共查询到20条相似文献，搜索用时 31 毫秒
1.
Abstract The action of tides on density‐driven circulation, internal gravity waves, and mixing was investigated in the St. Lawrence Estuary between Rimouski and Québec City. Time‐varying fields of water level, currents and density were computed under typical summer conditions using a three‐dimensional hydrostatic coastal ocean model that incorporates a second order turbulence closure submodel. These results are compared with current meter records and other observations. The model and the observations reveal buoyancy effects produced by tidal forcing. The semi‐diurnal tide raises the isopycnals over the sills at the head of the Laurentian Trough and English Bank, producing internal tides radiating seaward. Relatively dense intermediate waters rise from below 75‐m depth to the near surface over the sills, setting up gravity currents on the inner slopes. Internal hydraulic controls develop over the outer sills; during flood, surface flow separation occurs at the entrances of the Saguenay Fjord and the upper estuary west of Ilet Rouge Bank. Early during ebb flow (restratification), the surface layer deepens to encompass the tops of the sills. As the ebb current intensifies, the model predicts the formation of seaward internal jumps over the outer sills, which were confirmed from acoustic reflection observations. As the internal Froude number increases further, flow separation migrates up to sill height. As a result of these transitions, internal bores emanate from the head region one to two hours before low water. We find that the mixing of oceanic and surface waters near the sills is driven by the vertical shear produced during ebb in the channel south of Ilet Rouge, the shear produced in the bottom gravity flood currents, and, to a lesser extent, the processes over the sills. 相似文献
2.
为了研究三维静力适应过程的机理,推导三维静力适应方程组并导出声重力波的频散特征、解析解和能量转换关系以及位涡方程。结果表明,三维静力适应过程实际上就是三维声重力波和重力内波的频散过程,声重力波和重力内波的频率在水平方向上可以区分开来。声重力波的频率随着水平波数和垂直波数的增大而增大。取近轴近似,其解析解在空间上呈螺旋曲面,为大气中热通量和动量从一个区域向另一个区域的输送提供了一种机制,从而为研究大气提供了新的视角。垂直速度在动能与有效势能的转换、有效势能与有效弹性势能的转换中发挥着重要的作用,并且只发生在垂直方向上。在静力适应过程中总能量和位涡是守恒的。适应终态中有效势能比有效弹性势能大一个数量级。 相似文献
3.
Summary We have used a two dimensional version of a nonhydrostatic mesoscale model to simulate atmospheric gravity currents for different thermal stratification. The horizontal and vertical grid increments are chosen such, that the major features of a current like head and elevated nose are resolvable.When the density current propagates into a neutral stratified environment it was found, that frontspeed agrees well with an empirical formula. Also characteristic length scales like depth of the head or height of the following cold air body agree well with observations found in water tank experiments.When a stable atmosphere is adopted, the front moves faster and the generated gravity waves have a significant influence on the atmospheric variables ahead of the current. This results especially in a pressure increase before the front arrives, an effect, which was found in observations, too.Finally, it is shown, that an elevated inversion, embedded in a stable layer, intensifies the vertical velocities and therefore the mesoscale heat flux, which results in a stronger entrainment. For this case a remarkable decrease of front speed is simulated with time.With 7 Figures 相似文献
4.
Summary Some aspects of internal gravity waves in the multicell-type convective system are examined using a linear theory and a nonlinear numerical model. The basic-state wind is assumed to increase linearly with height and then remain constant.In the theoretical part, the two-dimensional, linear, steady-state response of a stably stratified atmosphere to specified cooling representing the evaporative cooling of falling precipitation in the subcloud layer is analytically considered. It is shown that there exist an updraft on the upstream side of the cooling and a downdraft on the downstream side. As the wind shear increases enough, the magnitude of the updraft decreases. This is because a large portion of the specified cooling is used to compensate for the positive vorticity associated with the positive wind shear and accordingly the effective cooling necessary to produce perturbations is reduced.In the numerical part, a two-dimensional version of the ARPS (Advanced Regional Prediction System) that is a nonhydrostatic, compressible model with detailed physical processes is employed. Results from the dry simulation, in which the steady cooling is specified in the model, show that the simulated quasi-steady field resembles the linear, steady-state solution field because the nonlinearity factor of thermally-induced waves in this case is small. For the moist simulation, the quasi-steady perturbations obtained from the dry simulation are used as initial conditions. It is shown that gravity waces can effectively initiate convection even with small amplitude and that updraft at the head of the density current somewhat resembles the linear, steady-state response of a stably straified flow to the specified cooling. The updraft, that is, forced internal gravity waves, at the head of the density current is responsible for the initiation of consecutive convective cells that move downstream and develop as a main convective cell. This study suggests that internal gravity waves play a major role in the initiation of consecutive convective cells in the multicell-type convective system and hence in its maintenance. 相似文献
5.
