Numerical investigation of the factors influencing the vertical profiles of current,salinity, and SSC within a turbidity maximum zone     

Yuanyang Wan  Li Wang 《国际泥沙研究》2017,32(1)

The flow-sediment interaction plays a considerable role on the vertical (internal) profiles of current,salinity and suspended sediment concentration (SSC) within a turbidity maximum zone (TMZ).Numerical modeling provides valuable insights into the complex estuarine physical processes.By combining numerical modeling with field observations,the influencing factors of fine sediment dynamics within the TMZ of Yangtze Estuary have been explored in this study.Firstly,during the neap tide,the measured data present that the current is too weak to break the density stratification,and the vertical flow structure is effectively altered.Secondly,a three-dimensional numerical model based on the Delft3D has been developed and a range of numerical sensitivity analyses were carried out to distinguish the dominant mechanisms and physical processes responsible for the phenomena observed from the measurement data.The numerical investigation highlights the following findings.(1) The vertical profile of currents within the TMZ is largely affected by saltwater intrusion,especially during lower currents when the baroclinic pressure gradient can significantly reshape the local vertical profiles of velocity.(2) The baroclinic effects are primarily determined by the stratification of salinity.(3) In addition to salinity,SSC also influences the local density stratification when its contribution to fluid density is comparable to that of salinity.(4) The settling velocity determines the overall sediment distribution and vertical profiles of the SSC in the water column.The SSC-dependent settling velocity (including the flocculation-induced acceleration and hindered settling deceleration phases) affects the longitudinal movement of the sediments.(5) The vertical profiles of current,salinity and SSC within the TMZ are highly associated with the turbulence determined by the model.The approach to modulate the vertical eddy viscosity in the model,based on the empirical dependency between Rig and Prt,may lead to a numerical instability in the stratified flow.In order to improve the stratification of SSC,additional turbulence damping effect is suggested to be implemented in the model.  相似文献   

      

P. Herzsprung  K. Friese  G. Packroff  M. Schimmele  K. Wendt-Potthoff  M. Winkler 《洁净——土壤、空气、水》1998,26(5):253-262

The investigated coal mining lakes (ML 111, ML 117, and ML 107) in the Lusatian lignite mining district are extremely acidic. The concentrations of iron in these geogenic acidified lakes are orders of magnitude higher than in acidic bog lakes and softwater lakes acidified by atmospheric deposition. For the most part ferric iron was the predominant species by a Fe(III) to Fe(II) ratio of more than 10. Density stratification of the water column leads to vertical concentration gradients of ferrous iron. Extremely high concentrations of ferrous iron were found in the anoxic layers above the sediment of ML 111 and ML 107. High concentrations of Fe(II) were correlated with high concentrations of carbon dioxide. A microbial potential to reduce ferric iron was found in the sediments. In ML 111 and ML 117 the concentrations of Fe(II) in the epilimnion were markedly higher than in the oxic hypolimnetic layers. It can be suggested that the occurence of ferrous iron in the epilimnion is of photochemical origin, as described for softwater lakes. Minimum concentrations of dissolved organic carbon in the epilimnion could be correlated to the increase of ferrous iron concentrations. Especially in springtime and summer the concentrations of Fe(II) in the epilimnion were higher than in the oxic hypolimnetic layer below.  相似文献   

Effects of stratification and mesoscale eddies on Kuroshio path variation south of Japan     

Kazunori Akitomo   《Deep Sea Research Part I: Oceanographic Research Papers》2008,55(8):997-1008

Bimodality of the Kuroshio current path south of Japan is investigated, focusing on the effects of stratification and mesoscale eddies. For this purpose, wind-driven numerical experiments are executed in barotropic and two-layered ocean models. Stratification has two effects on the path selection of the Kuroshio south of Japan. First, it makes an alongshore path stable at intermediate wind stress strength τ₀ by arresting an eddy southeast of Kyushu. This enables an alongshore path to appear in the entire experimental range of τ₀. Second, the upper limit of τ₀ which allows a meandering path decreases from ( in the Sverdrup transport at the Tokara Strait) to () as Δρ/ρ₀ increases from 2.0×10^-3 to 4.0×10^-3. While an anticyclonic eddy imposed upstream (southeast of Kyushu) can cause the transition from an alongshore to a meandering path, it occurs most easily when (). The transition from a meandering to an alongshore path requires an eddy imposed downstream (east of the meandering segment) which suppresses redevelopment of the meandering segment and breaks the balance between the advective and beta effects. Applicability of the results to previously observed path variations is discussed.  相似文献   

