Formation of seep bubble plumes in the Coal Oil Point seep field

The fate of marine seep gases (transport to the atmosphere or dissolution, and either bacterial oxidation or diffusion to the atmosphere) is intimately connected with bubble and bubble-plume processes, which are strongly size-dependent. Based on measurements with a video bubble measurement system in the Coal Oil Point seep field in the Santa Barbara Channel, California, which recorded the bubble-emission size distribution (Φ) for a range of seep vents, three distinct plume types were identified, termed minor, major, and mixed. Minor plumes generally emitted bubbles with a lower emission flux, Q, and had narrow, peaked Φ that were well described by a Gaussian function. Major plumes showed broad Φ spanning very small to very large bubbles, and were well described by a power law function. Mixed plumes showed characteristics of both major and minor plume classes, i.e., they were described by a combination of Gaussian and power law functions, albeit poorly. To understand the underlying formation mechanism, laboratory bubble plumes were created from fixed capillary tubes, and by percolating air through sediment beds of four different grain sizes for a range of Q. Capillary tubes produced a Φ that was Gaussian for low Q. The peak radius of the Gaussian function describing Φ increased with capillary diameter. At high Q, they produced a broad distribution, which was primarily described by a power law. Sediment-bed bubble plumes were mixed plumes for low Q, and major plumes for high Q. For low-Q sediment-bed Φ, the peak radius decreased with increasing grain size. For high Q, sediment-bed Φ exhibited a decreased sensitivity to grain size, and Φ tended toward a power law, similar to that for major seep plumes.     

冷泉活动区气泡羽状流数值模拟的进一步研究

以南海气泡羽状流赋存特征为参考,本文继续研究冷泉活动区气泡羽状流的地震响应。为使羽状流建模方案更合理,根据水中气泡在上升过程时其半径的变化情况,修改原建模方案,重新建立了羽状流水体模型。通过正演模拟得到了羽状流炮集地震记录,单炮记录上显示出明显的散射波场特征和模型的周期特点。通过叠前深度偏移处理炮集地震记录,得到边界收敛效果较好的成像剖面,且成像精度较高。以上研究又一次奠定了羽状流地震响应进一步研究的基础,也逐步探索出更适合羽状流地震资料的处理方法,为识别羽状流提供了理论指导。     

Modelling of underwater bubble plumes and gas dissolution with an Eulerian-Lagrangian CFD model

Underwater bubble plumes emerge from subsea releases of gas. They can be studied by mathematical models. Here an Eulerian-Lagrangian CFD model is presented. It accounts for relevant physics including buoyancy, turbulence, gas expansion and gas dissolution. The model is compared against three sets of experiments. There is consistency between model and experiments. Special focus is given to the effect of gas dissolution. It is demonstrated how the choice of mass transfer coefficient strongly affects the amount of gas dissolution. Since it is not apparent which mass transfer correlation is applicable, it is argued that more experimental data is needed.     

Spectral spreading from surface bubble motion

At low frequencies, surface bubbles contribute to acoustic backscattering in aggregate, and the motion of these bubble masses causes spectral spreading of the acoustic signals. This motion of the bubbles entrained in the surface waves is used to obtain the power spectrum of a low-frequency surface-scattered signal at a low grazing angle. A spectral distribution of the deterministic surface drift, augmented by breaking wave crests, is developed for the wave frequency components that are actively breaking. This motion is combined with the random motion in a wave cycle to predict the spectral widths of low-angle backscattered sound. To permit comparisons with measured data, convolutions of these spectra with simple square pulses of various durations are performed     

南海北部陆坡气泡羽状流的发现:多波束水体数据

利用2016年在南海西北部陆坡琼东南海域采集的多波束水体数据,发现了海底气体渗漏至海水中形成的羽状流。在多波束数据上, 羽状流成火焰状,直径大约为30~50 m,从1 380多米的海底延伸至大约650 m的深度,高度超过700 m。在经过羽状流的浅剖剖面上,存在显示浅层气存在的声学空白区域,并识别出断裂和裂隙区域,但在水体中并无明显的异常。这可能是由于浅剖数据的分辨率不够未能捕捉到水体异常,或者气体渗漏具有间歇性。该海域存在明显的似海底反射显示,气体渗漏可能与水合物系统之间存在复杂的相互作用。由于缺乏经过羽状流的多道地震数据,难以对羽状流的形成机制进行进一步的推测。南海北部陆坡羽状流的发现对于理解被动大陆边缘的甲烷渗漏机制、水合物的形成与分解具有重要的意义。     

