A three-dimensional surface wave–ocean circulation coupled model and its initial testing

A theoretical framework to include the influences of nonbreaking surface waves in ocean general circulation models is established based on Reynolds stresses and fluxes terms derived from surface wave-induced fluctuation. An expression for the wave-induced viscosity and diffusivity as a function of the wave number spectrum is derived for infinite and finite water depths; this derivation allows the coupling of ocean circulation models with a wave number spectrum numerical model. In the case of monochromatic surface wave, the wave-induced viscosity and diffusivity are functions of the Stokes drift. The influence of the wave-induced mixing scheme on global ocean circulation models was tested with the Princeton Ocean Model, indicating significant improvement in upper ocean thermal structure and mixed layer depth compared with mixing obtained by the Mellor–Yamada scheme without the wave influence. For example, the model–observation correlation coefficient of the upper 100-m temperature along 35° N increases from 0.68 without wave influence to 0.93 with wave influence. The wave-induced Reynolds stress can reach up to about 5% of the wind stress in high latitudes, and drive 2–3 Sv transport in the global ocean in the form of mesoscale eddies with diameter of 500–1,000 km. The surface wave-induced mixing is more pronounced in middle and high latitudes during the summer in the Northern Hemisphere and in middle latitudes in the Southern Hemisphere.     

Bed shear stress in the southern North Sea as an important driver for suspended sediment dynamics

This paper addresses the spatial and temporal patterns of drivers for sediment dynamics in coastal areas. The basic assumption is that local processes are dominating. The focus is put on the bed shear stress in the southern part of North Sea giving the basic control for deposition–sedimentation and resuspension–erosion. The wave-induced bed shear stress is formulated using a model based on the concept that the turbulent kinetic energy associated with surface waves is a function of orbital velocity, the latter depending on the wave height and period, as well as on the water depth. Parameters of surface waves are taken from simulations with the wave spectrum model WAM (wave model). Bed shear stress associated with currents is simulated with a 3D primitive equation model, Hamburg Shelf Ocean Model. Significant wave height, bed shear stress due to waves and currents, is subjected to empirical orthogonal functions (EOF) analysis. It has been found that the EOF-1 of significant wave height represents the decrease of significant wave height over the shallows and, due to fetch limitation, along the coastlines. Higher order modes are seesaw-like and, in combination, display a basin-scale rotational pattern centred approximately in the middle of the basin. Similar types of variability is also observed in the second and third EOF of bed shear stress. Surface concentrations of suspended matter derived from MERIS satellite data are analysed and compared against statistical characteristics of bed shear stress. The results show convincingly that the horizontal distribution of sediment can, to a larger extent, be explained by the local shear stress. However, availability of resuspendable sediments on the bottom is quite important in some areas like the Dogger Bank.     

主动空间试验中空间电荷波向电磁波的转换

本文理论分析了电子束沿地磁场穿越均匀、磁化等离子体密度跃变区域时,在弱磁场近似、哨声波激发、低频近似等几种典型情况下电子束流空间电荷波（Space charge wave）向电磁波的转换.先运用小信号假设求得电子束入射进均匀各向异性冷等离子体之后的色散关系和空间电荷波波数,然后借助于电磁波分量和电子束速度的边界条件,求解电子束在等离子浓度发生变化区域激发的波振幅,在几种典型情形下推导出空间电荷波转换为电磁波之间转换系数的近似解,给出了相应波辐射的坡印亭(Poynting)矢量表达式.结果表明,在渡越辐射（Transition radiation）情形下电子束可以在空间等离子体中激发出阿尔芬波（Alfven wave）和哨声波（Whistler wave）.所得结论可用于对主动空间试验结果的分析.     

