共查询到10条相似文献，搜索用时 93 毫秒
1.
本文利用二阶矩湍流模式着重讨论了有地形条件下Ekman螺旋的特征，结果表明，在北半球一定的环境地转风场结构下，斜坡Ekman螺旋可以变为反时针旋转。另外，还研究了地形对稳定边界层二阶矩湍流量的结构和垂直分布的影响。 相似文献
2.
A 1D model, including a time variation of eddy viscosity and mixed layer depth, is applied to study Ekman spirals. It simulates a weak velocity in the atmosphere but a jet in the upper oceanic mixed layer during daytime; and a strong velocity in the atmosphere but a weak, uniform velocity in the ocean at night. The mean spirals in both atmosphere and ocean are close to the average spirals at midday and midnight, they are not flat as suggested by previous studies but consistent with the observations of Polton et al (2013). Our results also show shorter length scale for magnitude decay than for rotation of mean velocity as observed in the ocean, which comes from the combined effects of the diurnal variation of PBL and the Coriolis force. The latter becomes more important away from the surface. In the upper oceanic mixed layer, the mean velocity mainly comes from the strong jets in the late afternoon and early evening. Near and below the depth of Ekman depth, the weak velocities change with time and cancel out each other if averaged timing is longer than the inertia period. It results in diminishing of magnitude of the mean velocity, but the amplitude of individual parcel oscillating can still be quite large near the Ekman depth. Meanwhile, the change of velocity angle from the surface is near or less than 90 degree. Hence, shorter length scale for magnitude decay than for rotation of the mean velocity is not controlled by viscosity alone. Meanwhile, the model does not need two viscosities as suggested previously.The results also show that either the diurnal variation of surface stress or eddy viscosity alone can create a diurnal oscillation of velocity in the ocean. The interactions between PBL force and the Coriolis force can create a weak instability in the atmosphere and ocean at 30° and 90°. This weak instability may explain the observed nocturnal LLJ near 30 °N on the lee of the Rocky Mountains and the intensification of mesoscale circulation simulated by Sun and Wu (1992). 相似文献
3.
Horizontal velocity spirals with a clockwise rotation(downward looking) rate of 1.7?m-1, on average, were observed in the western and northern Yellow Sea from December 2006 to February 2007. With the observed thermal wind relation,the beta-spiral theory was used to explain the dynamics of spirals. It was found that the horizontal diffusion of geostrophic vortex stretching is likely to be a major mechanism for generating geostrophic spirals. Vertical advection associated with surface/bottom Ekman pumping and topography-induced upwelling is too weak to support these spirals. Strong wind stirring and large heat loss in wintertime lead to weak stratification and diminish the effects of vertical advection. The cooling effect and vertical diffusion are offset by an overwhelming contribution of horizontal diffusion in connection with vortex stretching. The Richardson number-dependent vertical eddy diffusivity reaches a magnitude of 10-4 m^2 s-1 on average. An eddy diffusivity of 2870 m^2 s-1 is required for dynamic balance by estimating the residual term. This obtained value of 10-4 m^2 s-1 is in good agreement with the estimation in terms of observed eddy activities. The suppressed and unsuppressed diffusivities in the observation region are 2752 and 2881 m^2 s-1, respectively, which supports a closed budget for velocity rotation. 相似文献
4.
Daniel Reinert Volkmar Wirth Joachim Eichhorn Walter-Georg Panhans 《Boundary-Layer Meteorology》2007,125(1):109-132
This paper presents the dry version of a new large-eddy simulation (LES) model, which is designed to simulate air flow and
clouds above highly complex terrain. The model is three-dimensional and nonhydrostatic, and the governing equations are sound
filtered by use of the anelastic approximation. A fractional step method is applied to solve the equations on a staggered
Cartesian grid. Arbitrarily steep and complex orography can be accounted for through the method of viscous topography. The
dynamical model core is validated by comparing the results for a spreading density current against a benchmark solution. The
model accuracy is further assessed through the simulation of turbulent flow across a quasi two-dimensional ridge. The results
are compared with wind-tunnel data. The method of viscous topography is not restricted to moderately sloped terrain. Compared
to models using curvilinear grids, it allows this model to be applied to a much wider range of flows. This is illustrated
through the simulation of an atmospheric boundary-layer flow over a surface mounted cube. The results show that the dry model
version is able to accurately represent the complex flow in the vicinity of three-dimensional obstacles. It is concluded that
the method of viscous topography was successfully implemented into a micrometeorological LES model. As will be shown in Part
II, this allows the detailed study of clouds in highly complex terrain. 相似文献
5.