Compositional changes in natural gas bubble plumes: observations from the Coal Oil Point marine hydrocarbon seep field

Detailed measurements of bubble composition, dissolved gas concentrations, and plume dynamics were conducted during a 9-month period at a very intense, shallow (22-m water depth) marine hydrocarbon seep in the Santa Barbara Channel, California. Methane, carbon dioxide, and heavier hydrocarbons were lost from rising seep bubbles, while nitrogen and oxygen were gained. Within the rising seawater bubble plume, dissolved methane concentrations were more than 4 orders of magnitude greater than atmospheric equilibrium concentrations. Strong upwelling flows were observed and bubble-rise times were ~40 s, demonstrating the rapid exchange of gases within the bubble plume.     

Microscale gradients of planktonic microbial communities above the sediment surface in a mangrove estuary

The microscale (1 and 4 cm sampling resolution) distributions of chemical (O₂, NH₃, NO₃⁻, NO₂⁻, PO₄³⁻) and biological (Chl a, phytoplankton, bacterioplankton, viruses) parameters were measured in the 16 cm of water immediately overlaying the sediment-water interface (SWI) within a temperate mangrove estuary in South Australia during December 2003 and March 2004. Shear velocities (u_*) during the time of sampling were very low (<0.1 cm s⁻¹), and we consequently predict that resuspension of organisms and materials was negligible. In December 2003, profiles were often characterised by strong gradients in nutrients and organisms, with the highest concentrations often observed within 0.5 cm of the SWI. Microscale patterns in O₂, NH₃, NO₃⁻ and NO₂⁻ indicated that a variety of anaerobic and aerobic transformation processes probably occurred at the SWI and within profiles. Strong gradients in PO₄³⁻ were indicative of nutrient flux across the SWI as a consequence of degradation processes in the sediments. Pico- and nanophytoplankton concentrations were strongly correlated (p < 0.01) to PO₄³⁻, and exhibited 12- and 68-fold changes in abundance, respectively, with highest concentrations observed nearest to the SWI. Several bacterial subpopulations were discriminated using flow cytometry and significant shifts in the ‘cytometric structure’ of the bacterial community were observed within microscale profiles. Two populations of viruses were correlated to the phytoplankton and low DNA (LDNA) bacteria, and each exhibited elevated concentrations within 0.5 cm of the SWI. In March 2004, microscale distributions of O₂ and nutrients were more homogenous than in December 2003, and dissimilar microbial community structure and patterns were observed above the SWI. The patterns observed here support the prediction that benthic processes can strongly influence the ecology of planktonic communities in the overlaying water, and provide further evidence for the existence of microscale variability amongst communities of aquatic microorganisms.     

Measurements of sheet flow transport in acceleration-skewed oscillatory flow and comparison with practical formulations

Near-bed oscillatory flows with acceleration skewness are characteristic of steep and breaking waves in shallow water. In order to isolate the effects of acceleration skewness on sheet flow sand transport, new experiments are carried out in the Aberdeen Oscillatory Flow Tunnel. The experiments have produced a dataset of net transport rates for full-scale oscillatory flows with varying degrees of acceleration skewness and three sand sizes. The new data confirm previous research that net transport in acceleration-skewed flow is non-zero, is always in the direction of the largest acceleration and increases with increasing acceleration skewness. Large transport rates for the fine sand conditions suggest that phase lag effects play an important role in augmenting positive net transport. A comparison of the new experimental data with a number of practical sand transport formulations that incorporate acceleration skewness shows that none of the formulations performs well in predicting the measured net transport rates for both the fine and the coarser sands. The new experimental data can be used to further develop practical sand transport formulations to better account for acceleration skewness.     

