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1.
A new algebraic turbulent length scale model is developed, based on previous one-equation turbulence modelling experience in atmospheric flow and dispersion calculations. The model is applied to the neutral Ekman layer, as well as to fully-developed pipe and channel flows. For the pipe and channel flows examined the present model results can be considered as nearly equivalent to the results obtained using the standard k– model. For the neutral Ekman layer, the model predicts satisfactorily the near-neutral Cabauw friction velocities and a dependence of the drag coefficient versus Rossby number very close to that derived from published (G. N. Coleman) direct numerical simulations. The model underestimates the Cabauw cross-isobaric angles, but to a less degree than the cross-isobar angle versus Rossby dependence derived from the Coleman simulation. Finally, for the Cabauw data, with a geostrophic wind magnitude of 10 ms–1, the model predicts an eddy diffusivity distribution in good agreement with semi-empirical distributions used in current operational practice. 相似文献
2.
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.
Noor Afzal 《Boundary-Layer Meteorology》2009,132(2):241-259
The geostrophic Ekman boundary layer for large Rossby number (Ro) has been investigated by exploring the role played by the mesolayer (intermediate layer) lying between the traditional inner
and outer layers. It is shown that the velocity and Reynolds shear stress components in the inner layer (including the overlap
region) are universal relations, explicitly independent of surface roughness. This universality of predictions has been supported
by observations from experiment, field and direct numerical simulation (DNS) data for fully smooth, transitionally rough and
fully rough surfaces. The maxima of Reynolds shear stresses have been shown to be located in the mesolayer of the Ekman boundary
layer, whose scale corresponds to the inverse square root of the friction Rossby number. The composite wall-wake universal
relations for geostrophic velocity profiles have been proposed, and the two wake functions of the outer layer have been estimated
by an eddy viscosity closure model. The geostrophic drag and cross-isobaric angle predictions yield universal relations, which
are also supported by extensive field, laboratory and DNS data. The proposed predictions for the geostrophic drag and the
cross-isobaric angle compare well with data for Rossby number Ro ≥ 105. The data show low Rossby number effects for Ro < 105 and higher-order effects due to the mesolayer compare well with the data for Ro ≥ 103. 相似文献
4.
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
5.
The temperature and wind profiles in the planetary boundary layer (PBL) are investigated. Assuming stationary and homogeneous conditions, the turbulent state in the PBL is uniquely determined by the external Rossby number and the stratification parameters. In this study, a simple two-layer barotropic model is proposed. It consists of a surface (SL) and overlying Ekman-type layer. The system of dynamic and heat transfer equations is closed usingK theory. In the SL, the turbulent exchange coefficient is consistent with the results of similarity theory while in the Ekman layer, it is constant. Analytical solutions for the wind and temperature profiles in the PBL are obtained. The SL and thermal PBL heights are properly chosen functions of the stratification so that from the solutions for wind and temperature, the PBL resistance laws can be easily deduced. The internal PBL characteristics necessary for the calculation (friction velocity, angle between surface and geostrophic winds and internal stratification parameter) are presented in terms of the external parameters. Favorable agreement with experimental data and model results is demonstrated. The simplicity of the model allows it to be incorporated in large-scale weather prediction models as well as in the solution of various other meteorological problems. 相似文献
6.
By using simple barotropic boundary layer equations with constant eddy viscosity,the analytical solution is obtained under the initial condition that the distribution of wind for a given pressure is not the well-known Ekman flow.We have found that the wind will finally adjust to the Ekman flow at a rate faster than that of geostrophic adjustment.We have also found that the thinner the boundary layer,the faster the rate of adjustment. 相似文献
7.
If the steady-state geostrophic wind vector varies exponentially with height in the planetary boundary layer, calculated hodographs of the 24-hour mean wind resemble the classic Ekman spiral distorted by thermal effects. For such an assumed distribution, Lettau's (1967) conclusion that the effects of thermal influence on the steady-state boundary-layer winds can be linearly superimposed on the effects of internal friction is justified.The minimum value of the cross-isobar angle of the surface wind for a given magnitude of the thermal wind vector occurs when that vector points about 345° to the right of the surface geostrophic wind vector and the maximum value occurs when it points at an azimuth of about 120° relative to the surface geostrophic wind vector. The range of values of the cross-isobar angle is almost directly proportional to the magnitude of the thermal wind vector.Hodographs resulting from the assumed variation of the geostrophic wind have approximately the same shape as 24-hour mean hodographs at two locations over the Great Plains.This work is part of a thesis submitted to the University of Wisconsin in partial fulfillment of the requirements for the Ph.D. Degree, written under the supervision of Professor H. Lettau, Department of Meteorology. 相似文献
8.
The inertial coupling model of the surface shear stress at the sea surface (Bye, 1995) which takes account of the surface wavefield, has been applied to couple the Ekman layers of the ocean and atmosphere. We determine the surface shear stress and geostrophic drag coefficient, under barotropic conditions. The results are expressed in terms of the shear between the inertially weighted (i.e. velocity×square root of the density) relative geostrophic velocities in the two fluids, in which the reference velocity need not be specified, a priori. We find, in particular, that the deflection of the relative surface geostrophic wind to the surface shear stress in naturally occurring seastates, is about 9°. In the application of the analysis to general circulation models, it is argued that, since the inertially weighted relative geostrophic velocities in air and water are of similar magnitude, this implies that the surface shear stress can be significantly reduced by the current component of the inertially weighted geostrophic shear, with a corresponding reduction in importance of the Ekman transport. 相似文献
9.
