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1.
Recent observations from the South China Sea have demonstrated that semi-diurnal tides sometimes generate a double-frequency harmonic. Similar harmonic generation has been found in laboratory experiments and numerical simulations of internal wave beams refracting into a pycnocline. Here, a weakly nonlinear theory of internal wave refraction is applied to oceanic internal tides in an idealized stratification profile. The steady state harmonic amplitude is calculated as a function of the tidal frequency and the pycnocline characteristics. The results indicate that harmonic generation by nonlinear refraction of semi-diurnal tides is consistent with the South China Sea observations. 相似文献
2.
《Dynamics of Atmospheres and Oceans》2006,41(4):254-282
Coastal ocean numerical modeling is basically the representation of the dynamics of the coastal ocean in a chosen range of length scales and over an associated frequency band, including the modeling of both coherent processes and associated transient processes. The ocean dynamical features can be individually identified by combining wavelet analysis for time and frequency localization and principal component analysis to “decorrelate” physically consistent structures. In the present paper, the so-called WEof analysis is applied for the extraction of external gravity waves and internal gravity wave lower modes in a simple case of a flat bottom, constant Brunt-Väisälä ocean. It is shown that, with some well known restrictive assumptions, WEof analysis is an efficient candidate for the recognition of frequency localized dynamical processes. 相似文献
3.
‘Application of a New Spectral Theory of Stably Stratified Turbulence to the Atmospheric Boundary Layer over Sea Ice’ 总被引：7，自引：5，他引：2
A new spectral closure model of stably stratified turbulence is used to develop a K–ε model suitable for applications to the atmospheric boundary layer. This K–ε model utilizes vertical viscosity and diffusivity obtained from the spectral theory. In the ε equation, the Coriolis parameter-dependent formulation of the coefficient C 1 suggested by Detering and Etling is generalized to include the dependence on the Brunt-Väisälä frequency, N. The new K–ε model is tested in simulations of the ABL over sea ice and compared with observations from BASE as simulated in large-eddy simulations by Kosovic and Curry, and observations from SHEBA. 相似文献
4.
Internal waves incident on a sheared ocean pycnocline are studied using analytic and numerical methods. Linear analysis of the unstable modes of a sheared ocean pycnocline is used to demonstrate interactions between internal waves and shear instabilities. A new analytic solution for an asymmetric shear layer over a stratified layer is presented, illustrating modes which couple to internal waves, in addition to the well-known Holmboe modes. The robustness of these solutions is demonstrated using numerical methods for realistic shear profiles. Fully nonlinear numerical simulations illustrate the growth of these modes and demonstrate the excitation of shear instabilities by incident internal waves. The results may have implications for internal wave interactions with the ocean pycnocline and the local generation of internal solitary waves. 相似文献
5.
A Boundary-Layer Scaling for Turbulent Katabatic Flow 总被引：1，自引：1，他引：0
Scaling relationships are proposed for the turbulent katabatic flow of a stably stratified fluid down a homogeneously cooled planar slope—the turbulent analogue of a Prandtl-type slope flow. The \(\Pi \) Theorem predicts that such flows are controlled by three non-dimensional parameters: the slope angle, the Prandtl number, and a Reynolds number defined in terms of the surface thermal forcing (surface buoyancy or surface buoyancy flux), Brunt-Väisälä frequency, slope angle, and molecular viscosity and diffusivity coefficients. However, by exploiting the structure of the governing differential equations in a boundary-layer form, scaled equations are deduced that involve only two non-dimensional parameters: the Prandtl number and a modified Reynolds number. In the proposed scaling framework, the slope angle does not appear as an independent governing parameter, but merely acts as a stretching factor in the scales for the dependent and independent variables, and appears in the Reynolds number. Based on the boundary-layer analysis, we hypothesize that the full katabatic-flow problem is largely controlled by two rather than three parameters. Preliminary tests of the scaling hypothesis using data from direct numerical simulations provide encouraging results. 相似文献
6.
