An ocean current inversion accuracy analysis based on a Doppler spectrum model

Microwave remote sensing is one of the most useful methods for observing the ocean parameters. The Doppler frequency or interferometric phase of the radar echoes can be used for an ocean surface current speed retrieval,which is widely used in spaceborne and airborne radars. While the effect of the ocean currents and waves is interactional. It is impossible to retrieve the ocean surface current speed from Doppler frequency shift directly. In order to study the relationship between the ocean surface current speed and the Doppler frequency shift, a numerical ocean surface Doppler spectrum model is established and validated with a reference. The input parameters of ocean Doppler spectrum include an ocean wave elevation model, a directional distribution function, and wind speed and direction. The suitable ocean wave elevation spectrum and the directional distribution function are selected by comparing the ocean Doppler spectrum in C band with an empirical geophysical model function（CDOP）. What is more, the error sensitivities of ocean surface current speed to the wind speed and direction are analyzed. All these simulations are in Ku band. The simulation results show that the ocean surface current speed error is sensitive to the wind speed and direction errors. With VV polarization, the ocean surface current speed error is about 0.15 m/s when the wind speed error is 2 m/s, and the ocean surface current speed error is smaller than 0.3 m/s when the wind direction error is within 20° in the cross wind direction.

The effect of Coriolis-Stokes forcing on upper ocean circulation in a two-way coupled wave-current model

We investigated the Stokes drift-driven ocean currents and Stokes drift-induced wind energy input into the upper ocean using a two-way coupled wave-current modeling system that consists of the Princeton Ocean Model generalized coordinate system (POMgcs), Simulating WAves Nearshore (SWAN) wave model, and the Model Coupling Toolkit (MCT). The Coriolis-Stokes forcing (CSF) computed using the wave parameters from SWAN was incorporated with the momentum equation of POMgcs as the core coupling process. Experimental results in an idealized setting show that under the steady state, the scale of the speed of CSF-driven current was 0.001 m/s and the maximum reached 0.02 m/s. The Stokes drift-induced energy rate input into the model ocean was estimated to be 28.5 GW, taking 14% of the direct wind energy rate input. Considering the Stokes drift effects, the total mechanical energy rate input was increased by approximately 14%, which highlights the importance of CSF in modulating the upper ocean circulation. The actual run conducted in Taiwan Adjacent Sea (TAS) shows that: 1) CSF-based wave-current coupling has an impact on ocean surface currents, which is related to the activities of monsoon winds; 2) wave-current coupling plays a significant role in a place where strong eddies present and tends to intensify the eddy's vorticity; 3) wave-current coupling affects the volume transport of the Taiwan Strait (TS) throughflow in a nontrivial degree, 3.75% on average.

Study on short－range numerical forecasting of ocean current in the East China Sea－Ⅲ Three－dimensional baroclinic anomaly forecasting model And its application

A three-dimensional baroclinic numerical forecasting model for anomaly current field is developed forapplication in the Bohai Sea and the upper layer of the Huanghai Sea and the East China Sea. All the dynamical variables, including temperature and salinity, can be calculated predictively by using the model. The results of the numerical weather prediction are used as input fields,and various dynamic and thermodynamic boundary conditions areadopted. So, the model can be used as an operational numerical forecasting model for current fields. In this paper,the structure of the model is presented in detail, various tests for the performance of the model are made, and thedependence of the model on some parameters is discussed. The results of the numerical simulation using historicaldata and experimental forecasting tests are also presented.

Study on short-range numerical forecasting of ocean current in the East China Sea——Ⅰ Basic problems of ocean current forecasting and structure of the models

Ocean current forecasting is still in explorative stage of study. In the study, we face some problems that have not been met before. The solving of these problems has become fundamental premise for realizing the ocean current forecasting. In the present paper are discussed in depth the physical essence for such basic problems as the predictability of ocean current, the predictable currents, the dynamical basis for studying respectively the tidal current and circulation, the necessity of boundary model, the models on regions with different scales and their link. The foundations and plans to solve the problems are demonstrated. Finally a set of operational numerical forecasting system for ocean current is proposed.