Disturbance of meromixis in saline Lake Shira (Siberia,Russia): Possible reasons and ecosystem response     

《Limnologica》2017

Saline Lake Shira (Southern Siberia, Russia) was meromictic through the observation period 2002–2015. During the under-ice periods of 2015 and 2016, complete mixing of the water column was recorded for the first time, and hydrogen sulphide temporarily disappeared from the water column of the lake; i.e. in those years the lake turned to holomixis. In the summer of 2015, a sharp increase in chlorophyll a, organic carbon, zooplankton, and phytoflagellates was observed in the lake, which was probably due to the release of nutrients from the monimolimnion. Purple sulfur bacteria completely disappeared from the lake after the first mixing in 2015, and did not reappear despite the restoration of meromixis in 2017. Thus, it was demonstrated that purple sulfur bacteria are sensitive to the weakening of the stratification of Lake Shira. Based on the data of the seasonal monitoring of temperature and salinity profiles over the period 2002–2017, it was presumed that the main cause of deep mixing in 2015 was the weakening of the salinity gradient due to strong wind impact and early ice retreat in the spring of 2014. In addition, it was shown that in previous years a significant contribution to the maintenance of meromixis was made by an additional influx of fresh water, which caused a rise in the lake level in the period 2002–2007. Thus, we identified a relationship between the stratification regime of the lake and the change in its level, which provides valuable information both for the forecast of water quality and for reconstruction of the Holocene climate humidity in this region of Southern Siberia from the sediment cores of Lake Shira.  相似文献   

Long-term inter-annual variability of a cyclonic gyre in the western Irish Sea     

Agnieszka I. Olbert  Michael HartnettTomasz Dabrowski  Uwe Mikolajewicz 《Continental Shelf Research》2011,31(13):1343-1356

The western Irish Sea gyre (WISG) is a cyclonic baroclinic flow around a dome of stagnant water which develops each year during the heating season in the western Irish Sea. Research was carried out to determine long-term changes in the strength of stratification within WISG and associated changes in the gyre structure, circulation patterns and retentive properties. Model simulations were carried out for the 58-year period 1951-2008. The characteristics of the gyre were quantified by means of potential energy anomaly (PEA), measuring the strength of stratification, and total kinetic energy (KE), reflecting the strength of cyclonic circulation. Additionally, long-term changes in flushing rates within the gyre were assessed.Results show that stratification in the western Irish Sea consistently begins to develop in March, increases linearly from April till June, peaks at the beginning of July and remains at close to maximum level throughout the month of July, before a start of a sharp decline at the beginning of August. The strength of stratification is significantly correlated with averaged summer air temperatures and summer wind speeds. Trend analysis of PEA shows an increase in the stratification strength over the period considered; the increase of PEA peak value is accompanied by a shortening of the gyre duration and a delay in the timing of the peak value. There is also an increasing trend in the KE value, showing that the thermal stratification plays a crucial role in the hydrography of the region. Flushing analysis shows that the stronger the stratification the lower the residence time and thus the faster the removal of the material from the western Irish Sea. Residence time within WISG shortens on average by 8 days over the 58-year period.  相似文献   

Effect of stratification on the frequency of bounded Rossby modes over a non-flat bottom     

Giuseppe Colantuono  Robert Erdélyi  Michael S. Ruderman 《地球物理与天体物理流体动力学》2013,107(5):541-563

This work attempts to express and analyze the challenges, induced by stratification, affecting the Rossby-topographic eigenmodes of a closed domain with a general uneven bottom of arbitrary shape filled with a uniform fluid in the unperturbed configuration. The modified eigenmodes have been computed analytically: stratification is introduced in the mathematical form of a perturbation of a homogeneous fluid over a non-flat bottom. The eigenmodes lose their barotropic character and differences appear in the dynamical fields (velocity and pressure) from upper to lower layer, as expected. Expressions for the baroclinic and ageostrophic velocity components due to the perturbation are given. The analysis is carried out in the frame of linear shallow water approximation. All terms have been retained apart from nonlinear advection in the governing equations. We find that the frequencies of the eigenmodes change; an analytical expression of frequency correction as a function of layer density difference and interface depth is found. Initial results for some elementary geometrical settings with a waveguide bottom are determined and expressed in a concise, easily readable closed form. The results obtained in the shallow water approximation are expanded in series with respect to the Rossby number. Next, they are compared with the frequency correction obtained in an alternative framework in which the quasi-geostrophic approximation is used, and a purely baroclinic perturbation is imposed from the outset as the result of the introduction of stratification in the otherwise homogeneous fluid. In this scenario, reduced gravity and the ratio of upper to lower layer depth are, in turn, used as the expansion parameters in lieu of the Rossby number.  相似文献   