Areas of the global major river plumes

River plumes are the regions where the most intense river-sea-land interaction occurs, and they are characterized by complex material transport and biogeochemical processes. However, due to their highly dynamic nature, global river plume areas have not yet been determined for use in synthetic studies of global oceanography. Based on global climatological monthly averaged salinity data from the NOAA World Ocean Atlas 2009 (WOA09), and monthly averaged salinity contour maps of the East and South China Seas from the Chinese Marine Atlas, we extract the monthly plume areas of major global rivers using a geographic information system (GIS) technique. Only areas with salinities that are three salinity units lower than the average salinity in each ocean are counted. This conservative estimate shows that the minimum and maximum monthly values of the total plume area of the world’s 19 largest rivers are 1.72 × 10 6 km 2 in May and 5.38 × 10 6 km 2 in August. The annual mean area of these river plumes (3.72 × 10 6 km 2 ) takes up approximately 14.2% of the total continental shelves area worldwide (26.15 × 10 6 km 2 ). This paper also presents river plume areas for different oceans and latitude zones, and analyzes seasonal variations of the plume areas and their relationships with river discharge. These statistics describing the major global river plume areas can now provide the basic data for the various flux calculations in the marginal seas, and therefore will be of useful for many oceanographic studies.     

Detailed observation of the wind-exerted surface flow by use of flow visualization methods

Detailed observations were performed of the wind-exerted surface flow, before and after the generation of wind waves. As flow visualization techniques, 6 classes of polystyrene beads of from 0.33 mm to 1.93 mm in diameter, with a specific gravity of 0.99, and also, hydrogen bubble lines, were used. Experiments were carried out at three ranges of the wind speed: 4.0, 6.2 and 8.6ms^–1 in the mean in the wind-wave tunnel section, and the observations were made at 2.85 m in fetch. In the case of 6.2 m s^–1, when the initial surface skin flow attains 0.22 cm in the scale thickness and 16 cm s^–1 in the surface velocity in about 3 second from the onset of the wind, regular waves of about 1.7 cm in wave length appear on the water surface. In one second after that, the downward thrust of the surface flow and the consequent forced convection commences, and the transition of the surface layer to a turbulent state occurs. Ordinary wind waves begin to develop from this state. In developed wind waves the viscous skin flow grows on the windward side of the crests, frequently producing macroscopic skin flows, and these skin flows converge to make a downward thrust at the lee side, and the viscous skin layer disappears there. The velocity of the downward flow has a maximum at the phase of about 30, and the value is of the order of 10 cm s^–1 at 4-mm depth after the orbital velocity of the sinusoidal wave is subtracted. As the process through which the wind stress acts on the water surface, it is considered that the following particular one may be real: the skin friction concentrated at the windward side of the crest produces skin flows, which thrust into the inner region to make the forced convection, carrying the acquired momentum. The viscous shearing stress just before the generation of the surface undurations was about 1/4 of the total shearing stress under the existence of wind waves. It is considered that the increase of the wind stress by wind waves is caused by this mechanism.     

Sensitivity study of coastal plumes

INTRODUCTIONIn laboratory, Griffiths and Linden (1981 ) simulated the buoyancy-driven coastal currentsusing both a ring source and a point source in a rotating cylinder tank. The POint source was simifar to the river-forced plume in the coastal region. The coastal plume together with the gravitycoastal current moved along the coast, keeping the barrier on the right in the Northern Hemisphere. Stern et al. (1982) also conducted a similar experiment using a rectangular tank and carried ou…     

陆坡非旋转重力羽状流的数值模拟

借助MITgcm模式使用二维非静力近似在水平方向采用不等间距网格,模拟了陆坡非旋转重力羽状流的流动。模拟显示陆坡上的重力羽状流的运动比较复杂。通过数值实验,发现改变地形或调整冷源强度都会影响沿陆坡下沉的低温高密度水的羽状流形态,潮汐亦会对重力羽状流有一定的影响。经计算发现在陆坡处,Richardson数小于1/4,宜发生Kelvin-Helmholtz不稳定性,并由卷挟导致环境流体与高密度流体混合,沿着斜坡加速下滑。     

A note on surface wind-driven flow

A simple operationally oriented model of surface wind-driven currents is presented in which Lagrangian surface drift is assumed to be composed of a linear combination of a wave-induced Stokes drift plus a wind-driven Ekman drift. Using this approach, Stokes drift accounts for as much as half the total surface current magnitude. The Lagrangian current is predicted to be about 3.5% of the 10 m wind magnitude directed in the sense of an Ekman spiral about a 20° deviation angle. For comparison to this model, a second model is proposed that accounts for the interaction of Stokes current and Coriolis force. An inference drawn from this model is that there is only weak coupling between Coriolis force and Stokes drift. Such a conclusion, if correct, leads one to focus attention on the Lagrangian model for operationally oriented current estimates. Results of the Lagrangian model agree with observations of investigators for currents at the air-sea interface and may have application in the movement f oil slicks or surface drifters at sea under fetch or duration limited sea states.     

