Wave height distributions in bimodal sea states from offshore basins

The paper presents results for the distribution of wave heights from laboratory generated bimodal sea states. Data collected at the DHI offshore basin are analyzed and compared with results based on wave records from the MARINTEK offshore basin. The comparisons are done for three groups of mixed sea states: wind-sea dominated, swell-dominated and energy-equivalent, determined on the basis of the parameter sea-swell energy ratio (SSER), which have been generated according to the model of Guedes Soares (1984). In some sea states abnormal or freak waves have been observed.The quasi-determinism theory of Boccotti is used to expand some linear narrowband models to second order, thus providing validation of the adequacy of the equations to represent the linear components of the wave heights. Also, the data are compared with the predictions of a third order model using as a nonlinear correction the coefficient of kurtosis. Due to the coexistence of wind-sea and swell, the core of the autocovariance function in some cases demonstrates a global minimum which is the second local minimum in the sequence. This can affect the fitting ability of distributions whose parameters depend on the form of the autocorrelation function or its envelope.The results for MARINTEK and DHI data show similar patterns of fit between predicted and observed exceedance probabilities for the considered classes of bimodal spectra.     

基于多星融合高度计数据的中国海波浪能资源评估

Wave energy resources are abundant in both offshore and nearshore areas of the China's seas. A reliable assessment of the wave energy resources must be performed before they can be exploited. First, for a water depth in offshore waters of China, a parameterized wave power density model that considers the effects of the water depth is introduced to improve the calculating accuracy of the wave power density. Second, wave heights and wind speeds on the surface of the China's seas are retrieved from an AVISO multi-satellite altimeter data set for the period from 2009 to 2013. Three mean wave period inversion models are developed and used to calculate the wave energy period. Third, a practical application value for developing the wave energy is analyzed based on buoy data. Finally, the wave power density is then calculated using the wave field data. Using the distribution of wave power density, the energy level frequency, the time variability indexes, the total wave energy and the distribution of total wave energy density according to a wave state, the offshore wave energy in the China's seas is assessed. The results show that the areas of abundant and stable wave energy are primarily located in the north-central part of the South China Sea, the Luzon Strait, southeast of Taiwan in the China's seas; the wave power density values in these areas are approximately 14.0–18.5 k W/m. The wave energy in the China's seas presents obvious seasonal variations and optimal seasons for a wave energy utilization are in winter and autumn. Except for very coastal waters, in other sea areas in the China's seas, the energy is primarily from the wave state with 0.5 m≤H s≤4 m, 4 s≤T e≤10 s where H s is a significant wave height and T e is an energy period; within this wave state, the wave energy accounts for 80% above of the total wave energy. This characteristic is advantageous to designing wave energy convertors(WECs). The practical application value of the wave energy is higher which can be as an effective supplement for an energy consumption in some areas. The above results are consistent with the wave model which indicates fully that this new microwave remote sensing method altimeter is effective and feasible for the wave energy assessment.     

Correlation between successive wave heights and periods in mixed sea states

The degree of dependence between successive wave heights and periods is examined for sea states resulting from the combination of a remotely generated wave field and a locally generated wave system, based on simulated wave records. The sea states analysed represent situations that are swell dominated, wind–sea dominated or they have equivalent energy in the wind–sea and swell components. Results of the analysis of the simulated data have been compared with those expected from the theories for the joint distributions of consecutive wave heights and periods and with the results from a Pierson–Moskowitz target spectrum.     