The thermohaline structure of waters in the East Siberian Sea coastal zone in September 2000, 2003, and 2004 is investigated. It is found that the spatial variability of thermohaline characteristics was determined by various synoptic conditions observed over the East Siberian Sea during the summer-autumn season and by fluctuations in the river discharge. In surface layers, plume fronts are identified. Under conditions of cyclonic atmospheric circulation of 2003, fresh waters spread as a narrow jet along the coast. During anticyclonic circulation of 2004, some meridional fronts existed. Horizontal gradients of thermohaline characteristics were 0.01–0.02°C/km and 0.03–0.07 psu/km off the Indigirka River and 0.02–0.03°C/km and 0.06–0.09 psu/km near the Kolyma River mouth being an order of magnitude less than the vertical ones. The stratification of coastal waters was amplified as the river and ice-melted runoff increased. In the thermohalocline, the average Brunt-Väisälä frequency was 0.042 s?1 (in 2000), 0.068 s?1 (in 2003), and 0.074 s?1 (in 2004). 相似文献
7.
In Part I the dry version of a new large-eddy simulation (LES) model was presented that is specifically designed to simulate air flow and clouds above highly complex terrain. Here the implemented moisture physics are described and a new method for the generation of turbulent inflow conditions for meteorological LES is proposed. As a typical area of application the new model is applied to simulate banner clouds developing downwind of pyramidal mountain peaks. Banner clouds are shown to be primarily a dynamical phenomenon, and form in the lee of steep mountain peaks as a result of dynamically forced lee upslope flow. Due to the highly asymmetric flow field induced by the extreme orography, banner clouds can form even under horizontally homogeneous initial conditions regarding both moisture and temperature. Thus, additional leeward moisture sources, distinct air masses on both windward and leeward sides, or radiation effects are no prerequisite for banner-cloud formation. The probability of banner-cloud formation increases with increasing obstacle height and steepness and is, to a first approximation, independent of the pyramid’s orientation. Simulations with and without moisture physics reveal that, for the set-up chosen, moisture is of only secondary importance for banner-cloud dynamics. The reinforcement of lee upslope flow and corresponding cloud formation due to latent heat release turns out to be almost negligible. Nevertheless moisture physics are shown to induce a dipole-like structure in the vertical profile of the Brunt-Väisälä frequency, which in turn leads to a moderate increase in leeward turbulence. 相似文献
8.
在前文采用半地转近似和行波法来研究层结大气中的非线性波动的基础上,进一步探讨非线性方程及其级数近似方程的解的稳定性,波动解的存在条件和波动的一些特征。研究指出,在平衡点附近,将非线性方程用其级数近似方程代替是可行的、合理的。在初始拟能满足一定的条件下,椭圆余弦波和孤立波是存在的。文中还求得了椭圆余弦波解的周期、ｘ向波长和振幅以及孤立波的宽度和振幅。并指出,孤立波的宽度和振幅不但与波速有关,还与β因子和层结稳定度有关,而且在相同的某条件下,西行的混合Ｒｏｓｓｂｙ－重力孤立波比起东行的惯性重力孤立波来,宽度要小但振幅却大。 相似文献
9.
The analysis of katabatic flows is often complicated by heterogeneity in surface characteristics. This study focuses on an idealized type of katabatic flow driven by a simple form of inhomogeneous surface forcing: a buoyancy or buoyancy flux that varies down the slope as a top-hat profile (cold strip). We consider the two-dimensional Boussinesq system of governing flow equations with the slope angle, Brunt–Väisälä frequency, and coefficients of eddy viscosity and diffusivity treated as constants. The steady-state problem is solved analytically in a linearized boundary-layer framework. Key flow structures are a primary katabatic jet (essentially the classical one-dimensional Prandtl jet), a rotor-like feature straddling the upslope end of the strip, and two nearly horizontal jets: an inward jet of environmental air feeding into the primary jet on the upslope end of the strip and an outward jet resulting from the intrusion of the primary katabatic jet into the environment on the downslope end of the strip. Next, the corresponding nonlinear initial value problem is solved numerically until a steady state is reached at low levels. The main features of the linear solution are seen in the numerical results, but with some notable differences: (i) the primary jet in the numerical simulation requires a longer distance to attain a one-dimensional boundary-layer structure and extends further downslope off the strip before intruding into the environment; (ii) the numerically simulated outward environmental jet is narrower and more intense than the inward jet, and has a pronounced wave-like structure. 相似文献
10.
