Simulation of Typhoon-Driven Waves in the Yangtze Estuary with Multiple-Nested Wave Models

Typhoon-generated waves are simulated with two numerical wave models, the SWAN model for the coastal and Yangtze Estuary domain, nested within the WAVEWATCHIII （WW3） for the basin-scale East China Sea domain. Typhoon No. 8114 is chosen because it was very strong, and generated high waves in the Estuary. WW3 was implemented for the East China Sea coarse-resolution computational domain, to simulate the waves over a large spatial scale and provide boundary conditions for SWAN model simulations, implemented on a fine-resolution nested domain for the Yangtze Estuary area. The Takahashi wind model is applied to the simulation of the East China Sea scale （3-hourly） and Yangtze Estuary scale （1-hourly） winds. Simulations of significant wave heights in the East China Sea show that the highest waves are on the right side of the storm track, and maxima tend to occur at the eastern deep-water open boundary of the Yangtze Estuary. In the Yangtze Estuary, incoming swell is dominant over locally generated waves before the typhoon approaches the Estuary. As the typhoon approaches the Estuary, wind waves and swell coexist, and the wave direction is mainly influenced by the swell direction and the complex topography.     

Air-Sea Interactions under the Existence of Opposing Swell

Data of a comprehensive laboratory study on the coexistent system of wind waves and opposing swell (Mitsuyasu and Yoshida, 1989) have been reanalyzed to clarify the air-sea interaction phenomena under the coexistence of wind waves and swell. It is shown that the magnitude of the decay rate of swell due to an opposing wind is almost the same as that of the growth rate of swell caused by a following wind, as measured by Mitsuyasu and Honda (1982). The decay rate is much smaller than that obtained recently by Peirson et al. (2003), but the reason for the disagreement is not clear at present. The effect of an opposing swell on wind waves is very different from that of a following swell; wind waves are intensified by an opposing swell while they are attenuated by a following one. The phenomenon contradicts the model of Phillips and Banner (1974), but the reason for this is not clear at this time. The high-frequency spectrum of wind waves shows a small increase of the spectral density. Wind shear stress increases a little due to the effect of opposing swell. The intensification of wind waves by opposing swell and the small increase of the spectral density in a high-frequency region can be attributed to the increase of wind shear stress. Such organized phenomena lead to the conclusion that the hypothesis of local equilibrium for pure wind waves (Toba, 1972) can also be satisfied for wind waves that coexist with opposing swell. The recent finding of Hanson and Phillips (1999) can be explained by this mechanism.     

大西洋波高成分研究

利用Topex／Poseidon卫星高度计1992年10月到1998年12月连续75个月，230个重复周期的有效波高资料对南北大西洋的波高成分进行了分析，结果显示频数密度峰值对应的波高成分因子在夏季为1．4左右。冬季为1．2左右，其他季节为1．3左右。7，8，9月份大西洋含涌浪的波高成分占频数密度累积率的比例约为90％左右，其中涌浪成分占优约为65％左右；其余各月占85％左右，其中涌浪成分占优约为60％左右，表明大西洋海域常年有涌浪场存在。     

涌浪对风浪能量影响的实验研究

在实验室风浪水槽中进行纯风浪和混合浪波面位移观测,研究波长较长的规则波对风浪能量的影响.本文用混合浪和纯风浪中的风浪显著波的零阶谱矩之比代表混合浪中的风浪与纯风浪能量之比,并以此表征涌浪对风浪能量的影响.研究了该能量比随涌浪波陡S、风区x、波龄倒数u/C、涌浪频率与纯风浪谱峰频率之比fs/fwp的变化规律.结果表明,涌浪对风浪能量的抑制作用随涌浪波陡的增加、波龄倒数的增大及涌浪频率与纯风浪谱峰频率之比的增大而增强.发现该能量比依赖于无因次量R=(1+80(πS)2)1.9(fs/fwp)0.9(u/C)0.27,并拟合得到2者的经验关系.此外,本文实验还发现,在某些情况下,涌浪的存在使风浪能量增加.     

