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1.
With the Ekman momentum approximation,the influence of atmospheric baroclinity on the dynamics of boundarylayer is studied.Some new results are obtained.These results show that the atmospheric baroclinity plays an importantrole in altering the horizontal velocity of Ekman boundary layer and its angle with the horizontal wind velocity compo-nent near the surface.There are three different physical factors affecting the nonlinear Ekman suction,the vertical mo-tion at the top of boundary layer:first,barotropic geostrophic relative vorticity at the ground;second,the thermal windvorticity induced by the baroclinity;and third,the nonlinear interaction between the barotropic geostrophic relativevorticity and the baroclinic thermal wind vorticity.These results may provide a better physical basis for theparameterization of boundary layer and the interpretation of the numerical modeling results. 相似文献
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本文利用Ekman动量近似研究了斜压性对Ekman层动力学的影响,得到了一些新的结果。大气斜压性对Ekman层的水平风速分布及近地面的风速矢的水平分量夹角有重要的改变作用。斜压边界层顶部的非线性Ekman抽吸(垂直运动)由三个不同的物理因子决定,第一、正压性的地面地转涡度,第二、斜压性作用产生的热成风涡度,第三、正压性的地面地转涡度与斜压性的热成风涡度的非线性相互作用。这些理论结果为边界层的参数化及数值模拟结果的解释提供物理基础。 相似文献
3.
A model of the planetary boundary layer is used to determine the field of vertical motion over large-scale orography. This model represents Ekman boundary-layer dynamics modified by the inclusion of accelerations of the geostrophic wind under the geostrophic momentum approximation. The orography is represented by a circular mountain. The inviscid solution is provided by the sum of a constant translation and a steady, uniform potential vorticity, anticyclonic vortex. The boundary-layer solution vanishes on the mountain, but is matched to the inviscid solution as the top of the boundary layer is approached. The vertical velocity field at the top of the boundary layer is determined by integration of the continuity equation. The field of motion is largely determined by descent from above into the anticyclonic circulation, as in the classical Ekman model. Contributions that arise from the inclusion of accelerations are associated with boundary-layer advection and ageostrophic divergence that produce vorticity tendencies. Finally, the boundary-layer vertical motion is shown to be comparable in magnitude to the vertical motion forced by inviscid flow over the orography, although the distributions of each are significantly different. Effects of mountain asymmetry and a changing pressure field, that can be treated more fully by numerical model simulations, are not considered in the present study.On leave at the University of Colorado, 1990. 相似文献
4.
Numerical results indicate that advection of momentum in the boundary layer may significantly alter both the structure of the planetary boundary layer and its influence on the overlying free atmosphere. However, due to the nonlinearity of the inertial terms, it is always difficult to obtain the analytical solution of the boundary-layer model that retains the flow acceleration. In order to overcome this difficulty, the geostrophic momentum (hereafter GM) approximation has been introduced into boundary-layer models. By replacing the advected momentum with the geostrophic wind, the effect of the flow acceleration is partially considered and the original nonlinear partial differential equation set is converted to ordinary differential equations, the solutions of which can be obtained easily with standard techniques. However, the model employing GM fails to capture the features of the boundary layer when the spatio-temporal variation of the boundary-layer flow cannot be properly approximated by the geostrophic wind. In the present work, a modified boundary-layer model with the inertial acceleration in a different approximate form is proposed, in which the advecting wind instead of the advected momentum is approximated by the geostrophic wind (hereafter GAM).Comparing the horizontal velocity and boundary-layer pumping obtained from the classical Ekman theory, and the model incorporating (i) GM and (ii) GAM, it is found that the model with GAM describes most facets of the steady well-mixed layer beneath a north-westerly flow with embedded mesoscale perturbations that is considered in the present work. Inspection of the solution of the model with GAM shows that, within the limit of the validation of the model (i.e., the Rossby number RO is not very large and the drag coefficient CD is not too small), the horizontal convergence (divergence) is strengthened by the effect of the inertial acceleration in the region of maximum positive (negative) geostrophic vorticity. Consequently, the boundary-layer pumping there is intensified. It is found that the intensification is firstly strengthened and then weakened as RO or CD increases. 相似文献
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Ekman边界层动力学的理论研究 总被引：5，自引：0，他引：5
大气边界层及其与自由大气之间的相互作用具有明显的非线性特征,而这些特征是经典Ekman理论所不能描述的,因此,发展中等复杂程度(介于完全模式与经典Ekman模型之间)的大气边界层动力学模式,简称中间模式,对人们从理论上认识大气边界层动力学过程的非线性特征具有重要意义。本文对目前最具代表性的几个中间边界层模型:地转动量近似边界层模型、Ekman动量近似边界层模型以及弱非线性边界层模型进行了总结和分析,阐述了Ekman层主要动力学特征。通过分析上述各模型的理论框架,揭示了各模型的物理意义及其在描述Ekman边界层基本动力特征上的优点和局限性,并指出尽管在细节定量描述上有差异,但各中间模型对Ekman层动力学特征的定性描述具有很好的一致性。对于这些Ekman边界层近似理论模型的进一步应用问题,主要回顾和总结了利用上述模型探讨地形边界层结构、大气锋生过程、低层锋面结构和环流以及边界层日变化、低空急流形成等动力学问题的研究,并对这些研究所揭示的Ekman层动力学特征及其对自由大气低层运动的影响进行了分析,结果表明,这些Ekman边界层近似模型可以较好地揭示大气边界层动力学特征,在大气边界层动力学及其与自由大气相互作用的研究上具有重要价值。另外,还对目前Ekman边界层理论研究中存在的问题进行了一些分析,提出了有待进一步研究的科学问题。 相似文献
7.
