The use of a towed side-scan sonar to investigate processes near the sea surface

Side-scan sonars directed upwards from beneath the sea surface have been towed in waters off the west coast of Scotland. The sonographs show echoes from targets identified as subsurface bubble clouds produced by breaking wind-waves. In some conditions organised patterns are seen in the bubble clouds and, in particular, bubbles are accumulated in parallel rows by the effect of Langmuir circulations. The sonar is useful as a diagnostic tool to recognise and quantify these dynamical processes in the upper ocean boundary layer.     

Simulation of the stratospheric gravity waves generated by the Typhoon Matsa in 2005

The generation of stratospheric gravity waves(GWs) due to typhoon is simulated by using a meso-scale model(WRF) with a typhoon case,the Matsa in 2005.An 8-day model run that covers the major stages of the Matsa’s development reproduces the key features of the typhoon.For example,good agreements in the typhoon’s track,the intensity,and the spiral clouds,as well as mean state of stratosphere,are seen between the simulation results and the observation.Simulation results clearly show that with typhoon propagates northwestward,pronounced stratospheric GWs are generated continuously in the vicinity of Matsa.The GWs exhibit the typical curve-like wave fronts away from the Typhoon Matsa,and propagate preferentially in the upstream of the background winds.These characteristics reflect that the stratospheric GWs are closely associated with the typhoon,and thus the GWs are referred to as Tropical Cyclone related Gravity Waves(TC-GWs).The results also show that these waves should have a rather large horizontal scale so that the outmost wave fronts can be seen at the distance of ～1000 km to the typhoon center in the horizontal plane of 20 km.This is consistent with the phenomenon of stratospheric TC-GWs with ～1000 km horizontal scale disclosed by the previous observational analysis results.     

Ship-induced effects on bottom-mounted acoustic current meters in shallow seas

The echo-amplitude of a 23-m-deep bottom-mounted acoustic Doppler current profiler (ADCP) shows regular spikes up to 30 dB above background level when a ship passes nearby, due to deep penetration of bubble clouds. This is evidenced from regularly occurring spikes in echo-data that are simultaneous with ferry crossings in a narrow sea-strait. The bubbles can nearly reach the bottom and are comparable in magnitude to near-bottom scattering off suspended material in vigorous tidal currents exceeding 1 m s⁻¹ in magnitude. The bubble clouds mask the sea surface from the echo-amplitude, which hampers the use of an ADCP for estimating atmospheric parameters and near-surface currents, under such conditions. The echo-spikes associated with the ferry are confirmed with coinciding dips in bottom pressure up to 1200 N m⁻² and with deviations up to 10° in the ADCP's heading due to pressure waves and magnetic field disturbances from under the ferry and from its rear, respectively.     

Sea-floor-mounted rotating side-scan sonar for making time-lapse sonographs

A rotating side-scan sonar system was designed to make time-lapse sonographs of a circular area of the sea floor. To construct the system, the transducers of a commercial side-scan system (frequency 105 kHz; pulse length 0.1 ms; horizontal beam width 1°; vertical beam width 20°; beam depressed 10° with respect to horizontal) were mounted 2 m above the sea floor on a vertical shaft that had a rotation speed of 0.5 rpm.The radially collected sonar images are recorded linearly on a standard side-scan recorder. To convert the linear record to a radial record, the original moving record is photographed through a slit by a rotating camera, exposing a circular image on film.Records that are collected with this system offer several advantages over records that are collected with towed systems. Bottom features are presented in nearly true plan geometry, and transducer yaw, pitch, and roll are eliminated. Most importantly, repeated observations can be made from a single point, and bedform movements of <50 cm can be measured. In quiet seas the maximum useful range of the system varies from 30 m (for mapping ripples) to 200 m (for mapping 10-m wavelength sand waves) to 450 m or more (for mapping gravel patches).     

Sand wave migration near the southeastern corner of Martha's Vineyard,Massachusetts, USA

Sand waves of approximately 2 m in height were observed to migrate nearly 40 m with counterclockwise rotation between two bathymetric surveys performed three months apart near the southeastern corner of Martha's Vineyard, Massachusetts. The region is characterized by strong tidal currents, intermittent energetic surface wave events, and shallow water with local depth ranging from 2 to 7 m. This study uses the process-based model, Delft3D, with a three-dimensional approach to examine the sand wave dynamics by incorporating surface waves, winds, currents, and bathymetric observations. The model successfully simulates sand wave migration in comparisons to observations. Model sensitivity analyses show that the sand wave migration reduces by 65% with the absence of the surface waves. The modeled sand wave migration speed is correlated with the tidal current Shields parameter, and sharp increases in migration speed occur when the wave-driven Shields parameter increases in response to energetic surface wave events. The combined effect of tides, surface waves, and bathymetry is the origin of the rotational aspect of the sand wave, using the Shields parameter as an indicator of tidal currents and surface wave influence on sand wave dynamics.     