海冰对海浪影响研究综述

受全球气候变化的影响,极区海浪尤其是北极海浪在过去几十年发生了显著的变化,使得海冰边缘区海冰与海浪的相互作用愈发显著。本文从物理海洋学的角度出发,较系统地总结了海冰对海浪作用研究的国内外现状,从理论和实测的角度分别探讨了海冰对海浪能量的耗散及其引起的波动频散关系的变化,同时分析了当前海冰覆盖海域海浪的数值模拟与现场观测研究,指出了未来开展有冰海域海浪数值模拟与预报所面临的主要问题,并对该方向今后的研究做出展望。总体来看,尽管海冰对海浪作用的机理复杂且与海冰类型高度相关,但是海冰对海浪能量的衰减与传播距离基本呈指数关系,并且海冰会一定程度上影响海浪的传播速度。未来依然需要更多不同海冰类型下海浪的观测数据以开展进一步的机理分析、模型检验和参数校准,进而实现高精度的业务化预报。     

Retrieval of surface wave parameters from sar images and their validation in the coastal seas around Japan

We have developed a scheme to retrieve surface wave parameters (wave height and wave propagation direction) from European Remote-Sensing Satellite (ERS) Synthetic Aperture Radar (SAR) image mode data in coastal seas around Japanese coastlines. SAR spectra are converted to surface wave spectra of swell-dominated or wind-wave dominated cases. The SAR spectrum and SAR-derived wind speed are used to derive the surface wave spectrum. The wind-wave dominated case and swell-dominated case are differentiated by a wind speed of 6 m/s, and processed in different ways because of their different degree of nonlinearity. It is indicated that the cutoff wavelength for retrieval of the wind-wave dominated spectrum is proportional to the root of significant wave height, which is consistent with the results of previous studies. We generated 66 match-ups using the SAR sub-images and the in-situ surface wave parameters, which were measured by wave gauges installed in near-shore seas. Among them, there are 57 swell-dominated cases, and 9 wind-wave dominated cases. The significant wave heights derived from SAR and from in-situ observation agree with the bias of 0.09 m, the standard deviation of 0.61 m and the correlation coefficient of 0.78. The averaged absolute deviation of wave propagation directions is 18.4°, and the trend of the agreement does not depend on the wave height. These results demonstrate that the SAR surface wave spectrum retrieved by the present system can be used to observe the surface wave field in the coastal seas around Japan.     

"苏拉"和"达维"双台风过程中台风浪和海温的数值模拟

本文基于FVCOM-SWAVE耦合模型,以双台风"苏拉"和"达维"的台风过程为例,研究了台风过程中海浪和海温的变化,通过与高度计和Argo资料的对比,发现耦合模型能较准确的模拟出有效波高和海表面温度。由于双台风风场相互作用,风场结构和最大风速位置发生改变,影响着有效波高的分布,台风"苏拉"产生的最大有效波高位于台风后侧。海表面温度的降低与风场、浪场分布密切相关,强风强浪处的降温现象更明显,"苏拉"产生的降温区域位于路径附近,"达维"产生的降温区域位于路径右侧。台风对海表面温度的降低与初始的混合层厚度、温跃层强度存在相关性,具体表现为初始的混合层越薄、温跃层强度越大,降温越明显。     

推算波浪多年一遇波高的新方法

采用卫星遥感推算多年一遇波高的新方法，对中国近海多年一遇波高进行推算研究。运用卫星遥感散射计得到的风资料推算了整个渤海区域波浪多年一遇的波高分布。通过与有实测资料4个点推算的结果比较表明：卫星资料和实测计算的误差随着重现期的增大而减小，最大误差百年和50年一遇波高为20cm；而百年和50年一遇波高正是工程中最为关心的，说明运用卫星遥感散射计风资料推算渤海多年一遇波高可行且结果合理。本研究方法为进一步运用和完善卫星遥感资料推算中国近海多年一波高场提供了一条新途径。     

Wave measurements by pressure transducers using artificial neural networks

Underwater ultrasonic acoustic transducers are frequently used in ocean wave measurements as they measure surface level using acoustic waves. However, their effectiveness can be severely affected in rough sea conditions, when bubbles generated by breaking waves interfere with their acoustic signals. When the seas are rough, one therefore often has to rely on a pressure transducer, which is generally used as a back-up for the acoustic wave gauge. A pressure transfer function is then used to obtain the surface wave information. Alternatively, the present study employed an artificial neural network to convert the pressure signal into significant wave height, significant wave period, maximum wave height, and spectral peakedness parameter using data obtained from various water depths. The results showed that, for water depths greater than 20 m, the wave parameters obtained from the artificial neural network were significantly closer to those obtained by the acoustic measurements than those obtained by using a linear pressure transfer function. Moreover, for a given water depth, the wave heights estimated by the network model from pressure data were not as good as those estimated by linear wave theory for large wave heights (above a 4 m significant wave height in this study). This can be improved if the training data set has more records with large wave heights.     

