Modeling vertical motion at ocean fronts: Are nonhydrostatic effects relevant at submesoscales?

Through a suite of three-dimensional, high-resolution numerical modeling experiments, we examine the role of nonhydrostatic effects on O(1 km) submesoscale processes at ocean fronts, with particular focus on the vertical velocity field. Several differences between nonhydrostatic and hydrostatic models are pointed out using a framework that enables precise comparison, but it is difficult to identify categorical differences between the model solutions at the grid resolutions afforded. The instantaneous vertical velocity structure is sensitive to the model choice and, even more so, to grid resolution, but the average vertical flux is similar in both hydrostatic and nonhydrostatic cases.When a frontal region with horizontal density gradients is perturbed by wind, a profusion of submesoscale, O(1 km), secondary circulation features develops in the upper 50 m. Narrow, elongated cells of intense up- and down-welling are found to occur close to the surface, overlying broader regions of weaker up- and down-welling associated with the mesoscale meanders of the baroclinically unstable front. The submesoscale down-welling is considerably stronger than up-welling and is concentrated in 1–2 km width filaments within which velocities can attain magnitudes as high as 200 m day⁻¹. The submesoscale features are found to be robust at horizontal grid resolutions varying between 1 and 0.25 km and exist even in the hydrostatic model. Submesoscale circulation is difficult to observe or resolve in coarser resolution circulation models, but is likely to play a significant role in the exchange of energy and properties between the surface ocean and thermocline. Possible mechanisms for the generation of these features are investigated in a follow-on paper.     

南海西部风驱离岸急流次中尺度锋面的动力学分析

本文利用卫星观测资料和500 m分辨率数值模拟结果,结合理论分析,对南海西部夏季风场驱动的离岸急流海域次中尺度锋面及其不稳定对背景流场的动力学影响进行了研究。卫星观测和模拟结果表明,南海西部（WSCS）存在侧向尺度为O（1-10）km的次中尺度锋面,在地转和非地转运动的共同作用下,次中尺度密度锋面具有一阶Rossby（Ro）和Richardson（Ri）数。锋面诊断结果显示,沿锋面急流方向的风场强迫引起了显著的跨锋面Ekman净输送,有效地在跨锋面方向将表层冷水平流输送至暖水侧,导致海表浮力损失。减弱的垂向层结和增强的水平浮力梯度使得锋面海域出现负Ertel位涡（PV）,表明该密度锋面易受次中尺度对称不稳定（SI）的影响。次中尺度锋面不稳定引起的跨锋面次级环流能够显著增强垂向速度,其最大值可达100 m·d^-1。能量评估结果表明,次中尺度湍流的两个主要能量源,即地转剪切项（GSP）和垂向浮力通量（BFLUX）在锋面海域显著增强表明在沿锋面急流方向的风场强迫作用下,大尺度地转流的地转剪切动能和锋面有效位能能有效地通过锋面不稳定向次中尺度过程传递。因此,次中尺度锋面及其不稳定有助于增强局地垂向交换和正向串级地转能量,可以为夏季WSCS高叶绿素浓度的相干结构和锋面地转能量的正向传递提供新的动力解释。     

南海西部夏季上升流锋面的次中尺度特征分析

利用卫星遥感资料和区域海洋数值模式ROMS(regional ocean modeling system)高分辨率数值模拟结果, 对南海西部夏季上升流锋面的次中尺度特征及其非地转过程进行了探讨。高分辨率卫星遥感观测和数值模拟结果显示, 南海西部夏季锋面海域存在活跃的次中尺度现象, 其水平尺度约为1~10km, 且具有O(1)罗斯贝数(Rossby number, Ro)的典型次中尺度动力学特征。进一步的诊断分析表明, 在夏季西南风的驱动下, 沿锋面射流方向的风应力(down-front wind stress)引起的跨锋面埃克曼输运有利于将海水由锋面冷水侧向暖水侧输运, 减小了锋面海域的垂向层结和Ertel位涡, 加剧了锋面的不稳定, 并形成跨锋面的垂向次级环流。高分辨率模拟结果显示, 锋面海域最大垂向流速可达100m?d ^-1, 显著增强了上层海洋的垂向物质交换。因此, 活跃在锋面海域的次中尺度过程可能是增强南海西部上升流海域垂向物质交换的重要贡献者。     

