Determination of Ocean Wave Period from Altimeter Data Using Wave-Age Concept

This study makes use of the concept of wave age in estimating ocean wave period from space borne altimeter measurements of backscattering coefficient and significant wave height. Introduction of wave age allowed better accounting of the difference between swells and wind waves. Using two years (1998 and 1999) data of TOPEX/Poseidon altimeter and ocean data buoy observations in the Indian Ocean, coefficients were generated for wave period, which were subsequently tested against data for the years 2000 and 2001. The results showed the wave period accuracy to be of the order of 0.6 sec (against 1.3 sec obtained with the semiempirical approach, reported earlier).     

赤道中、东太平洋表层流速20d振荡特征分析

通过对1985年1月1日—1986年12月31日沿赤道5个锚定浮标站表层流速资料的分析,发现在140°W与108°W之间表层流速v存在一周期约为20d的显著振荡。该振荡是由波长约2000km、以1.15-1.23m／s的波速向西传播的波动引起的。该波动被推断为第二斜压模态混合Rossby惯性重力波。带通滤波和低通滤波结果表明,以110°W测站为例,20d振荡流速构方根为21.8cm／s（纬向）和22.1cm／s（经向）;单一流速振幅的特征值为30-50cm／s,最大振幅可达70cm／s;u季节变化的均方根小于17cm／s,v无明显季节变化。年平均流速通常小于5cm／s。以上各统计量表明,20d周期波动引起的v振荡在赤道东太平洋表层流速变化中非常显著。     

Generating Velocities in Precursor Soliton Generation in a Straight Channel with Arbitrary Cross Section

利用准三维的ｆＫｄＶ方程及四区间划分法,理论上求得了先锋孤立子生成诸速度—先锋孤立子的运动速度、压水区中流的速度及尾波列第一个跨零点的速度。所述理论是Ｘｕ等人的先锋孤立子生成理论的一个推广。作为应用的例子,运用作者得到的理论结果,对具有相同截面面积的等腰三角形和正方形截面的直渠中先锋孤立子生成的诸速度进行了理论预报和数值比较。比较指出,理论和数值结果吻合得很好。     

畸形波作用下四立柱张力腿平台动力响应研究

介绍了畸形波生成的方法并对各种线性合成畸形波的效率进行了对比。选用改进的随机波加瞬态波的方法模拟强非线性波———"新年波"的波形。采用微元法对张力腿平台的立柱和浮筒进行离散,编制程序对张力腿平台在强非线性波作用下的耦合动力响应进行了数值模拟,重点比较分析了张力腿平台在随机波及畸形波中所受波浪力、平台动力响应、系泊系统张力特性及浪向角对平台运动的影响。研究表明:在"新年波"作用下,0°浪向时,平台在x和z方向所受波浪力较随机波增长了约1/4,纵荡、垂荡及纵摇响应值分别增加了33%,38%和12%,张力腿张力幅值增加约20%,位于平台四角的张力腿张力有所差异,相邻两根张力腿的张力差别不大,浪向角对平台运动响应、张力腿张力的影响畸形波大于随机波的作用。为今后考虑强非线性波浪影响时张力腿平台的设计提供了借鉴和参考。     

Interaction of flexural gravity waves with shear current in shallow water

In the present study, the effect of shear current on the propagation of flexural gravity waves is analyzed under the assumptions of linearized shallow-water theory. Explicit expressions for the reflection and transmission coefficients associated with flexural gravity wave scattering by a step discontinuity in both water depth and current speed are derived. Further, trapping and scattering of flexural gravity waves by a jet-like shear current with a top-hat profile are examined and certain limiting conditions for the waves to exist are derived. The effects of change in water depth, current speed, incident wavelength and the angle of incidence on the group and phase velocities as well as on the reflection and transmission characteristics are analyzed through different numerical results.     

Analytical method of radiation by a wave energy device with dual rectangular floating bodies

The system with one floating rectangular body on the free surface and one submerged rectangular body has been applied to a wave energy conversion device in water of finite depth. The radiation problem by this device on a plane incident wave is solved by the use of an eigenfunction expansion method, and a new analytical expression for the radiation velocity potential is obtained. The wave excitation force is calculated via the known incident wave potential and the radiation potential with a theorem of Haskind employed. To verify the correctness of this method, an example is computed respectively through the bound element method and analytical method. Results show that two numerical methods. are in good agreement, which shows that the present method is applicable. In addition, the trends of hydrodynamic coefficients and wave force are analyzed under different conditions by use of the present analytical method.     

