On the origin of the inverted-barometer effect at subinertial frequencies

Summary An analytical model, simulating the frictionless response of the sea contained in a rotating, rectangular channel of arbitrary width to air pressure waves travelling at varying directions, is developed. Since planetary atmospheric waves are of primary interest as forcing agents, a solution is found for subinertial frequencies. For an atmospheric wave travelling along a channel whose width is close to the Rossby deformation radius, the model predicts sea levels and currents organized in two coastal waves and a geostrophic current system prevailing in mid-basin. The right-hand coastal wave is more pronounced than the left-hand wave. The structure is coupled to the atmospheric wave, and is resonantly driven when the phase velocity of the forcing wave approaches the Kelvin wave velocity. Along the coasts a quasi-static adjustment occurs under off-resonant conditions. When the atmospheric wave is moving across the channel at a sharp angle, the response of the sea is enhanced for the apparent along-channel velocities below those of free shallow-water waves, due to reflections at channel boundaries. For the atmospheric wave that travels at right angle across the channel, the resonance is not possible, and the sea level undershoots a simple inverted-barometer response. Both travelling and standing waves appear in the channel. In the narrow-channel limit only a standing wave remains, with a nodal line in the middle of the channel. In the central part of the channel the currents are almost geostrophic at very low frequencies. The model is used to interpret some aspects of the response of the Mediterranean Sea to planetary-scale atmospheric forcing. In particular, it is shown that resonant transfer of energy from the atmosphere to the sea is most unlikely, since planetary atmospheric waves are rather slow and they travel along the main axis of the Mediterranean basin.     

Characteristics of the reverberation signal at a vertical array with tonal insonification of a shallow-water basin

The angular and spectral characteristics of reverberation signals on a fixed path are studied by using vertical receiving arrays with a tonal insonification of the basin. The choice of the frequency range F ～ 200–300 Hz is caused, first, by the low propagation loss and the availability of high-power acoustic transducers for this range and, second, by the possibility to study the phenomena of sound scattering by the sea surface for comparable wavelengths of surface and acoustic waves. The data of experiments on two paths are considered: the first path is in the Barents Sea with propagation conditions corresponding to a uniform waveguide with sea depths H ～ 110–120 m, whereas the second path is in the White Sea with propagation conditions corresponding to a nonuniform waveguide where the sea depth varies from 30 to 250 m.     

Experimental and theoretical study of the vertical coherence of the sound field in a shallow sea

The combined effect of multiple scattering by random inhomogeneities of a waveguide and bottomcaused sound absorption is known to be a fundamental factor that governs the formation of the sound field in a shallow sea [1, 2]. A number of publications [2–6] present statistical analyses of the fluctuation phenomena that accompany the sound propagation in shallow sea regions. In these publications, most attention is paid to studying the evolution of the intensities of normal waves under the effect of scattering by the random field of internal waves. It is shown that in natural conditions, in addition to the inhomogeneities of the water column, one should take into account the irregular boundaries of the sound channel, which can also affect the correlation characteristics of the sound field. In this paper, we present experimental data on the vertical coherence of the sound field on a fixed path in the Barents Sea. We also compare the experimental data with theoretical calculations based on a model of sound scattering by the rough sea surface.     

Long-range reverberation in a randomly inhomogeneous shallow water with the use of focused radiation

The low-frequency bottom reverberation in a randomly inhomogeneous shallow water is investigated within the framework of a numerical experiment using vertical transmitting arrays focusing the acoustic field at various distances from the sea bottom. It is assumed that the main source of sound velocity fluctuations in the medium is represented by background internal waves. To focus the field, a phase conjugation of acoustic waves from a probe source positioned at the focusing point is used. It is demonstrated that the reverberation level is mainly determined by the presence of internal waves and may vary by 5–20 dB as the distance from the focusing point to the sea bottom increases up to H/2, where H is the channel depth.     

The effect of slope of island reef on the deep sea ambient noise

Considering the effect of the slope of reef area,the phenomenon that the noise level in the deep sea area of the South China Sea increases with depth is analyzed theoretically.The experiment on ambient noise in the deep sea area of the northern South China Sea shows that the noise spectrum level changes little with depth(160 m to 800 m) for most of the time,but at some time,the phenomenon that the noise spectrum level increases with depth would arise.Considering the effect of topography on sound propagation in the South China Sea,the ambient noise model of deep sea area in the South China Sea is developed.When a ship passes the sea around the reef or island,the ship noise spreads into the deep sea sound channel due to the terrain near the reef or island such that the distant noise spectrum level increases with depth.The distribution of noise spectrum level with depth can be computed by using the noise model,and it has good agreement with the experimental data.So the variation characteristics of deep sea noise affected by the slope of island reef in the South China Sea are verified.     