Centrifuge modelling of SCR vertical motion at touchdown zone

Steel catenary risers (SCR) connect seabed pipelines and flow lines to floating structures used for oil and gas production in deep waters. Waves and currents induce motions of the structure and the risers. The repeated motions of the risers at the touchdown zone in turn induce loads on the seabed soil and might eventually cause fatigue damage to the risers. The analysis of riser fatigue damage is heavily dependent on the soil model. Soil behaviour at touchdown zone such as soil remoulding, stiffness degradation and deformation of the seabed at the touchdown zone further complicate the accurate assessment of riser fatigue damage, which is currently not appropriately quantified in existing design methods. This paper presents centrifuge model tests simulating the repeated vertical movement of a length of riser on clay seabed with increasing undrained shear strength with depth. During the tests, the pipe was subject to cyclic motion over fixed vertical displacement amplitude from an invert embedment of 0.5-3.5 pipe diameters into the soil. The test results show a significant progressive degradation of soil strength and diminution of excess pore water pressure with increasing number of riser penetration/uplift cycle. In view of the different types of environment loadings experienced by floating platforms and various soil conditions, tests were also conducted to investigate the effect of soil strength, riser displacement rate and loading mode on riser-soil interaction during repetitive penetration/uplift motion of the riser.     

海工结构物桩基础竖向稳定性的可靠度分析

应用可靠度理论对渤海海域中海工结构物桩基础的稳定性进行了可靠度分析,阐述了各个设计指标的变化对分项系数及可靠度指标的影响规律,并进一步对各指标的灵敏度进行比较分析。分析结果表明,在各个设计参数中,厚土层的强度指标和设计荷载是两个相对重要的参数,在渤海海域中应用API标准中规定的荷载抗力系数进行设计是适用的。     

我国周边海域典型海洋锋特征建模研究

基于海洋锋空间位置、水平分布结构和垂直扩展特征等时空特征参数,结合海洋锋空间结构几何模型,建立了区域海洋锋温盐三维结构快速重构特征模型,对黄海西部沿岸锋和东海黑潮中段锋锋区温度场进行了仿真计算,并与实测数据进行了比较分析,实验结果表明：仿真结果与实测数据符合较好,实验结果验证了特征模型的有效性和可推广性。海洋锋区声速具有明显的水平梯度变化,对声纳的水下探测和反探测产生显著影响,因此,需要建立实时估计获取锋区水下温、盐结构的方法。海洋锋特征模型能够快速有效地重构海洋锋区温度场,为实时获取海洋锋水下结构特征提供了方法。     

波浪驱动式海洋要素垂直剖面持续测量搭载系统

新型海洋要素观测平台能够提升人类对海洋进行持续现场监测的能力.设计并实现了一种借助于波浪能量进行驱动的海洋要素垂直剖面测量搭载系统.对系统的总体构成与工作原理、各部件的设计制作过程、以及系统海试的情况作了论述.海试结果表明,系统设计初步达到了预期的目的,实现无人值守情况下对海洋要素垂直剖面的持续长期观测.     