Effects of cross-anisotropic soil behaviour on the wave-induced residual liquefaction in the vicinity of pipeline buried in elasto-plastic seabed foundations

In this paper, a two-dimensional integrated numerical model is developed to examine the influences of cross-anisotropic soil behaviour on the wave-induced residual liquefaction in the vicinity of a pipeline buried in a porous seabed. In the wave model, the RANS (Reynolds Averaged Navier–Stokes) equation is used to govern the wave motion. In the seabed model, the residual soil response in the vicinity of an embedded pipeline is considered with the 2-D elasto-plastic solution, where the phase-resolved shear stress is used as a source for the build-up of residual pore pressure. Classical Biot׳s consolidation equation is used for linking the solid-pore fluid interaction. The validation of the proposed integrated numerical model is conducted by the comparisons with the previous experimental data. Numerical examples show that the pore pressures can accumulate to a large value, thus resulting in a larger area of liquefaction potential in the given anisotropic soil compared to that with isotropic solution. The influences of anisotropic parameters on the wave-induced residual soil response in the vicinity of pipeline are significant. A high rate of pore pressure accumulation and dissipation is observed and the liquefaction potential develops faster as the anisotropic parameters increase. Finally, a simplified approximation based on a detailed parametric investigations is proposed for the evaluation of maximum liquefaction depth (z_L) in engineering application.     

Near-surface flow structure over wind-generated water waves,part I: wave-induced flow characteristics

We report on an experimental study conducted to investigate the influence of small-scale wind waves on the airflow structure in the immediate vicinity of the air–water interface. PIV technique was used to measure the two-dimensional velocity fields at wind speeds of 3.7 and 4.4 m?s^?1 and at a fetch of 2.1 m. The flow structure was analyzed as a function of wave phase. In the near-surface region, significant variations were observed in the flow structure over the waveform. The phase-averaged profiles of velocity, vorticity, and Reynolds stress showed different behavior on the windward and leeward sides of the wave in the near-surface region. The influence of wave-induced velocity was restricted within a distance of three significant wave heights from the surface, which also showed opposite trends on the windward and leeward sides of the crest. The results also show that the turbulent Reynolds stress mainly supports downward momentum transfer whereas the wave-induced Reynolds stress is responsible for the upward momentum transfer from wave to wind. In the immediate vicinity of the air–water interface, the momentum is transferred from waves to wind along the windward side, whereas, the momentum transfer is from wind to waves along the leeward side.     

Near-surface flow structure over wind-generated water waves,part II: characteristics of separated and non-separated flows

We report on an experimental study conducted to investigate the influence of small-scale wind waves on the airflow structure in the immediate vicinity of the air–water interface. PIV technique was used to measure the two-dimensional velocity fields at wind speeds of 3.7 and 4.4 m s⁻¹ and at a fetch of 2.1 m. The flow structure was analyzed as a function of wave phase. In the near-surface region, significant variations were observed in the flow structure over the waveform. The phase-averaged profiles of velocity, vorticity, and Reynolds stress showed different behavior on the windward and leeward sides of the wave in the near-surface region. The influence of wave-induced velocity was restricted within a distance of three significant wave heights from the surface, which also showed opposite trends on the windward and leeward sides of the crest. The results also show that the turbulent Reynolds stress mainly supports downward momentum transfer whereas the wave-induced Reynolds stress is responsible for the upward momentum transfer from wave to wind. In the immediate vicinity of the air–water interface, the momentum is transferred from waves to wind along the windward side, whereas, the momentum transfer is from wind to waves along the leeward side.     

南海北部雾状层分布和特征的地震海洋学研究

利用地震海洋学方法在南海北部陆架和上陆坡区域发现了15个雾状层.这些雾状层的延伸长度从几千米到几十千米,厚度十几米到一百米,其顶界所处水深在135 m至715.5 m范围之间.雾状层在地震海洋学剖面上表现为强反射特征.不同于其他传统声学或光学方法,地震海洋学方法分辨率高,且能在短时间内对整个水体进行成像,可以记录到雾状层的时空变化特征,实现对雾状层的"四维"观测.南海北部上陆坡区域是内孤立波浅化、能量耗散集中的区域,在此过程中内孤立波会导致较大的波致流速,侵蚀海底使得表面沉积物再悬浮,进入水体,形成和维持雾状层的存在.     

