Entrainment of fine-grained surface deposits into new ice in the southwestern Kara Sea,Siberian Arctic     

Dirk Dethleff  Gesa Kuhlmann 《Continental Shelf Research》2009

The entrainment of bottom deposits (silt and clay) into newly formed ice was investigated in the Amderma/Vaygach flaw lead in the southwestern Kara Sea, Siberian Arctic. Fine-grained bottom deposits and sea ice sediments (SIS) were analyzed by granulometry, scanning electron microscopy and X-ray diffractometry. On average, SIS contain by a factor of four times more silt than the shelf deposits (66.7% vs. 16.3%), and the SIS clay percentage is more than three-fold of the bottom value (31.2% vs. 9.1%). Sand-sized particles are significantly less abundant in SIS compared to bottom sediment (2.1% vs. 74.6%). The preferred entrainment of silt into ice is underpinned by the enhanced silt-to-clay-ratio in SIS compared to bottom deposits. Though silt is preferably entrained into SIS, no evidence was found for preferential ice-entrainment of any silt sub-fraction (coarse, medium or fine). However, sub-angular- and angular-discoidal silt particles are favorably entrained into local sea ice. Clay mineral assemblages in SIS and shelf surface sediments match very well revealing that no individual clay mineral is preferably enriched in SIS or reduced at the bottom. The general textural, compositional and statistical match of fine-grained shelf surface deposits and SIS proves that bottom sediment is the principle source for ice-entrained material in the study area. We propose e.g. wave action and thermohaline convection to take sediment particles upward from the bottom nepheloid layer into the well-mixed 10–40 m deep water column of the Amderma/Vaygach flaw lead, and the turbulent process of suspension freezing to bring sediment particles and frazil crystals into contact, finally leading to the formation of sediment-laden ice. The role of SIS entrainment and export for local/regional shelf erosion and coastal retreat is of minor importance in the SW Kara Sea compared to other circum-Arctic shelf seas. However, the characteristic clay mineral assemblage of local SIS and bottom deposits can help identify the origin of SIS both on regional and Arctic-wide scales.  相似文献   

液化天然气接收站取排水对渔业资源的影响研究——以温州LNG接收站工程为例          下载免费PDF全文

曹静  王志富  丰卫华  许恒韬  赵沧海  林柏  陈立红 《海洋开发与管理》2020,37(11):45-49

液化天然气接收站取排水过程的卷载效应、冷冲击效应和余氯效应均会对渔业资源造成一定影响。文章以温州LNG接收站工程为例,根据2017年11月在工程附近海域的渔业资源现状调查资料,定性分析了取排水对渔业资源的影响,并对卷载效应和余氯排放造成的渔业资源损失进行定量估算。温州LNG接收站工程建成投产近期运行期间,将造成鱼卵损失量为1.45×107 ind./a,仔鱼损失量为1.66×107ind./a,鱼类幼体损失量为7.56×104 ind./a,虾类幼体损失量为8.68×104 ind./a,蟹类幼体损失量为0.04×103 ind./a。  相似文献   

Initial motion and pivoting characteristics of sand particles in uniform and heterogeneous beds: experiments and modelling     

Wallbridge  Voulgaris  Tomlinson  & Collins 《Sedimentology》1999,46(1):17-32

Experiments are described in which the threshold conditions for sediment entrainment are measured for uniform and mixed sand beds beneath both steady and combined steady/oscillatory flows. Derived critical shear stresses are compared with the mixed bed entrainment model of Wiberg & Smith (1987). As predicted by the model, coarser grains within a sand mixture are entrained at lower bed shear stresses than progressively finer grains. Entrainment occurs generally at lower shear stresses than predicted by the model, especially under unidirectional flows. This may be the result of grains resting in unusually unstable positions during the experiments because the beds are ‘unworked’ at the start of the experiments. The model of Wiberg and Smith predicts threshold conditions more accurately for the mixed beds if the bed pivoting angle is correctly defined. The pivoting angles of the beds used here are measured using a new technique designed specifically for comparison with the threshold data. The measured angles repeat the finding that the coarse grains are more mobile than the finer fractions of a mixture. The results are poorly described by the pivoting angle model presented by Wiberg & Smith (1987) and are better represented by a model of the form Φ = αD^γ(D_i/D₅₀)^β (after 21 ), where α, γ and β are empirical constants. The threshold model is found to be more effective using the improved pivoting relationship. The entrainment of grains is found to be easier beneath unidirectional flows than combined flows, in accordance with previous authors’ findings. A suggestion that this result is caused by a change in the erosion mechanism beneath wave flows is made. Wave boundary layers may act as an extended laminar sublayer over bed grains and reduce the erosive efficiency of the overlying current flow. The results of the experiment have implications for the natural sorting mechanisms of sediment beds being deposited in near-threshold flows.  相似文献   

