共查询到20条相似文献，搜索用时 31 毫秒
1.
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
2.
The roles of different mechanisms related to the tide-induced fronts in the Yellow Sea in summer 总被引：2，自引：0，他引：2
In summer, the Yellow Sea Cold Water Mass （YSCWM） is a stable water mass of low temperature lying at the bottom of the central Yellow Sea （YS）. It is fringed by some typical tidal fronts, which separate deep, stratified water on the offshore side from the well-mixed, shallow water on the inshore side. Three striking fronts--Subei Bank Front （SBF）, Shandong Peninsula Front （SPF）, and Mokpo Front （MKF; a front off the southwestern tip of the Korean Peninsula）--have been iden- tified by various studies from both satellite observations and model results. Tide plays an important role in the formation and maintenance of these fronts. However, it is still a matter of debate as to the roles these two kinds of mechanisms of upwelling and tidal mixing play, and how importance they are in the maintenance processes of the above three fronts. Basing a nested high-resolution model HYCOM （the Hybrid Coordinate Ocean Model）, this study focuses on the different mechanisms of tidal effects on the thermal fronts in the YS in summertime. Through comparative experiments with and without tidal forcing, the results indicate that the MKF is mainly driven by tide-induced upwelling. For the SPF, tidal mixing is the dominant factor, when lower cold water is stirred upwards along the sloping topography of the western YS. Meanwhile, the combined effect of upwelling and tidal mixing is the main cause of the formation of the SBF. Diagnostic analysis of thermal balance shows that horizontal nonlinear advection induced by strong tidal currents also contributes to the thermal balance of frontal areas. 相似文献
3.
Water mass modification in surface-trapped, near-field river plumes is examined using a 1.5-layer reduced gravity model and a three-dimensional numerical model. Solutions to the layer model are shown to be qualitatively similar to previous observations and three-dimensional simulations of near-field plumes. Analytic analysis of the layer model demonstrates how the near-field plume is controlled by the competing processes of mixing and spreading. The two models are then used to explore the parameter space dependence of density changes within the near-field plume and their associated cross-shore length scales. Both the magnitude of density changes and their length scales are proportional to either estuarine discharge or fresh water discharge; density changes are also inversely proportional to the estuary mouth width. One surprising feature of the parameter space solutions is that the density of water exiting the near-field plume, a measure of the net dilution of the entire near-field plume, is shown to be inversely proportional to local mixing rates. This is because when local mixing is lower, the influence of plume spreading becomes greater; this spreading accelerates the plume, requiring more net mixing to bring the plume back to subcritical flow. 相似文献
4.
Takeyoshi Nagai Amit Tandon Nicolas Gruber James C. McWilliams 《Dynamics of Atmospheres and Oceans》2008,45(3-4):229
Subduction, upwelling, and phytoplankton blooms are commonly observed features at oceanic fronts. This study isolates the role of vertical mixing for enhanced production and water mass subduction near fronts, considering the time-developing problem with a Semi-Geostrophic circulation model coupled to a planktonic ecosystem model. Our model results show that vertical mixing in the surface boundary layer strongly modifies the time evolution of the front and of its associated biology. Ageostrophic flows caused by the combined effects of confluence and vertical mixing enhance primary production on the less dense side and increase water mass subduction on the dense side of the front. Confluence alone results in the intensification of the front by the same advective response, while the phytoplankton bloom on the less dense side does not arise without vertical mixing. Vertical mixing alone slumps the front near the surface and provides weak subduction on the dense side and uplift of the isopycnals at the center of the front. We find that it is possible to sustain an isolated phytoplankton patch above the domed isopycnals at the center of the front with the nutrients supplied by the secondary circulations arising due to vertical mixing. These results suggest that the phytoplankton bloom and patches found on the less dense side of fronts in many field observations are likely caused by fine-scale along-isopycnal upwelling of nutrients forced by adiabatic confluence in the meander trough of fronts and further pumping and entrainment of nutrients by the secondary circulation due to vertical mixing. Isolated patches observed at the center of the front in many frontal surveys could be caused by secondary flows due to vertical mixing. 相似文献
5.
