共查询到16条相似文献，搜索用时 609 毫秒
1.
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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β-GMA与边界层动力学特征 总被引：1，自引：0，他引：1
本文研究了β平面中的地转动量近似(β-GMA)及其成立条件.利用β-GMA得到了一个描述Ekman动力学特征的动力学方程组.研究了β项和地转动量(GM)对Ekman层动力学的共同作用.结果表明,β-GMA下的解由两部分组成,一部分即为f-GMA下的解,另一部分为β项与GM的相互作用解.这样,一方面,β-GMA解由于β项存在,对一个定常纬向对称的高度场具有纬向非对称结构,而f-GMA解是纬向对称的.利用一个定常、轴对称的圆形涡旋例子计算了Ekman层的水平风速和边界层顶部的垂直速度,并与经典解、f-GMA解 相似文献
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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
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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
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本文首先利用变分方法,考察了边界层运动能量的变化,指出经典Ekman流是在不可压缩条件下,能量积分达最小值时的一种平衡运动。这对Ekman层运动的物理本质有了进一步的认识。其次,讨论了Ekman动量近似下的Ekman层的平衡风场特征,研究了该平衡风场附近扰动的稳定性问题,结果表明,在自由大气气压场不发生扰动条件下,Ekman层中存在一类新的与惯性不稳定相类似的动力不稳定,且其不稳定性可与Ekman抽吸相联系,还讨论了一般性条件下的扰动不稳定性问题。 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
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文中求解了锋面存在时地转动量近似下的大气边界层运动方程 ,得到了边界层内冷锋流场的一些特征 ,如冷锋坡度随地转涡度增加而增加 ,随地转风速时间倾向的增加而增加 ,随沿锋面传播方向的热成风分量的减少而增加。而边界层内冷锋面上下的流场与锋面坡度、地转风及其时空变化特征有关 ,共同特点是在冷锋面高度以下有下滑运动 ,而其上有一层上滑运动区。 相似文献
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西南低涡Ekman层流场特征分析 总被引：6，自引：3，他引：6
本文利用大气边界模式，对1979年7月24－25日的一次西南低涡暴雨进行了Ekman层的流场分析。在理论计算的前提下，通过加入有限的实测风资料进行动力学调整，不仅在Ekman层中能较好地模拟出高层（850，700hPa）气旋环流特征，且发现西南低涡笔星边界层具有中尺度扰动的特征。在整个气旋环流中有局部的反气旋环流出现，这是850，700hPa 天气图上得不到的，且与边界层诊断分析相符合。 相似文献
9.
本文在地转动量[1]近似下，采用与Agee[2]公式相近的分段常数来描述湍流粘性系数K及其一阶导数（湍流摩擦力所包含的两项和均得到考虑），求得了层结大气中的边界层风场分布和边界层顶垂直速度的解析表达式。计算结果表明；不稳定层结的K比稳定层结的大，且Ekman抽吸作用强；在边界层低层，不稳定层结的全风速比稳定层结的大，而在中层以上则相反。还有，摩擦力中的一阶导数项和二阶导数项量级相同，以往忽略一阶导数项的做法会导致在边界层低层，不稳定层结的全风速反而较小，这与事实不符。因而考虑一阶导数项将进一步完善边界层动力学的理论和应用。 相似文献
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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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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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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
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The general Ekman momentum approximation boundary-layer model(GEM) can be effectively used to describe the physical processes of the boundary layer. However, eddy viscosity, which is an approximated value, can lead to uncertainty in the solutions. In this paper, stochastic eddy viscosity is taken into consideration in the GEM, and generalized polynomial chaos is used to quantify the uncertainty. The goal of uncertainty quantification is to investigate the effects of uncertainty in the eddy viscosity on the model and to subsequently provide reliable distribution of simulation results. The performances of the stochastic eddy viscosity and generalized polynomial chaos method are validated based on three different types of eddy viscosities, and the results are compared based on the Monte Carlo method. The results indicate that the generalized polynomial chaos method can be accurately and efficiently used in uncertainty quantification for the GEM with stochastic eddy viscosity. 相似文献
16.
William Blumen 《Boundary-Layer Meteorology》2001,99(3):509-519
The traditional Ekman boundary-layer parameterization is introduced into the quasigeostrophic Eady baroclinic instability model and into the deformation flow model, to couple the planetary boundary layer with the inviscid interior flow aloft. An explicit time-dependent version of this parameterization is then introduced into an unbalanced zero potential vorticity model to evaluate the initial transient response. It is noted that the adaptation of the geostrophic flow to the same parameterization is different in each of the balanced models. The characteristic flow response reflects thedifferent constraints imposed by each model. Further, the zero potential vorticity condition constrains the evolution of the baroclinic geostrophic part of the flow, which leads to an unphysical flow response when the Ekman boundary-layer parameterization is employed with this unbalanced model. The barotropic part of the flow does, however, evolve in a physically consistent manner spinning down to reflect the introduction of low momentum air pumped into the interior from the boundary layer. Moreover, the transient spin-up processis shown to have an insignificant effect on this spin-down process. 相似文献