对流槽湍流涡旋结构特征的小波分析   总被引：7，自引：1，他引：6       下载免费PDF全文

陈子赟  孙鉴泞  袁仁民  蒋维楣 《地球物理学报》2004,47(6):964-970

为研究夹卷层的湍流特性,用对流槽模拟大气对流边界层并做湍流温度测量.能谱分析表明,混合层湍流谱幂律接近“-5/3”次律;而夹卷层湍流谱幂律有明显的分段特征.利用离散正交小波对其湍流脉动信号进行去噪和尺度分解,将原始信号分成均匀各向同性小尺度涡旋成分和大尺度涡旋成分,并对分解后的信号做能谱分析.结果表明,不论是在混合层还是在夹卷层,小涡都能很好地满足Kolmogorov的“-5/3”次律;而大尺度涡旋则在混合层和夹卷层中表现出不同的特征.  相似文献   

Measurements Of Fine-Scale Structure At The Top Of Marine Stratocumulus     

Donald H. Lenschow  Mingyu Zhou  Xubin Zeng  Lianshou Chen  Xiangde Xu 《Boundary-Layer Meteorology》2000,97(2):331-357

During the Dynamics and Chemistry of the MarineStratocumulus (DYCOMS) experiment in July–August 1985, the NCAR Electra aircraft flew a series of flight legs just at the top of the marinestratocumulus cloud decks that cap the mixed layer off the coast of southernCalifornia. Because of the corrugated structure of the cloud-top, the aircraft, which was flown at a nearly constant level and adjusted only to maintain its altitude at the average cloud-top height, was alternately within and above the clouds – roughly half the time in each domain. These legs were used to examine the structure of the cloud-top by compositing the segments on either side of the cloud/clear-air interface, which was identified by the transitions of liquid water measured by the Forward Scattering Spectrometer Probe (either increasing or decreasing) through a threshold of 0.04 × 10^-3 kg m^-3.An equivalent vertical distance (EVD) from the cloud-top was obtained from the horizontal flight legs by estimating the average slope of the cloud-top from the cloud-top radiation temperature. The results show that a near discontinuity occurs in variables across cloud top over an EVD of 0.3 m, but that above this, the air has already been modified by boundary-layer air. Thus, cloud-top is not the limit of mixing of boundary-layer air. This mixing may extend to tens of metres or more. The bulk Richardson number in the vicinity of cloud-top increases from near zero within the cloud to about 1.2 at an EVD of 3–6 m above cloud. Fluctuations of the three velocity components within cloud are nearly equal; above cloud the vertical component structure function is about half the horizontal components. The scalar structure functions are about an order of magnitude higher above cloud than in cloud. The structure parameters of temperature and humidity measured just below cloud-top agree reasonably well with predicted values based on a previously-developed model for the clear convective boundary layer. Above cloud, the scalar structure parameters are much larger, but their interpretation is questionable, since this region does notcontain isotropic turbulence.  相似文献   

A reappraisal of the significance of obstacle clasts in cluster bedform dispersal     

Carmen De Jong 《地球表面变化过程与地形》1991,16(8):737-744

The dispersal of cluster bedforms is examined in a short, partially braided reach of the River Quoich, northeast Scotland, after minor flood events in 1989 and 1990. In all cases, the eventual dispersal of clusters occurred without prior removal of the obstacle clast. Since none of the obstacle clasts was dispersed in 1989 and only two-thirds by 1990, obstacle clasts probably form a less significant delay to sediment entrainment than previously assumed and represent a relatively immobile component of bed sediment.  相似文献   