Effects of wave-induced forcing on a circulation model of the North Sea

The effect of wind waves on water level and currents during two storms in the North Sea is investigated using a high-resolution Nucleus for European Modelling of the Ocean (NEMO) model forced with fluxes and fields from a high-resolution wave model. The additional terms accounting for wave-current interaction that are considered in this study are the Stokes-Coriolis force, the sea-state-dependent energy and momentum fluxes. The individual and collective role of these processes is quantified and the results are compared with a control run without wave effects as well as against current and water-level measurements from coastal stations. We find a better agreement with observations when the circulation model is forced by sea-state-dependent fluxes, especially in extreme events. The two extreme events, the storm Christian (25–27 October 2013), and about a month later, the storm Xaver (5–7 December 2013), induce different wave and surge conditions over the North Sea. Including the wave effects in the circulation model for the storm Xaver raises the modelled surge by more than 40 cm compared with the control run in the German Bight area. For the storm Christian, a difference of 20–30 cm in the surge level between the wave-forced and the stand-alone ocean model is found over the whole southern part of the North Sea. Moreover, the modelled vertical velocity profile fits the observations very well when the wave forcing is accounted for. The contribution of wave-induced forcing has been quantified indicating that this represents an important mechanism for improving water-level and current predictions.     

有限水域近水面气流演化特征试验研究

气流作为湖泊、湿地等有限水域的主要驱动力之一,其演化特征是研究水-气间能量与物质传递的基础,决定了水域水环境与水生态格局,具有重要研究意义.采用室内风洞水槽,通过设置不同试验风速,研究近水面气流与风速和吹程的响应关系与演化特征,并将光滑壁面条件下的气流特征作为试验对照组,分析时间域内风速垂向分布形式、摩阻风速、特征粗糙...     

Observation-based evaluation of surface wave effects on currents and trajectory forecasts

Knowledge of upper ocean currents is needed for trajectory forecasts and is essential for search and rescue operations and oil spill mitigation. This paper addresses effects of surface waves on ocean currents and drifter trajectories using in situ observations. The data set includes colocated measurements of directional wave spectra from a wave rider buoy, ocean currents measured by acoustic Doppler current profilers (ADCPs), as well as data from two types of tracking buoys that sample the currents at two different depths. The ADCP measures the Eulerian current at one point, as modelled by an ocean general circulation model, while the tracking buoys are advected by the Lagrangian current that includes the wave-induced Stokes drift. Based on our observations, we assess the importance of two different wave effects: (a) forcing of the ocean current by wave-induced surface fluxes and the Coriolis–Stokes force, and (b) advection of surface drifters by wave motion, that is the Stokes drift. Recent theoretical developments provide a framework for including these wave effects in ocean model systems. The order of magnitude of the Stokes drift is the same as the Eulerian current judging from the available data. The wave-induced momentum and turbulent kinetic energy fluxes are estimated and shown to be significant. Similarly, the wave-induced Coriolis–Stokes force is significant over time scales related to the inertial period. Surface drifter trajectories were analysed and could be reproduced using the observations of currents, waves and wind. Waves were found to have a significant contribution to the trajectories, and we conclude that adding wave effects in ocean model systems is likely to increase predictability of surface drifter trajectories. The relative importance of the Stokes drift was twice as large as the direct wind drag for the used surface drifter.

     

On the sensitivity of the sedimentary system in the East Frisian Wadden Sea to sea-level rise and wave-induced bed shear stress