Long-range sound propagation in the shallow-water part of the Sea of Okhotsk

Experimental data on the long-range propagation of explosion-generated sound signals in the shallow-water northern part of the Sea of Okhotsk are analyzed. The propagation conditions in this region are characterized by a fully-developed underwater sound channel that captures the rays crossing the channel axis at angles lower than 3°. The experimental data reveal a small increase in the duration of the sound signal in proportion to the range with the proportionality factor lower than 0.00025 s/km. The frequency dependence of attenuation exhibits a pronounced minimum whose position on the frequency axis is close to the critical frequency of the first “water” mode (about 160 Hz). The increase in the attenuation coefficient at lower frequencies is confirmed by the field calculations performed with the wave-field computer code and is explained by the sound energy loss in the bottom sediments. At frequencies higher than 200 Hz, as in the Baltic Sea, the most probable reason for the attenuation to exceed the absorption in sea water is sound scattering by internal waves.     

不同海面风速对量子卫星星舰通信性能的影响

量子卫星星舰通信是量子保密通信的重要应用场景之一,在海面上,由于不同风速所引起的气溶胶粒子浓度发生剧烈变化,而气溶胶粒子浓度的剧变,必然导致星舰量子链路性能的剧烈衰减.然而,有关不同海面风速与量子卫星星舰通信信道参数关系的研究,迄今尚未展开.本文根据海面风速与气溶胶的Gras模型,分别建立了风速与星舰量子信道误码率、信道容量和信道平均保真度的定量关系.仿真结果表明,当风速分别为4 m/s和20 m/s时,海洋大气信道误码率、信道容量、信道平均保真度分别依次为4.62×10^-3和4.91×10^-3、0.957和0.65、0.999和0.974.由此可见,风速对海上量子通信性能有显著的影响.因此,为了提高通信的可靠性,应根据风速大小,自适应调整系统的各项参数.     

岛礁区域斜坡地形对深海环境噪声级深度分布的影响

考虑岛礁区域斜坡地形的影响,从理论上分析了南海深海区域噪声级随深度增加而变大的现象。南海北部深海区域环境噪声测量实验表明:大部分时间内噪声谱级随深度变化较小(160～800 m),但是在一些时间里,噪声谱级会出现随深度增加而变大的现象。考虑南海岛礁区域斜坡地形对声传播的影响,建立了南海深海区域的环境噪声模型,当航船经过岛礁附近时,航船噪声由于岛礁附近斜坡地形的作用进入深海声道,使远距离噪声谱级随深度增加而变大。利用模型计算噪声谱级随深度的分布,结果与实验数据符合较好。验证了南海深海区域噪声在岛礁斜坡地形影响下的复杂的变化特性。     

Tides and sea level evolution at Alexandria (Egypt)

Summary The harmonic constants of the tides at Alexandria, in the Eastern Mediterranean Sea, have been recalculated by means of a new method. Long period evolution of the sea level in the years 1985–1986 has been also considered.     

GNSS Reflectometry of the Black Sea Level in the Experiments at the Stationary Oceanographic Platform

The results of sea level estimation by GNSS reflectometry (GNSS-R) in the Black Sea in the experiments at the stationary oceanographic platform are presented. The influence of the layout of the antennas and the characteristics of the wind waves on the GNSS-R sea level estimates is studied with the use of multi-antenna GPS/GLONASS receiver and wire resistance wave gauge. For the selected geometry of the experiment, the threshold amplitudes and frequencies of the wind waves at which estimation of the sea level by GNSS reflectometry becomes impossible are determined.     

Chemical waves as a result of instability in reaction-diffusion systems

Many-component reaction-diffusion systems are shown to be able to undergo the instability, leading to the spontaneous formation of waves. In the one-dimensional case the general equations, governing the dynamics in the neighbourhood of the wave-type instability point, are derived. The investigation of all steady state solutions of these equations shows that depending on the magnitude of only one essential parameter stable are either a uniform running wave and “leading center” regime or a uniform standing wave. All other solutions are unstable.     

深海不完整声道下反转点会聚区研究

近期南海远程声传播实验数据的处理分析表明在深海不完整声道中声道轴以下存在一种会聚区,该会聚区相比于海面附近的上反转点会聚区在远距离处具有更高的会聚增益.本文利用射线简正波理论确定了水中反转型焦散线和海面反射型焦散线位置,对比发现实验中观测到的深海大深度会聚区位置与水中反转型焦散线位置一致,证明该会聚区是由大量简正波同相叠加形成的下反转点会聚区,其在深海声道轴以下的一定深度范围内都具有会聚效应,研究了该会聚区的形成条件以及声源深度变化对会聚区焦散结构的影响,对比了远距离处上下反转点会聚区的传播损失以及会聚区宽度,分析表明第七个下反转点会聚区的会聚增益仍不小于10 dB,研究了声速垂直结构变化对下反转点会聚区的影响,理论分析结果与实验数据吻合较好.     

Numerical time reversal of waves

A numerical time reversal of waves is proposed instead of the conventional time reversal of wave procedure used in underwater acoustics. In the numerical method, the test sound source and the receiving arrays are used, as in the conventional method, but the transmission of the received signals after their time reversal into the same medium, as well as the measurement of the field obtained in this way at the point of the test source, is replaced by computations. To use the proposed technique for obtaining the same results as those provided by the conventional time reversal of waves, the teset source should be placed at different depths. A simplified numerical algorithm with the test source operating at a single depth is proposed and justified. This version of the time reversal of waves is successfully applied to the experiment in the Barents Sea. In contrast to the conventional method, the proposed technique allows one to study the stability of the sea medium with currents.     