A Mixed-Layer Model Of The Well-Mixed Stratocumulus-Topped Boundary Layer     

J. L. Pelly  S. E. Belcher 《Boundary-Layer Meteorology》2001,100(1):171-187

Recently a range of sophisticated large-eddy simulations of thecloud-topped boundary layer have been intercompared and furthercompared with observations and single column models. Here we comparethese results with perhaps the simplest model of the cloud-toppedboundary layer, namely a mixed-layer model. Results from the model aredescribed with two aims in mind. Firstly, the good results act as areminder of the success of simple models, and, secondly, we suggestthat a simple mixed-layer model could be used as a baseline for futuremodel intercomparisons.The mixed-layer model is based on two assumptions that follow previousstudies. Firstly, the liquid-water potential temperature and the total waterspecific humidity are assumed to be constant with height in the boundarylayer. Secondly, turbulence entrains air across the inversion into the boundarylayer at a rate that is assumed to be proportional to the jump in radiative flux at the cloud top and inversely proportional to the jump in buoyancy at the inversion. The constant of proportionality is called the entrainment efficiency.Results from the model for the entrainment rate and height evolutionof the boundary layer are compared with the observations and modelsconsidered in a EUCREM intercomparison study. Thepresent mixed-layer model accurately predicts the observed heightevolution of the boundary layer, but over-estimates the entrainmentrate to a similar degree as the large-eddy simulations. We show that,if the subsidence rate is reduced to the value given by observationsrather than the value used in the EUCREM intercomparison study,then the model agrees well with observed value of the entrainment rateif the entrainment efficiency is taken to be 0.6. With this value, themodel also agrees well with a further case study byBechtold et al. An entrainment efficiency of 0.6 is a little higherthan suggested by large eddy simulations, but such simulations do notcurrently resolve the entrainment events explicitly. Hence this pointdeserves further study.  相似文献   

Seasonal variation of average phytoplankton concentration in the Kattegat—a periodical point model     

Aleksander Toompuu  Jacob Carstensen  Brbel Müller-Karulis 《Journal of Sea Research》2003,49(4):323

Seasonal variations in primary production, phytoplankton biomass, chlorophyll-a, dissolved inorganic phosphorus and nitrogen concentrations in the upper 10 m of the Kattegat were analysed by means of monitoring data from 1993–1997. Spatial optimal analysis, based on a stochastic model, was used to reconstruct weekly constituent fields onto a spatial grid. The reconstructed fields were spatially integrated, resulting in a relatively smooth seasonal variability of the average variables. A simple dynamical model, set up as a periodical boundary problem, is suggested for the average phytoplankton concentration, dissolved inorganic nitrogen and entrainment depth as state variables. The model is forced by the solar radiation, nitrogen load from the land sources and atmosphere as well as by nitrogen supply from the lower nutrient-rich layer. The latter process is modelled proportional to the water entrainment into the upper euphotic layer and is driven by atmospheric forcing, river runoff and the Baltic water inflow. Four model coefficient values were fitted by minimising the root mean square difference between the integrated monitoring data and the model output. The suggested diagnostic model reflects the main features in seasonal variability of phytoplankton and nitrogen concentrations by average values, including the magnitude and timing of such dynamic events as the spring and late summer phytoplankton blooms. The importance of different forcing factors is quantified and estimates of unobserved components such as new primary production can be computed.  相似文献   

Bedload entrainment,travel lengths,step lengths,rest periods—studied with passive (iron,magnetic) and active (radio) tracer techniques     

