A method for evaluating statistical errors associated with logarithmic velocity profiles

A logarithmic velocity profile is often fitted to velocity data in order to calculate the friction velocity (u _*) and typify the surface texture by a roughness length (z _o ). A method is given for estimating the errors in these parameters as calculated by this method. An example is given in which the size of the error is compared with the fluctuations that typically occur in the time seriesu _*(t) andz _o (t).     

实验水槽常规流速模拟的缩尺方法

提出了一种实验室水槽中流速模拟的方法,该方法可根据需要提供合适的模型缩尺比,用来完成相关海洋工程的模型水动力性能实验,可以近似地模拟出实际设备在水中的工作状态,为在实验水槽中模拟水流动力参数提供有效的参考,方便水槽中的科学实验的开展。     

海岸盐沼冠层水流平均流速分布的实验研究

利用室内水槽测量海岸盐沼植物冠层水流平均流速分布，对a)不同属种,b)同属种不同观测位置和流速；c)同属种不同高度;d)同属种不同密度的海岸盐沼植物冠层湍流结构作了分析，产探讨了冠层植物本身及其水流结构对粘性泥沙运动的可能影响；实验结果揭示了冠层上，下水流的复杂性；1）明显的三层结构，即冠层底部，过渡层，冠层上部水体层；2）由于植物的柔弹性，冠层顶部与上层水体相互作用可能产生界面波，导致一个次级流速最大值，在冠层内出现流速反转梯度；3）冠层顶部以上的水体水流切变流速u,（切应力τb)大于冠层内的ux(切应力τb)，冠层内（底部）水流切变流速（切应力τb)的减小有利于粘性泥沙的沉积；4）植物冠层影响湍流强度和扩散。     

A new method for weakening the combined effect of residual errors on multibeam bathymetric data

Multibeam bathymetric system (MBS) has been widely applied in the marine surveying for providing high-resolution seabed topography. However, some factors degrade the precision of bathymetry, including the sound velocity, the vessel attitude, the misalignment angle of the transducer and so on. Although these factors have been corrected strictly in bathymetric data processing, the final bathymetric result is still affected by their residual errors. In deep water, the result usually cannot meet the requirements of high-precision seabed topography. The combined effect of these residual errors is systematic, and it’s difficult to separate and weaken the effect using traditional single-error correction methods. Therefore, the paper puts forward a new method for weakening the effect of residual errors based on the frequency-spectrum characteristics of seabed topography and multibeam bathymetric data. Four steps, namely the separation of the low-frequency and the high-frequency part of bathymetric data, the reconstruction of the trend of actual seabed topography, the merging of the actual trend and the extracted microtopography, and the accuracy evaluation, are involved in the method. Experiment results prove that the proposed method could weaken the combined effect of residual errors on multibeam bathymetric data and efficiently improve the accuracy of the final post-processing results. We suggest that the method should be widely applied to MBS data processing in deep water.     

An effective method for shear-wave velocity prediction in sandstones

Marine Geophysical Research - Shear-wave velocity is essential in various seismic exploration applications, including seismic modeling, amplitude variation with offset analysis, multicomponent...     

Three-dimensional instability of shear flows with inflection-free velocity profiles in stratified media with a high Prandtl number

In stably stratified media with a Prandtl number Pr ≫ 1, vertical scales of the density (ℓ) and horizontal velocity variation (L) are quite different, ℓ/L = O(Pr^−1/2) ≪ 1, and this influences the flow stability. In particular, shear flows without inflection points on the velocity profile are unstable even in an ideal incompressible fluid. The maximum instability growth rate for sufficiently small ℓ/L is of the same order as in homogeneous mixing layers, with mainly three-dimensional rather than two-dimensional oscillations increasing in a wide range of parameters. This paper focuses on the three-dimensional instability of such flows. It is shown that the spectrum of unstable oscillations is essentially anisotropic in the case of a relatively weak stratification when the bulk Richardson number J ≤ O[(ℓ/L)^3/2]. The results of the asymptotic analysis are illustrated by calculations for a model flow in a two-layer medium (ℓ = 0) as well as for flows with values of ℓ/L corresponding to a temperature or salinity stratification of the water.     

Semi-probabilistic method for evaluating system redundancy of existing offshore structures

Various forms of recently appeared concepts of structure system redundancy are studied. A new measure for existing structure redundancy is proposed. By introducing the concept of structure system failure at the mechanism level and the concept of equivalent reliability index, the evaluation of safety criteria for existing offshore structures is developed. By employing the method presented in this paper, system redundancy has been evaluated for some numerical examples and satisfactory results have been drawn.     