Wave-current interaction during Typhoon Nuri(2008)and Hagupit(2008):an application of the coupled ocean-wave modeling system in the northern South China Sea

The northern South China Sea(SCS) is frequently affected by typhoons. During severe storm events, wave-current interactions produce storm surges causing enormous damage in the path of the typhoon. To evaluate the influence of wave-current interactions on storm surge, we used a coupled ocean-atmospherewave-sediment transport(COAWST) modeling system with radiation-stress and vortex-force formulations to simulate two typically intense tropical storms that invaded the SCS, namely Typhoons Nuri(2008) and Hagupit(2008), and compared results with observations from the Hong Kong Observatory. Both radiationstress and vortex-force formulations significantly improved the accuracy of the simulation. Depending on which typhoon and the topography encountered, the influence of surface waves on the oceanic circulation showed different characteristics, including the differences of range and intensity of storm surge between vortex-force and radiation-stress experiments. During typhoon landing, strong sea-surface elevation in concert with wave set-up/set-down caused the adjustment of the momentum balance. In the direction perpendicular to the current, but especially in the cross-shore direction, the pressure gradient and wave effects on the current dominated the momentum balance.

Ocean Data Assimilation Using Intermittent Analyses and Continuous Model Error Correction

A new data insertion approach is applied to the Derber and Rosati ocean data assimilation (ODA) system, a system that uses a variational scheme to analyze ocean temperature and provide ocean model corrections continuously. Utilizing the same analysis component as the original system, the new approach conducts analyses to derive model corrections intermittently at once-daily intervals. A technique similar to the Incremental Analysis Update (IAU) method of Bloom et al. is applied to incorporate the corrections into the model gradually and continuously. This approach is computationally more economical than the original.A 13-year global ocean analysis from 1986 to 1998 is produced using this new approach and compared with an analysis based on the original one. An examination of both analyses in the tropical Pacific Ocean shows that they have qualitatively similar annual and interannual temperature variability. However, the new approach produces smoother monthly analyses. Moreover, compared to the independent

Water exchange through the Kerama Gap estimated with a 25-year Pacific HYbrid Coordinate Ocean Model

Variations in water exchange through the Kerama Gap(between Okinawa Island and Miyakojima Island) from 1979 to 2003 were estimated with the 0.08° Pacific HYbrid Coordinate Ocean Model(HYCOM). The model results show that the mean transport through the Kerama Gap(KGT) from the Pacific Ocean to the East China Sea(ECS) was 2.1 Sv, which agrees well with the observed mean KGT(2.0 Sv) for 2009–2010. Over the time period examined, the monthly KGT varied from-10.9 Sv to 15.8 Sv and had a standard deviation of ? 5.0 Sv. The water mainly enters the ECS via the subsurface layer(300–500 m) along the northeastern slope of the Kerama Gap and mainly flows out of the ECS into the southwest of the Kerama Gap. The seasonal and interannual variations of the KGT and the Kuroshio upstream transport were negatively correlated. The Kuroshio upstream transport was largest in summer and smallest in autumn while the KGT was smallest in summer(1.02 Sv) and largest in spring(2.94 Sv) and autumn(2.44 Sv). The seasonal and interannual variations in the Kuroshio downstream(across the PN-line) transport dif fered significantly from the Kuroshio upstream transport but corresponded well with the KGT and the sum of the transport through the Kerama Gap and the Kuroshio upstream, which indicates that information about variation in the KGT is important for determining variation in the Kuroshio transport along the PN-line.

An extended variable-grid global ocean circulation model and its preliminary results of the equatorial Pacific circulation

To investigate the interaction between the tropical Pacific and China seas a variable-grid global ocean circulation model with fine grid covering the area from 20°S to 50°N and from 99° to 150°E is developed. Numerical computation of the annually cyclic circulation fields is performed. The results of the annual mean zonal currents and deep to abyssal western boundary currents in the equatorial Pacific Ocean are reported. The North Equatorial Current,the North Equatorial Countercurrent, the South Equatorial Current and the Equatorial Undercurrent are fairly well simulated. The model well reproduces the northward flowing abyssal western boundary current.From the model results a lower deep western boundary current east of the Bismarck-Solomon-New Hebrides Island chain at depths around 2 000 m has been found. The model results also show that the currents in the equatorial Pacific Ocean have multi-layer structures both in zonal currents and western boundary currents, indicating that the global ocean overturning thermohaline circulation appears of multi-layer pattern.

Lateral Dynamic Analysis and Motion Control of a Jet Trencher in Ocean Currents

In order to control the lateral motion of a jet trencher which is important for stable trenching operation,the oscillation characteristics of the jet trencher are researched. The jet trencher is simplified into a single degree of freedom model with restoring and damping force. The nonlinear mathematical model of the trencher laterally oscillating in ocean currents is established,and its approximate analytical solution is obtained.Results show that the analytical solution has small differences with numerical solution based on the fourth-order Runge-Kutta method and can effectively describe the underwater oscillation. A double-loop PID controller is designed to control the lateral motion displacement of the trencher to return to the center of the pipeline route which is effective and robust for the propulsion system.