Effect of tide on sound propagation in the shelf zone of the Sea of Japan

Experimental and numerical studies of the effect of surface and internal tides on 315-Hz sound waves propagating along fixed paths, 260 m to 23 km in lengths, oriented across the shelf of the Sea of Japan, are discussed. The measurements are performed using self-contained radio-hydroacoustic receiving stations, which are equipped with hydrophones and scalar-vector receivers, and two vertical acoustic-hydrophysical measuring systems. For the sound signals propagating along the longer paths, the intensity fluctuations are shown to loose their linear relation to the tide-caused changes in the waveguide parameters because of the refraction by the sound speed inhomogeneities induced by different hydrodynamic processes. However, it is established that the phase variations can serve as quantitative indicators of the integral changes in the waveguide parameters.     

Effects of wind waves on horizontal array performance in shallow-water conditions

We analyze the influence of statistical effects of the propagation of an acoustic signal excited by a tone source in a shallow-water channel with a rough sea surface on the efficiency of a horizontal phased array. As the array characteristics, we consider the angular function of the array response for a given direction to the source and the coefficient of amplification of the signal-to-noise ratio (array gain). Numerical simulation was conducted in to the winter hydrological conditions of the Barents Sea in a wide range of parameters determining the spatial signal coherence. The results show the main physical effects of the influence of wind waves on the array characteristics and make it possible to quantitatively predict the efficiency of a large horizontal array in realistic shallow-water channels.     

阻抗劈绕射对破碎波后向散射特性的影响

海浪的破碎区会导致海面电磁散射特性发生很大改变,导致海尖峰现象的产生.本文结合阻抗劈结构模型分析了劈绕射对破碎波后向散射特性的影响.首先利用基尔霍夫近似求解破碎波的物理光学场;基于Maliuzhinets方法,从波动方程及精确阻抗边界条件出发,由谱函数的积分形式得到阻抗劈的一致性绕射系数,结合物理光学绕射系数导出阻抗劈等效边缘电磁流;利用边缘绕射场修正物理光学场,得到考虑劈绕射效应的破碎波散射总场.数值结果表明,阻抗劈的绕射场在Keller锥内出现HH极化大于VV极化的现象,因此计入绕射场的影响会使得破碎波生长到临近坍塌阶段时,小擦地角逆风观测出现总场的后向散射截面HH极化大于VV极化的现象,说明劈绕射是造成海尖峰现象产生的原因之一.     

宽度缓慢均匀变化水槽中的非传播表面孤波

用多重尺度微扰理论导出了宽度均匀缓变水糟中非传播孤波所服从的非线性方程及其解析解。结果指出，孤波恒向宽度较窄的一端近加速移动。加速度正比于宽度的变化率．本文所用的方法原则上可以推广于其他均匀缓变波导中线性波和非线性波的理论研究。 关键词：     

浅海涌浪对表面声道声传播的影响

受海面强风和海-气相互作用影响,表面声道普遍存在于冬季海洋环境中,是一种天然有利于声传播的波导.但是海面波浪使得海表形成粗糙界面,会严重破坏这种优良性能.本文利用南海北部海区的一次冬季声传播实验数据,研究表面声道声传播特性.研究表明,海底底质对表面声道内声传播的影响较弱,当海面风较小时,涌浪造成的影响为主要原因.实验数据显示,考虑涌浪后的粗糙海面给70km远处带来了10dB的传播损失增长.因此在考察南海北部海区冬季声场特性时,不仅要考虑海面风浪的影响,更需要考虑周围海域传来的涌浪的影响.研究涌浪存在时的声传播特性对提升声纳设备在海况较差时的使用性能具有重要意义.     

Nonlinear internal waves over the inclined bottom: Observations with the use of an acoustic profiler

Experimental data on propagation of solitary internal waves on the shelf of the Sea of Japan are presented and analyzed. The data are obtained by using an acoustic Doppler current profiler (ADCP). The process of propagation of the waves over the inclined bottom, up to their coming out to the coastal zone, is discussed for the case in which the pycnocline is closer to the surface than to the bottom. By using the approximation of an adiabatic invariant, it is shown that the height of the internal wave increases as it becomes closer to the overturning point. Qualitative agreement between the experimental data and the calculations is found.     

Freak waves in random oceanic sea states.

Freak waves are very large, rare events in a random ocean wave train. Here we study their generation in a random sea state characterized by the Joint North Sea Wave Project spectrum. We assume, to cubic order in nonlinearity, that the wave dynamics are governed by the nonlinear Schr?dinger (NLS) equation. We show from extensive numerical simulations of the NLS equation how freak waves in a random sea state are more likely to occur for large values of the Phillips parameter alpha and the enhancement coefficient gamma. Comparison with linear simulations is also reported.