末次冰期最盛时期以来中国东部边缘海潮波系统演变过程的模拟研究

在系统地研究了中国东部边缘海区引潮力、入射潮波、科氏力、水下地形、海岸形态及磨擦作用等因素对区域潮波系统形成影响的基础上,确定了中国东部边缘海岸轮廓是区域潮波系统形成的主导因素,认为模拟末次冰期最盛时期以来中国东部边缘海区潮波系统的形成过程需要3个重要的控制性条件,即入射潮波、海岸线变迁过程和海面变化曲线。利用中国东部边缘海潮波系统与海岸演变模拟支持系统,对末次冰期最盛时期以来30个时期中国东部边缘海区潮波系统的形成与演变过程进行了模拟再现研究,系统地恢复了在不同的海面高程、不同的岸线形态条件下全日潮、半日潮潮汐分布特征。模拟结果显示,末次冰期最盛时期以来,12400aB.P.和7500aB.P.两个时间界限分别代表了旋转潮波系统－无潮点开始出现、中国东部边缘海潮波系统现代格局基本奠定这两个特征事件的发生时间,将中国东部边缘海潮波系统的演变过程划分为3个发展时期。     

基于ECMWF数据的中国近海低空波导特征研究

基于1988-2017年高分辨率的欧洲中尺度天气预报中心再分析数据,本文对中国近海的低空大气波导进行了统计分析.结果表明:该海域整体大气波导概率为22％,其中悬空波导占60％以上;春季最容易发生大气波导,其次是夏季、秋季和冬季.区域时空分布上,中国近海大气波导特征具有明显的月变化和区域分布特征.大气波导发生概率北部海域...     

北极海域海面风场和海浪遥感观测能力分析

卫星遥感是开展北极海域海面风场和海浪分布特征与变化规律研究的重要手段。本文基于在轨多源卫星遥感数据,从遥感观测空间覆盖、时间覆盖和多源卫星遥感数据融合等方面开展北极海域海面风场与海浪遥感观测能力分析,研究主要结果为:基于ASCAT和HY-2A散射计可实现北极海域海面风场遥感观测,通过多星联合观测可获取北极海域时空分辨率优于12 h和0.1°的海面风场遥感融合数据;基于HY-2A、CryoSat-2、SARAL和Sentinel-3高度计可实现北极海域海浪遥感观测,同样通过多星联合观测可获取北极海域时空分辨率优于1 d和0.25°的海浪有效波高遥感融合数据;基于2016年北极海面风场和海浪遥感融合数据,分析得出北极海域海面风场和海浪在2月处于极大值,然后逐渐减小,7月最小,随后开始逐渐增大。本研究表明,基于多源散射计和高度计遥感观测可实现北极海域海面风场和海浪的高时空分辨率遥感业务化监测。     

Relation between Gravity Field Feature and Tectonics and Earthquakes in Taiwan and Its Adjacent Seas

Short wave gravity anomaly is correlated to sea floor topography in the gravity field of Taiwan and its adjacent seas. Gravity values of 200 × 10-5ms-2 at Yushang and -160 × 10-5ms-2 at Liuqiu sea trench are respectively the maximum and minimum gravity values in this area.Bouguer gravity anomaly reflects not only Moho interface undulation, but also fault distribution.The inflexion of gradient belt of Bouguer gravity anomaly is a spot liable to earthquakes. Middlelong wave geoid is the best data to invert crustal thickness. We calculate crustal thickness by using geoid data, and the maximum value is 38km; the minimum value is 12km in Taiwan and its adjacent seas.     

