), as large as disturbances based on the linear normal modes. However, linear horizontal shear always reduces the amplification factors. The mechanism is that the shear confines the disturbance meriodionally and therefore limits the energy conversion from the zonal available potential energy to eddy energy. The effect of barotropic shear on the transient growth is not changed much in the presence of either thermal damping or Ekman pumping. Nonmodal integrations of baroclinic wave lifecycles show that the energy level reached by eddies is not very sensitive to the structure of the initial disturbance if the amplitude of the initial disturbance is small. Although in some cases the eddy kinetic energy level reached by the wave integrated from nonmodal disturbance can be 25–150% larger than the normal mode integrations, barotropic shear, characterized by large shear vorticity with small horizontal curvature, always reduces the eddy kinetic energy level reached by the wave, confirming the results of normal mode studies. 相似文献
20.
A class of non-linear instabilities of a vertically sheared zonal flow is discussed. This is a type of baroclinic instability that lies outside the purview of a linear eigenmode analysis of baroclinic instability problems. The form taken by the instability is that of an ensemble of three neutral Rossby waves whose amplitudes are slowly modified by their mutual non-linear interactions. For a triad of small amplitude, these interactions introduce a weak, vertical variation of phase to the structure of the individual waves. This allows the generation of rectified heat fluxes and an exchange of energy with the mean flow.This instability exhibits explosive growth and spans a range of horizontal wavenumbers that exceeds the range that is unstable in the corresponding linear model. It is shown that the type of instability discussed can only occur when the model used admits unstable eigenmodes as well as neutral Rossby waves.The mechanism for the non-linear instability discussed here is believed to be fairly general and should exist also in the context of a horizontally sheared flow where it would take the form of a barotropic instability. 相似文献