A numerical study of the dynamics of the wave-driven circulation within a fringing reef system

The circulation driven by wave breaking, tides and winds within a fringing coral reef system (Ningaloo Reef) in Western Australia was investigated using the ocean circulation model ROMS two-way coupled to the wave model SWAN. Currents within the system were dominantly forced by wave breaking, with flow driven over the shallow reefs and towards the lagoon, which returned to the ocean through channels in the reef. Hindcast model simulations were compared against an extensive field dataset, revealing that the coupled wave–circulation model could accurately predict the waves and currents throughout this morphologically complex reef–lagoon system. A detailed momentum budget analysis showed that, over the reef, a dominant cross-shore balance was established between radiation stress gradients and a pressure (mean water level) gradient (similar to a beach). Within the lagoon, alongshore currents were primarily balanced by alongshore gradients in wave setup, which drove flow towards (and ultimately out) the channels. The importance of these wave-driven currents to Ningaloo Reef was quantified over a full seasonal cycle, during periods when wave and wind conditions significantly differed. These results showed that wave breaking still overwhelmingly dominated the circulation and flushing of Ningaloo Reef throughout the year, with winds playing an insignificant role.     

Why ripples become sweiis,and swells in shallow water decay rapidly

Abstract

In an ocean with a horizontal bottom where no wind is blowing it is shown that the spin (angular momentum) of the ocean is conserved. Thus, when energy is dissipated, at least one of three things will happen: i) Wave spectra may move towards lower frequencies. ii) The directional distribution may be changed towards long-crested waves. iii) Shear currents may be generated. By neglecting ii) and iii), the frequency shift of a spectrum is calculated due to molecular dissipation. When all energy transforming phenomena as e.g. wave breaking and turbulence generation are taken into account, the conservation of spin seems to be able to explain the frequency shift of wave spectra. In shallow water it is shown that there is energy transfer from the waves to shear currents.     

Interactions among waves,current, and mud: Numerical and laboratory studies

Interactions between waves, current, mud and turbulence are very complicated in the coastal and estuarine turbid waters. It is still necessary to improve our understanding of the fundamental physical processes governing the cohesive sediment transport in the coastal and estuarine waters. A numerical model is developed to study the interactions among waves, current, and mud. An eddy viscosity model for wave and current is proposed in order to close the equations of wave motion or of current motion in a combined flow, respectively. The equations of mud transport are derived based on the visco-elastic properties of mud. Coupling the equations of wave motion or of current motion for water layer with those of mud layer can give (1) wave height; (2) distributions of current velocities in the water layer; (3) distributions of transport velocities at the water–mud interface; and (4) distributions of mass transport velocities within the mud layer. These modeled results are in a reasonable agreement with experimental results. Results suggest that (1) the rate of wave attenuation increases in the opposing currents (currents against in the direction in which the waves propagate) and decreases in the following currents (currents in the same direction as that in which the waves propagate); (2) the opposing currents would have more significant effects on the rate of wave height attenuation than the following currents; (3) the effect of current on the rate of wave attenuation on the muddy bottom is larger than that on the rigid bottom; (4) mud transport rate increased in the following currents but decreased in the opposing currents; and (5) the rate of wave height attenuation on the mud bottom is one order of magnitude larger than that on the rigid bottom.     

The influence of background winds and attenuation on the propagation of atmospheric gravity waves

The influence of background winds and energy attenuation on the propagation of atmospheric gravity waves is numerically analyzed. The gravity waves, both in the internal and ducted forms, are included through employing ray-tracing method and full-wave solution method. Background winds with different directions may cause ray paths of internal gravity waves to be horizontally prolonged, vertically steepened, reflected or critically coupled, all of which change the accumulation of energy attenuation along ray paths. Only the penetrating waves propagating against winds can easily reach the ionospheric height with less energy attenuation. The propagation status of gravity waves with different periods and phase speeds is classified into the cut-off region, the reflected region and the propagating region. All the three regions are influenced significantly by winds. The area of the reflected region reduces when gravity waves propagate in the same direction of winds and expands when propagating against wind. In propagating region, the horizontal attenuation distances of gravity waves increase and the arrival heights decrease when winds blow in the same direction of gravity waves, while the attenuation distances decrease and the arrival heights increase when gravity waves propagate against winds. The results for ducted gravity waves show that the influence of winds on waves of lower atmospheric modes is not noticeable for they propagate mainly under mesosphere, where the wind field is relatively weak. However, strong winds at thermospheric height lead to considerable changes of dispersion relation and attenuation distance of upper atmospheric modes. Winds against the wave propagating direction support long-distance propagation of G mode, while the attenuation distances decrease when winds blow in the same direction of the wave. The distribution of TIDs observed by HF Doppler array at Wuhan is compared with the simulation of internal gravity waves. The observation of TIDs shows agreement with our numerical calculations.     

