波生流对海岸污染物输移的影响

通过物理模型实验对海岸波浪作用下污染物运动特性进行了分析,重点分析了质量输移流、沿岸流、沿岸流不稳定运动及破波带内旋涡运动等海岸水动力因素的影响.实验中坡度分别取为1:100和1:40,实验中采用CCD摄像机记录墨水的运动轨迹,同步测量流体质点速度以及波面升高.实验表明,在破碎带外污染物主要受波浪非线性引起的质量输移流的影响;在破碎带内主要受沿岸流的影响,同时还受沿岸流不稳定运动及大尺度旋涡运动的影响.     

Estimating wave crest distributions using the method of L-moments

The design of fixed or floating offshore structures requires accurate information of the met-ocean data at the intended offshore site. In the design process it is recognized that this environmental data is modified in the near-field by the interaction with the particular geometrical configuration of the offshore structure. This transformation of the incident wave field around and beneath an offshore structure presents a challenge for ocean engineers when specifying the wave gap elevation to avoid impact loads on the underside of the deck and inundation of the topsides. Thus, the accurate estimation of the wave crest distributions from measurements at various locations near and under the offshore structure during model test studies is essential. A semi-empirical approach is presented herein that builds upon the findings of previous studies and introduces the Method of L-moments. A three parameter model for a wave crest probability distribution function is presented and explicit relationships between the parameters of the distribution and its’ first three L-Moments are established. Furthermore, three narrow-band models from earlier research studies are reviewed and compared with the new model. Wave measurements from a mini-TLP model test program are used as the basis for comparison of the four distributions. The root-mean-square error is used as a metric to quantify the overall fit of the data and its accuracy in the high end tail of the data. The L-Moment model is shown to be more robust in representing the data in both the far-field and beneath the deck of the mini-TLP where the wave field demonstrates increased non-linear behavior.     

A GPU accelerated Boussinesq-type model for coastal waves

This study presents an efficient Boussinesq-type wave model accelerated by a single Graphics Processing Unit (GPU). The model uses the hybrid finite volume and finite difference method to solve weakly dispersive and nonlinear Boussinesq equations in the horizontal plane, enabling the model to have the shock-capturing ability to deal with breaking waves and moving shoreline properly. The code is written in CUDA C. To achieve better performance, the model uses cyclic reduction technique to solve massive tridiagonal linear systems and overlapped tiling/shared memory to reduce global memory access and enhance data reuse. Four numerical tests are conducted to validate the GPU implementation. The performance of the GPU model is evaluated by running a series of numerical simulations on two GPU platforms with different hardware configurations. Compared with the CPU version, the maximum speedup ratios for single-precision and double-precision calculations are 55.56 and 32.57, respectively.     

转换波精确无迭代速度分析

提出了一种随着速度比与转换点深度的变化而动态抽道的转换波速度分析方法,实现了扫描叠加速度、转换点深度与共转换点道集的一一对应,是一种真正意义上的共转换点无迭代速度分析技术,理论模型试算与实际地震资料处理都证明了此方法的正确性与有效性.     

Instability of planetary waves in a high vertical resolution model

Abstract

A high vertical resolution model is used to examine the instability of a baroclinic zonal flow and a finite amplitude topographically forced wave. Two families of unstable modes are found, consisting of zonally propagating most unstable modes, and stationary unstable modes. The former have time scale and spatial structure similar to baroclinic synoptic disturbances, but are localized in space due to interaction with the zonally asymmetric forcing. These modes transport heat efficiently in both the zonal and meridional directions. The second family of stationary unstable modes has characteristics of modes of low frequency variability of the atmosphere. They have time scales of 10 days and longer, and are of planetary scale with an equivalent barotropic vertical structure. The horizontal structure resembles blocking flows. They are maintained by available potential energy of the basic wave, and have large zonal heat fluxes. The results for both families of modes are interpreted in terms of an interaction between forcing and baroclinic instability to create favoured regions for eddy development. Applications to baroclinic planetary waves are also considered.     

Open-ocean response and normal mode excitation in an eddy-resolving general circulation model

Abstract

Analysis of a two-layer, flat-bottom, steady-wind driven, eddy-resolving general circulation model reveals a distinct separation in frequency of baroclinic and barotropic motion in the region distant from the model Gulf Stream. The far-field motions at periods less (greater) than about 100 days are predominantly barotropic (baroclinic), unlike the near-field, eddy-generating, free-jet region which contains barotropic and baroclinic energy throughout the modei frequency range. The far-field barotropic energy produces a peak in the model sea-level spectra between 25 and 50 days with a magnitude comparable to energy levels observed in spectra of sea level from oceanic island tide gauges. The far-field barotropic motion is clearly composed of large-scale, resonant, barotropic normal modes drive by mesoscale activity of the turbulent, free-jet region. Oceanic mesoscale turbulence may therefore provide for planetary normal modes an excitation mechanism distinct from atmospheric forcing. The open-ocean, barotropic, model response is very similar to that of a fluctuating-wind driven model, which suggests that atmospheric and intrinsic forcing of mid-ocean eddies may be of comparable importance.     