In this paper, Wu and Blumen’s boundary layer geostrophic momentum approximation model (Wu and Blumen, 1982) is applied to baroclinic and non-neutral PBL, the motion equations for the PBL under the geostrophic momentum approximation are solved, in which the eddy transfer coefficient is a function of the distributions of the wind and temperature. The results are compared with those in barotropic and neutral conditions with the geostrophic momentum approximation. It is found that in the baroclinic condition, the wind distribution has both the characteristics of a steady, homogeneous and baroclinic PBL and those caused by the geostrophic momentum approximation. Those in non-neutral conditions show that they retain the intrinsic characteristics for the wind in non-neutral PBL, at the same time, the effects of the large-scale advection and local variation are also included. We can predict the wind in the non-neutral and baroclinic PBL by use of the geostrophic mo-mentum approximation when the temporal and spatial distributions of the geostrophic wind, as well as the po-tential temperatures and their variation rates at the upper and lower boundary of the PBL are given by large-scale model. Finally, the model is extended to the case over sea surface. 相似文献
10.
Nonlinear permanent form solutions have been found for the barotropic, quasi-geostrophic divergenceless vorticity equation describing large scale, rotating flows over zonal relief. In the linear limit these solutions are topographic Rossby waves. The analytical procedure is an expansion in two small dimensionless parameters, an amplitude parameter (the Rossby number) and the aspect ratio between North-South (cross-relief) and East-West length scales. Permanent form solutions exist when these two parameters, and the related effects of dispersion and nonlinearity, mutually balance. By the same expansion procedure, an analytical linearized stability theory has been formulated which proves the neutral stability of these solutions to infinitesimal, two-dimensional perturbations. 相似文献
11.
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
12.
Dynamics of nonlinear baroclinic Ekman boundary layer 总被引：2，自引：0，他引：2
By the geostrophic momentum approximation, the wind structure and vertical motion within the non-linear baroclinic Ekman layer matching with the surface layer are determined. A comparison of the Ekman solution with the classical one is made. It is demonstrated that the contributions of baroclinity, stratification and nonlinear effects to the wind profile within the layer are all of definite importance. 相似文献
13.
A. Wiin-Nielsen 《Boundary-Layer Meteorology》1974,6(3-4):459-476
The Ekman-Taylor problem for the planetary boundary layer is solved in the case of a thermal wind which varies linearly with height. The upper boundary condition is a vanishing ageostrophic wind, while the lower boundary condition is continuity of the stress vector across the interface between the planetary boundary layer and the surface layer. The latter condition is used to determine the magnitude and the direction of the wind at the bottom of the Ekman layer.Theoretical hodographs are compared with observed hodographs based on five years of ohservations from Ship N in the Pacific, giving fair agreement.The divergence, the vorticity, and the vertical velocity are calculated through the Ekman layer with emphasis on differences between the classical barotropic and the baroclinic cases; these differences are significant, especially in the vertical velocities as compared to the standard approximation.An extension of the present study to include thermal stratification is desirable. 相似文献
14.
15.
An analysis tool for the study of wind speed profiles over the water has been developed. The profiles are analysed using a
modified dimensionless wind speed and dimensionless height, assuming that the sea surface roughness can be predicted by Charnock’s
roughness length model. In this form, the roughness dependency on wind speed is extracted and the variations on the wind profile
are due solely to atmospheric stability. The use of the Charnock’s non-dimensional wind profile is illustrated using data
collected from a meteorological mast installed in the Danish North Sea. The best fit with the observed mean non-dimensional
wind profile under neutral atmospheric conditions is found using a value of 1.2 × 10−2 for Charnock’s parameter. The stability correction on the neutral wind profile suggested by the Businger-Dyer relations was
found to perform well over the sea. 相似文献
16.
17.
Y. L. McHall 《大气科学进展》1993,10(2):169-180
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. 相似文献
18.
The impact of the Wangara experiment 总被引：1，自引：0，他引：1
19.
E. L. Deacon 《Boundary-Layer Meteorology》1973,5(3):321-340
Data on the relationship of the surface wind to the geostrophic wind at Porton Down, Salisbury Plain, are presented for various
stability conditions and analysed in the light of the Rossbynumber similarity theory. For near-neutral conditions, the geostrophic
drag coefficients for geostrophic wind speeds 5 to 15 m s-1 are close to those found by other workers but at higher speeds the values are low. Comparisons of geostrophic and radar wind
speeds for ⋍900-m height, suggest that undetectably small mean cyclonic curvatures of the trajectories of the air are responsible
for this departure.
A value of the geostrophic drag coefficient for the open sea at wind speeds around 8 m s-1 (neutral conditions) is deduced from recent observations of the drag in relation to the surface wind, combined with the ratios
of 900-mb radar wind to surface wind obtained from the North Atlantic weather ship data tabulations of Findlater et al. (1966). 相似文献
20.
在大尺度系统中,由于Rossby数很小,大气呈准地转运动。但在低纬度地区或中小尺度系统中,Rossby数不是小参数,运动的非地转特征就很明显了。为了避免非线性问题中数学处理的复杂性,可以使用半地转近似,这不但可以使问题得到线性化,而且其结果又有非线性的主要特征。本文在半地转假定下,当风压场处于平衡状态时,我们得到了一种简单的表示式,并将它与Rossby近似和Phillips近似进行了比较。文中还对扰动的稳定性进行了研究,在某些条件下,例如风场发生扰动而气压场未受扰 相似文献