Ekman动量近似下中间边界层模式中的风场结构 总被引：2，自引：0，他引：2
发展了一个准三维的、中等复杂的边界层动力学模式，该模式包含了EKman动量近似下的惯性加速度和Blackadar的非线性湍流粘性系数，它进一步改进了Tan和Wu(1993）提出的边界层理论模型。该模型在数值计算复杂性上与经典Ekman模式相类似，但由于包含了Ekman动量近似下的惯性项，使得该模式比传统Ekman模式更近于实际过程。中详细地比较了该模式与其他简化边界层模式在动力学上的差异，结果表明：在经典的Ekman模式中，由于忽略了流动的惯性项作用，导致在气旋性切变气流（反气旋性切变气流）中风速和边界层顶部的垂直速度的高估（低估），而在半地转边界层模式中，由于高估了流动惯性项的作用，结果与经典Ekman模式相反。同样，该模式可以应用于斜压边界层，对于Ekman动量下的斜压边界层风场同时具有经典斜压边界层和Ekman动量近似边界层的特征。 相似文献
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An Analytical Study of the Diurnal Variations of Wind in a Semi-geostrophic Ekman Boundary Layer Model 总被引：1，自引：0，他引：1
A time-dependent semi-geostrophic Ekman boundary-layer model based on the geostrophic momentum approximation is used to study the diurnal wind variation in the planetary boundary layer (PBL) and the evolution of the low-level nocturnal jet (LLJ). The coefficient of eddy viscosity varies periodically with time, varies linearly with height in the surface layer and is constant above the surface layer. The influence of horizontal advection of momentum on the diurnal wind variation in the PBL, the development of inertial oscillations (IOs) and the formation of the LLJ are examined.In comparison with the Ekman solutions, the diurnal wind variation in semi-geostrophic Ekman boundary-layer dynamics has the following features: (1) the phase angle of the diurnal wind wave shifts with height, the rate of shifting is increased in anticyclonic regions and decreased in cyclonic regions, (2) the time of occurrence of the low-level maximum wind speed is later in anticyclonic regions and earlier in cyclonic regions, (3) the height of occurrence of the maximum wind speed is higher in the anticyclonic and lower in cyclonic regions, (4) the wind speed maximum and the amplitude of the diurnal wind variation are larger in anticyclonic and smaller in cyclonic regions, (5) the period of IOs is larger in anticyclonic regions and smaller in cyclonic regions, (6) anticyclonic vorticity is conducive to the generation of LLJ in the PBL. These features are interpreted by means of the physical properties of semi-geostrophic Ekman boundary-layer dynamics and inertial oscillation dynamics. 相似文献
9.
本文研究了斜压效应对地转动量Ekman流的影响。利用两变量奇异摄动方法求得了边界层中风场及顶部垂直速度的前二级一致有效渐近解析解,解中明显地反映了斜压情形地转风随高度变化(即热成风)的影响,尤其是其中一级近似解完全由热成风影响所致。在边界层顶垂直速度的解中导出了三种由斜压效应引起的Ekmon抽吸新物理因子,即热成风形变、热成风涡度及热成风涡度交叉项等抽吸因子。分析表明,这些因子只在具有水平温度梯度不均匀的系统(譬如锋区)中方能出现。文中还对锋区内上述三种抽吸因子的动力特征作了具体的分析,指出在锋区这样的强斜压系统中,此三种抽吸因子的贡献是显著的。下一文中,我们将利用本文所得理论解对斜压效应进行具体的定量计算。 相似文献
10.