Simulation of the 1953 storm surge in the North Sea

The 1953 North Sea floods, the Big Flood, was one of the worst natural disasters in Europe in modern times and is probably one of the most studied severe coastal floods. Several factors led to the devastating storm surge along the southern North Sea coast in combination of strong and sustained northerly winds, invert barometric effect, high spring tide, and an accumulation of the large surge in the Strait of Dover. However, the storm waves and their roles during the 1953 North Sea storm surge are not well investigated. Therefore, the effect of wave setup due to breaking waves in the storm surge processes is investigated through numerical experiments. A coupled process-based tide-wave-surge model was used to investigate and simulate the storm surge in the North Sea during January 31–February 1, 1953 and validated by comparing with historical water level records at tide gauges and wave observations at light vessels in the North Sea. Meteorological forcing inputs for the period, January 27–February 3, 1953 are reproduced from ERA-20C reanalysis data with a constant correction factor for winds. From the simulation results, it is found that, in addition to the high water due to wind setup, wave setup due to breaking waves nearshore play a role of approximately 10% of the storm surge peaks with approximately 0.2 m. The resulting modeling system can be used extensively for the preparedness of the storm surge and wave of extreme condition, and usual barotropic forecast.     

Influenza meteomorfolitologica inerente alla caratterizzazione del moto ondoso in mare,in correlazione alla piattaforma litorale,alfetch ed alla disintegrazione del-l'energia potenziale dell'onda d'oscillazione in energia cinetica del frangente

Summary After a short reference made to the utility, having regard on maritime structures, and particularly on vertical wall breakwaters, of knowledge on the centurial observations on the storm wave height and the depth of water breaking wave or breaking hydrogeological on the bottom of Mediterranean Sea, because of the subaqueous source, are here shown the correlations among the waves characteristics and the morphological configuration of the continental-shelf, of the fetch, of the winds, of the sedimentation, of the stereophotogrammetry at the stormy sea surface. It is here likewise stated the analogical parallelism between the atomical disintegration and the transformation from potential energy of the oscillatory wave till kinetic energy of breaking waves.     

Equatorial Plasma Bubbles: Effect of Thermospheric Winds Modulated by DE3 Tidal Waves

A hypothesis about the effect of the tropospheric source on the longitudinal distributions of the equatorial plasma bubbles observed in the topside ionosphere was proposed earlier. It was supposed that this influence is transferred mainly by the thermospheric winds modulated by the DE3 tropospheric tidal waves. This conclusion was based on the discovered high degree correlation (R ? 0.79) between the variations of the longitudinal distribution of the plasma bubbles and the neutral atmospheric density. In this work, the hypothesis of the effect of the thermospheric tidal waves on the plasma bubbles at the stage of their generation is subjected to further verification. With this purpose, the longitudinal distributions of the frequency of the plasma bubble observations at the different ionospheric altitudes (~600 km, ROCSAT-1; ~1100 km, ISS-b) are analyzed; their principal similarity is revealed. Comparative analysis of these distributions with the longitudinal profile of the deviations of the zonal thermospheric wind (~400 km, CHAMP) modulated by the DE3 tidal wave is carried out; their considerable correlation (R ? 0.69) is revealed. We conclude that the longitudinal variations of the zonal wind associated with DE3 tidal waves can effect the longitudinal variations in the appearance frequency of the initial “seeding” perturbations, which further evolve into the plasma bubbles.     