Peter C. Smith 《大气与海洋》2013,51(1):181-209
Abstract Analysis of current, temperature and salinity records in the nearshore region of the Scotian Shelf during the Canadian Atlantic Storms Program (CASP), reveals that the inertial wave field is highly intermittent, with comparable amplitudes in the surface and deep layers. Clockwise current energy in the surface layer is concentrated at a frequency slightly below inertial, consistent with Doppler shifting by the strong mean current and/or straining by the mean flow shear, whereas the spectral peak in deep water is at the local inertial frequency. Clockwise coherence is high (γ2 ≥ 0.8) horizontally over the scale of the array (60 km × 120 km) and in the vertical, with upward phase propagation rates of 0.15–0.50 × 10?12 ms?1, inversely proportional to the local value of the Brunt Väisälä frequency. Clockwise current energy decreases in the onshore direction and appears to be completely inhibited on the 60‐m isobath. A case study of the response to the CASP IOP 14 storm indicates that the inertial waves may be generated by a strong wind shift propagating onshore at a speed of 10 ms?1. On the eastern side of the array (Liscomb line), clockwise current oscillations propagate onshore in the surface layer at a rate (8.1 ± 0.9 m s?1) comparable with the speed of the atmospheric front, while waves in the pycnocline move offshore at a lower (internal wave) speed (1.8 m s?1). Furthermore the temperature and salinity fluctuations are in (out) of phase with longshore current in the deep (surface) layer. However, on the western side of the array (Halifax line), the inertial waves are more complex. A sharp steepening of phase lines at the coast indicates that the phase speed of clockwise current oscillations is considerably reduced and the evidence for offshore propagation of internal waves is less clear. The discrepancies between observations on the two lines suggest that the internal wave field is three‐dimensional. Results of simple mixed‐layer models indicate that the inertial response near the surface is sensitive to the accurate definition of the local wind field, but not to certain model physics, such as the form of the decay term. The observations also show some qualitative similarities with models for two‐dimensional response to a moving front (e.g. Kundu, 1986), but the actual forcing terms are more complicated, based on IOP 14 wind measurements. 相似文献
11.
青藏高原上空云宏观参数的日变化受大尺度环流、当地太阳辐射和地表过程的联合作用,对辐射收支、辐射传输及感热、潜热的分布等有重要影响。由于缺乏持续定量的观测,对各类天气系统云宏观参数日变化特征的了解还十分不足。多波段多大气成分主被动综合探测系统APSOS（Atmospheric Profiling Synthetic Observation System）的Ka波段云雷达是首部在青藏高原实现长期观测云的雷达。本文基于2019年全年APSOS的Ka波段云雷达资料,采用统计和快速傅里叶变换方法研究了西风槽、切变线和低涡三类重要天气系统影响下的有云频率、单层非降水云或者降水云非降水时段的云顶高度、云底高度和云厚日变化的时域和频域特征,得到了统计回归方程。主要结论有:（1）西风槽系统日均有云频率为56.9%,切变线系统为50.8%,低涡系统达73%。（2）尽管西风槽和切变线系统的成因不同,但两类系统云宏观参数的日变化趋势和主要谐波周期相似:日变化趋势基本为单峰单谷型,日出前最低,日落前最高。有云频率表现为日变化和半日变化,单层云云顶高度、云底高度和云厚主要表现为日变化。（3）低涡系统云宏观参数的日变化特征与前两类系统明显不同:日变化趋势表现为多峰多谷型,虽然有云频率和单层云云顶高度、云底高度主要谐波中均以日变化振幅最大,但频谱分布分散,云厚主要变化中振幅最大的是周期为4.8 h的波动。（4）得到了各系统有云频率、单层云云顶高度、云底高度和云厚日变化的统计回归方程。 相似文献
12.