Taylor-Görtler vortices expected in the air flow on sea surface waves—I

Taylor-Grörtler vortices are longitudinal vortices resulting from a centrifugal instability. They are generated in the flow having a curved streamline with an increasing velocity in the direction of decreasing curvature.It is shown that the air flow above wind waves and swells also satisfies locally the condition of the centrifugal instability. Numerical calculations indicate the possibility of generation of Taylor-Görtler vortices on the trough of sea waves. For example, when a wind of about 12.2 m/s at 10-m level is blowing over sea waves of the wave length of 15 m like the swell, the critical water wave height beyond which the vortices may be generated is about 0.5 m, and the critical wave length and the height of center of the generated vortices are about 24 m and 3.7 m, respectively. Further, about the relations between the generation of vortices and wind waves, it is shown that the condition of their generation is satisfied at the trough of waves for early stages of the wave generation.In conclusion, it is expected that the Taylor-Görtler vortices change the wind profile along the sea surface, and also, play some part in the growth of wind waves, especially in the formation of their three dimensional structure.     

Impact of swell on simulations using a regional atmospheric climate model

When long, fast swell waves travel in approximately the same direction as the wind, the surface stress is reduced compared with under wind-sea conditions. Using measurements from the Östergarnsholm site in the Baltic Sea, new expressions of the roughness length were developed for wind sea and swell. These new expressions were implemented in the RCA3 regional climate model covering Europe. A 3-year simulation and two case studies using the wavefield from the ECMWF reanalysis (ERA-40) were analysed using the improved formulations. Wind-following swell led to a significant reduction of mean wind stress and heat fluxes. The mean surface layer wind speed was redistributed horizontally and the marine boundary layer cooled and dried slightly. This cooling was most pronounced over North Sea and the Norwegian Sea (almost 0.2 °C annually on average) whereas the drying was most pronounced over the Mediterranean Sea (almost 0.4 g kg⁻¹). Somewhat less convective precipitation and low-level cloudiness over the sea areas were also indicated, in particular over the Mediterranean Sea. The impact on the atmosphere, however, is significantly locally greater in time and space.     

Characteristics of storm waves off the Mediterranean coast of Israel

A two-year series of directional wave measurement off the Eastern Mediterranean coast of Israel reveals an abundance of high storm waves. Some of these waves have significant height in excess of 5 meters and periods as long as 15 sec.The evolution of the storm waves is described and related to the growth and paths of the storm fronts in Mid-Mediterranean. Shorter-period waves are found to always lead the arrival of longer-period swell. This characteristic is explained by a short decay distance and/or a high migration velocity of the storm front.The scatter plot of significant wave height vs period for the recorded events of each storm describes an open-loop time sequence. The difference in period between that of the peak height event and the period of a fully arisen sea of the same height is found to be indicative of the true decay distance the waves have travelled.     

Ocean wave characteristics around New Zealand

Nearly 17 years wave records from deep water and shore‐based stations are used to describe the ocean wave characteristics around New Zealand. The wave environment is dominated by west and southwest swell and storm waves generated in the temperate latitude belt of westerly winds. As a result, the west and south coasts are exposed, high energy shores, the east coast is a high energy lee shore, and the northern coast from North Cape to East Cape is a low energy lee shore sheltered from these winds and waves. South of New Zealand, wave energies are extremely high; the prevailing deep water wave is 3.5–4.5 m high and has a 10–12 s period, with a slight increase in wave heights in winter.

The west coast wave environment is mixed, and consists of locally generated westerly and southerly storm waves, and swell waves generated to the south. The prevailing wave is t.0–3.0 m and 6–8 s period. There are no strong seasonal rhythms, only shorter period cycles of wave height (5 day) associated with similar quasi‐rhythmic cycles in the weather.