Wave- and current-induced bottom shear stress distribution in the Gulf of Lions

Simulations of both currents and waves were performed throughout the year 2001 to assess the relative contribution of each to their overall erosive potential on the Gulf of Lions shelf. Statistical analysis of bottom shear stress (BSS) was compared to sediment grain-size distribution on the bottom. The hydrodynamic features of the bottom layer coincide with the distribution of surficial sediments, and three areas with different hydro-sedimentary characteristics were revealed. (i) The sandy inner shelf (<30 m) area is a high-energy-wave dominated area but may be subjected to intense current-induced BSS during on-shore winds along the coast and during continental winds mainly in the up-welling cells. (ii) The middle shelf (30–100 m) is a low-energy environment characterised by deposition of cohesive sediments, where the wave effect decreases with depth and current-induced BSS cannot reach the critical value for erosion of fine-grained sediments. (iii) The outer shelf, which has a higher bottom sand fraction than the middle shelf, may be affected by strong south-westward currents generated by on-shore winds, which can have an erosive effect on the fine-grained sediments.     

On the modifications of near-inertial waves at fronts: implications for energy transfer across scales

In the ocean, wind-generated kinetic energy (KE) manifests itself primarily in balanced currents and near-inertial waves. The dynamics of these flows is strongly constrained by the Earth’s rotation, causing the KE in balanced currents to follow an inverse cascade but also preventing wave-wave interactions from fluxing energy in the near-inertial band to lower frequencies and higher vertical wavenumbers. How wind-generated KE is transferred to small-scale turbulence and dissipated is thus a non-trivial problem. This article presents a review of recent theoretical calculations and numerical simulations that demonstrate how some surprising modifications to internal wave physics by the lateral density gradients present at ocean fronts allow for strong interactions between balanced currents and near-inertial waves that ultimately result in energy loss for both types of motion.     

Hydrodynamics and sediment-transport in the nearshore of Poverty Bay, New Zealand: Observations of nearshore sediment segregation and oceanic storms

Nearshore regions act as an interface between the terrestrial environment and deeper waters. As such, they play important roles in the dispersal of fluvial sediment and the transport of sand to and from the shoreline. This study focused on the nearshore of Poverty Bay, New Zealand, and the processes controlling the dispersal of sediment from the main source, the Waipaoa River. Hydrodynamics and sediment-transport in water shallower than 15 m were observed from April through mid-September 2006. This deployment afforded observations during 3-4 periods of elevated river discharge and 5 dry storms.Similar wind, river discharge, wave, current, and turbidity patterns were characterized during three of the wet storms. At the beginning of each event, winds blew shoreward, increasing wave heights to 2-3 m within Poverty Bay. As the cyclonic storms moved through the system the winds reversed direction and became seaward, reducing the local wave height and orbital velocity while river discharge remained elevated. At these times, high river discharge and relatively small waves enabled fluvially derived suspended sediment to deposit in shallow water. Altimetry measurements indicated that at least 7 cm was deposited at a 15 m deep site during a single discharge event. Turbidity and seabed observations showed this deposition to be removed, however, as large swell waves from the Southern Ocean triggered resuspension of the material within three weeks of deposition. Consequently, two periods of dispersal were associated with each discharge pulse, one coinciding with fluvial delivery, and a second driven by wave resuspension a few weeks later. These observations of nearfield sediment deposition contradict current hypotheses of very limited sediment deposition in shallow water offshore of small mountainous rivers when floods and high-energy, large wave and fast current, oceanic conditions coincide.Consistently shoreward near-bed currents, observed along the 10 m isobath of Poverty Bay, were attributed to a combination of estuarine circulation, Stokes drift, and wind driven upwelling. Velocities measured at the 15 m isobath, however, were directed more alongshore and diverged from those at the 10 m isobath. The divergence in the currents observed at the 10 and 15 m locations seemed to facilitate segregation of coarse and fine sediment, with sand transported near-bed toward the beach, while suspended silts and clays were exported to deeper water.     

Shallow water sea-floor morphologies around Asinara Island (NW Sardinia,Italy)

Side scan sonar, high-resolution seismic and multibeam data collected off the coast of Asinara Island in the northernmost part of Sardinia have revealed the occurrence of four distinct acoustical patterns that can be related to four main sea-bed types. The four facies represent fine sand (Type A), medium to coarse sand (Type B), patchy sand cover between basement outcrops (Type C) and a predominantly sandy sea-floor colonized by the seagrass Posidonia Oceanica (Type P). The distribution of the sea-bed types indicates that the study area is characterized by a rather complex morphological setting. A remarkable physiographic difference exists between the eastern and western sector of the island, with the latter being affected by strong, westerly winds, mainly the so-called “Mistral”. The wave climate in this area is dominated by wind waves, which can reach a maximum height of 8 m during winter storms. Sediments are mobilized at water depths greater than 30 m from the sea surface during these energetic oceanographic regimes. We infer that dune fields and sand ribbons are deposited by strong offshore flows along the sea-bed resulting as a consequence of wave- and wind-induced onshore water transport.     

