Oceanic pycnocline depth retrieval from SAR imagery in the existence of solitary internal waves

Oceanic pycnocline depth is usually inferred from in situ measurements. It is attempted to estimate the depth remotely. As solitary internal waves occur on oceanic pycnocline and propagate along it, it is possible to retrieve the depth indirectly in virtue of the solitary internal waves. A numerical model is presented for retrieving the pycnocline depth from synthetic aperture radar （SAR） images where the solitary internal waves are visible and when ocean waters are fully stratified. This numerical model is constructed by combining the solitary internal wave model and a two-layer ocean model. It is also assumed that the observed groups of solitary internal wave packets on the SAR imagery are generated by local semidiurnal tides. A case study in the East China Sea shows a good agreement with in situ CTD （conductivity-temperature-depth） data.

A study of SAR remote sensing of internal solitary waves in the north of the South China Sea:Ⅰ.Simulation of internal tide transformation

For settlement of the well-known problem of contemporary radar imaging models, i. e. , the problem of a general underestimation of radar signatures of hydrodynamic features over oceanic internal waves and underwater bottom topography in tidal waters at high radar frequency bands （ X-band and C-band）, the impact of the ocean surface mixed layer turbulence and the significance of strat- ified oceanic model on SAR remote sensing of internal solitary waves are proposed. In the north of the South China Sea by utilizing some observed data of background field the nonlinearity coefficient, the dispersion coefficient, the horizontal variability coefficient and the phase speed in the generalized K-dV equation are determined approximately. Through simulations of internal tide transfor- mation the temporal evolution and spatial distribution of the vertical displacement and horizontal velocity of internal wave field are obtained. The simulation results indicate that the maximum amplitudes of internal solitary waves occur at depth 35 m, but the maximum current speeds take place at depth 20 m in this area of the sea （about 20°30＇N, 114°E） in August. It was noticed that considering the effects of flood current and ebb current respectively is appropriate to investigate influence of the background shear flow on coefficients of the K-dV equation. The obtained results provide the possibility for the simulation of SAR signatures of internal solitary waves under considering the impact of ocean surface mixed layer turbulence in the companion paper.

A study of SAR remote sensing of internal solitary waves in the north of the South China Sea:Ⅱ.Simulation of SAR signatures of internal solitary waves

A new model developed from the full-spectrum model of Lyzenga and Bennett （ 1988 ） is built up by us preliminarily through considering the impact of the ocean surface mixed layer turbulence on SAR remote sensing of internal solitary waves. In the partial differential equation of the action spectral density of the surface gravity-capillary waves the source function representing the contribution of the turbulence is added besides the usual source function representing the contribution of the wind. The source function is determined by applying the κ - ε model and adopting the Nasmyth spectrum of oceanic turbulence （ Nasmyth, 1970; Oakey, 1982; Fan, 2002） on the basis of the previous simulation results of internal tide transformation obtained in the companion paper （Fan et al. ,2008）. Either under relatively high wind speed, or under low wind speed, our model predicts significant large modulations of radar backscatter at all three bands （ L, C and X bands） for both VV and HH polarization. These results prove that considering the impact of ocean surface mixed layer turbulence on SAR remote sensing of internal solitary waves is reasonable and appropriate for settlement of the well-known problem of contemporary radar imaging models.

Study on characteristics of internal solitary waves in the Malacca Strait based on Sentinel-1 and GF-3 satellite SAR data

The study of the characteristics of internal solitary waves happened in the Malacca Strait is an urgent problem for submarine, ship navigation and marine engineering. Based on SAR remote sensing data obtained from the high spatial resolution Sentinel-1 and GF-3, the internal solitary wave characteristics of the Malacca Strait are investigated. By use of 20 Sentinel-1 SAR images from June 2015 to December 2016 and 24 GF-3 images from April2018 to March 2019, the spatial distribution characteristics of internal solitary wave s are statistically analyzed. It is found that the internal solitary waves are usually in the form of wave packets and single solitary waves, and the maximum crest length of the leading wave can reach 39 km. The amplitude and group velocity of internal solitary wave s can be calculated by the inversion model of high-order nonlinear Schrodinger(NLS) equation, and the calculated amplitude of the internal solitary wave s and the propagation group velocity of the wave packets range from 4.7 m to 23.9 m and 0.12 m/s to 0.40 m/s, respectively. The range of phase velocity of single internal solitary waves obtained by Kd V equation is 0.26 m/s to 0.60 m/s. In general, the amplitude and the velocity of internal solitary wave s in Malacca strait are related to the topography.