The paper addresses the individual and collective contribution of different forcing factors (tides, wind waves, and sea-level rise) to the dynamics of sediment in coastal areas. The results are obtained from simulations with the General Estuarine Transport Model coupled with a sediment transport model. The wave-induced bed shear stress is formulated using a simple model based on the concept that the turbulent kinetic energy (TKE) associated with wind waves is a function of orbital velocity, the latter depending on the wave height and water depth. A theory is presented explaining the controls of sediment dynamics by the TKE produced by tides and wind waves. Several scenarios were developed aiming at revealing possible trends resulting from realistic (observed or expected) changes in sea level and wave magnitude. The simulations demonstrate that these changes not only influence the concentration of sediment, which is very sensitive to the magnitude of the external forcing, but also the temporal variability patterns. The joint effect of tides and wave-induced bed shear stress revealed by the comparison between theoretical results and simulations is well pronounced. The intercomparison between different scenarios demonstrates that the spatial patterns of erosion and deposition are very sensitive to the magnitude of wind waves and sea-level rise. Under a changing climate, forcing the horizontal distribution of sediments adjusts mainly through a change in the balance of export and import of sediment from the intertidal basins. The strongest signal associated with this adjustment is simulated North of the barrier islands where the evolution of sedimentation gives an integrated picture of the processes in tidal basins.     

Interaction of waves,currents and tides,and wave-energy impact on the beach area of Sylt Island

Erosion due to waves is an important and actual problem for most coastal areas of the North Sea. The objective of this study was to estimate the impact of wave action on the coastline of Sylt Island. From a 2-year time series (November 1999 to October 2001) of hydrological and wave parameters generated with a coupled wave–current modelling system, a period comprising storm ‘Anatol’ (3–4 December 1999) is used to investigate the effects of waves on currents and water levels and the input of wave energy into the coastline. The wave-induced stress causes an increase of the current velocity of 1 m/s over sand and an additional drift along the coast of about 20 cm/s. This produces a water level increase of more than 20 cm in parts of the tidal basin. The model system also calculates the wave energy input into the coastline. Scenario runs for December 1999 with a water level increase of 50 cm and wind velocity increased by 10% show that the input of the wave energy into the west coast of Sylt Island increases by 30% compared to present conditions. With regard to the forecasted near-future (Woth et al., Ocean Dyn 56:3–15, 2006) increase of strong storm surges, the scenario results indicate an increased risk of coastal erosion in the surf zone of Sylt Island.     

Wave climate and long-term changes for the Southern North Sea obtained from a high-resolution hindcast 1958–2002

An analysis of the extreme wave conditions in 1958–2002 in the North Sea as obtained from a regional model hindcast is presented. The model was driven by hourly wind fields obtained from a regional atmosphere model forced with reanalysis data from the National Center for Environmental Prediction (NCEP/NCAR). Furthermore, observed sea ice conditions from the Norwegian Meteorological Institute have been accounted for in the simulation. It is shown that the model is capable of reproducing extreme wave height statistics at a reasonable degree of approximation. The analysis of severe wave height events reveals that for much of the Southern North Sea, their number has increased since the beginning of the simulation period (1958), although the increase has attenuated later and leveled off around about 1985. On the other hand, the intensity and duration of severe wave height events decreased within the last few years of the simulation so that annual 99%-ile wave heights have also reduced since about 1990–1995. For the UK North Sea coast, a different behavior was found characterized by a reduction in severe wave conditions over much of the hindcast period.     

地震数据的反射波动方程最小二乘偏移

基于反射波动方程,本文提出了一种估计地下反射率分布的地震数据最小二乘偏移方法.高频近似下,非齐次的一次反射波动方程的源项是由反射率与入射波场的时间一阶导数相互作用产生的.根据反射波动方程,利用线性最小二乘反演方法由地震反射数据重建出地下产生反射波的反射源,再结合波场正演计算出的地下入射波场,得到地下反射率分布的估计.在地下反射源的线性最小二乘反演重建中,我们采用迭代求解方法,并以地震波的检波器单向地下照明强度作为最小二乘优化问题中Hessian矩阵的近似.

