白洋淀水陆下垫面上行星边界层空间变动研究

利用三维非静力中尺度模式ARPS,对白洋淀地区水陆非均匀下垫面上大气边界层结构及其变动进行了模拟研究.结果表明,晴天弱背景下,白洋淀地区陆地大气比湿高于水域的情况发生在白天大部分时间内,是普遍存在的一种现象,该现象首先出现在近地层,然后不断向高空延伸,与水陆两种不同的下垫面特征密切相关.由于水陆下垫面的差异,导致水域感热和潜热通量全天较小,变化平稳,而陆地日间感热和潜热通量较大,且潜热高于感热.水陆两地近地层湍流运动状态存在较大差异,湍流输送在水陆比湿空间变动过程中起决定性作用.形成的局地环流同样影响着水陆比湿的空间变动,主要表现在环流上下支对近地层湍流发展以及水汽的垂直输送的影响.陆地上空水汽主要来源于土壤植被的蒸发蒸腾,而来自水域的仅占很少一部分.     

Mathematische Modelle zur Simulation von Zirkulationsströmungen in einem See

The three-dimensional, finite-difference model of R.L. Street (Stanford University) has been adapted at the simulation of wind-induced currents in the northern basin of the Lake of Lugano. The model makes use of complete Navier-Stokes equations and a dynamical algorithm to calculate the eddy viscosity. The model predictions are compared with data collected in the measure campaign of 1979. A further application of the model is the cold inflow simulation in a stratified basin with inclusion of turbulent vertical exchange processes. A two-dimensional version of the model, which assumes an hydrostatic pressure distribution, is utilized to simulate the thermal sommer development of the Lake of Lugano, during 40 days. The model includes the thermical interactions and the wind influence. The results obtained show the possible application of the three-dimensional model to the currents simulation in lake basins. The two-dimensional model may be utilized to predict the development of a basin subject to a thermal load.      

利用ERA-Interim资料对平流层Brewer-Dobson环流变化趋势的分析

选用每天12∶00UTC时次的逐日ERA-Interim再分析资料,根据transformed Eulerian-mean(TEM)方程通过积分剩余速度珔v*,研究了1979—2011年间Brewer-Dobson(BD)环流的时空演变规律.并将其与downward control(DC)原理研究的结果进行比较,同时还探讨了平流层温度与BD环流之间的相互联系.结果表明,由TEM方程通过积分剩余速度珔v*估算的BD环流与利用DC原理估算的环流相比较,在热带地区的形势更加明显.环流在热带对流层中上层上升至平流层中下层,最高可达1hPa等压面附近.然后在热带外向极向下运动,最后在中高纬度下沉回到对流层.BD环流的上升中心及质量通量均随季节的变化产生变动,环流在冬半球的形势显著地强于夏半球.在春季和秋季期间,环流呈现出南北两半球的对称形势.从全球尺度物质输送的角度来看,在过去的33a间平流层BD环流的长期变化趋势是减弱的,且在平流层中下层减弱是明显的.环流的减弱趋势与纬向平均温度的长期变化趋势相匹配.     

Hydrodynamics of Lake Lugano

The hydrodynamic processes in the Lake of Lugano (Lago di Lugano) have been the object of three major measuring campaigns. The first during summer 1979 in the North Basin, the second in the autumn of 1984 in the South Basin and the last in 1989 again in the North Basin. It is expected that analysis of the collected data in conjunction with theoretical concepts will yield conclusive findings on the driving forces and hydrodynamics of the lake. Numerical models have been applied to the lake and the results have been compared to the measurements. Some models calculate the periods and distributions of amplitude, as well as surface velocities and internal seiches. Other models calculate wind-induced water circulation. Evidence is given to the peculiarities of the North Basin and the difficulties related with the depth-width ratio and with its deep chemical stratification.     