A set of new parameterizations for the friction velocity and temperature scale over gently
sloped terrain and in calm synoptic conditions are theoretically derived. The friction velocity is
found to be proportional to the product of the square root of the total accumulated heating in the
boundary layer and the sinusoidal function of the slope angle, while the temperature scale is
proportional to the product of the boundary layer depth, the sinusoidal function of the slope angle
and the potential temperature gradient in the free atmosphere. Using the new friction velocity
parameterization, together with a parameterization of eddy diffusivity and an initial potential
temperature profile around sunrise, an improved parameterization for the thermally induced upslope
flow profile is derived by solving the Prandtl equations. The upslope flow profile is found to be
simply proportional to the friction velocity. 相似文献
6.
台风移动规律的研究 Ⅱ.小地形与边界层的动力作用 总被引：1，自引：1，他引：1
从支配台风中心移动的基本方程出发，着重分析了小地形（地形高度与台风系统的的垂直厚度相双为小量）的抬升作用和边界层的摩擦作用对台风移动影响的定性特征。结果表明，较高地势对台风有“吸收”作用；边界层摩擦辐合引起的艾克曼抽吸有利于台风产生沿局地流场引导速度方向的加速度。 相似文献
7.
Impact of the Diurnal Cycle of the Atmospheric Boundary Layer on Wind-Turbine Wakes: A Numerical Modelling Study 总被引：1，自引：1，他引：0
The wake characteristics of a wind turbine for different regimes occurring throughout the diurnal cycle are investigated systematically by means of large-eddy simulation. Idealized diurnal cycle simulations of the atmospheric boundary layer are performed with the geophysical flow solver EULAG over both homogeneous and heterogeneous terrain. Under homogeneous conditions, the diurnal cycle significantly affects the low-level wind shear and atmospheric turbulence. A strong vertical wind shear and veering with height occur in the nocturnal stable boundary layer and in the morning boundary layer, whereas atmospheric turbulence is much larger in the convective boundary layer and in the evening boundary layer. The increased shear under heterogeneous conditions changes these wind characteristics, counteracting the formation of the night-time Ekman spiral. The convective, stable, evening, and morning regimes of the atmospheric boundary layer over a homogeneous surface as well as the convective and stable regimes over a heterogeneous surface are used to study the flow in a wind-turbine wake. Synchronized turbulent inflow data from the idealized atmospheric boundary-layer simulations with periodic horizontal boundary conditions are applied to the wind-turbine simulations with open streamwise boundary conditions. The resulting wake is strongly influenced by the stability of the atmosphere. In both cases, the flow in the wake recovers more rapidly under convective conditions during the day than under stable conditions at night. The simulated wakes produced for the night-time situation completely differ between heterogeneous and homogeneous surface conditions. The wake characteristics of the transitional periods are influenced by the flow regime prior to the transition. Furthermore, there are different wake deflections over the height of the rotor, which reflect the incoming wind direction. 相似文献
8.
Li Chongying 《大气科学进展》1985,2(1):72-80
By using a two-dimensional axisymmetrical PEM in which two physical processes (the Ekman pumping and the vertical transportation of cumulus momentum) are included, the genesis and development of typhoons have been simulated. The results of numerical simulation show that the generation and structure of the typhoon simulated by the model involving both the physical processes are much close to a real one in the atmosphere as compared with that involving either the Ekman pumping or the cumulus momentum transport. Therefore, it can be suggested that the cumulus momentum transport and Ekman pumping together play an important role in the genesis and development of typhoons through the CISK mechanism. 相似文献
9.
The vertical velocity at the top of Ekman layer caused by katabatic winds is proposed and deduced. By com-puting actual data we jet a distribution of the velocities over Antarctica. The distribution plays a positive role in maintaining the cyclone and anticyclone over Antarctica. 相似文献
10.
本文采用二维数值模式模拟了一个坡地上夜间边界层的发展过程。所得到的结果与1974—1976年在北京北部山区得到的观测资料做了比较。模拟出的夜间辐射逆温层的厚度和形状以及下坡风的廊线都与观测事实大致相符。模式进一步改进后似可做为解决中、小尺度复杂地形上夜间边界层演变的工具。 相似文献