Finite volume method on unstructured grids in 3D with applications to the simulation of gravity waves 总被引：2，自引：0，他引：2
Summary ?A code for the simulation of atmospheric flows in 3D is presented. The underlying mathematical model is fully compressible,
it takes gravity into account but Coriolis forces, turbulence and viscosity are neglected. The general numerical code consists
of a finite volume discretization on unstructured hexahedral grids in 3D. The code is presently being investigated on applications
to the calculation of atmospheric gravity waves on a mesh which has a structured type and is locally refined near the orography.
We develop two schemes, the main difference between them lies in the different discretizations for the mass fluxes. We show
that both schemes resolve typical structures of gravity waves in potential flow, linear hydrostatic motion and nonlinear non-hydrostatic
regime. We compare advantages and disadvantages of the developed schemes.
Received April 20, 2001; revised September 10, 2001 相似文献
6.
建立了一个二维非静力平衡模式，通过坐标变换的方法使得模式低层有较高的分辨率。对重力流的模拟结果显示；本模式模拟出的重力流冷锋主要特征，如抬高的头部、头部较强的上升运动以及等位温线密集带与实际大气观测很一致。此外，模式还成功地模拟出了重力流头部后面底层的高速潜流层。应用模拟结果与理论结果对重力流的移动速度进行了对比分析。当考虑地形时，重力流在山前受阻，其头部比不考虑地形时高，且产生了向上游传播的“涌浪”；重力流过山后其头部降低，位温梯度减弱，头部后面的水平速度减小。此外，还对非静力平衡与静力平衡条件下重力流的差异进行了分析讨论。 相似文献
7.
8.
André Tremblay 《大气与海洋》2013,51(3):567-603
Abstract Numerical simulations of the severe squall line of 14 July 1987 are discussed within the context of semi‐Lagrangian and semi‐implicit integrations. The fully compressible non‐hydrostatic Euler set of equations constitutes the basic dynamical framework of the numerical model. With elementary precipitation physics and with a generalized treatment of lateral boundary conditions, nested integrations simulate the observed structure of the squall line. The numerical solution and observations show a well organized precipitation system including a mesoscale fast‐propagating prefrontal squall line and a slow‐propagating system moving with the synoptic wave. The prefrontal squall line is seen to be a manifestation of the organization of an inertia‐gravity wave and has characteristics of a wave‐CISK mechanism. Owing to the interaction between the upper jet stream dynamics and moisture, the prefrontal perturbation is initiated locally and subsequently irradiates away from its point source. The high computational efficiency and the accuracy of the model emphasize its potential and demonstrate its value as an interesting tool for mesoscale modelling. 相似文献
9.
Summary A two-dimensional nonhydrostatic numerical model was used to investigate the behaviour of a cold air gravity current, moving along complex terrain. It is found, that the model with a high horizontal and vertical resolution and with a closure scheme, using the turbulent kinetic energy, is suitable to simulate currents, which have the main features of those found in laboratory experiments.Simulations are presented for different orographic structures (mountain and valley), for varying thermal stratification of the environmental atmosphere (neutral, stable and stable with an elevated inversion) and for different heights of the cold air reservoir.The major effect of a hill on the advance of a gravity current is a reduction of the front speed upstream as well as (even stronger) downstream of the obstacle, where the amount of this decrease depends on thermal stratification. Near surface blocking of the air flow on the windward side occurs for all simulations. However, for small depths of the oncoming cold air, the current cannot surmount the hill and remains on the lee side.With 11 Figures 相似文献
10.