Modelling hydrodynamics in Yachiyo Lake using a non‐hydrostatic general circulation model with spatially and temporally varying meteorological conditions

In this study, a three‐dimensional (3D) non‐hydrostatic circulation model was applied to study the thermal structure, its evolution and water circulation of Yachiyo Lake in Hiroshima, Japan. The simulations were conducted for 1 month during July 2006. The meteorological forcing variables such as wind stress, surface atmospheric pressure and heat flux transfer through the lake surface were provided by an atmospheric mesoscale model run. The vertical mixing process of the lake was calculated using the Mellor‐Yamada turbulence model. The 1‐month numerical simulation revealed the wind‐induced currents of the lake, two gyres in the mid‐layer, and depth‐averaged monthly mean currents. Further numerical experiments studying the mechanism of the two gyres in the lake showed the important role of topography in gyre formation. The thermal structure of the lake and its evolution both in space and in time as predicted by the model showed very good agreement with the observed values and characteristics of Yachiyo Lake. The internal gravity waves, which are crucial for mixing in the stratified lake, are depicted by the vertical fluctuation of isotherms. Using the non‐dimensional gradient Richardson number, Yachiyo Lake was determined to be stable under strong stratification during the study period, and therefore very sensitive to wind stress. Copyright © 2009 John Wiley & Sons, Ltd.     

Numerical modelling of stratified flow: a spectral approach

A numerical formulation is developed to solve the three-dimensional hydrodynamic equations which describe flow in a stratified sea.Arbitrary continuous physically realistic variations of density and eddy viscosity can be included in the model, which is sufficiently flexible to be applicable to sea areas of any horizontal extent and depth. A continuous current profile from sea surface to sea bed, is computed with the model. A method for expanding computed current profiles in terms of vertical modes is proposed and the contribution of these modes to the current profiles is considered.The time variation of the wind-induced circulation of a stratified lake in response to a suddenly imposed and maintained wind stress is examined. Calculations show that the wind-driven surface current is modulated by the internal seiche motion of the lake.     

One-dimensional simulation model for steady transcritical free surface flows at short length transitions

A numerical experiment is carried out to investigate the suitability of a Boussinesq-type momentum model for simulating transcritical flows at short length transitions in open channel flow measuring structures. Two one-dimensional Boussinesq-type equation models, which incorporate different degrees of dynamic pressure corrections, are considered for this purpose. A finite difference method is employed to discretise and solve the equations. The models are then applied to simulate different test cases for flows in such channels with predominant non-hydrostatic pressure distribution effects. A comparison of the computed results with the corresponding experimental data is presented. Results of this study reveal that the proposed model, which includes a higher-order correction for the effect of the centrifugal pressure, describes well even relatively abrupt changes from sub- to super-critical flow state.     

Tidal and wind-induced circulation within the Southeastern limit of the Bay of Biscay: Pasaia Bay,Basque Coast

The Basque coastal area, in the southeastern Bay of Biscay, can be characterised as being more influenced by land climate and inputs, than other typically ‘open sea’ areas. The influence of coastal processes, together with the presence of irregular and steep topography, complicate greatly the water circulation patterns. Water movement along the Basque coastal area is not well understood; observations are scarce and long-term current records are lacking. The knowledge available is confined to the surface currents: the surface water circulation is controlled mainly by wind forcing, with tidal and density currents being weak. However, there is a lack of knowledge available on currents within the lower levels of the water column; likewise, on the main time-scales involved in the water circulation. This study quantifies the contribution of the tidal and wind-induced currents, to the overall water circulation; it identifies the main time-scales involved within the tidal and wind-induced flows, investigating difference in such currents, throughout the water column, within Pasaia Bay (Basque coast). Within this context, extensive oceanographic and meteorological data have been obtained, in order to describe the circulation. The present investigation reveals that the circulation, within the surface and the sub-surface waters, is controlled mainly by wind forcing fluctuations, over a wide range of meteorological frequencies: third-diurnal, semidiurnal and diurnal land–sea breezes; synoptic variability; frequencies, near fortnightly periods; and seasonal. At the lower levels of the water column, the main contribution to the water circulation arises from residual currents, followed by wind-induced currents on synoptic time-scales. In contrast, tidal currents contribute minimally to the overall circulation throughout the water column.     