Magnetic Rossby waves in a stably stratified layer near the surface of the Earth's outer core

Abstract

The stratification profile of the Earth's magnetofluid outer core is unknown, but there have been suggestions that its upper part may be stably stratified. Braginsky (1984) suggested that the magnetic analog of Rossby (planetary) waves in this stable layer (the ‘H’ layer) may be responsible for a portion of the short-period secular variation. In this study, we adopt a thin shell model to examine the dynamics of the H layer. The stable stratification justifies the thin-layer approximations, which greatly simplify the analysis. The governing equations are then the Laplace's tidal equations modified by the Lorentz force terms, and the magnetic induction equation. We linearize the Lorentz force in the Laplace's tidal equations and the advection term in the magnetic induction equation, assuming a zeroth order dipole field as representative of the magnetic field near the insulating core-mantle boundary. An analytical β-plane solution shows that a magnetic field can release the equatorial trapping that non-magnetic Rossby waves exhibit. A numerical solution to the full spherical equations confirms that a sufficiently strong magnetic field can break the equatorial waveguide. Both solutions are highly dissipative, which is a consequence of our necessary neglect of the induction term in comparison with the advection and diffusion terms in the magnetic induction equation in the thin-layer limit. However, were one to relax the thin-layer approximations and allow a radial dependence of the solutions, one would find magnetic Rossby waves less damped (through the inclusion of the induction term). For the magnetic field strength appropriate for the H layer, the real parts of the eigenfrequencies do not change appreciably from their non-magnetic values. We estimate a phase velocity of the lowest modes that is rather rapid compared with the core fluid speed typically presumed from the secular variation.     

Nonlinear wave propagation on a sphere: Interaction between Rossby waves and gravity waves; stability of the Rossby waves

Abstract

An analytical spectral model of the barotropic divergent equations on a sphere is developed using the potential-stream function formulation and the normal modes as basic functions. Explicit expressions of the coefficients of nonlinear interaction are obtained in the asymptotic case of a slowly rotating sphere, i.e. when the normal modes can be expressed as single spherical harmonics.     

Shallow Water Dissipation Processes for Wind Waves off the Mackenzie Delta

This study focuses on two physical processes for waves in shallow waters off the Mackenzie Delta: bottom friction and depth-induced breaking terms. We use field observations of winds and waves, the state-of-the-art Simulating Waves Nearshore (SWAN) model, and reanalysis wind and wave data. The two field observation periods are an August 2008 field experiment, during which in situ field data were collected, and an Arctic storm when data were recorded by buoy measurements from 4 to 6 August 1991. Wind and wave development processes are analyzed during these two periods with comparisons to observed winds and waves. Our analyses show that bottom friction is the main shallow water physical process during the August 2008 field experiment, whereas depth-induced breaking is the dominant shallow water physical process during the 4–6 August 1991 storm, in conjunction with the effects of bottom friction. The SWAN wave model is used to investigate the shallow water physical processes during these two observation periods. Simulation results indicate that the model can give reasonable results, with an appropriate Collins coefficient of 0.006 and a wave breaking parameter of 0.55 to represent bottom friction and depth-induced breaking physics, respectively.

RÉSUMÉ?[Traduit par la rédaction] Cette étude porte sur deux processus physiques concernant les vagues dans les eaux peu profondes au large du delta du Mackenzie : les termes du frottement contre le fond et du déferlement lié à la profondeur. Nous utilisons des observations du vent et des vagues, le modèle d'avant-garde SWAN (Simulating Waves Nearshore) et des données de vent et de vagues réanalysées. Les deux périodes d'observations sont une expérience sur le terrain réalisée en août 2008, au cours de laquelle des données de terrain ont été recueillies, et une tempête arctique lors de laquelle des mesures faites par bouée du 4 au 6 août 1991 ont été enregistrées. Nous analysons les processus dévolution du vent et des vagues durant ces deux périodes, et comparons avec le vent et les vagues observées. Nos analyses montrent que le frottement contre le fond est le processus physique en eaux peu profondes le plus important durant l'expérience sur le terrain d'août 2008, alors que le déferlement lié à la profondeur est le processus physique en eaux peu profondes dominant pendant la tempête arctique du 4 au 6 août 1991, en combinaison avec les effets du frottement contre le fond. Nous nous servons du modèle de vagues SWAN pour étudier les processus physiques en eaux peu profondes durant ces deux périodes d'observations. Les résultats des simulations indiquent que le modèle peut donner des résultats raisonnables, avec un coefficient de Collins approprié de 0,006 et un paramètre de déferlement de 0,55 pour représenter la physique du frottement contre le fond et du déferlement lié à la profondeur, respectivement.     

The formation and evolution of an isolated submarine valley in the North Channel,Irish Sea: an investigation of Beaufort's Dyke

Beaufort's Dyke is a submarine depression located in the North Channel of the Irish Sea. With a maximum depth of 312 m, the dyke is one of the deepest areas within the European continental shelf. Integration and interpretation of 450 km of sparker seismic data and full‐coverage bathymetric data derived from multi‐beam echo sounder surveys allow for the investigation of the formation processes of Beaufort's Dyke and the evolution of geomorphological features within it. The dyke, formed by composite subglacial processes dominated by subglacial meltwater discharge, is interpreted as a tunnel valley. The regional isolation of Beaufort's Dyke may be explained by the bounding of the North Channel by the bedrock masses of Ireland and Scotland, coupled with the exploitation of structural weakness along a fault plane and presence of halite in the eroded substrate enhancing the erosive potential of the overlying glacier. Beaufort's Dyke has probably been maintained as an open feature by strong rectilinear tidal currents. The morphology of lunate sediment waves and a large parabolic bedform towards the south of the dyke contradict the observed dominant S–N mean hydrodynamic flow recorded within the North Channel, suggesting an alternative hydrodynamic regime either within the dyke or during bedform creation. Copyright © 2011 John Wiley & Sons, Ltd.