Verification of an operational ocean circulation-surface wave coupled forecasting system for the China's seas

An operational ocean circulation-surface wave coupled forecasting system for the seas off China and adjacent areas(OCFS-C) is developed based on parallelized circulation and wave models. It has been in operation since November 1, 2007. In this paper we comprehensively present the simulation and verification of the system, whose distinguishing feature is that the wave-induced mixing is coupled in the circulation model. In particular, with nested technique the resolution in the China's seas has been updated to(1/24)° from the global model with(1/2)°resolution. Besides, daily remote sensing sea surface temperature(SST) data have been assimilated into the model to generate a hot restart field for OCFS-C. Moreover, inter-comparisons between forecasting and independent observational data are performed to evaluate the effectiveness of OCFS-C in upper-ocean quantities predictions, including SST, mixed layer depth(MLD) and subsurface temperature. Except in conventional statistical metrics, non-dimensional skill scores(SS) is also used to evaluate forecast skill. Observations from buoys and Argo profiles are used for lead time and real time validations, which give a large SS value(more than 0.90). Besides, prediction skill for the seasonal variation of SST is confirmed. Comparisons of subsurface temperatures with Argo profiles data indicate that OCFS-C has low skill in predicting subsurface temperatures between 100 m and 150 m. Nevertheless, inter-comparisons of MLD reveal that the MLD from model is shallower than that from Argo profiles by about 12 m, i.e., OCFS-C is successful and steady in MLD predictions. Validation of 1-d, 2-d and 3-d forecasting SST shows that our operational ocean circulation-surface wave coupled forecasting model has reasonable accuracy in the upper ocean.     

Statistics of Wind and Waves off Tsuyazaki, Fukuoka, in the Eastern Tsushima Strait

The variability of the sea surface wind and wind waves in the coastal area of the Eastern Tsushima Strait was investigated based on the hourly data from 1990 to 1997 obtained at a station 2 km off Tsuyazaki, Fukuoka. The annual mean wind speed was 4.84 m s⁻¹, with strong northwesterly monsoon in winter and weak southwesterly wind in summer. Significant wave heights and wave periods showed similar sinusoidal seasonal cycles around their annual means of 0.608 m and 4.77 s, respectively. The seasonal variability relative to the annual mean is maximum for wave heights, medium for wind speeds, and minimum for wave periods. Significant wave heights off Tsuyazaki turned out to be bounded by a criterion, which is proportional to the square of the significant wave period corresponding to a constant steepness, irrespective of the season or the wind speed. For terms shorter than a month, the significant wave height and the wave period were found to have the same spectral form as the inshore wind velocity: white for frequencies less than 0.2 day⁻¹ and proportional to the frequency to the −5/3 power for higher frequencies, where the latter corresponds to the inertial subrange of turbulence. The spectral levels of wave heights and wave periods in that inertial range were also correlated with those of the inshore wind velocity, though the scatter was large. This revised version was published online in August 2006 with corrections to the Cover Date.     

Correlation between individual waves in a real sea state

The correlation between individual waves in a real sea state has a central role in existing theories of wave grouping. The attractive Kimura (1980) theory has two critical assumptions, that the sequence of individual wave heights follows a Markov process and that the joint distribution of consecutive wave heights follows a bivariate Rayleigh form. Analysis of measured water surface records suggests that sequences of individual waves can reasonably be described as a first order mixed autoregressive, moving-average or ARMA process, though a distinction among ARMA (1,0), ARMA (0,1) and ARMA (1,1) models was beyond the resolution of the data. These include the Markov or ARMA (1,0) model. The decisive detail, the joint distribution of consecutive wave heights in the sea state, was evaluated by a simulation methodology that is consistent with the Gaussian random wave model. The estimates are dependent on spectral shape and are consistently narrower and more sharply focussed at the peak than the corresponding bivariate Rayleigh estimate. The resulting predictions of run and group length statistics differ from the Kimura theory, though not by a sufficient margin to displace the Kimura theory as a pragmatic choice for wave grouping.     