定常风对鄱阳湖水动力的影响

鄱阳湖属大风区,风场作为仅次于流域"五河"倾泻和长江顶托作用的另一重要驱动力,或在某些时刻影响局部区域的水流结构,进而影响局部水体中泥沙、污染物、营养盐等物质的输移和扩散.基于鄱阳湖二维水动力数学模型,模拟定常风场条件下的鄱阳湖流场分布及环流形式,并与无风条件下的水流时空结构进行对比.结果表明:3.03 m/s的NE向和SSW向定常风对湖泊水位影响微弱;对流速的影响主要集中在7月中旬至9月底的"湖相"期;其影响区域主要分布在湖区中部大湖面偏西岸及东部湖湾,约占湖泊最大水面积的16%;上述区域出现明显环流,环流结构具有时空异质性特点,环流区流速普遍增至无风时的两倍以上;NE向和SSW向风场产生的环流位置相近,方向相反.相比于以往鄱阳湖水动力研究中对风场的忽略,本次研究揭示了定常风场对鄱阳湖的重点影响区域、影响程度及影响形式,可为泥沙及污染物输移模拟中对风场条件的处理及可能带来的误差与误差的空间分布提供重要依据.     

A 3D unstructured non-hydrostatic ocean model for internal waves

A 3D non-hydrostatic model is developed to compute internal waves. A novel grid arrangement is incorporated in the model. This not only ensures the homogenous Dirichlet boundary condition for the non-hydrostatic pressure can be precisely and easily imposed but also renders the model relatively simple in its discretized form. The Perot scheme is employed to discretize horizontal advection terms in the horizontal momentum equations, which is based on staggered grids and has the conservative property. Based on previous water wave models, the main works of the present paper are to (1) utilize a semi-implicit, fractional step algorithm to solve the Navier-Stokes equations (NSE); (2) develop a second-order flux-limiter method satisfying the max–min property; (3) incorporate a density equation, which is solved by a high-resolution finite volume method ensuring mass conservation and max–min property based on a vertical boundary-fitted coordinate system; and (4) validate the developed model by using four tests including two internal seiche waves, lock-exchange flow, and internal solitary wave breaking. Comparisons of numerical results with analytical solutions or experimental data or other model results show reasonably good agreement, demonstrating the model’s capability to resolve internal waves relating to complex non-hydrostatic phenomena.     

21世纪以来泛北极湖泊水位变化时空特征及原因探讨

在全球气候变暖和极端气候事件增加的背景下,流域水文循环过程受到的影响越来越强烈,导致湖泊水位变化表现出复杂的时空特征。而泛北极地区是地球上湖泊数量与面积分布最为集中的区域之一,该地区湖泊对气候变化响应非常敏感。因此,了解这些湖泊近期水文变化特征十分必要。本研究共搜集了36个泛北极大型湖泊(>500 km²)基于遥感或站点观测的近20年水位数据,分析其时空变化特征。本文使用线性回归模型来估算湖泊水位的变化趋势,进而利用皮尔逊相关分析了其主要水文影响变量和大气环流机制,并运用Mann-Kendall突变检验法探讨了水位突变的原因。结果表明,泛北极湖泊的水位整体上呈现不同程度上升(平均速率为0.013 m/a),有23个(64%)湖泊的水位呈上升趋势;研究湖泊中有10个通过90%统计显著性检验。其中,水位上升速率最大的湖泊是位于哈萨克斯坦的腾吉兹湖,上升速率为0.078 m/a。泛北极湖泊水位的波动主要与径流有关,有19个(53%)湖泊的水位波动与径流的增加更为相关;相比而言,位于亚洲的极地湖泊水位的上升与流域蒸发的降低显著相关,尤其是库苏古尔湖。从区域大气环流影响来看,泛北极湖泊水位变化主要与厄尔尼诺-南方涛动有关,其次是北极涛动和北大西洋涛动。本研究有助于加深对泛北极湖泊近20年水位变化规律及气候影响特征的科学理解。     

Modelling the growth of cyanobacteria (GrowSCUM)