The Scaling Behaviour of a Turbulent Kinetic Energy Closure Model for Stably Stratified Conditions 总被引：2，自引：1，他引：1
We investigate the scaling behaviour of a turbulent kinetic energy (TKE) closure model for stably stratified conditions. The mixing length scale for stable stratification is proportional to the ratio of the square root of the TKE and the local Brunt–Väisälä frequency, which is a commonly applied formulation. We analyze the scaling behaviour of our model in terms of traditional Monin–Obukov Similarity Theory and local scaling. From the model equations, we derive expressions for the stable limit behaviour of the flux–gradient relations and other scaling quantities. It turns out that the scaling behaviour depends on only a few model parameters and that the results obey local scaling theory. The analytical findings are illustrated with model simulations for the second GABLS intercomparison study. We also investigate solutions for the case in which an empirical correction function is used to express the eddy diffusivity for momentum as a function of the Richardson number (i.e. an increasing turbulent Prandtl number with stability). In this case, it seems that for certain parameter combinations the model cannot generate a steady-state solution. At the same time, its scaling behaviour becomes unrealistic. This shows that the inclusion of empirical correction functions may have large and undesired consequences for the model behaviour. 相似文献
13.
By using the symmetric equations of atmospheric dynamics in y-z plane with vertical and horizontal shear of wind, the nonlinear ordinary differential equation is derived with the method of travelling wave.Its stability is discussed by using the theory of nonlinear stability and the KDV equation is solved. The effects of linear CISK, nonlinear CISK, inertial stability and vertical shear of wind on the amplitude and the width of isolated inertial gravitational wave are discussed. In order to understand deeply the formation and development of meso-scale synoptic systems, such as the squall line, MCC, the cold surge of Asia high and typhoon, the factors of development of the isolated inertial gravity wave are analysed. 相似文献
14.
The height of the atmospheric boundary layer (ABL) obtained with lidar and radiosondes is compared for a data set of 43 noon (12.00 GMT) cases in 1984. The data were selected to represent the synoptic circulation types appropriately. Lidar vertical profiles at 1064 nm were used to obtain three estimates for the ABL height (h lid), based on the first gradient in the back-scatter profile, namely, at the beginning, middle and top of the gradient. The boundary-layer height obtained with the radiosondes (h s) was determined with the dry-parcel-intersection method in unstable conditions. As a first guess for near-neutral and stable conditions, the height of the first significant level in the potential temperature profile was taken. Overall, the boundary-layer thickness estimates agree surprisingly well (regression lineh lidb=hs:cc.=0.93 and the standard error=121 m). However, in 10% of the cases, the lidar estimate was significantly lower (difference>400 m) than the routinely inferredh s. These outliers are discussed separately. For stable conditions, an estimate of ABL height (h N) is also made based on the friction velocity and the Brunt-Väisälä frequency. The agreement betweenh Nandh lidbis good. Discrepancies between the two methods are caused by:
- rapid growth of the boundary layer arround the measurement time;
- the presence of a deep entrainment layer leading to a large zone in which quantities are not well mixed;
- a large systematic error of 100–200 m in the estimate of boundary-layer height obtained from the radiosonde due to the way that profiles are recorded, as a series of significant points.
15.
Yih-Ho Pao 《Boundary-Layer Meteorology》1973,5(1-2):177-193
A turbulent stratified shear flow is generated in a towing tank by towing a grid or a circular cylinder through a tank of stratified salt water. The internal waves and turbulence generated in these flows are visualized with shadowgraphs and measured with quartz-coated hot-film probes (up to four probes for velocity fluctuations) and single-electrode conductivity probes (up to four probes for salinity fluctuations) which are towed at the same speed as the obstacle. The velocity and salinity signals are recorded on magnetic tapes. A portion of these signals is processed directly-on-line with a digital computer. From these shadowgraphs and probe measurements, we observe that
- Far downstream of the obstacle where the turbulence has already subsided, the stratified fluid always has a layered structure. This layered structure persists for a long time, and is a result of the convection of turbulently mixed layers by the mean flow. These results indicate that in the regions of a stably stratified atmosphere and ocean where the turbulence has subsided, one could often find layered structure.
- There are spectral peaks and valleys in the measured velocity and salinity autospectra when the stratifications are sufficiently strong. Under certain conditions, these spectral peaks tend to lift up the spectral curves to show substantialf ?5/3 subranges, although the turbulence Reynolds numbers are too low for the flows to have recognizable inertial subranges. This anomalousf ?5/3 subrange demonstrates the pitfalls of using spectral measurements in thef ?5/3 subrange to predict the turbulent energy dissipation rate through the Kolmogorov hypothesis.