The east coast also has a mixed wave climate with southerly swells, originating in the westerlies south of New Zealand, and locally generated southerly and northerly storm waves. The prevailing wave is 0.5–2.0 m and 7–11 s period. A short period rhythmic cycle, similar to that on the west coast, is superimposed on a weak seasonal cycle. The seasonal, cycle results from an increase in the frequency of local northerly waves in summer.

The prevailing wave on the north coast is a northeasterly, 0.5–1.5 m high and 5–7 s period. Subtropical disturbances and southward‐moving depressions generate a mixed wave environment and a possible seasonally reflecting a winter increase in. storminess.     

Influence of the type of sea waves on the backscattered radar cross section at medium incidence angles

We consider the influence of the sea surface state on the backscattered radar cross section and the accuracy of the wind speed retrieval from the scatterometer data. We used a joint set of radars and buoys to determine the type of sea waves. Three types of sea waves were distinguished: developing wind waves, fully developed wind waves, and mixed sea. It is shown that the retrieval error of the near surface wind speed using a one-parameter algorithm is minimal in the case of fully developed wind waves. We compared these data with the results of radio-altimeter data analysis and showed that in both cases underestimation of the retrieval wind speed exists for developing wind waves and overestimation occurs for mixed sea. A variety of swell parameters (length of the dominating wave, swell height, swell age) significantly influence the backscattered radar cross section, leading to a growth in the mean square error of the retrieved wind speed during vertical sounding (radio-altimeter data), and only slightly influence the mean square error of the scatterometer data (medium incidence angles). It is necessary to include the information about the parameters of sea waves in the algorithms and take into account the regional wave properties to increase the accuracy of wind speed retrieval.     

划分风浪与涌浪的一个新判据——海浪成份及其在南海的应用

使用风浪谱的零阶矩（Ｍ０ｗ）和混合浪谱的零阶矩（Ｍ０）定义的混合浪能量成份因子，作为划分风浪与涌浪的一个新判据，给出了混合浪能量成份因子和混合浪波高成份因子的计算公式。根据混合浪波高成份因子的计算公式，使用ＧＥＯＳＡＴ卫星高度计５０个重复周期的资料，计算了南海海域波高成份因子及其月变化规律，资料的样本长度是１个月。计算结果表明：该海域的混合浪波高成份因子具有明显的时间变化规律。海浪在１１、１２、１、２月份和５、６、７、８月份，混合浪波高成份因子的概率密度分布形状高而窄，而在３、４月份和９、１０月份，混合浪波高成份因子的概率密度分布形状低而宽。在１１、１２、１和２月份，最可能出现的混合浪波高成份因子等于１．２，有７０％的波浪含有涌浪成份，在整个海域涌浪占主导地位。在５、６、７月份，最可能出现的混合浪波高成份因子位于０．３～０．４之间，有６０％的波浪只含有风浪成份，在整个海域风浪占主导地位。其它月份，最可能出现的混合浪波高成份因子介于冬夏两季之间，亦即风浪和涌浪出现的概率几乎是相同的     

Influence of discrete step size on wind setup component of storm surge

Open coast storm surge water levels consist of wind setup due to wind shear at the water surface; a wave setup component caused by wind induced waves transferring momentum to the water column; an atmospheric pressure head component due to the atmospheric pressure deficit over the spatial extent of the storm system; a Coriolis forced setup or setdown component due to the effects of the rotation of the earth acting on the wind driven alongshore current at the coast; a possible seiche component due to resonance effects initiated by moving wind system, and, if astronomical tides are present, an astronomical tide component (although the tide is typically considered to be a forced astronomical event and not really a direct part of the external wind-driven meteorological component of storm surge). Typically the most important component of a storm surge is the wind setup component, especially on the U.S. East Coast and the Gulf of Mexico shorelines. In many approaches to storm surge modeling, a constant depth approximation is invoked over a limited step size in the computational domain. The use of a constant depth approximation has received little attention in the literature although can be very important to the resulting magnitude of the computed storm surge. The importance of discrete step size to the wind setup storm surge component is considered herein with a simple case computation of the wind setup component on a linear slope offshore profile. The present study findings show that the constant depth approximation to wind setup storm surge estimation is biased on the low side (except in extremely shallow water depths) and can provide large errors if discrete step size is not sufficiently resolved. Guidance has been provided on the error that one might encounter for various step sizes on different slopes.     