轨道运动对BiSAR海浪成像的影响

速度聚束调制是研究BiSAR海浪成像机理的一个重要方面,而海浪的轨道运动是速度聚束调制产生的根源。在平飞非等速情况下,本文以单色波为例,建立了含有轨道速度和加速度扰动的海面回波模型,推导了BiSAR海浪图像强度的表达式。当收发平台取适当的参数时,数值计算表明BiSAR是可以对一定条件下的海浪成像,并比较了单站SAR和双站SAR海浪图像的异同点。     

Performance bounds for acoustic tomography in a vertical ocean slice

Reciprocal travel-time data along acoustic multipaths between transceivers in the ocean sound channel can be inverted to form estimates of sound speed and horizontal current in the vertical plane of the transceiver pair. We investigate the quality of such estimates in a range-independent environment. Since most of the information gathered along an acoustic path is concentrated near its turning depths, inversions are characterized by strong sidelobe contamination. Estimates at depths in the upper ocean are contaminated by fields in the lower ocean and vice versa. Therefore estimates can be improved by use of a priori knowledge of contaminating fields at the position of the sidelobe. The Backus-Gilbert method is adapted to allow for use of a priori knowledge and three measures of system performance are defined, viz., resolution length, stochastic error, and ratio of sidelobe to primary lobe acceptance. Trade-off relations among these quantities are calculated for 17 eigenrays in a Munk canonical profile for various orders of a priori knowledge. The limit of perfect knowledge gives an absolute bound on system performance. Numerical examples indicate poor performance in the lower ocean; but good estimates of sound speed and acceptable estimates of horizontal current are feasible in the upper ocean.     

An investigation on the vertical motion sickness characteristics of a high-speed catamaran ferry

Low frequent motions of vessel may cause motion sickness in rough seas. These undesirable effects induce fatigue of crews during the navigation. The motion sickness is always an important criterion for the high-speed craft design. Modern ferry designs have been marketed with a great emphasis on the seakeeping performance. This research has been carried out by investigating the results on the vertical motion sickness incidence (MSI) study for a 40 m wave-piecing catamaran at seas. The primary purpose of this research is to investigate the vertical motion sickness characteristics of a high-speed catamaran ferry. Two mathematical models, three-dimensional translating–pulsating source distribution technique and three-dimensional pulsating source distribution technique, are used for predicting the vertical acceleration responses of the wave-piecing catamaran in oblique waves. The comparison between numerical predictions and experimental data shows a good agreement except that around the pitch resonance region in FP vertical acceleration motions. Based on the experimental observation, the discrepancies may be caused by the nonlinear effects of centre bow during large pitch motions in waves. The comfort assessments are based on the ISO-2631/1997 standard with the hydrodynamic analysis for determining the acceleration levels in different locations on the vessel. The effects of seating location, wave heading and duration of motion exposure on seasickness are discussed.     

Thermohydrodynamics of the ocean generation of seiches by moving baric fronts in bounded basins

We consider a plane problem of barotropic seiches generated by a front of atmospheric pressure moving over a bounded basin. A system of nonlinear equations of long waves is solved by the finite-difference method with regard for the bottom friction and Earth's rotation. The numerical analyses are performed for two basins with distributions of depths typical of the Black Sea. It is shown that the passage of a baric front over the basin leads to the generation of lower seiches. The oscillations of level and the corresponding currents are especially intense in the shallow-water zones of the basins. The seiches become more intense as the velocity of transfer of the atmospheric front increases and the width of the front decreases. Earth's rotation leads to the generation of longshore currents and promotes the process of weakening of residual oscillations of the fluid following the passage of the front. The influence of nonlinearity on seiches is small for the analyzed basins. __________ Translated from Morskoi Gidrofizicheskii Zhurnal, No. 2, pp. 3–18, March–April, 2008.     

海洋模式中垂直混合参数化方案介绍

介绍了海洋垂直混合过程参数化方案的发展,以及不同参数化方案在海洋模式中的应用情况。首先,介绍不同垂直混合参数化方案的物理问题、理论依据、数学表达和特征,并对不同参数化方案进行了比较。其次,针对中尺度涡、亚中尺度涡以及波浪、潮流混合参数化的最新研究进展进行了总结并对垂直混合参数化的未来发展提出了一些建议。     

An integrated dynamic model of ocean mining system and fast simulation of its longitudinal reciprocating motion