Obtaining the distribution of quiescent periods directly from the power spectral densities of Sea waves

There is a growing practical interest in the ability to increase the sea states at which marine operations can be safely undertaken by exploiting the quiescent periods that are well known to exist under a wide range of sea conditions. While the actual prediction of quiescent periods at sea for the control of operations is a deterministic process, the long term planning of future maritime tasks that rely on these quiescent periods is a statistical process involving the anticipated quiescence properties of the forecasted sea conditions in the geographical region of interest. It is in principle possible to obtain such data in tabular form either large scale simulation or from field data. However, such simulations are computationally intensive and libraries of appropriate field data are not common. Thus, it is clearly attractive to develop techniques that exploit standard wave spectral models for describing the quiescence statistics directly from such spectra. The present study focuses upon such techniques and is a first step towards the production of a computationally low-cost quiescence prediction tool and compares its efficacy against simulations. Two significant properties emerge for a large class of wave spectral models that encompasses the ubiquitous Neumann and Pierson Moskowitz or Bretschneider forms. Firstly, the auto-correlation function of the wave profile that are required to produce the quiescence property can be obtained analytically in terms of standard special functions. This considerably reduces the computational cost making desktop computer-based planning tools a reality. Secondly, for each class of these parametric spectra, the probability of a given number of consecutive wave heights (normalised to the significant wave heights) less than some critical value is in fact independent of absolute wave height. Thus, for a broad class of practically interesting wave spectra all that is required to obtain the statistical distribution of the quiescent periods is simple rescaling.     

A maximum-entropy compound distribution model for extreme wave heights of typhoon-affected sea areas

A new compound distribution model for extreme wave heights of typhoon-affected sea areas is proposed on the basis of the maximum-entropy principle.The new model is formed by nesting a discrete distribution in a continuous one,having eight parameters which can be determined in terms of observed data of typhoon occurrence-frequency and extreme wave heights by numerically solving two sets of equations derived in this paper.The model is examined by using it to predict the N-year return-period wave height at two hydrology stations in the Yellow Sea,and the predicted results are compared with those predicted by use of some other compound distribution models.Examinations and comparisons show that the model has some advantages for predicting the N-year return-period wave height in typhoon-affected sea areas.     

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。     

北冰洋公海渔业权益争端的博弈分析——基于环北极国家视角

北冰洋公海尚处于冰封状态,但气候变暖、海冰加速融化使未来公海出现新渔场成为可能,其战略价值因而得到国际关注。当前,环北极国家已率先展开渔业权益争夺。为分析北冰洋公海渔业事务的国际走势,文章基于博弈论思想,构建非对称懦夫博弈模型及古诺模型来模拟和探究环北极国家在渔业利益争夺中的合作或冲突行为,研究得出:现阶段北冰洋公海渔业权益争端主要表现为渔业管理权的争夺,在该博弈中,非北冰洋沿岸国采取退让策略,沿岸国采取进攻策略,可以达到进化稳定均衡;一旦公海具备捕鱼条件,争端将演化为开发模式选择的博弈,此时,北冰洋沿岸国采取合作开发策略可以有效保护公海渔业资源,并实现国家的长期利益。     

Wind wave measurements and modelling in a fetch-limited semi-enclosed lagoon

The south-west reef lagoon of New Caledonia is a semi-enclosed basin where, on first approximation, dominating sea state component corresponds to locally generated wind waves. This study aims to evaluate the ability of the wave model WAVEWATCH III to simulate wind wave distribution in this particular fetch-limited context, with a given parameterisation. In order to evaluate the consistency of the simulation results, wave parameters were measured in situ by a wave and tide recorder (WTR9 Aanderaa) and by an acoustic Doppler velocimeter (ADV Sontek). This study underlines specific constrains for the deployment of instruments to assess the characteristic parameters of low amplitude and high frequency wind-waves. Special care was taken in the comparison step as, on one hand the wave model did not simulate the propagation of low-frequency oceanic waves inside the lagoon, and on the other hand the measured spectra bear an intrinsic limitation for high frequencies. The approximation of a sea state dominated by wind waves is verified on the study site. The accuracy of the simulation results is discussed with regards to the wind forcing applied to the model.