Impact of the 1997–1998 El Niño and 1999 La Niña on nutrient supply in the Gulf of Alaska     

F. A. Whitney  D. W. Welch 《Progress in Oceanography》2002,54(1-4)

Nutrient surveys of the Gulf of Alaska, from 1997 through 1999, show that coastal waters of British Columbia and southern Alaska experienced nitrate depletion each spring and summer. Through the 1997–1998 El Niño, waters with less than 1 μM NO₃ covered 250,000 km² area greater than 1999. Silicate levels as low as 0.2 μM were observed in coastal waters, suggesting that diatom growth may have been nutrient limited both in 1998 and 1999. Detailed sampling off the southern coast of British Columbia revealed that 1998 nitrate levels were only half the average of that during the 1970s winter, were depleted 1 month earlier in spring and remained low throughout the summer. Satellite images show that, compared to 1997 and 1999, chlorophyll levels were much lower in the spring of 1998 throughout the coastal waters of the Gulf of Alaska. Conditions changed dramatically during the 1999 La Niña, with ocean-mixed layer depths increasing by 20 m in winter and 40 m in spring when compared to that during 1997–1998 El Niño. Winter nutrient levels increased and summer upwelling returned. Over the past several decades, a trend towards greater stratification of coastal waters appears to be affecting the supply of nutrients to the mixed layer. The effects of stratification were especially obvious during the 1998 El Niño.  相似文献   

江苏北部全新世风暴沉积研究   总被引：1，自引：0，他引：1  

杨宝国  邓伟栋 《海洋科学》1997,21(6):28-32

  相似文献   

阿克苏地区陆面蒸发的数值研究   总被引：13，自引：0，他引：13  

沈卫明  姚德良 《地理学报》1993,48(5):457-467

本文的试验地区——塔里木盆地阿克苏地区气候干旱,雨量稀少。本文详细地介绍了在土壤湿润不足情况下蒸发的计算方法,但由于人工灌溉的原因使试验地区的地下水位上升到1.5m至2m,故对小麦地的蒸散研究,找们不能完全采用干旱情况下的蒸发公式来处理,本文采用大气湍流边界层阻力的空气动力学方法来计算,并通过与波文比方法的比较,给出了小麦地总蒸散的时变化状况。同时利用土壤分层模式,给出了土壤表层与深层的体积含水量的模拟结果,并与实测值进行了比较,发现二者是基本一致的。  相似文献   

Modelling the tidal mixing fronts and seasonal stratification of the Northwest European Continental shelf     

Jason Holt  Lars Umlauf 《Continental Shelf Research》2008

We investigate mixing processes under stratified conditions on the Northwest European Continental shelf using a numerical model (POLCOMS). Our results indicate that convection induced by vertical shearing of horizontal density gradients (‘shear-induced convection’) is a regularly occurring feature in the bottom and surface boundary layers in this open shelf-sea situation. Two types of turbulence models are investigated to study their capability for reproducing the observed location of tidal mixing fronts, and the physical processes occurring in seasonally stratified waters. The first model is a one-equation variant of the Mellor–Yamada model, whereas the second model combines a more recent second-momentum closure with a two-equation model. It is found that generally mean frontal positions (as estimated from ICES data) are predicted more accurately by the two-equation model. The one-equation model reproduces the mean frontal locations to 18.1 km (<3 grid spacings) and the two-equation model to 17.1 km; although in the Celtic Sea the accuracy is ∼33 and ∼12 km, respectively. Comparison with historical tide gauges, current metres, CTD stations, and thermistor chain data from the North Sea Project all show an improvement in accuracy when the two-equation model is used. This is particularly apparent in the model's ability to reproduce the spring–neap variability during stratification. We find that in the presence of shear-induced convection the routinely applied clipping of the turbulent length-scale, previously thought to be a minor ingredient in a turbulence model, has a dramatic effect on the results: if the length-scale clipping is not applied, substantial over-mixing is observed to occur. The causes and possible remedies of this effect are investigated. Overall our results demonstrate a sensitivity to the details of the turbulence model that is significantly greater than previously thought.  相似文献