Toxic cyanobacteria have become a common nuisance in freshwater lakes and reservoirs throughout the world, sometimes resulting in the closure of sites with high amenity value. Cyanobacteria are able to regulate their buoyancy state in response to changing photosynthetic rates. Additionally, the cyanobacteria are liable to become entrained within wind-induced near-surface turbulent currents, resulting in mixing and mass transport. These movement processes have been modelled. A mathematical function is presented which describes light- and nutrient-limited cyanobacterial growth. The growth model is integrated with a previous movement model (SCUM: simulation of cyanobacterial underwater movement) as movement patterns and wind-induced lake mixing strongly affect the intensity and duration of light received by the cyanobacteria and thereby determine the photosynthetic potential. Results of the model suggest that cyanobacteria are resistant to periods of lake mixing and continue to increase their biomass, but at a depressed rate. Growth is most rapid under calm conditions. The results agree well with field-based findings, confirming the validity of the growth function.     

A non-hydrostatic numerical model for calculating free-surface stratified flows

A three-dimensional non-hydrostatic numerical model for simulation of the free-surface stratified flows is presented. The model is a non-hydrostatic extension of free-surface primitive equation model with a general vertical coordinate and horizontal orthogonal curvilinear coordinates. The model equations are integrated with mode-splitting technique and decomposition of pressure and velocity fields on hydrostatic and non-hydrostatic components. The model was tested against laboratory experiments on the steep wave transformation over the longshore bar, solitary wave impact on the vertical wall, the collapse of the mixed region in the thin pycnocline, mixing in the lock-exchange flows and water exchange through the sea strait. The agreement is generally fair.Responsible Editor: Hans Burchard     

Multi-block lattice Boltzmann simulations of solute transport in shallow water flows

We report a two-dimensional multi-block lattice Boltzmann model for solute transport in shallow water flows, which is developed based on the advection–diffusion equation for mass transport and the shallow water equations for the flows. A weighting factor is included in the centered scheme for improved accuracy. The model is firstly verified by simulating three benchmark tests: wind-driven circulation in a dish-shaped lake, jet-forced flow in a circular basin, and flow formed by two parallel streams containing different uniform concentrations at the same constant velocity; and then it is applied to a practical wind-induced flow, Baiyangdian Lake, which is characterized by irregular geometries and complex bathymetries. The numerical results have shown that the model is able to produce accurate and detailed results for both water flows and solute transport, which is attractive, especially for flows in narrow zones of practical terrains and certain areas with largely varying pollutant concentrations.     

Effects of spatial wind inhomogeneity and turbulence anisotropy on circulation in an elongated basin: A simplified analytical solution

The response of a closed basin to wind forcing has been studied extensively, but the role of turbulence anisotropy coupled with spatial wind inhomogeneity has never been considered explicitly. An analytical solution is presented for steady state hydrodynamics, considering the central part of a constant-depth, elongated basin with a laterally varying wind, and a homogeneous, yet anisotropic, eddy viscosity tensor. The solution is derived both for a single-layer and for a two-layer basin, which is representative of a stratified lake with a well-developed thermocline. Since the focus is on the short-term barotropic reaction to wind forcing, which determines the type of lake circulation, baroclinic effects are neglected as a first approximation. In this case, the development of planimetric (depth-averaged) circulation superimposed on circulation in the vertical plane can be determined as a function of wind lateral variation and a few dimensionless parameters. The relevance of such an analytical solution is twofold. Firstly, the knowledge of the prevailing circulation can help in the choice of the best type of numerical model (three-dimensional vs. two-dimensional, depth- or lateral-averaged). Secondly, it shows the importance of correct estimates of both vertical and horizontal eddy viscosity, whereas the latter is not usually considered as an important parameter in lake hydrodynamics modeling.     