Zhe-Min Tan 《Boundary-Layer Meteorology》2001,98(3):361-385
The WKB method has been used to develop an approximate solutionof the semi-geostrophic Ekman boundary layer with height-dependenteddy viscosity and a baroclinic pressure field. The approximate solutionretains the same simple form as the classical Ekman solution. Behavioursof the approximate solution are discussed for different eddy viscosityand the pressure systems. These features show that wind structure inthe semi-geostrophic Ekman boundary layer depends on the interactionbetween the inertial acceleration, variable eddy viscosity and baroclinicpressure gradient. Anticyclonic shear has an acceleration effect on theair motion in the boundary layer, while cyclonic shear has a decelerationeffect. Decreasing pressure gradient with height results in a super-geostrophicpeak in the wind speed profile, however the increasing pressure gradient withheight may remove the peak. Anticyclonic shear and decreasing the variableeddy viscosity with height has an enhanced effect on the peak.Variable eddy viscosity and inertial acceleration has an important role in thedivergence and vorticity in the boundary layer and the vertical motion at the top of the boundary layer that is called Ekman pumping. Compared to the constanteddy viscosity case, the variable eddy diffusivity reduces the absolute value ofEkman pumping, especially in the case of eddy viscosity initially increasing with height. The difference in the Ekman pumping produced by different eddy diffusivity assumptions is intensified in anticyclonic flow and reduced in cyclonic flow. 相似文献
11.
D. Seidov 《Dynamics of Atmospheres and Oceans》1996,25(1):25-55
The design and purposes of an intermediate model are discussed along with fundamentals of the model and results of numerical experiments. The main purposes of the model are reconstructions of the schemes of the ocean large-scale circulation and paleocirculation. For these problems numerical effectiveness is the key factor. A novel feature is the parameterization of the side wall Ekman boundary layers which was introduced to enable the use of geostrophy for calculating baroclinic velocity. This approach of connecting the side frictional layers with a non-viscous interior is not model-specific and can be used in any model employing geostrophy in the interior. The method can facilitate the no-flux and no-slip boundary conditions at the side walls in such models. Preliminary numerical experiments with simple basin geometry and idealized forcing aimed at a comparison with primitive equation and planetary geostrophic models are carried out. A direct comparison with the Geophysical Fluid Dynamics Laboratory (GFDL) primitive equation model was performed for a quantitative test of the proposed model. The results of the experiments are discussed in the context of the applicability of intermediate models for studying ocean climate dynamics. 相似文献
12.
利用最小能量原理讨论了非地转流经过调整后的平衡态的情况。对于两维模型来说，不平衡流通过调整获得的平衡终态依赖于初始不平衡场的分布特点。如果不平衡流的初始位涡是非均匀的，则其通过适应过程而达到的平衡状态不一定满足热成风关系；对于初始位涡是均匀的或为零的不平衡流来说，如果初始密度的水平变化不是非常显著，则运动最后可以达到热成风平衡状态，反之，最后的平衡态则不满足成风关系，对于这种情况，我们利用广义动量坐标系进行了详细讨论。 相似文献
13.
G.W. Wake G.N. Ivey J. Imberger N.R. McDonald 《Dynamics of Atmospheres and Oceans》2005,39(3-4):189-210
A laboratory study in a rotating stratified basin examines the instability and long time evolution of the geostrophic double gyre introduced by the baroclinic adjustment to an initial basin-scale step height discontinuity in the density interface of a two-layer fluid. The dimensionless parameters that are important in determining the observed response are the Burger number S=R/R0 (where R is the baroclinic Rossby radius of deformation and R0 is the basin radius) and the initial forcing amplitude (H1 is the upper layer depth). Experimental observations and a numerical approach, using contour dynamics, are used to identify the mechanisms that result in the dominance of nonlinear behaviour in the long time evolution, τ>2−1 (where τ is time scaled by the inertial period TI=2π/f). When the influence of rotation is moderate (0.25≤S≤1), the instability mechanism is associated with the finite amplitude potential vorticity (PV) perturbation introduced when the double gyre is established. On the other hand, when the influence of rotation is strong (S≤0.1), baroclinic instability contributes to the nonlinear behaviour. Regardless of the mechanism, nonlinearity acts to transfer energy from the geostrophic double gyre to smaller scales associated with an eddy field. In the lower layer, Ekman damping is pronounced, resulting in the dissipation of the eddy field after only 40TI. In the upper layer, where dissipative effects are weak, the eddy field evolves until it reaches a symmetric distribution of potential vorticity within the domain consisting of cyclonic and anticyclonic eddy pairs, after approximately 100TI. The functional dependence of the characteristic eddy lengthscale LE on S is consistent with previous laboratory studies on continuously forced geostrophic turbulence. The cyclonic and anticyclonic eddy pairs are maintained until viscous effects eventually dissipate all motion in the upper layer after approximately 800TI. The outcomes of this study are considered in terms of their contribution to the understanding of the energy pathways and transport processes associated with basin-scale motions in large stratified lakes. 相似文献
14.