Effects of winds,tides and storm surges on ocean surface waves in the Sea of Japan

Ocean surface waves are strongly forced by high wind conditions associated with winter storms in the Sea of Japan. They are also modulated by tides and storm surges. The effects of the variability in surface wind forcing, tides and storm surges on the waves are investigated using a wave model, a high-resolution atmospheric mesoscale model and a hydrodynamic ocean circulation model. Five month-long wave model simulations are inducted to examine the sensitivity of ocean waves to various wind forcing fields, tides and storm surges during January 1997. Compared with observed mean wave parameters, results indicate that the high frequency variability in the surface wind filed has very great effect on wave simulation. Tides and storm surges have a significant impact on the waves in nearshores of the Tsushima-kaihyō, but not for other regions in the Sea of Japan. High spatial and temporal resolution and good quality surface wind products will be crucial for the prediction of surface waves in the JES and other marginal seas, especially near the coastal regions.     

海浪双峰方向分布的一种物理解释

用 １８个波高计组成的直径为 ４０ｃｍ的圆形阵列在大型风浪槽内系统地测量了风浪和涌浪方向谱。用两种分辨力较高的方向谱估计方法最大似然法（ＭＬＭ）和贝叶斯方法（ＢＤＭ）分析的结果表明：风浪高频域出现的依赖于估计方法的双峰方向分布是一种物理假象;在较成长的涌浪低频域,得到跟传播方向对称、两峰间隔大约６０°－９０°非常规则的双峰方向分布,它跟频率和涌浪的成长状态有关,而跟估计方法无关,这种现象可以用非线性波－波相互作用过弱,在不同方向之间不能有效传递能量来解释。     

太平洋涌浪分布特征及其对海-气边界层的影响

本文对海-气边界层波致风机制的相关理论进行了阐述,并利用ERA-40再分析资料给出了太平洋谱峰速度、波龄、波陡等描述涌浪和波致风机制物理量的年际和季节空间分布特征。分析表明:东太平洋赤道地区等海域涌浪速度最大且涌浪由南向北传播明显;太平洋波边界层高度基本呈现出东高西低的分布形势;波致风机制主要发生在赤道热带海域,北半球夏季波致风机制偏强,冬季偏弱,南半球反之;北半球北部海域夏季更易发生波致风机制,赤道附近海域相反;南海为风浪与涌浪组成的混合浪,对其波候等相关研究有必要分开进行讨论。     

Identifying storm impacts on an embayed, high-energy coastline: examples from western Ireland

Large sections of the western Irish coast are characterised by a highly compartmentalised series of headland-embayment cells in which sand and gravel beaches are backed by large vegetated dune systems. Exposure to modally high-energy swell renders most of these beaches dissipative in character. A mesotidal range (c. 3.5–4.5 m) exists along much of the coast. Analysis of instrumental wind records from three locations permitted the identification of a variety of storm types and the construction of storm catalogues. Few individual storms were recorded at all three stations indicating a lack of regional consistency in storm record. Of the total storms recorded, only a small percentage are potentially damaging (onshore directed) and even fewer span a high tide and thus potentially induce a measurable morphological response at the coast.

Through a combination of historical records, meteorological records, field observations and wave modelling we attempt to assess the impact of storms. Quantifiable records of coastal morphology (maps, air photos and beach profiles) are few in number and do not generally record responses that may be definitely attributed to specific storms. Numerical wave simulations and observations at a variety of sites on the west Irish coast, however, provide insights into instantaneous and medium term (decadal) storm responses in such systems.