Wind effects on the lateral structure of density-driven circulation in Chesapeake Bay

The response of the density-driven circulation in the Chesapeake Bay to wind forcing was studied with numerical experiments. A model of the bay with realistic bathymetry was first applied to produce the density-driven flow under average river discharge and tidal forcing. Subsequently, four spatially uniform wind fields (northeasterly, northwesterly, southwesterly, and southeasterly) were imposed to examine the resulting cross-estuary structure of salinity and flow fields. In general, northeasterly and northwesterly winds intensified the density-driven circulation in the upper and middle reaches of the bay, whereas southeasterly and southwesterly winds weakened it. The response was different in the lower bay, where downwind flow from the upper and middle reaches of the bay competed with onshore/offshore coastal flows. Wind remote effects were dominant, over local effects, on volume transports through the bay entrance. However, local effects were more influential in establishing the sea-level slopes that drove subtidal flows and salinity fields in most of the bay. The effect of vertical stratification on wind-induced flows was also investigated by switching it off. The absence of stratification allowed development of Ekman layers that reached depths of the same order as the water depth. Consequently, bathymetric effects became influential on the homogeneous flow structure causing the wind-induced flow inside the bay to show a marked transverse structure: downwind over the shallow areas and upwind in the channels. In the presence of stratification, Ekman layers became shallower and the wind-induced currents showed weaker transverse structure than those that developed in the absence of stratification. In essence, the wind-driven flows were horizontally sheared under weak stratification and vertically sheared under stratified conditions.     

风场对藻类在太湖中迁移影响的动力学研究

在Ｗｅｂｓｔｅｒ等人工作的基础上,以三维湖流为背景,考虑了波浪及藻类自身浮力的影响,建立了一个太湖梅梁湾三维藻类迁多模型,以研究在不同几场作用下藻类在湖泊中的迁移过程。模拟结果表明,不同风场对于藻类在湖泊中的水平及垂直分布影响很大,并且存在着一临界风速,其范围在２－３ｍ／ｓ之间,当几速小于临床风速时,水面可以拟看作水动力学光滑,没有波浪产生,在水表面藻类顺着风向迅速的向迎风岸的边漂移,形成藻类大量     

Numerical studies of flow over a sill: sensitivity of the non-hydrostatic effects to the grid size

A non-hydrostatic terrain-following model in cross sectional form is applied to study the processes in the lee of a sill in an idealized stratified fjord during super-critical tidal inflow. A sequence of numerical studies with horizontal grid sizes in the range from 100 to 1.5625 m are performed. All experiments are repeated using both hydrostatic and non-hydrostatic versions of the model, allowing a systematic study of possible non-hydrostatic pressure effects and also of the sensitivity of these effects to the horizontal grid size. The length scales and periods of the internal waves in the lee of the sill are gradually reduced and the amplitudes of these waves are increased as the grid size is reduced from 100 down to 12.5 m. With a further reduction in grid size, more short time and space scale motions become superimposed on the internal waves. Associated with the internal wave activity, there is a deeper separation point that is fairly robust to all parameters investigated. Another separation point nearer to the top of the sill appears in the numerical results from the high-resolution studies with the non-hydrostatic model. Associated with this shallower separation point, an overturning vortex appears in the same set of numerical solutions. This vortex grows in strength with reduced grid size in the non-hydrostatic experiments. The effects of the non-hydrostatic pressure on the velocity and temperature fields grow with reduced grid size. In the experiments with horizontal grid sizes equal to 100 or 50 m, the non-hydrostatic pressure effects are small. For smaller grid sizes, the time mean velocity and temperature fields are also clearly affected by the non-hydrostatic pressure adjustments.     

Non-hydrostatic model of airflow over irregular topography — Theoretical bases

Summary This article deals with some problems connected with the formulation of a non-hydrostatic mesoscale model of airflow in the atmosphere. Due to an irregular surface a terrain-following coordinate system is used and the equations of the model are transformed into this system. Sound waves are eliminated by the use of the anelastic approximation. The influence of boundaries is minimized by the use of open boundary conditions at the lateral boundaries of the computational domain and of the absorbing layer beneath the upper boundary.     

The SOMS model and its application to Lake Neuchâtel

A three-dimensional numerical circulation model (SOMS) based on primitive equations is described. The algorithm, by which Coriolis and vertical diffusion terms are treated implicitly while mass is still conserved exactly (algebraically), is discussed in detail. The model is applied to Lake Neuchâtel (Switzerland), to determine the general circulation under influence of the most prevailing wind.