利用SAR图像计算内波深度和振幅的可行性研究

根据星载合成孔径雷达(SAR)内波成像机理,建立了包括KdV方程、作用量谱平衡方程和Bragg散射模型在内的内波深度和振幅探测模型,更为严谨地推导出用SAR图像计算内波深度的方法,并首次推导出内波振幅的计算方法。利用该方法对台湾东北部海域的两幅ERS-1SAR内波图像进行了实例研究,结果表明,该方法是合理且可行的。

南海北部内孤立波SAR遥感反演的初步研究

应用海洋内孤立波2层流体模型的SAR遥感反演技术,对南海北部内孤立波SAR遥感反演进行了初步研究。对于不存在由同一波源生成的2个内孤立波波包的SAR遥感图像资料,可以利用Levitus等的月平均温盐资料确定跃层深度和约化重力加速度,进而确定内波波速并进行内波振幅的反演。这样能够充分利用宝贵的SAR图像资源。

从内波SAR图像中提取跃层深度和内波振幅的非线性方法

针对SAR图像进行内波参数提取方法的不足:非线性方法将跃层深度作为已知量,将跃层深度作为未知量却用了线性方法。本文将根据非线性内波KdV方程的频散关系,结合半日潮假定并利用从SAR图像上得到的半波宽度,提出了非线性框架下同时从内波SAR图像反演内波振幅及跃层深度的方法。然后利用实测数据和内波SAR图像数据对此方法进行了分析,所得结果较理想。最后根据倾斜底地形内波波包的周期验证了文中所用半日潮假定的正确性。

南海东北部深水海域大振幅内孤立波SAR遥感仿真研究

为了克服基于两层海洋的内孤立波SAR遥感仿真模型的缺陷,使用基于连续分层海洋模型的GK-dV方程,在南海东北部深水海域进行了大振幅内孤立波传播模拟,模拟输出内孤立波振幅91.0m,半波宽度262.0m。然后使用新建立的基于连续分层海洋模型的内孤立波SAR遥感仿真模型进行了内孤立波反演,反演出内孤立波半波宽度251.5m,相对输入误差为4.0%,比旧模型的34.2%有了较大改进。

地转及均匀背景流场对混合层深度SAR遥感反演的影响

以考虑了地转和均匀背景流场影响的两层流体界面内波频散关系模型为基础,得到新的利用SAR遥感图像计算混合层深度的方法。利用该方法对一幅南海北部SAR内波图像进行了实例研究,并且和时空同步的CTD资料进行了对比。结果表明,加入地转和均匀背景流场影响的模型更为合理,为更准确地反演混合层深度奠定了基础。

利用SAR遥感卫星资料研究东海内孤立波时空分布特征

通过分析欧洲空间局的地球资源卫星(ERS-1/2)从1995—2010年的合成孔径雷达(SAR)图像,研究了东海内孤立波的时间分布特征,绘制了东海内孤立波的空间分布图,并且反演了个别海区内孤立波的传播速度。研究结果表明:东海内孤立波主要分布在两个海域,台湾东北部海域和长江口、杭州湾以及外舟山群岛附近海域。在这两个海域,大潮期间的强潮流有利于内孤立波的出现。在长江口及附近海域,跃层强时,内孤立波出现的概率相对较高。台湾东北部内孤立波波速约为0.6—0.8 m/s,长江口及附近海域内孤立波波速约为0.6 m/s左右。

Effects of westward shoaling pycnocline on characteristics and energetics of internal solitary wave in the Luzon Strait by numerical simulations

An internal gravity wave model was employed to simulate the generation of internal solitary waves(ISWs)over a sill by tidal flows.A westward shoaling pycnocline parameterization scheme derived from a three-parameter model was adopted,and then 14 numerical experiments were designed to investigate the influence of the pycnocline thickness,density difference across the pycnocline,westward shoaling isopycnal slope angle and pycnocline depth on the ISWs.When the pycnocline thickness on both sides of the sill increases,the total barotropic kinetic energy,total baroclinic energy and ratio of baroclinic kinetic energy(KE)to available potential energy(APE)decrease,whilst the depth of isopycnal undergoing maximum displacement and ratio of baroclinic energy to barotropic energy increase.When the density difference on both sides of the sill decreases synchronously,the total barotropic kinetic energy,ratio of baroclinic energy to barotropic energy and total baroclinic energy decrease,whilst the depth of isopycnal undergoing maximum displacement increases.When the westward shoaling isopycnal slope angle increases,the total baroclinic energy increases whilst the depth of turning point almost remains unchanged.When the depth of westward shoaling pycnocline on both sides of the sill reduces,the ratio of baroclinic energy to barotropic energy and total baroclinic energy decrease,whilst the total barotropic kinetic energy and ratio of KE to APE increase.When one of the above four different influencing factors was increased by 10%while the other factors keep unchanged,the amplitude of the leading soliton in ISW Packet A was decreased by 2.80%,7.47%,3.21%and 6.42%respectively.The density difference across the pycnocline and the pycnocline depth are the two most important factors in affecting the characteristics and energetics of ISWs.