Development of a High-Resolution Coastal Circulation Model for the Ocean Observatory in Lunenburg Bay

An advanced ocean observatory has been established in Lunenburg Bay of Nova Scotia, Canada as part of an interdisciplinary research project of marine environmental prediction. The development of a high-resolution coastal circulation model is one of important components of the observatory. The model horizontal resolution is 60 m and the vertical resolution is about 1 m. The coastal circulation model is used to simulate the semi-diurnal tidal circulation and associated nonlinear dynamics with the M2 forcing specified at the model open boundaries. The model is also used to simulate the storm-induced circulation in the bay during Hurricane Juan in September 2003, with the model forcing to be the combination of tides and remotely generated waves specified at the model open boundaries and wind stress applied at the sea surface. The model results demonstrate strong interactions between the local wind stress, tidal forcing, and remotely generated waves during this period. Comparison of model results with the surface elevation and current observations demonstrates that the coastal circulation model has reasonable skills in simulating the tidal and storm-induced circulation in the bay.

Numerical study of current fields near the Changjiang Estuary and impact of Quick-EnKF assimilation

A 30-d current numerical simulation is running for the Yangshan Port,the Changjiang Estuary,the Hangzhou Bay and their adjacent seas using a finite volume coastal ocean model （FVCOM）,with Changjiang River runoff and wind effect being considered.At the open boundary,this model is driven by the water level obtained from prediction including eight main partial tides.After the harmonic analysis,the cotidal chart and the iso-amplitude line as well as the current ellipse distribution map are displayed to illustrate the propagation property of a tidal wave.Horizontal velocity of both the U and V components coincides with the actual measurement,which shows that the model result is credible to describe the hydrodynamic pattern in this sea area.On this basis,real-time current data from high-frequency radar is assimilated with the implementation of quick ensemble Kalman filter,which takes the variation tendency of the state vector to compute the analysis field,instead of integrating the field for N （the number of ensemble） times as it used to in the standard EnKF,aiming at raising the efficiency of computation,reducing the error of prediction and at the same time,improving the forecast effect.

Test Study on Current Force on Mooring Ships

In respect of an offshore berthing pillar, the test study result of current force acting on mooring ships is described in this paper. Empirical and semi-empirical relationships of current force coefficient are given according to the angle between the flow direction and the ship's longitudinal axis, which are coincident with the result of theoretical analysis.

UMERICAL SIMULATIONS OF OCEANIC ELEMENTS IN THE SCS AND ITS NEIGHBORING SEA REGIONS FROM JANUARY TO AUGUST IN 1998

在华南海(SCS ) 和邻近的水的海洋的条件上的研究对在东亚的夏天季风的彻底的理解有用。有海洋的水流怎么变化的一幅 3-dimensional 图画，在华南海(SCS ) 的海洋的元素和它在 1 月 1998 年 8 月的附近的海区域被在这份报纸使用改进普林斯顿大学海洋模型(POM ) 模仿了。主要结果在对海洋调查和另外的模拟的好同意。结果证明自台湾东面的菲律宾海流向日本的暖流水流的 SCS 分支是在从 1 月的北方 SCS 的重要部分到 8 月；除了在冬季， SCS 温暖的水流在所有月内清楚地被复制；总是在那里存在在到 8 月的从 1 月的自台湾东面的菲律宾海流向日本的暖流水流的右的一个大规模逆旋风的旋涡。在模型领域， SCS 的表面水流与一个明显的季节的变化与季风有最靠近的关系。另外，在在夏天季风发作前后的 SCS 和它的附近的海区域的 SST 的发展中的特征被改进 POM 很好也模仿。那些是为开发 a coupled 的基础地区性的海洋大气的模型系统。

An Assessment of a Nowcast/Forecast System for the Straits of Florida/Florida Current Regime

The Florida Current (FC) largely fills the Straits of Florida and is variable on a broad spectrum of time and space scales. Some portions of the variability are due to variable forcing by tides, winds, heating/cooling, and throughflow; other portions are due to intrinsic instabilities of the FC. To predict, as well as to better understand this complex regime, a nowcast/forecast system (East Florida Shelf Information System (EFSIS)) has been implemented and assessed (http://efsis. rsmas. miami. edu). EFSIS is based on an implementation of the Princeton Ocean Model (POM) with mesoscale - admitting resolution on a curvilinear grid. It is forced by a mesoscale numerical weather prediction system (called Eta) run operationally by the National Centers for Environmental Prediction (NCEP), eight tidal constituents from a global tidal model, and lateral boundary conditions from an operational global ocean prediction model, i.e., the Navy Coastal Ocean Model (NCOM).Real-time observations of coastal sea level, coastal sea surface temperature, coastal HF radar-derived surface current maps,and FC volume transport are used to verify and validate EFSIS. EFSIS is part of an evolving strategy for real-time predictive coastal ocean modeling methodology, and for fostering the understanding of the variability of the regime on several time and space scales. Here, some of the verification and validation results are provided, as well as diagnostic analyses of dynamical servations and numerical circulation models to yield a credible sequence of synoptic views of coastal ocean circulation for the first time.