AREM（Advanced Regional Eta-coordinate Model）对中国暴雨、台风等中尺度天气系统的模拟、预报能力突出。但是伴随模式分辨率的提高,制约该模式发展的一个问题日渐突出,即"静力平衡近似"的约束。本文通过对原静力平衡系统进行修正,引入高阶订正参数定义第三运动方程来构建该模式的非静力动力框架。我们基于Euler原始方程组,有效结合原静力平衡模式的标准层结扣除及IAP（Institute of Atmospheric Physics）变换方法,推导出了球面余纬坐标下的非静力框架,并在E网格和η坐标下进行了时空离散。采用时间两部分离技术进行积分运算以提高计算效率,并通过"追赶法"结合迭代法计算声波。此框架可方便地继承静力平衡框架的特点,最大限度地保留静力平衡框架的优势。理论推导和数值试验表明,当非静力框架退化为静力平衡框架后,方程形式及其模拟结果一致。在文章第二部分将通过理想和实例试验检验非静力模式性能。 相似文献
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12.
W. C. Ulrich 《Meteorology and Atmospheric Physics》1993,52(3-4):137-145
Summary The effect of simple quasi-nonhydrostatic approximations in a numerical model of a gravity current are discussed. Two methods for including non-hydrostatic effects are explored. The reasons for the limited success of a first method is discussed briefly.With 9 Figures 相似文献
13.
14.
在尺度分析的基础上,利用大气垂直运动方程中各项量级的分布特点,提出了多级扰动法.通过它可以使该方程中量级最大的垂直气压梯度力项(VPGF)和重力项(G)的相应扰动项的量级随级次n的增加明显减小,而其他项的量级不变;而且,方程中最大的垂直截断误差的量级也随n的增加而减小,直到高级扰动项不再是扰动方程中仅有的最大项时为止.... 相似文献
15.
Modeling nonhydrostatic atmospheric flow requires the solution of the vertical equation of motion and a prognostic or diagnostic equation for pressure. If the nonhydrostatic components of the flow are relatively small, they can be approximated and incorporated into a purely hydrostatic model, which usually is conceptually simpler and computationally more efficient. A method to do this for a linear model of local thermally-induced circulations is further developed and adapted to a non-linear numerical model of the neutral atmospheric boundary layer. A hydrostatic model and the quasi-nonhydrostatic version were used to simulate neutral flow over simple terrain features. One set of observations taken over a simple change in roughness and another set taken over a change in both roughness and terrain were simulated by both models to assess the capabilities of the quasi-nonhydrostatic technique.It is found that (as expected) the pressure deviation from the hydrostatic state is negligible for the roughness change, but it is an important aspect of neutral flow over terrain. Thus, for flow encountering a simple roughness change, the hydrostatic approximation is good, even for small horizontal scales. However, the quasi-nonhydrostatic model qualitatively produces the features in the observations for flow over a terrain change that the hydrostatic model cannot produce.Journal Paper No. J-12737 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 2779. 相似文献
16.
T. Kato 《Meteorology and Atmospheric Physics》1997,63(1-2):39-51
Summary Quantitatively comparative experiments of moist convection using hydrostatic and non-hydrostatic models are reviewed and a further study is made of the suitability of the hydrostatic approximation for a high-resolution model when the grid size falls below 20 km. Idealized moist convection is treated, and then the torrential rain that occurred on 6 August 1993 in Kagoshima, southern Kyushu, Japan is simulated by each model. An explicit warm-rain process predicting cloud water and rainwater and the scheme of moist convective adjustment are individually or conjunctively employed in the model. The effect of hydrostatic water loading is also examined in detall.For the simulation of idealized convection, the hydrostatic simulation tends to overestimate and overexpand precipitation in comparison with the non-hydrostatic counterpart, and the drag effect of hydrostatic water loading is more significant for convective development than the non-hydrostatic effect. In the 20-km simulations, however, the hydrostatic simulation with hydrostatic water loading produces results that are comparable to the nonhydrostatic counterpart. For the simulation with real data, the comparative results well correspond to those of idealized convection. Furthermore, the 5 km hydrostatic simulation overestimates total precipitation more than that of dealized convection. On the basis of these results, when developing 1020 km numerical weather prediction (NWP) models, hydrostatic water loading should be evaluated in preference to adopting non-hydrostatic models, and a non-hydrostatic model with hydrostatic water loading is thought to be recommendable for a high-resolution NWP model.With 7 Figures 相似文献
17.