Karl-Heinz Schmidt  Peter Ergenzinger 《地球表面变化过程与地形》1992,17(2):147-165

This paper reports results from bedload transport investigations with active (radio) and passive (iron, magnetic) tracers in the Lainbach, a step-pool mountain river, in Bavaria, Southern Germany. The spatial distributions of the iron tracers after flood events can be best described by exponential or Gamma distributions. There is some indication of a tendency of size-selective transport of the iron tracers, but there is also a considerable amount of scatter in the correlations between weight (size) and travel length owing to the masking influence of other variables, such as the shape of the particles and different positions in the river bed. The experiments with artificial magnetic tracers showed that elongated pebbles (rods) had the longest mean transport distance, platy ones (discs) remained relatively close to the starting points. The particles from the pool showed the greatest transport lengths and a 100 per cent chance of being eroded. The Pebble Transmitter System (PETSY) consists of transmitters implanted into individual pebbles, a computerized receiver, a stationary antenna system with an antenna switchboard, and a data logging system. The particles do not move continuously but in a series of transport steps and non-movement intervals. A single value for a given size-class is not adequate to describe the critical conditions of entrainment under natural circumstances. A probability approach is much more suitable. The critical unit discharges (total discharge divided by active channel width) along the measuring reach are dependent on river bed morphology. In the steps bedload needs higher unit discharges to be entrained. Once entrained, the transport of bedload is stochastic in nature and the single particle transport is controlled by the step lengths and the duration of rest periods. The distributions of both parameters can be approximated by exponential functions. Applying the stochastic concept proposed by Einstein the mean values of the random variables [step length] and [duration of rest period] measured with the PETSY technique were used for the simulation of spatial distributions of bedload particles from point sources. More field and laboratory data are needed to include varying flow and roughness conditions with tracers representing different particle characteristics.  相似文献   

Studies on settling, traction and entrainment of larger benthic foraminiferal tests: implications for accumulation in shallow marine sediments   总被引：1，自引：0，他引：1  

ELZA K. YORDANOVA  JOHANN HOHENEGGER 《Sedimentology》2007,54(6):1273-1306

Settling and traction velocities were measured on optimally preserved tests of larger foraminifera using a settling tube and flume tank. Within larger foraminifera with porcelaneous tests, the peneroplids, Peneroplis antillarum, P. planatus, P. pertusus and Dendritina cf. D. zhengae, are distinguished by low test densities (ca 1·2) that do not change with growth. Buoyancy is high because of low Reynolds numbers and increases in large individuals because of the allometric change of test shape. The fusiform Alveolinella quoyi, with test densities ca 1·6, is characterized by high Reynolds numbers, inducing the weakest buoyancy within porcelaneous larger foraminifera. The highest buoyancy was recorded for the three soritids, Parasorites orbitolitoides, Sorites orbiculus and Amphisorus hemprichii, because of their low test densities (ca 1·25) and the extremely flat, biconcave, plate‐like shape. Flat tests, however, reduce traction and entrainment from smooth surfaces. Within hyaline larger foraminiferat, the amphisteginids show thick‐lenticular (Amphistegina lobifera, A. radiata) to thin‐lenticular tests (A. bicirculata, A. papillosa), influencing buoyancy. Here, high test densities (ca 1·8) decrease with growth in A. lobifera, A. lessonii and A. bicirculata, and remain constant in A. radiata and A. papillosa. Minimum velocities required for entrainment are lower for thick‐lenticular tests and higher for thin‐lenticular tests. Test densities remain constant with growth in the calcarinid Baculogypsina sphaerulata (ρ ∼ 1·78) and decrease slightly in Calcarina gaudichaudii and Neorotalia calcar (starting at ρ ∼ 1·85), all living under extreme hydrodynamic conditions. Density decreases the most in Baculogypsinoides spinosus (starting at ρ ∼ 1·8), resulting in higher buoyancy through low Reynolds numbers. Traction is promoted in spherical tests of Baculogypsina and Baculogypsinoides. Within nummulitids, the thick‐lenticular Palaeonummulites venosus (test density decreasing with size; starting at 1·78) is less buoyant, expressed in high Reynolds numbers, but easily entrained. Thick‐lenticular juveniles and extremely flat adults distinguish Operculinella cumingii, Heterostegina depressa and the giant Cycloclypeus carpenteri. Test densities increase during growth, starting from ca 1·6 and attaining a maximum of 1·8. Buoyancy is low in small tests and high in large tests, while entrainment velocities are reduced as the tests flatten. High buoyancy is also a characteristic of the entirely flat tests in Operculina ammonoides (from deeper regions) and Planostegina operculinoides, which is expressed in the lowest Reynolds numbers within larger foraminifera.  相似文献