- A diagnostic method is developed for distinguishing internal waves from turbulence, utilizing their phase characteristics. The phase characteristics can be conveniently examined from the cospectra and quadrature spectra measurements of: (a), two vertically separated velocity probes; (b), two vertically separated density probes; and (c), a velocity probe and a density probe. This method is demonstrated to be useful in the laboratory and can be applied directly to atmospheric and oceanic measurements to distinguish internal waves from turbulence.
- From the coherency measurements, it is found that the entire turbulent stratified wake is actually whipping up and down at a frequency corresponding to the Brunt-Väisälä frequency. This indicates that similar stratified shear flows in the atmosphere and in the ocean, such as the jet streams in the atmosphere and the Cromwell current in the ocean, may oscillate vertically, which in turn can induce horizontal oscillation and meandering.
16.
V. Andreev 《Boundary-Layer Meteorology》1973,3(3):273-283
The changes with timet of a temperature deviation δT(t,α) and of a vertical velocityW i(t,α) of an isolated dry thermal have been investigated theoretically. Solutions for the functionW i(t, α) have been derived for stable and unstable environmental stratifications. Comparing these solutions with the corresponding ones for the rise of an adiabatic thermal yield some interesting conclusions. Firstly, there is the evident relation between the rate of entrainment of environmental air (expressed by the parameter α=(1/M i) dM i/dz whereM i is the mass of the thermal) and the vertical velocity of the thermal: an increase in α decreases the velocity. Two similar thermals in stably stratified surroundings, one of them moving adiabatically (α=0) the other nonadiabatically (α>0), would rise for the same length of timet 2=π/N, whereN is a typical Brunt-Väisälä frequency, but with different velocities and to different heights: the ascent timet 2 depends only on environmental parameters. In an unstable stratification, the vertical non-adiabatic velocity of the thermal, instead of increasing without limit, tends towards a finite asymptotic velocity $$W_t (\infty ) = (\sqrt { - \mathcal{N}^2 } )/\alpha $$ the value of which depends upon both the stratification of the surroundings and upon the entrainment rate α. In a real atmosphere, where additional retarding forces exist, the motion will certainly be damped. 相似文献
17.
利用2014年6月-2017年9月的秒级探空资料,选取四川地区5个代表性站点研究重力波在对流层（2~10 km）和平流层（18~25 km）的时空特征。选取结果表明:重力波能量在四川地区各个高度均存在明显的季节变化,冬季强,夏季弱;在对流层由于地形影响,川西和川北高原地区的能量小于其他地区。垂直波长没有明显的时空变化,在对流层和平流层分别集中分布于1.5~3 km和1.5~3.5 km;水平波长则差别较大,分别分布于0~300 km和100~700 km,平均值分别为100 km和350 km。重力波固有频率在对流层有较大的区域差异,表现为在四川西北部的高原地区固有频率平均值为3f（f为地转参数）,其他地区则仅为2.4f;平流层则没有明显的差异存在,均约为2f。四川地区重力波的垂直传播方向特征基本相同,在对流层约有50%的波动向上传播,平流层则有90%以上的波动向上传播。水平传播则存在明显的不确定性,特别是对流层;平流层水平传播方向存在明显的季节变化,表现为夏季重力波多向偏东方向传播,而其他季节则向偏西方向传播。 相似文献
18.