William H. Snyder 《Boundary-Layer Meteorology》1972,3(1):113-134
Similarity criteria for modeling atmospheric flows in air and water are reviewed. It is shown that five nondimensional parameters plus a set of nondimensional boundary conditions must be matched in model and prototype. The neglect of the Rossby number can lead to serious errors in modeling of diffusion in a prototype with a length scale greater than about five kilometers. The Reynolds number, the Peclet number and the Reynolds-Schmidt product criteria may be neglected if the model flow is of sufficiently high Reynolds number. The Froude number criterion appears to be the most important. The complete specification of boundary conditions is found to be nebulous, but is discussed in some detail. Over-roughening of the model surface may be necessary to satisfy a roughness Reynolds number criterion. Both air and water appear to be suitable fluids to use as modeling media.On assignment from the National Oceanic and Atmospheric Administration, United States Department of Commerce. 相似文献
6.
The effects of precipitation and river runoff in a coupled ice-ocean model of the Arctic 总被引：1，自引：0，他引：1
A coupled ice-ocean model of the Arctic is developed in order to study the effects of precipitation and river runoff on sea
ice. A dynamic-thermodynamic sea ice model is coupled to an ocean general circulation model which includes a turbulent closure
scheme for vertical mixing. The model is forced by interannually varying atmospheric temperature and pressure data from 1980–1989,
and spatially varying mean monthly precipitation and river runoffs. Salinity and fresh water fluxes to the ocean from ice
growth, snow melt, rain, and runoffs are computed, with no artificial constraints on the ocean salinity. The modeled ice thickness
is similar to the observed pattern, with the thickest ice remaining against the Canadian Archipelago throughout the year.
The modeled ice drift reproduces the Beaufort gyre and Transpolar drift exiting through Fram Strait. The stable arctic halocline
produced by the vertical mixing scheme isolates the surface from the Atlantic layer and reduces the vertical fluxes of heat
and salinity. A sensitivity experiment with zero precipitation results in rapidly decreasing ice thickness, in response to
greater ocean heat flux from a weakening of the halocline, while an experiment with doubled precipitation results in a smaller
increase in ice thickness. A zero-runoff experiment results in a slower decrease in ice thickness than the zero-precipitation
case, due to the decadal time scale of the transport of runoff in the model. The results suggest that decadal trends in both
arctic precipitation and river runoffs, caused either by anthropogenic or natural climatic change, have the potential to exert
broad-scale impacts on the arctic sea ice regime.
Received: 6 February 1996 / Accepted: 4 April 1996 相似文献
7.
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
8.
We investigated the turbulent intensities and Reynolds shear stress at high Reynolds number $({Re_\tau = 5 \times 10^{6}})$ in the atmosphere surface layer (ASL) through analyzing observations in near-neutral stratified conditions. The results show that with increasing Reynolds number the streamwise turbulent intensity increases linearly, and the peak of the Reynolds shear stress extends to a higher non-dimensional height, which means that the thickness of the logarithmic region increases. Furthermore, our results provide evidence for treating the ASL as a canonical turbulent boundary layer, the results of which can be extended and applied to higher Reynolds number wall turbulence in the ASL. 相似文献
9.
10.
J. R. Chasnov 《Dynamics of Atmospheres and Oceans》1996,23(1-4)
The decay of statistically homogeneous velocity and density fluctuations in a stably stratified fluid is considered. Over decay times long compared with the turbulence time scale but short compared with the period of internal gravity waves, three distinct high Reynolds number similarity states may develop. These. similarity states are a consequence of the invariance of the low wavenumber coefficients of the three-dimensional kinetic or potential energy spectrum, and their preferential development depends on the relative magnitudes of the initial kinetic and potential energy per unit mass of the fluid. When the turbulence has decayed over a time comparable with the period of the gravity waves, the three similarity states mentioned above are disrupted. Evidence will be presented of a new similarity state which then develops asymptotically. In this similarity state, the time decay exponent of the total energy per unit mass of the turbulence is reduced by a factor of two from its value for decaying isotropic turbulence, and the associated vertical integral scale approaches a constant independent of time. 相似文献
11.