地震体波斜入射下的断层台阶地震反应分析

应用显式有限元法对倾斜断层台阶在地震体波斜入射下的地震反应进行了数值模拟。模拟结果表明，P波入射时的竖向运动分量和SV波入射时的水平向运动分量在台阶上角点处存在极大值、在台阶下角点处存在极小值，同时在上下台面还出现由角点激发、向台阶外传播的转换体波与面波。这一结果说明，在断层台阶场地建设重大工程时应离开上角点一定距离，并考虑上下水平表面地面运动的差异性。     

Near-inertial currents in the DeSoto Canyon region

Near-inertial currents in the DeSoto Canyon region are described using current and wind observations taken between April 1997 and March 1998 for the “DeSoto Canyon Eddy Intrusion Study”. Distinct energy peaks are present at near-inertial frequencies for the clockwise spectrum and there is little energy at the same frequencies for the counterclockwise current spectrum. In this region, amplitudes of the near-inertial currents can be as high as 40 cm s⁻¹. These currents are surface-intensified and display an increase in amplitude from the shelf break to offshore. Between November 1997 and March 1998, they were effectively generated by shifting winds accompanying passages of cold fronts. For this time period, near-inertial currents are reasonably well-simulated by a mixed-layer model forced by observed winds. During summer 1997, however, enhanced near-inertial motions often resulted from resonance between winds and existing currents.     

基于含气泡液体声波方程的海底冷泉数值模拟

海底冷泉羽状流与海底天然气水合物的分布密切相关,对水合物稳定带的边界具有指示作用,是未来能源勘探的重要领域.研究海底冷泉羽状流的地震响应特征,对确定天然气水合物的储集区域及成藏环境等均有重要意义.当前获得海底冷泉羽状流的地震响应主要通过数值模拟进行,然而该过程所依据的含气泡介质声速模型及随机介质理论不能完整描述海底冷泉的物理性质,采用的声波方程也不适用于高频地震波数值模拟.为了准确地实现海底冷泉羽状流地震波数值模拟,精确分析其地震响应特征,本文提出利用Keller-Miksis气泡振动模型来描述气泡在声波作用下的运动状态,同时考虑气泡间的相互作用,建立海底冷泉气泡模型.在此基础上,本文创新性地采用含气泡液体声波方程进行海底冷泉高频地震波数值模拟.数值模拟结果表明,本文提出的方法能够实现海底冷泉羽状流地震响应的高精度数值模拟.     

The influence of gas bubbles on sediment acoustic properties: in situ, laboratory, and theoretical results from Eckernförde Bay, Baltic sea

Acoustic turbidity caused by the presence of gas bubbles in seafloor sediments is a common occurrence worldwide,but is as yet poorly understood. The Coastal Benthic Boundary Layer experiment in the Baltic off northern Germany was planned to better characterize the acoustic response of a bubbly sediment horizon. In this context, in situ measurements of compressional wave speed and attenuation were made over the frequency range of 5–400 kHz in gassy sediments of Eckernförde Bay. Dispersion of compressional speed data was used to determine the upper limit of the frequency of methane bubble resonance at between 20 and 25 kHz. These data, combined with bubble size distributions determined from CT scans of sediments in cores retained at ambient pressure, yield estimates of effective bubble sizes of 0.3–5.0 mm equivalent radius. The highly variable spatial distribution of bubble volume and bubble size distribution is used to reconcile the otherwise contradictory frequency-dependent speed and attenuation data with theory. At acoustic frequencies above resonance (>25 kHz) compressional speed is unaffected by bubbles and scattering from bubbles dominates attenuation. At frequencies below resonance (<1 kHz) ‘compressibility effects’ dominate, speed is much lower (250 m s^-1) than bubble-free sediments, and attenuation is dominated by scattering from impedance contrasts. Between 1.5 and 25 kHz bubble resonance greatly affects speed and attenuation. Compressional speed in gassy sediments (1100–1200 m s^-1) determined at 5–15 kHz is variable and higher than predicted by theory (<250 m s^-1). These higher measured speeds result from two factors: speeds are an average of lower speeds in gassy sediments and higher speeds in bubble-free sediments; and the volume of smaller-sized bubbles which contribute to the lower observed speeds is much lower than total gas volume. The frequency-dependent acoustic propagation is further complicated as the mixture of bubble sizes selectively strips energy near bubble resonance frequencies (very high attenuation) allowing lower and higher frequency energy to propagate. It was also demonstrated that acoustic characterization of gassy sediments can be used to define bubble size distribution and fractional volume.