基于分段模型预测控制的亚中尺度观测器垂向控制研究

为获取海洋亚中尺度过程在水平固定位置垂直剖面的连续时间序列数据,本文采用分段模型预测控制的方法,对亚中尺度观测器的垂向运动进行精确的控制。在建立亚中尺度观测器精确模型的基础上,对模型预测控制(MPC)算法的预测步长、控制步长、约束和成本函数开展设计。针对海流不确定性所导致的模型变化问题,对系统模型、控制算法进行了分段处理。通过对比分析剖面运动的仿真结果和试验数据,验证了模型和算法的有效性。仿真结果表明对于垂向运动的控制,分段式模型预测控制相对于传统的MPC具有调节误差小和稳定时间短的优点,能够减少亚中尺度观测器水平漂移量约7.9%,对亚中尺度过程的数据获取和认知具有现实意义。

基于AR模型的多普勒散射计回波功率谱估计

当采样点数少或流速较小时,采用周期图法估计的多普勒散射计(Doppler scatterometer,DopSCAT)回波功率谱分辨率低,多普勒频移提取精度低。对此,提出了一种基于自回归(autoregressive,AR)模型的DopSCAT回波功率谱估计方法。该方法为DopSCAT回波信号建立含有未知参数的AR模型,采用赤池信息量准则自适应确定模型的最优阶数;对定阶后的AR模型采用Burg算法计算模型参数,利用得到的AR模型估计回波功率谱;对功率谱进行峰值检测提取多普勒频移计算径向流,采用两个观测方位向的径向流合成得到海面流场。利用OSCAR海流数据进行了回波功率谱估计与海流反演实验。分析结果表明,与周期图法相比,该方法能够显著提高回波功率谱分辨率和多普勒频移的提取精度,进而提高了海流的反演精度。

基于强化学习的无人水面艇能耗最优路径规划算法

针对无人水面艇路径规划时存在洋流、障碍物等外部干扰的问题,提出一种改进的基于强化学习的无人水面艇能耗最优路径规划算法。首先,建立多个随机涡流组成的二维洋流模型和无人水面艇平面运动学模型;其次,依据洋流和无人水面艇相对速度关系,计算路径点是否可达;然后,利用改进的奖励函数、动作集和状态集求解全局最优路径,并采用B样条法进行平滑;最后,在2种典型环境下进行了数值仿真。仿真结果表明,该算法可规划出一条能耗最优且平滑的路径,验证了算法的有效性和最优性。

海流环境下二关节机械臂动力学建模及其跟踪控制

水下机械臂的水动力性能受海流载荷的影响较大,目前针对水下机械臂控制的相关研究中将水下环境做了静水的假设或将海流载荷仅仅作为一种简单的随机扰动来考虑,控制精度较低.该文分别基于Lagrange法和Newton-Euler法推导出均匀海流环境下二关节机械臂的动力学模型,考虑海流和机械臂的相对运动,引入Morison公式计算海流对机械臂的水阻力和惯性力.基于此动力学模型,利用滑模控制策略实现机械臂理想轨迹的精确跟踪.对比PD(proportional-derivative)控制,仿真结果表明滑模控制具有更优的控制效果.

偏心圆柱绕流柱体运动特征及能量转化特性的研究

为研究偏心圆柱绕流柱体的运动特征及其转化海流能等低流速流体能量的特性,开展了偏心圆柱绕流试验研究,测量了角度摆动、扭矩变化规律,分析了偏心圆柱的运动特征、水动力学性能,以及偏心圆柱绕流摆动动力学系统的混沌特性,计算了平均输出功率与能量转化效率。结果表明:偏心圆柱绕流摆动需要一定的启动流速,来流雷诺数大于一定数值时,发生偏心圆柱的摆动频率锁定现象;偏心圆柱绕流转化流体能量的效率与阻尼呈线性正比关系,随来流雷诺数的增大而减小,最大转化效率可达37%。偏心圆柱绕流摆动系统为一弱混沌系统,其运动规律可近似由线性方程拟合。依据斯特劳哈尔相似准则和拟合的运动规律预测了原型偏心圆柱绕流的运动规律和能量转化效率。研究成果可为海流能开发利用装备的设计研发提供理论基础和试验依据。