黄海夏季潮汐锋区环流的数值研究 总被引：6，自引：0，他引：6
基于Blumberg＆Mellor三维非线性环流模型(ECOM)并结合Mellor＆Yamada的湍封闭模型，对黄海M2分潮、密度环流、锋区环流等进行了的数值模拟：潮汐模拟结果较好的体现了黄、渤海M2潮波传播系统，验证了模型的可行性；模拟密度环流的量阶和方向均与夏季实测环流保持一致，体现了密度环流是夏季总环流的重要组成部分；两个断面锋区环流的模拟结果显示锋区位置不同环流特征也不相同，锋区对应的上升流特征与锋区表面通常观测到冷水现象吻合，锋区上层水体沿潮汐锋方向流速较强。另外，数值试验结果显示了地形和潮混合对潮汐锋的形成及锋区环流有重要影响。 相似文献
12.
13.
Richard P. Mied Arnold L. Cooper Gloria J. Lindemann Mark A. Sletten 《Dynamics of Atmospheres and Oceans》2002,36(1-3)
This work deals with the propagation and evolution of disturbances which move along freely propagating two-dimensional gravity current fronts. Examples of evolving perturbations on fronts are displayed in real-aperture radar images of gravity currents in the coastal zone. The theory of Cooper et al. (2001), which is based upon the ray tube formulation of Whitham (1974), is employed to simulate disturbances of the sort seen in this imagery and in the larger body of literature. Initial anomalies in both shape and velocity are introduced and allowed to evolve, and several types of new and interesting behaviors emerge. Shape perturbations of the form x=a sech δy evolve into two anomalies, which separate in time as they propagate in opposite directions along the front. When the value of a is increased, the disturbances, which propagate along the gravity current, can break, forming breaking frontal waves (BFWs). These manifest themselves as sharp angular features to either side of the main bulge. Two types of velocity disturbances are employed. The first has the form U=U0(1+â sech δy), and evolves to preserve a single frontal bulge that increases in amplitude and width as it propagates. Here again, large values of â result in BFWs. In this case, they replicate the general behavior present in the imagery. The second type of velocity perturbation used is U=U0(1+â cos δy). The smallest values of a generate no BFWs, but yield fronts which oscillate in space and time. Larger values produce a string of BFWs which are qualitatively similar to the cusp-and-trough morphology observed so frequently in nature. The largest values of a allow the gravity current to form a string of large, bulbous structures which intersect one another as they propagate forward and spread laterally. And finally, we make an effort to correlate the results of the simulations with the shapes seen in radar and visible imagery in the literature. 相似文献
14.
《Dynamics of Atmospheres and Oceans》2006,41(2):103-120
The effects of Earth's rotation on convection into stratified fluid under uniform surface cooling are investigated using a large-eddy simulation (LES) model. The initial mixed layer depth varies by a factor of 40 and temperature gradient below the mixed layer varies by three orders of magnitude. At the end of integration (typically 20–40 inertial periods), the so-called natural Rossby number for the rotating experiments varies from 0.06 to 2. The wide range of conditions used is designed to extract scaling laws of rotating convection and to shed light on the importance of Earth's rotation on convection. It is found that the effects of rotation can be characterized by a series of hyperbolic tangent functions of the natural Rossby number. The effects of rotation are most pronounced when Ro is order 0.1 or less. For Ro ≥ 1, the effects of rotation become small. Comparison of Lagrangian statistics of numerical floats reveals that horizontal mixing is suppressed in the presence of rotation. This result is consistent with the finding that integral length scale and turbulent intensity decrease when rotation is included, in contrast to the conclusion of an early study that argued for increased horizontal mixing in the presence of rotation. 相似文献
15.
Laboratory experiments were carried out to investigate the interaction between turbulent line buoyant plumes and sharp density interfaces, with the aim of using the results to interpret oceanic observations pertinent to crack openings in the polar ice-cap (leads). These openings take the form of long narrow channels, and are often modeled as line bouyant plumes. The plumes descend as in a homogenoous fluid, impinge on the density interface, and then spread horizontally as gravity currents. Depending on the Richardson number
, where Δb is the buoyancy jump across the interface, lD is the half-width of the plume before the impingement and q0is the buoyancy flux per unit length of the source, different flow patterns were identified. When Ri < 0.5, the plumes penetrate deep into the bottom layer, deflect horizontally and then spread while showing little vertical rise. When 0.6 < Ri < 5, the penetration is significant, but the fluid bounces back after entraining heavy fluid from the lower layer and then spreads horizontally above the interface as a gravity current. Appreciable mixing between this current and the lower layer was detected when Ri <1. When Ri > 10, the penetration was small and a sharp-nosed gravity current emerged some time after the impact. Measurements were made on the penetration depth, the velocities of the gravity current and the subsurface flow towards the plume, the entrainment rate and other wave parameters. Possible implications of the results for oceanic cases are also discussed. 相似文献
16.