Laboratory simulation of the Kolmogorov flow on a spherical surface

The first results of a laboratory simulation of the Kolmogorov flow on a spherical surface are described. The primary laminar regime was found to be a system of zonal laminar jets of alternating directions. When the first critical value is passed, the primary regime loses its stability, and on its background a secondary vortex quasi-periodic regime with low frequency is formed. With a further increase in the Reynolds number and when the second critical value is passed, this vortex regime becomes unstable and self-excited oscillations emerge in the flow. Specifically, it was found that, if the spherical layer radius is chosen as a length scale, the wavelengths of perturbations in the vortex regime fall in the range of maximum intensity in the spectrum of the horizontal component of wind speed at the tropopause level. We explain the maximum peak shift in the wind spectrum on synoptic time scales when the observational height increases from 3000 km in the surface layer up to 8000?C10000 km in the upper troposphere and lower stratosphere.     

Measurements of boundary-layer flow and boundary roughness over Campeche Bank, Yucatan

Aseries of boundary-layer measurements was made at three anchor stations on Campeche Bank, Yucatan. Measurements consisted of current speed at six levels within 1.5 m of the bed, current direction, and stereophotographs of boundary. Analysis of the drag coefficient revealed that the boundary-layer flow was hydrodynamically transitional and the magnitude of the drag coefficient varied between 2·10⁻³ and 6·10⁻² as a function of the flow conditions and bed configuration. These data appear to be representative of flows of an intermediate nature, not only in the observed variations of the drag coefficient with respect to Reynolds number but in the way that the drag coefficient correlates with other measurements made in low- and high-velocity environments, respectively.     

Simulation of turbulent flow over periodic bottom surface

A mathematical model is developed for the simulation of 2D turbulent flow over a periodic surface. An asymptotic solution is found which allows the estimation of the velocity field and tangential stress at a preset periodic form of a surface and rough peak heights. The solution allows the detection of the flow separation points and maximal tangential stress on the surface. The solution found for the near-bottom tangential stress is compared with experimental data on pressure regions of bottom waves and shows their good agreement.     

Surface buoyant plumes from melting icebergs in the Labrador Sea

Canada׳s Department of Fisheries and Oceans (DFO) conducts annual surveys in the Labrador Sea along the repeat hydrography line AR7W. The occupation of the AR7W line in May 2013 was followed by the experiment aimed at resolving the imprint of melting drifting icebergs on the upper layer thermohaline characteristics in the Labrador Sea. We present high-resolution observations around two icebergs conducted with the towed undulating platform Moving Vessel Profiler (MVP). The first iceberg drifted in relatively warm water of Atlantic origin (~2.5–3.1 °C) off Greenland, while the second iceberg was on the Labrador shelf in cold water below 0 °C. Both icebergs had a lengthscale of O(100 m). In both cases surface buoyant plumes fed by melt water and attached to the iceberg were observed. The plumes were evident in the anomalous thermohaline characteristics of the seawater. Their density anomalies were sufficiently strong to produce visible frontal structures, which imply a development of the intrinsic dynamics associated with a plume. The first plume formed over a time interval of ~10 h, while the second plume formed over several days and extended for more than 1 km (tenfold the iceberg׳s size). Strong vertical displacements of the pycnocline were observed near the second iceberg. They are interpreted as the internal wave wake. This interpretation is based on the temporal scale of these oscillations (local buoyancy frequency), as well as on the spatial orientation of these waves with respect to the iceberg drift relative to the pycnocline. The observed internal waves partially overlapped with the plume and affected its structure. The saline seawater splashing by swell contributed to the surface melting of the icebergs. Scaling analysis of the second plume suggests that it could be in the “rotational” dynamic regime with recirculating anticyclonic flow.     

Numerical calculation for the flow of submerged bodies under a free surface

The flow field generated by a Rankine body moving under a free surface in afinite-depth water is calculated by potential theory. Velocity field generated by a source located at the origin is calculated first by using highly efficient and adaptive quadratures of the QUADPACK library. This solution is used for generating the flow around a Rankine body by locating a source and an equal strength sink along the body axis. Results agree well with the existing literature.