     

Observation-based evaluation of surface wave effects on currents and trajectory forecasts

Knowledge of upper ocean currents is needed for trajectory forecasts and is essential for search and rescue operations and oil spill mitigation. This paper addresses effects of surface waves on ocean currents and drifter trajectories using in situ observations. The data set includes colocated measurements of directional wave spectra from a wave rider buoy, ocean currents measured by acoustic Doppler current profilers (ADCPs), as well as data from two types of tracking buoys that sample the currents at two different depths. The ADCP measures the Eulerian current at one point, as modelled by an ocean general circulation model, while the tracking buoys are advected by the Lagrangian current that includes the wave-induced Stokes drift. Based on our observations, we assess the importance of two different wave effects: (a) forcing of the ocean current by wave-induced surface fluxes and the Coriolis–Stokes force, and (b) advection of surface drifters by wave motion, that is the Stokes drift. Recent theoretical developments provide a framework for including these wave effects in ocean model systems. The order of magnitude of the Stokes drift is the same as the Eulerian current judging from the available data. The wave-induced momentum and turbulent kinetic energy fluxes are estimated and shown to be significant. Similarly, the wave-induced Coriolis–Stokes force is significant over time scales related to the inertial period. Surface drifter trajectories were analysed and could be reproduced using the observations of currents, waves and wind. Waves were found to have a significant contribution to the trajectories, and we conclude that adding wave effects in ocean model systems is likely to increase predictability of surface drifter trajectories. The relative importance of the Stokes drift was twice as large as the direct wind drag for the used surface drifter.     

Wave energy input into the Ekman layer

This paper is concerned with the wave energy input into the Ekman layer, based on 3 observational facts that surface waves could significantly affect the profile of the Ekman layer. Under the assumption of constant vertical diffusivity, the analytical form of wave energy input into the Ekman layer is derived. Analysis of the energy balance shows that the energy input to the Ekman layer through the wind stress and the interaction of the Stokes-drift with planetary vorticity can be divided into two kinds. One is the wind energy input, and the other is the wave energy input which is dependent on wind speed, wave characteristics and the wind direction relative to the wave direction. Estimates of wave energy input show that wave energy input can be up to 10% in high-latitude and high-wind speed areas and higher than 20% in the Antarctic Circumpolar Current, compared with the wind energy input into the classical Ekman layer. Results of this paper are of significance to the study of wave-induced large scale effects.     

Dynamic response of a porous seabed around pipeline under three-dimensional wave loading

The evaluation of the wave-induced pore pressure around a buried pipeline is particularly important for pipeline engineers involved in the design of offshore pipelines. Most previous investigations of the wave-induced dynamic response around an offshore pipeline have limited to two-dimensional cases. In this paper, a three-dimensional model including buried pipeline is established, based on the existing DYNE3WAC models. Based on the proposed numerical model and poro-elastic soil material assumption, the effects of wave and soil characteristics, such as wave period, water depth, shear modulus and permeability, and configuration of pipelines, such as pipeline radius and pipeline buried depth, on the wave-induced excess pore pressure will be examined. Numerical results indicated that the normalized excess pore pressures versus z/h near the pipeline increase as the obliquity angle, wave period and water depth increase, and they decrease as the burial depth and radius of pipeline increase above the pipeline. Soil permeability has obvious influence on the wave-induced normalized excess pore pressure, and different soil material will result in distinct computation results.     

苏鲁交界至南黄海地区应力场的动态变化与地震活动

利用地震记录的初至波符号使用格点尝试法处理了苏鲁交界至南黄海地震重点监视区及其分区１９６９－１９９１年８月逐时段滑动小震综合震源机制解，研究了该区应力场的基本特征及其动态变化与中强地震活动的关系．发现该区应力场总体特征与华北大区域应力场基本一致，但稳定性较差，可能存在一定的垂向作用．该区中强地震多发生在综合机制解Ｐ、Ｔ轴仰角增大时段，震前多表现为矛盾符号比高值异常．这一现象对今后该区中强地震预报具有现实意义．     

Experiments on wave-soil interaction and wave-driven soil transport in clay beds

Wavelength and wave damping of water waves propagating over soft clay beds are measured in a soil-wave tank. Mudline motion and wave-induced mass transport in the clay beds are also measured. It is confirmed experimentally that the wave dispersion is uniquely governed by the mudline motion. Mass transport develops in a clay bed when shear strain amplitude exceeds the limit shear strain of about 5%. The rate of wave-induced mass transport in a clay bed is found to be proportional to the rate of wave energy dissipation in the clay bed. This paper presents a complete report of the measurements and data.