在程锐等（2018）中,我们完成了非静力AREM（Advanced Regional Eta-coordinate Model）模式动力框架设计。本文将通过理想和实例试验检验其模拟能力。设计理想试验并通过与国际成熟的中尺度非静力框架比较,直接检验非静力AREM三维动力框架在细致分辨率（约1 km）下的模拟性能。可以看出,非静力AREM与ARPS（Advanced Regional Prediction System）、WRF（Weather Research and Forecasting Model）模拟出类似的积云对流结构及演变特征,从而基本确证了本文发展的非静力框架的正确性。结合原静力平衡模式的初始化和物理参数化过程,形成非静力AREM模式系统。台风实例模拟表明,粗分辨率下静力、非静力AREM模式性能接近;但在高分辨率下,非静力明显优于静力模式。我们还开展了批量降水试验检验,对非静力AREM模式性能进行了进一步的验证。 相似文献
18.
Impact of a sea breeze on the boundary-layer dynamics and the atmospheric stratification in a coastal area of the North Sea 总被引：1，自引：0，他引：1
Charles Talbot Patrick Augustin Céline Leroy Véronique Willart Hervé Delbarre Georgui Khomenko 《Boundary-Layer Meteorology》2007,125(1):133-154
In-situ sodar and lidar measurements were coupled with numerical simulations for studying a sea-breeze event in a flat coastal
area of the North Sea. The study’s aims included the recognition of the dynamics of a sea-breeze structure, and its effects
on the lower troposphere stratification and the three-dimensional (3D) pollutant distribution. A sea breeze was observed with
ground-based remote sensing instruments and analysed by means of numerical simulations using the 3D non-hydrostatic atmospheric
model Meso-NH. The vertical structure of the lower troposphere was experimentally determined from the lidar and sodar measurements,
while numerical simulations focused on the propagation of the sea breeze inland. The sea-breeze front, the headwind, the thermal
internal boundary layer, the gravity current and the sea-breeze circulation were observed and analysed. The development of
a late stratification was also observed by the lidar and simulated by the model, suggesting the formation of a stable multilayered
structure. The transport of passive tracers inside the sea breeze and their redistribution above the gravity current was simulated
too. Numerical modelling showed that local pollutants may travel backward to the sea above the gravity current at relatively
low speed due to the shearing between the landward gravity current and the seaward synoptic wind. Such dynamic conditions
may enhance an accumulation of pollutants above coastal industrial areas. 相似文献
19.
A. S. Broad 《Meteorology and Atmospheric Physics》2000,72(1):1-11
Summary. ?A hydrostatic numerical model is used to simulate the lee wave event IOP3 (0000 GMT to 1200 GMT 15th October 1990) from the PYREX mountain experiment. Results from integrations at different horizontal resolutions are used
to investigate the effect on surface pressure drag and the vertical flux of horizontal momentum due to orographically forced
gravity waves. In particular, results showing the dependence on resolution of the partitioning between resolved and parametrized
wave drag and fluxes are presented. With the model horizontal gridlength changing from 50 km to 10 km the majority of wave
momentum flux changes from being parametrized to becoming resolved. More significantly, there is a change in the profile of
flux with height. At 50 km resolution the largest inferred mean flow decelerations are at lower stratospheric level due to
the parametrization scheme. At 10 km resolution this is shifted, with less deceleration high up and more wave deceleration
lower down in the troposphere. Numerical weather prediction models are now beginning to take account of such low level drag
with beneficial results.
Received March 2, 1999/Revised July 15, 1999 相似文献
20.
Surface meteorological observations, associated with gust fronts produced by thunderstorm outflows over Tehran, an area surrounded by mountains, have been analyzed. Distinctive features are sudden drop in air temperature, up to 10℃, sharp increase in wind speed, up to 30 m s-1, with wind shift, to northwesterly, ressure jump, up to 4 hPa, humidity increase, up to 40%, and rain after some 20 min. Gust fronts which often occur in spring time, have a typical thickness of about 1.5 km and produce vertical wind shear of the order of 10-2s-1. Although these features seem to be common for most of the events, their intensities differ from one event to another, indicating that the gust fronts may occur in different sizes and shapes. Apart from a dominant effect on the formation of the original thunderstorms, topography appears to break up the frontal structure of the gust fronts. The internal Rossby radius of deformation for these flows is small enough (～ 100 km) for rotational effects to be minor.A laboratory model of the gust front (gravity current) also shows that it initially has a distinctive head with a turbulent wake, and can be broken up by topography. It is shown that when the environment is stratified, turbulence due to lobes and clefts instabilities near the nose of the current is suppressed. When the ground is rough, these instabilities are highly amplified and the internal Froude number of the flow is reduced. The bottom slope in the presence of rough topography leads to the break up of the current head and produces a broad and highly non-uniform head, recognized in the density signals. 相似文献