The tendency for rising sea levels, combined with changes in the frequency and intensity of extreme storm events raises the potential for flooding and inundation of coastal locations such as Halifax Harbour, in the context of climate change. In this study, we consider three scenarios for extreme high water levels based on climate change scenarios and estimates for land subsidence, rising mean sea levels, and return period analysis for extreme events (from previous studies). We also investigate the effect of ocean waves on these estimates for extreme high water levels. Because the most damaging storm to make landfall in Nova Scotia over the last century was Hurricane Juan (2003), it was chosen to simulate the extreme case of storm-generated waves and wave run-up in Halifax Harbour. To simulate waves generated by Hurricane Juan, a nested-grid system consisting of two modern state-of-the-art operational wave models was used. High quality winds were used to drive the wave models, and the simulations used recently updated high resolution coastal bathymetry. Observed water elevation changes in Halifax Harbour were used in wave model simulations of Hurricane Juan. The Federal Emergency Management Agency's (FEMA) run-up model is used to estimate wave run-up elevation, which is validated with recorded high water observations along the coastline. Simulated waves and wave run-up elevations for Hurricane Juan suggest that the maximum significant wave heights at the mouth of the Harbour were 9.0 m, and the wave run-up was as high as 2.0 m along the shoreline of Halifax Harbour. In this way, we estimated the impact of waves and wave run-up on extreme high water elevations for three climate change scenarios in Halifax Harbour, under worst-case conditions. The sensitivity of these estimates is analyzed for different water level variations, wave propagation directions and shore slope profiles. [Traduit par la rédaction] La tendance à la hausse du niveau de la mer de pair avec les changements dans la fréquence et l'intensité des événements de tempêtes extrêmes augmentent le risque d'inondation et de submersion à des emplacements côtiers comme le port d'Halifax dans le contexte du changement climatique. Dans cette étude, nous examinons trois scénarios de niveaux de hautes eaux extrêmes basés sur des scénarios de changement climatique et estimations de subsidence du terrain, l’élévation du niveau moyen de la mer ainsi que l'analyse de la période de retour d’événements extrêmes (faite lors d’études antérieures). Nous examinons aussi les effets des vagues de l'océan sur ces estimations de niveaux de hautes eaux extrêmes. Étant donné que la tempête la plus dévastatrice à avoir touché terre en Nouvelle–Écosse au cours du dernier siècle a été l'ouragan Juan (2003), c'est celle-ci que nous avons choisie pour simuler le cas extrême de vagues produites par une tempête et de remontée de vagues dans le port d'Halifax. Pour simuler les vagues produites par Juan, nous nous sommes servis d'un système à grilles imbriquées consistant en deux modèles de vagues opérationnels à la fine pointe de la technologie. Les modèles de vagues étaient pilotés par des données de vent de haute qualité et les simulations disposaient d'une bathymétrie côtière à haute résolution récemment mise à jour. Les changements observés d’élévation de l'eau dans le port d'Halifax ont été utilisés dans les simulations de l'ouragan Juan par les modèles de vagues. Nous utilisons le modèle de remontée des vagues de la Federal Emergency Management Agency (FEMA) pour estimer la hauteur des remontées, qui est validée par rapport aux observations de hautes eaux enregistrées le long de la côte. Les vagues et les hauteurs de remontée des vagues simulées pour l'ouragan Juan suggèrent que la hauteur significative maximale des vagues à l'entrée du port était de 9,0 m et la remontée des vagues atteignait 2,0 m le long de la côte du port d'Halifax. Nous avons de cette manière estimé l'impact des vagues et de la remontée des vagues sur l’élévation des hautes eaux extrêmes pour trois scénarios de changement climatique dans le port d'Halifax dans les conditions les plus défavorables. Nous analysons la sensibilité de ces estimations à divers changements de niveau d'eau, directions de propagation et profils de pente côtière. 相似文献
19.
Abstract An intercomparison of the Väisälä MicroCora system used in the Automated Shipboard Aerological Program (ASAP) and the Atmospheric Environment Service upper‐air system (GMD/ADRES) was conducted in May‐June 1983. Thirty‐three paired ascents were made. The ASAP system dry‐bulb temperatures averaged 0.3°C warmer. For the lowest 100 mb, the dew‐point temperature difference (ASAP ‐ GMD/ADRES) was near ‐1°C whereas for the 780–500 mb layer, the difference was 1°C. The wind component mean differences averaged were small but with a 4 m s?1 standard deviation. 相似文献
20.
Plumb三维波作用通量和EP剖面图——对定常波模式输出结果的诊断分析 总被引：1，自引：0，他引：1
雷兆崇 《南京气象学院学报》1991,(1)
本文利用Plumb(1985年)所导出的线性、准地转定常波的三维波作用通量和Eliassen-Palm(EP)剖面图,对于一个线性和一个非线性的关于大气定常波的斜压初始方程三维谱模式的若干输出结果进行了诊断分析。分析结果表明,Plumb通量和EP剖面图,不仅可以用来诊断波作用在三维空间的传播、波动与纬向平均气流之间的相互作用,而且也可以用来诊断非线性作用的显著性,在利用它们对于观测资料进行诊断分析时,应当充分注意到非线性作用的影响。 相似文献