在边界层动力学中，涡动粘性系数是影响边界层风场结构的一个重要参数。本文利用边界层动力学中的Ekman动量近似理论，给出了涡动粘性系数随高度缓变条件下的Ekman动量近似边界层模式解，着重讨论了边界层的风场结构、水平散度、垂直涡度以及边界层顶部的垂直速度。结果分析表明：与常值涡动粘性系数情况相比，在边界层低层随高度增加的涡动粘性系数可以导致低层边界层风速随高度迅速增加，即风速垂直切变增加，同时风速矢与地转风之间的夹角减小。惯性项作用可以导致上述作用在气旋性区域减小、而在反气旋性区域增大。随高度增加的涡动粘性系数导致水平散度绝对值、垂直涡度绝对值以及边界层顶部的垂直速度绝对值在气旋性区域减小，而在反气性旋区域增大。涡动粘性系数与惯性之间的非线性相互作用是边界层动力学中重要过程。 相似文献
15.
A three-dimensional model of the mesoscale surface boundary layer of the open ocean is developed through scale analysis of the primitive equations with mixing included. A set of surface boundary-layer equations appropriate for a broad range of oceanic and atmospheric scales is thereby derived. The essential basis of the model is a coupling between quasigeostrophic dynamics away from the boundary layer and arbitrary mixing models within the mixed layer. The coupling consists of advection of the boundary layer by the horizontal and vertical components of the interior quasigeostrophic flow and forcing of the interior by the boundary layer in the form of divergence within the boundary layer which leads to vortex stretching/compression in the interior. The divergence is generalized for mesoscale wind-driven flows and includes nonlinear interaction between the directly wind-driven boundary-layer flow and the interior flow in the form of interior relative vorticity advection by the wind-driven flow. The nature of the equations leads us to apply a numerical algorithm to their solution. This algorithm is calibrated through application to idealized problems to determine the temporal and spatial grid requirements. The model is initialized with a realistic ocean flow having the properties of the Gulf Stream. 相似文献
16.
Wu Rongsheng 《大气科学进展》1985,2(1):1-7
In this paper, the influences of orography on the boundary layer flow with the approximation of geostrophic momentum are studied. The wind velocity at the lower boundary will not always be zero when the orography exists. So the structure of the boundary layer flow, as well as the vertical velocity at the top of the boundary layer, is affected. There are three factors affecting the vertical motion at the top of the boundary layer: lifting due to orography; divergence due to Ekman flow, and advection of the geostrophic momentum. These effects and the features of the flow within the boundary layer are discussed in detail. 相似文献
17.
Ralph C. Foster 《Boundary-Layer Meteorology》2009,131(3):321-344
We present similarity solutions for the mean boundary-layer profiles under an axisymmetric vortex that is in gradient wind
balance; the similarity model includes the nonlinear momentum advection and curvature terms. These solutions are a generalization
of the Ekman layer mean flow, which is the canonical boundary-layer basic state under a uniform, geostrophically-balanced
flow. Near-surface properties such as inflow angle, surface wind factor, diffusive transport of kinetic energy into the surface
layer and dissipational heating are calculated and shown to be sensitive to the choice of turbulence parameterization. 相似文献
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地形与Ekman边界层中的气流 总被引：1，自引：3，他引：1
利用σ坐标讨论地形与边界层气流是有很多方便的地方,因为,在此坐标中,下边界条件较为简单。在本工作中,首先将混合长理论加以推广并将它用于σ坐标,于是导得了用以描述地形上空边界层气流的控制方程,对边界层气流的特征,特别是对于Ekman抽吸作用进行了详细分析。指出有三种因子影响边界层顶部的垂直运动,第一种因子是边界层内涡度分布,这是与边界层中由于摩擦作用所引起的辐合辐散有直接联系;第二种因子是由于边界层顶部的气流爬坡运动所引起的;第三种是由于边界层中跨越等压线的分量爬坡所引起的,它出现于当等压线与地形等高线相平行时,或地转风呈现绕流情况时,这一作用最为明显。 相似文献
20.
边界层动力学中的Ekman动量近似 总被引：3，自引：3，他引：3
自由大气中,大气运动的基本状态是地转风,近年来发展的地转动量近似,是为了进一步研究非均匀地转流的动力学问题,然而,在边界层大气中,运动的基本状态是经典的Ekman流,所以对边界层运动来说,地转动量近似是不合适的,需作一推广。本文提出了一种所谓Ekman动量近似,它相似于自由大气中的地转动量近似,并讨论了Ekman动量近似的物理基础,对边界层的风场结构及边界屋顶部的垂直速度也作了详细分析。 相似文献