We argue that beaches and dunes that are attuned to modally high-energy regimes require extreme storms to cause significant morphological impact. The varying orientation of beaches, a spatially nonuniform storm catalogue and the need for a storm to occur at high water to produce measurable change, impart site-specific storm susceptibility to these embayments. Furthermore, we argue that long-period wave energy attenuation across dissipative shorefaces and beaches reduces coastal response to distant storms whereas short-period, locally generated wind waves are more likely to cause major dune and beach erosion as they arrive at the shoreline unrefracted.

This apparently variable response of beach and dune systems to storm forcing at a decadal scale over a coastline length of 200 km urges caution in generalising regarding regional-scale coastal responses to climatic change.     

Variations in the Baltic Sea wave fields

The surface waves in the Baltic Sea are hindcast with the spectral wave model HYPAS during a 12-month period. The model results show a strong temporal and spatial variation in the wave field due to the physical dimensions of the different basins and the predominant wind field. The highest waves in the area are found in the outer part of Skagerrak, as well as in the central and southern parts of the Baltic Proper. To get significant waves above 6 m high, strong winds (15–20 m/s) must have been blowing for 6 to 24 h from a favourable direction over a deep area.     

The Needed Minimum Submergence Depth When a Submarine Runs into Typhoon

In this paper, the influence depths of all levels of wind power produced waves are reckoned according to the measured swaying degree when a submarine meets with a tbrce 11 strong tropical storm and has to submerge; Then the minimum submergence depth is given when a submarine is faced with weather systems with force ≥ 6 wind and the swaying degree of less than 12 degree is assured as reference for a submarine working and sailing in big strong winds and waves.     

Buoy observations of the influence of swell on wind waves in the open ocean

The influence of longer (swell) on shorter, wind sea waves is examined using an extensive database of directional buoy measurements obtained from a heave-pitch-roll buoy moored in deep water in the South Atlantic. This data set is unique for such an investigation due to the ubiquitous presence of a young swell component propagating closely in direction and frequency with the wind sea, as well as a longer, opposing swell. Our results show, within the statistical limits of the regressions obtained from our analysis when compared to measurements in swell free environments, that there is no obvious influence of swell on wind sea growth. For operational purposes in ocean engineering this means that power-laws from fetch limited situations describing the wind sea growth can be applied in more realistic situations in the open sea when swell is present.     

世界大洋长历时局地风速和有效波高的统计与分析

本文基于美国海军测地卫星高度计提供的全球范围长历时局地平均风速和有效波高资料进行统计分析,结果表明,世界大洋长历时局地平均风速和有效波高有明显的相关性,其散布点系统地位于Wilson提出的深水充分成长风浪平均风速和有效波高经验曲线之上;并从能量叠加平衡方程,近似定量估计出大洋中长历时涌浪有效波高与局地平均风速的关系。     

太平洋东边界波浪输运

通过计算2000年涌浪指标(swell index)的全球分布,发现太平洋东边界赤道附近区域存在涌浪池.利用ECMWF再分析波浪资料,计算出2000年全球月平均波浪体积输运.比较2000年全球月平均波浪体积输运和2000年QUICKSCAT月平均风场,发现在赤道太平洋东边界涌浪池区域内的波浪输运方向和风向存在很大的差别,两者方向相差大约90°.这进一步验证了该地区涌浪池存在的真实性.研究发现,赤道太平洋东边界涌浪主要来源于北太平洋和南太平洋的西风带对应的海区.在涌浪池区域内分别在2.5°S和2.5°N取两条边界(边界起点为125°W,终点为美洲大陆西边界),计算通过这两条边界进入赤道区域涌浪的Stokes体积净输运量.结果表明,不同月份通过南、北两条边界波浪的净输运量与当月南、北太平洋西风带的风浪强度密切相关.同时指出了,涌浪的体积输运将会对大洋环流系统产生潜在的重要影响.