Mixing in a two-layer stably stratified fluid by a turbulent jet was studied by a laboratory experiment. A non-swirling jet was discharged vertically downwards in a confined fluid system consisting initially of a top layer of fresh water and a bottom layer of salt water. In total, 16 experimental cases were considered, where the diameter and exit velocity of the jet were varied together with the density difference between the top and bottom layer. Vertical density profiles were determined from conductivity measurements. A three-layer density structure developed in all cases with an intermediate layer that grew in size with time elapsed as fresh and salt water were mixed. The mixing efficiency, defined as the percentage of the supplied kinetic jet energy that is used for increasing the potential energy of the fluid system, was related to a densimetric Froude number based on the intermediate layer depth. Overall, the calculated jet mixing efficiency displayed higher values than comparable efficiencies for destratification with air-bubble plumes. 相似文献
17.
Results are presented from a study of blocked flow (practically stagnant or recirculating light winds) in periodic valleys in thermally stably stratified ambient conditions. Inviscid and turbulent diffusion cases were modelled numerically to clarify the effects of turbulence on the blocking. The reflection of gravity waves from the top boundary of the hydrostatic model atmosphere was avoided by employing the radiation condition given by Klemp and Durran (1983). The dissipative numerical results are compared with new laboratory experiments which utilized the technique of Baines and Hoinka (1985) to simulate a semi-infinitely deep region.A criterion for the occurrence of blocked flow cannot be defined for the inviscid case except when the Froude number, Fr, based on the peak-to-trough ridge amplitude is less than about 0.4: then blocking is clearly identifiable before wave-breaking occurs. Breaking of waves is evident for Fr as large as 0.75, in agreement with analytical results given by Lilly and Klemp (1979).At small Froude number (Fr 0.5) in the dissipative flow simulations, blocked flow (stagnation) is present in the valleys, but a lee rotor (complete stagnation) is not evident. For order unity Froude numbers, blocking is a wave phenomenon, resulting from wave steepening and overturning or turbulent mixing. A finite thickness is brought to rest or participates in a recirculating flow when it first appears. A strong upward flow appears ahead of the rotor in the valleys, and the downslope wind over the windward side of the valleys is strengthened. Thus the present study shows that conditions for the onset of a rotor, and of stagnant flow, in periodic valleys are different.When blocked flow exists, the amplitudes of gravity waves in the upper layer are only 15% (Fr = 0.3) to 80% (Fr = 1.5) of those given by linear theory; this is supported by observations. 相似文献
18.
大气边界层湍流运动是地球大气运动最重要的能量输送过程之一.当数值模式分辨率接近活跃含能湍涡长度尺度时,湍流运动被部分解析,被称为"灰色区域",传统的边界层方案不适合此时模式湍流问题的描述.为了提高模式边界层方案在包括"灰色区域"的不同网格尺度上的描述能力,适应不同分辨率模式的需要,在雷诺平均湍流理论基础上,修正Mell... 相似文献
19.
本文采用多重尺度分析方法讨论了在波动之间的弱非线性相互作用时锋面附近的垂直环流的特征,发现:当基本风场随高度分布有极小值点并且这种分布曲线的曲率半径不大于10公里时,此垂直环流呈孤立重力波特征。 相似文献
20.
The vortex structure formed in front of and behind a hemispheric surface hump has been studied in a water recirculating flume. In the recirculation zone behind the hemisphere, arched vortex tubes were formed. Up to a certain critical Reynolds number, each tube was shed individually from the recirculation zone. However, when the Reynolds number exceeded a critical value, several of the tubes coalesced in the recirculation zone before shedding. In front of the hemisphere, some parabolic vortex tubes formed. Their number increased with Reynolds number below its critical value and decreased above this value. The dimensionless height of the parabolic vortex tube decreased with Reynolds number. 相似文献