On generation source sites of internal waves in the Luzon Strait

This effort aims to determine the generation source sites in the Luzon Strait for energetic, long-crest, transbasin internal waves （IW） observed in the northern South China Sea （NSCS）. The roles of islands distributed on eastern side of the strait, Kuroshio, submarine ridges, shoaling thennocline, and strait configuration played in the IW generation are examined using the cruise data analysis, satellite data interpretation, and dynamical analysis. The islands and channels on eastern side of the strait are excluded from a list of possible IW source sites owing to their unmatched horizontal dimensions to the scale of IW crest line length, and the relative low Reynolds number. The Kuroshio has a potential to be a radiator for the long-crest IW disturbances, meanwhile, the Kurosbio west （east） wing absorbs the eastward （westward） propagating IW disturbance. Namely, the Kuroshio blockades the outside west-east propagating IW disturbances. The 3-D configuration of the Luzon Strait is characterized by a sudden, more than one order widening of the cross-section areas at the outlets on both sides, providing a favorable condition for IW type initial disturbance formation. In the Luzon Strait, the thermocline is featured by a westward shoaling all the year around, providing the dynamical conditions for the amplitude growth （declination） to the westward （eastward） propagating IW type disturbance. Thus, the west slope of western submarine ridge at the western outlet of the Luzon Strait is a high possibility source sites for energetic, long-crest, transbasin IWs in the NSCS. The interpretation results of satellite SAR images during a 13 a period from 1995 to 2007 provide the convincing evidence for the conclusions.

Modification on Lynett and Liu’s model for internal solitary wave propagation over variable bathymetry

A two-dimensional, depth-integrated model proposed by Lynett and Liu (2002) was checked carefully, and several misprints in the model were corrected after detailed examination on both the theory and the numerical program. Several comparisons were made on wave profile, system energy and maximum wave amplitude. It is noted that the modified model can simulate the propagation of the internal solitary waves over variable bathymetry more reasonably to a certain degree, and the wave pro-files obtained based on the modified model can better fit the experiment data reported by Helfrich (1992) than those from original model.

跃层厚度对内孤立波能量耗散的影响

内孤立波破碎混合是陆架地形上海洋混合过程重要的能量汇,为了探究内孤立波在连续跃层密度分层中能量变化及能量耗散规律,本文使用OpenFOAM建立数值水槽,基于双曲正切曲线设置连续跃层密度分层开展了一系列工况的模拟,针对跃层厚度对内孤立波能量传递及其耗散进行了详细分析。结果表明跃层厚度与跃层处流速剪切存在着负相关关系,随着跃层厚度的增加能量耗散先减小后增大。跃层厚度较小时,跃层处流速剪切强,理查德森数小,易产生开尔文-亥姆霍兹(Kelvin–Helmholtz)不稳定现象;随着跃层厚度的增大,流速剪切减小,理查德森数增大,开尔文-亥姆霍兹不稳定消失;跃层厚度达到临界值后,层结稳定性减小,理查德森数减小,流场的翻转混合过程加强,能量耗散也明显增强。

内波环境下水下滑翔机流场与载荷特性分析

为保障水下滑翔机的作业安全,本文采用k-Epsilon湍流模型,结合自定义场函数建立了密度分层流体中水下滑翔机的流场与载荷特性分析的CFD模型。结果表明:水下滑翔机在工作时不能忽略内波的影响。当滑翔机经过内孤立波时,滑翔机所处的流场流速会增大,涡量也会明显增大;当滑翔机接近交界面时,滑翔机受到的升力和俯仰力矩变化趋势增大,远离交界面时,阻力会逐渐增大;当滑翔机的俯仰角增大时,其受到的阻力幅值会减小,升力幅值变化较小,俯仰力矩的幅值增大。本文揭示了内波环境下水下滑翔机的流场与载荷特性,对经典式水下滑翔机的设计以及作业时遭遇内孤立波的水动力响应预测具有重要的理论和工程意义。

引入强分层近似对含密度跃层情况下的大幅内孤立波计算误差研究

At present,studies on large-amplitude internal solitary waves mostly adopt strong stratification models,such as the twoand three-layer Miyata–Choi–Camassa(MCC)internal wave models,which omit the pycnocline or treat it as another fluid layer with a constant density.Because the pycnocline exists in real oceans and cannot be omitted sometimes,the computational error of a large-amplitude internal solitary wave within the pycnocline introduced by the strong stratification approximation is unclear.In this study,the two-and three-layer MCC internal wave models are used to calculate the wave profile and wave speed of large-amplitude internal solitary waves.By comparing these results with the results provided by the Dubreil–Jacotin–Long(DJL)equation,which accurately describes large-amplitude internal solitary waves in a continuous density stratification,the computational errors of large-amplitude internal solitary waves at different pycnocline depths introduced by the strong stratification approximation are assessed.Although the pycnocline thicknesses are relatively large(accounting for 8%–10%of the total water depth),the error is much smaller under the three-layer approximation than under the two-layer approximation.