数值模式与统计模型相耦合的近岸海浪预报方法

针对数值模式和统计模型预报近岸海浪存在的局限性,构建了数值模式和统计模型相耦合的近岸海浪预报框架,在模式计算格点和近岸预报目标点之间定义一个海浪能量密度谱传递系数,通过经验正交函数分解和卡尔曼滤波方法建立传递系数的统计预报模型并与数值模式进行耦合。经过对近岸波浪观测站1a的预报试验表明:该方法能够提高近岸海浪有效波高预报精度,有效波高的均方根误差降低了约0.16m,平均相对误差降低约9%。进一步试验和分析发现,该方法的预报有效时间小于24h,将海浪能量密度谱经过分解后得到的基本模态反映了近岸波侯的主要特征,海浪能量密度谱传递系数的变化体现了波侯的季节变化特点。     

The statistical distributions for the elevation,velocity and acceleration of the surface of wind waves

Water surface elevations(t), vertical surface velocities and vertical surface acceleration of wind-generated waves have been measured in a laboratory wind wave channel by using resistance-type wave gauges combined with an electronic differentiation circuits. Probability distributions of the values of(t), , and have been determined from the wave records.In an initial stage of wave generation,i.e., when wind waves are generated at short fetches and low wind speeds, the observed distributions for(t), and are appreciately good fit to the distributions given by successive sum of a Gram-Charlier series, which has been derived following the formulation ofLonguet-Higgins (1963), by taking the weakly nonlinear effect into account.However, when wind waves develop with increasing wind speeds and fetches, the observed distributions deviate gradually from the Gram-Charlier series. Particularly, the deviations are remarkable for the distribution of .When the wind speed increases, the observed distributions of(t), and show the following characteristics: (i) the skewnesses of the distributions of(t) and decrease slightly, (ii) the skewness of changes, at some wind speed, from positive small values to relatively large negative values, (iii) the kurtosis of the distribution of(t) decreases slightly but that of increases slightly and these characteristics seem to depend not so much on fetches, (iv) the kurtosis of the distribution of increases rapidly.     

Correction method for full-depth current velocity with lowered acoustic Doppler current profiler (LADCP)

A new method is presented to process and correct full-depth current velocity data obtained from a lowered acoustic Doppler current profiler (LADCP). The analysis shows that, except near the surface, the echo intensity of a reflected sound pulse is closely correlated with the magnitude of the difference in vertical shear of velocity between downcast and upcast, indicating an error in velocity shear. The present method features the use of echo intensity for the correction of velocity shear. The correction values are determined as to fit LADCP velocity to shipboard ADCP (SADCP) and LADCP bottom-tracked velocities. The method is as follows. Initially, a profile of velocity relative to the sea surface is obtained by integrating vertical shears of velocity after low-quality data are rejected. Second, the relative velocity is fitted to the velocity at 100–800 dbar measured by SADCP to obtain an “absolute” velocity profile. Third, the velocity shear is corrected using the relationship between the errors in velocity shears and echo intensity, in order to adjust the velocity at sea bottom to the bottom-tracked velocity measured by LADCP. Finally, the velocity profile is obtained from the SADCP-fitted velocity at depths less than 800 dbar and the corrected velocity shear at depths greater than 800 dbar. This method is valid for a full-depth LADCP cast throughout which the echo intensity is relatively high (greater than 75 dB in the present analysis). Although the processed velocity may include errors of 1–2 cm s⁻¹, this method produced qualitatively good current structures in the Northeast Pacific Basin that were consistent with the deep current structures inferred from silicate distribution, and the averaged velocities were significantly different from those calculated by the Visbeck (2002) method.     

Numerical analysis of water exit for a sphere with constant velocity using the lattice Boltzmann method

In this paper, the three-dimensional water exit of a sphere with different vertical velocities is investigated numerically using the lattice Boltzmann method (LBM). In this method, the liquid-gas two-phase flow is simplified as a single-phase free surface flow. To capture the free surface, a mass tracking algorithm is incorporated into the LBM. The gravity as a body force is introduced in the form of calculating the equilibrium distribution with an altered velocity, while the surface tension is neglected. Besides, the employed bounce-back boundary conditions are used for a moving sphere. What’s more, the Wall-Adapting Local Eddy (WALE) viscosity model is employed to capture the turbulent structures of the flow and stabilize the simulation. The accuracy of the numerical results is demonstrated through comparisons with the previous numerical and experimental results in the literature. The results show that the spike height is significantly influenced under the Froude number (Fr) below 4.12 and slightly affected under the Fr varying from 4.12 to 8.24. After the sphere exits water totally, the evolution of the free surface waterfall can be described as two phases and becomes more intense with the Froude number increasing. The non-uniform distribution of velocity results in the breaking of the free surface after the sphere completely exits the water. Moreover, the Reynolds number greatly affects the wake dynamics and hydrodynamics acting on the sphere when it moves beneath the water surface.     

Alkalinity determination in hypersaline solutions: a method for overcoming glass electrode errors

The non-ideality of commercially available glass pH electrode normally used for alkalinity determination by the Gran-type titration has been examined and its effect on the accuracy of the estimated alkalinity evaluated. It is found that slope errors of the glass electrode could be a limiting factor to the accuracy that can be obtained. This error source could be eliminated altogether by applying the proposed ‘slope correction’ algorithm which uses the titration data for in situ slope calibration of the glass electrode. Application of the proposed method eliminates the need for calibration of the glass electrode and the requirement for thermostating the test solution.     