Onshore sandbar migration at Tairua Beach (New Zealand): Numerical simulations and field measurements

We observed the onshore migration (3.5 m/day) of a nearshore sandbar at Tairua Beach, New Zealand during 4 days of low-energy wave conditions. The morphological observations, together with concurrent measurements of waves and suspended sediment concentrations, were used to test a coupled, wave-averaged, cross-shore model. Because of the coarse bed material and the relatively low-energy conditions, the contribution of the suspended transport to the total transport was predicted and observed to be negligible. The model predicted the bar to move onshore because of the feedback between near-bed wave skewness, bedload, and the sandbar under weakly to non-breaking conditions at high tide. The predicted bathymetric evolution contrasts, however, with the observations that the bar migrated onshore predominantly at low tide. Also, the model flattened the bar, while in the observations the sandbar retained its steep landward-facing flank. A comparison between available observations and numerical simulations suggests that onshore propagating surf zone bores in very shallow water (< 0.25 m) may have been responsible for most of the observed bar behaviour. These processes are missing from the applied model and, given that the observed conditions can be considered typical of very shallow sandbars, highlight a priority for further field study and model development. The possibility that the excess water transported by the bores across the bar was channelled alongshore to near-by rip-channels further implies that traditional cross-shore measures to judge the applicability of a cross-shore morphodynamic model may be misleading.     

mKdV-sine-Gordon方程丰富的相互作用解

基于mKdV-sine-Gordon方程的Wronsk解的形式和结构,提出了Wronsk形式展开法,通过这一方法求得了该方程的丰富的相互作用解,该方法的主要特征是不要求Wronsk行列式元素满足线性偏微分方程组。     

The statistical distribution of nearbed wave orbital velocity in intermediate coastal water depth

The statistical distribution of wave orbital velocity in intermediate coastal water depth has been quantitatively determined from the comprehensive field velocity data collected near the seabed in this study. Two ocean ADV current meters, which were mounted at 0.5 m above the seabed on two separate stainless steel tripods sitting on the seabed, were used to measure instantaneous water particle velocities at a 2 Hz sampling rate for 17.07 min every hour in two coastal water depths of 11 m and 23 m in nine field deployments over a period of 2 years. The zero-crossing method is applied to analyse the field velocity data collected in each field deployment to obtain a large sample of wave orbital velocity amplitudes of individual waves. Based on the collected field velocity data, it is found that the histogram of instantaneous wave orbital velocities perfectly follows the Gaussian distribution as commonly assumed, while the histogram of wave orbital velocity amplitudes is less accurately described by the Rayleigh distribution than the modified Rayleigh and the Weibull distribution. It is also found that large orbital velocity amplitudes are generally overestimated by the Rayleigh distribution, but well predicted by the modified Rayleigh and the Weibull distribution. The expected value of maximum orbital velocity in a velocity record of finite size is also derived from the three distributions and found to agree well with the present field data.     

Reevaluation of historical ocean heat content variations with time-varying XBT and MBT depth bias corrections

As reported in former studies, temperature observations obtained by expendable bathythermographs (XBTs) and mechanical bathythermographs (MBTs) appear to have positive biases as much as they affect major climate signals. These biases have not been fully taken into account in previous ocean temperature analyses, which have been widely used to detect global warming signals in the oceans. This report proposes a methodology for directly eliminating the biases from the XBT and MBT observations. In the case of XBT observation, assuming that the positive temperature biases mainly originate from greater depths given by conventional XBT fall-rate equations than the truth, a depth bias equation is constructed by fitting depth differences between XBT data and more accurate oceanographic observations to a linear equation of elapsed time. Such depth bias equations are introduced separately for each year and for each probe type. Uncertainty in the gradient of the linear equation is evaluated using a non-parametric test. The typical depth bias is +10 m at 700 m depth on average, which is probably caused by various indeterminable sources of error in the XBT observations as well as a lack of representativeness in the fall-rate equations adopted so far. Depth biases in MBT are fitted to quadratic equations of depth in a similar manner to the XBT method. Correcting the historical XBT and MBT depth biases by these equations allows a historical ocean temperature analysis to be conducted. In comparison with the previous temperature analysis, large differences are found in the present analysis as follows: the duration of large ocean heat content in the 1970s shortens dramatically, and recent ocean cooling becomes insignificant. The result is also in better agreement with tide gauge observations. On leave from the Meteorological Research Institute of the Japan Meteorological Agency.