An integrated dynamic model of China’s deep ocean mining system is developed and the fast simulation analysis of its longitudinal reciprocating motion operation processes is achieved. The seafloor tracked miner is built as a three-dimensional single-body model with six-degree-of-freedom. The track-terrain interaction is modeled by partitioning the track-terrain interface into a certain number of mesh elements with three mutually perpendicular forces, including the normal force, the longitudinal shear force and the lateral shear force, acting on the center point of each mesh element. The hydrodynamic force of the miner is considered and applied. By considering the operational safety and collection efficiency, two new mining paths for the miner on the seafloor are proposed, which can be simulated with the established single-body dynamic model of the miner. The pipeline subsystem is built as a three-dimensional multi-body discrete element model, which is divided into rigid elements linked by flexible connectors. The flexible connector without mass is represented by six spring-damper elements. The external hydrodynamic forces of the ocean current from the longitudinal and lateral directions are both considered and modeled based on the Morison formula and applied to the mass center of each corresponding discrete rigid element. The mining ship is simplified and represented by a general kinematic point, whose heave motion induced by the ocean waves and the longitudinal and lateral towing motions are considered and applied. By integrating the single-body dynamic model of the miner and the multi-body discrete element dynamic model of the pipeline, and defining the kinematic equations of the mining ship, the integrated dynamic model of the total deep ocean mining system is formed. The longitudinal reciprocating motion operation modes of the total mining system, which combine the active straight-line and turning motions of the miner and the ship, and the passive towed motions of the pipeline, are proposed and simulated with the developed 3D dynamic model. Some critical simulation results are obtained and analyzed, such as the motion trajectories of key subsystems, the velocities of the buoyancy modules and the interaction forces between subsystems, which in a way can provide important theoretical basis and useful technical reference for the practical deep ocean mining system analysis, operation and control.     

Intrusion layering and vertical diffusion in the ocean owing to tidal mixing at a sloping bottom

The empirical and analytical relationships between the vertical scale of intrusions, which are formed owing to the layering of the mixed boundary layer at a sloping bottom, and the determining parameters are obtained on the basis of field measurements in the coastal zone and energy considerations. An estimate of the vertical diffusion coefficient is given which allows us to determine the role of tidal mixing at the rigid boundaries as a mixing factor for the entire world's oceans.Translated by Mikhail M. Trufanov.     

A model of ocean current flow and drifter motion

A minor generalization of the theory of random walk is used as a basis for a model of ocean current flow. The model is then applied in a computer simulation of drifter motion. The results of simulation indicate that the geometry of a coastline can have significant impact on the distribution of drifter landings.     

An ocean model for climate variability studies

Models of the time dependent ocean circulation can be simplified considerably by filtering out all short term, small scale motions which are unimportant for climatic processes. For time scales large compared with a day and space scales large compared with the internal Rossby radius of deformation (～50 km), the currents in most of the interior ocean can be determined diagnostically as quasi-equilibrium fields, so that only the salinity and temperature fields need be treated prognostically.Regions of closed f/h contours, however, represent exceptions. Here trapped vorticity gyres exist as free flow solutions without external forcing, and in the presence of forcing the barotropic velocity field must therefore be determined prognostically through a potential vorticity equation for the gyres.Lateral boundary layers and the equatorial regions also require separate treatment. These were not considered specifically, but it is suggested that integrated (parametrical) models analogous in structure to mixed-layer models or the integrated boundary layer models of aerodynamics may be the most appropriate technique for coupling these regions to the interior ocean in a comprehensive ocean model suitable for climate studies.A coupled multi-region model of the global ocean circulation based on these scale considerations could be sufficiently cost-effective to permit systematic investigation of the role of the oceanic heat storage and transport in climate variability studies over a wide spectrum of space and time scales.The analysis of the seasonal variations of the interior ocean circulation represents a simple example in which the filtered model yields considerably simpler and more readily interpretable results than a fully three-dimensional, unfiltered model.