基于遗传BP神经网络的海底沉积物声速预报

在海底沉积物声速预报中,针对传统经验公式存在预测精度差、适用范围窄、缺乏物理意义等问题,在已有BP神经网络预测的基础上,运用遗传算法优化其初始权值和阈值的方法,构建出基于含水量、孔隙度的声速预报模型。将南沙海域采集得到的海底沉积物样品分为两部分,抽取120组涵盖陆架、陆坡、海槽等地貌单元的样品作为训练数据,另外剩余6组作为测试数据。经试验对比后发现,在对本区域进行声速预报时,宜采用遗传算法优化的BP神经网络,其要优于传统的单参数、双参数回归拟合预报方法和国内外其他学者所得到的经验公式。此种预报方法具有一定的科学依据和广泛的应用前景,可在今后为建立明确、统一的声速预报模型提供参考。     

大气垂直速度计算中校正方法的探讨

本文对大气运动中垂直速度ｗ计算中的几种校正方法作了分析比较，并在前人工作的基础上，对校正方法作了改进，提出了进行低通滤波、散度校正和法线风分量校正的方法。通过这些校正，使计算出的平均散度和垂直速度的垂直廓线能维持原计算曲线的形式，并能满足整个气层连续方程积分为零的条件，也符合误差随高度增大的情况。     

A new method of evaluating the mineralization of particulate and dissolved photoassimilated organic matter

A new method of evaluating the rate of mineralization of photoassimilated organic matter is described. This method enables us to compare the rate of direct mineralization of particulate organic carbon (POC) to CO₂ with the rate of solubilization of photoassimilated organic carbon followed by the mineralization of the resultant dissolved organic carbon (DOC) under the same conditions. The direct mineralization of photoassimilated carbon from POC to CO₂ is a more significant process compared with the mineralization of extracellular released organic carbon. The first-order rate coefficients range from 0.132 to 0.434 day^–1 for direct mineralization and 0.034 to 0.189 day^–1 for solubilization.     

Validation of volume continuity method for estimation of cross-shore swash flow velocity

The use of flow volume continuity to estimate depth-averaged cross-shore swash flow velocity is validated using appropriate field data. Swash surface and beach face elevation were measured by a network of ultrasonic altimeters mounted through the swash zone of a sandy beach. Estimates of cross-shore flow velocity derived from these data compare well with concurrent measurements obtained from a vertical array of four electromagnetic current meters (EMCMs). Significantly, the continuity technique allows swash velocity to be estimated throughout the full swash cycle. This is not possible using fixed current meters and the new measurements indicate that swash velocity is more negatively skewed than previously observed.     

A statistical method to get surface level air-temperature from satellite observations of precipitable water

Ten-day mean surface level air-temperature from SSMI precipitable water (SSMI-T _a ) has been derived and compared with the temperature from two ocean data buoys (Buoy-T _a ) of Japan Meteorological Agency (JMA) for a period of six months (July–December, 1988). Statistical relations between air-temperature and mixing ratio, using data from ocean data buoys are used to derive air-temperature from mixing ratio, obtained from SSMI precipitable water. For getting the mixing ratio from precipitable water, regional mixing ratio-precipitable water relations have been used, instead of global relation proposed by Liu (1986). The rms errors (standard deviation of the difference between SSMI-T _a and Buoy-T _a ) for two buoy locations are found to be 1.15 and 1.12°C, respectively. Surface level temperature for the two buoy locations are also derived using direct regression relation between Buoy-T _a and precipitable water. The rms errors of the SSMI-T _a , in this case are found to be reduced to 1.0°C.     

A parametric model for barred equilibrium beach profiles

Environment predictions for locations for which bathymetric data is missing, poor or outdated requires the use of some sort of representative bathymetric form, usually one that is concave up but monotonic. We propose and test a parametric form that superimposes realistic sand bars (Ruessink et al., 2003b) on a background profile that mixes a concave up nearshore with a planar far field behavior. Implementation at any new site involves estimation of five parameters, three that can be found from approximate information from climatology or old offshore charts, one that can be estimated by almost any remote sensing modality and one, h_sea that is less well understood but mostly affects deeper bathymetry that has little impact on the resulting surf zone hydrodynamics. Tests against several hundred surveys at three diverse locations show that bathymetry is better estimated by the new barred form than with a previous monotonic profiles in about 80% of cases. The remaining cases are usually associated with the parametric prediction of bars that look realistic but are out of phase. The presence of parametric bars has an even greater impact on predicted hydrodynamics since wave breaking is concentrated at sand bar locations. Modeled cross-shore transects of alongshore current and wave height over the measured survey profile are well represented by modeled transects over the barred parametric form but not for results over a Dean profile. The peak alongshore current strength and location are particularly sensitive to the presence of a sand bar.