两层模式下内波K-dv方程的频散关系

为了更好地了解内孤立波的特性,分别利用椭圆余弦波方法和摄动展开方法,对两层模型下内孤立波K-dv方程的频散关系进行了推导。结果显示:利用后一种方法得到的两层模式下内孤立波K-dv方程的频散关系是一种方便、实用的解析式。据此解析式可进一步得到内孤立波半波宽度与内波波数间的关系,从而可将内孤立波理论与内波合成孔径雷达(SAR)图像联系起来。用此解析式计算得到的内孤立波频率和相速度与实测结果具有很好的一致性,因而为进一步进行分析和利用数据估计内孤立波的部分参数提供了理论依据。对解析式中频散项和非线性项量级的进一步分析认为,非线性项的作用在从SAR图像中提取内波振幅等参数时最好不予忽略。

内波流场中悬链线立管涡激振动数值研究

基于内波流场的eKdv理论并结合尾流振子模型和挠性立管的非线性振动方程建立内波流场中悬链线立管计算模型,研究了内波流场中悬链线立管升力方向运动的响应问题,并分析了内孤立波波高和密度跃层位置对悬链线立管升力方向响应的影响。结果表明:内孤立波经过悬链线立管时会引起突然增大的涡激振动,其中高阶模态占主导;涡激振动最大位移发生于流体密度跃层位置附近;内孤立波波高增大以及密度跃层位置加深会使立管转变为更高阶模态振动,并且密度跃层位置加深会使立管涡激振动位移明显减小。

南海北部陆坡海域第二模态内孤立波统计特征

根据南海北部陆坡海域潜标观测数据,对该海域第二模态内孤立波统计特征进行分析。观测期间共发现72个第二模态内孤立波,包含101个第二模态内孤立波孤立子。该海域第二模态内孤立波以凸型为主,且最大振幅多为下凹振幅,占79.2%。KdV方程波速计算结果表明,波速可用三角函数描述,整体表现为夏季大、冬季小,且第二模态内孤立波流速呈明显的三层结构。最大流速深度及人工判别得到的上下层转向深度的统计结果表明:上层内波流流速转向平均深度约为97.7 m,中层内波流最大流速深度约为134.6 m,下层内波流转向平均深度约为204.2 m,内波流最大流速多发生在中层,流向主要为西北向,流速主要分布于0.2~0.8 m·s^(-1)区间范围内,占82.2%;上层流向主要为东南向,流速主要分布于0~0.6 m·s^(-1)区间范围内,占98.0%;中层流向主要为西北向,流速主要分布于0.2~0.8 m·s^(-1)区间范围内,占93.1%;下层流向主要为东南向,流速主要分布于0~0.4 m·s^(-1)区间范围内,占94.1%。本文系统性地给出了南海北部陆坡海域第二模态内孤立波统计特征,可为相关研究、水下航行及工程应用提供重要的资料和参考。

连续密度跃层中第二模态内孤立波及其波致流场实验研究

该文在大型分层流水槽中开展了连续密度跃层中第二模态内孤立波及其波致流场的实验研究。利用分层流体重力塌陷原理,该文实验模拟了连续密度跃层中“上凸下凹”型的第二模态内孤立波,利用电导率探头阵列和粒子图像测速技术测量第二模态内孤立波及其波致流场,获得了连续密度跃层中充分发展第二模态内孤立波的几何与波-流场非均匀结构,以及波幅变化对第二模态内孤立波的波要素关系和波致流场特征的影响。研究结果表明:连续密度跃层中,第二模态内孤立波波形几何特征表现为上凸下凹的卵形波包状,波形非均匀垂向结构特征表现为波幅和波长随深度的非均匀变化;第二模态内孤立波波包上凸与下凹域分别形成逆时针和顺时针环流,两者组合为一对上下反对称的环流并共同向前传播;第二模态内孤立波波速随波幅的增大而增大,波长则随波幅的增大而减小;水平和垂向流速强度与特征波幅呈正相关关系,且水平流速强度大于垂向流速强度。该文提升了对实际海洋层化条件下第二模态内孤立波及其波致流场的认识。