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1.
Decadal long simulations of mesoscale structures in the northern North Sea/Skagerrak using two ocean models 总被引：2，自引：2，他引：0
We consider results from two 27-year-long simulation pairs derived using two different ocean models. We focus on the Skagerrak/North
Sea area. Each pair consists of the two terrain-following coordinate models ROMS and MIPOM. The first pair utilizes an eddy-permitting
grid, that is, a grid in which the Rossby radius is barely resolved. The second pair utilizes an eddy-resolving grid in which
the Rossby radius is truly resolved. The goal is to compare the quality of the two models and the two pairs. To this end we
derive statistical properties such as probability density functions and compare them with similar statistics derived from
observations. Thereby we obtain insight into whether a truly eddy-resolving model is required to realistically capture the
mesoscale statistics. We find that eddy resolution is critical to get the mesoscale statistics correct, in particular, the
strength of the current jets. Our results also indicate that the improvement gained by employing the eddy-resolving grid is
mostly due to a better resolved topography. In particular, we find that this is the case in areas exhibiting prominent topographic
features, such as the deep Norwegian Trench cutting into the heart of the northern North Sea/Skagerrak area. The results also
highlight the advantage of first performing quality assurance investigations when implementing a new model for a new area. 相似文献
2.
Arne Melsom 《Ocean Dynamics》2005,55(3-4):338-350
A set of two simulation ensembles of the ocean circulation in the North Sea, the Skagerrak and bordering seas has been run
for the ten year period that started in January 1992. The ensembles differed only in the horizontal grid resolution. The main
purposes of this investigation are (1) to quantify the variability that can be expected in multi-year simulations due to noise-like
perturbations in the initial fields, and (2) to examine the robustness of model results for mesoscale features that form on
the front between the Norwegian Coastal Current and water masses that are of an Atlantic Ocean origin. It is shown that the
model resolution has a substantial impact on the ensemble variability, and that the role of small perturbations become more
significant as the grid mesh is refined. Nevertheless, it is demonstrated that in a region to the west of the southern tip
of Norway, eddies are occasionally found in the same positions at the same time in the results from all members of the ensembles.
This is particularly the case in the aftermath of outbreak events of low salinity water masses from the Skagerrak into the
North Sea. 相似文献
3.
In this paper we explore the optimum assimilation of high‐resolution data into numerical models using the example of topographic data provision for flood inundation simulation. First, we explore problems with current assimilation methods in which numerical grids are generated independent of topography. These include possible loss of significant length scales of topographic information, poor representation of the original surface and data redundancy. These are resolved through the development of a processing chain consisting of: (i) assessment of significant length scales of variation in the input data sets; (ii) determination of significant points within the data set; (iii) translation of these into a conforming model discretization that preserves solution quality for a given numerical solver; and (iv) incorporation of otherwise redundant sub‐grid data into the model in a computationally efficient manner. This processing chain is used to develop an optimal finite element discretization for a 12 km reach of the River Stour in Dorset, UK, for which a high‐resolution topographic data set derived from airborne laser altimetry (LiDAR) was available. For this reach, three simulations of a 1 in 4 year flood event were conducted: a control simulation with a mesh developed independent of topography, a simulation with a topographically optimum mesh, and a further simulation with the topographically optimum mesh incorporating the sub‐grid topographic data within a correction algorithm for dynamic wetting and drying in fixed grid models. The topographically optimum model is shown to represent better the ‘raw’ topographic data set and that differences between this surface and the control are hydraulically significant. Incorporation of sub‐grid topographic data has a less marked impact than getting the explicit hydraulic calculation correct, but still leads to important differences in model behaviour. The paper highlights the need for better validation data capable of discriminating between these competing approaches and begins to indicate what the characteristics of such a data set should be. More generally, the techniques developed here should prove useful for any data set where the resolution exceeds that of the model in which it is to be used. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
4.
In cases when an equivalent porous medium assumption is suitable for simulating groundwater flow in bedrock aquifers, estimation of seepage into underground mine workings (UMWs) can be achieved by specifying MODFLOW drain nodes at the contact between water bearing rock and dewatered mine openings. However, this approach results in significant numerical problems when applied to simulate seepage into an extensive network of UMWs, which often exist at the mine sites. Numerical simulations conducted for individual UMWs, such as a vertical shaft or a horizontal drift, showed that accurate prediction of seepage rates can be achieved by either applying grid spacing that is much finer than the diameter/width of the simulated openings (explicit modeling) or using coarser grid with cell sizes exceeding the characteristic width of shafts or drifts by a factor of 3. Theoretical insight into this phenomenon is presented, based on the so-called well-index theory. It is demonstrated that applying this theory allows to minimize numerical errors associated with MODFLOW simulation of seepage into UMWs on a relatively coarse Cartesian grid. Presented examples include simulated steady-state groundwater flow from homogeneous, heterogeneous, and/or anisotropic rock into a vertical shaft, a horizontal drift/cross-cut, a ramp, two parallel drifts, and a combined system of a vertical shaft connected to a horizontal drift. 相似文献
5.
The seismic behavior of a school gymnasium, whose steel grid roof was heavily damaged during the Mw6.6 Lushan earthquake in 2013, is simulated through nonlinear dynamic analysis. The simulated damage is compared with field observations to validate the numerical model, based on which a parametric study was performed to provide insight into the failure process and damage patterns of steel grids. The results suggest that the grid damage is strongly related to roofsubstructure interactions. These include not only the substructure's amplification of the vibration, but the uncoordinated displacement of the substructure's columns which support the grid also play an equally important role. In particular, the latter effect may significantly alter the internal force distribution in the steel grid and lead to unexpected buckling of members that are proportioned as tension-only members. While such interactions are generally not accounted for in the design practice for grid structures in China, similar seismic damage may be expected for other existing grid roofs in future earthquakes. As is also demonstrated in this study, seismic isolation of the roof is a promising solution to protect grid roof structures by mitigating the detrimental effects of roof-substructure interactions. 相似文献
6.
Sub-grid scale parameterisation of 2D hydrodynamic models of inundation in the urban area 总被引：1，自引：0，他引：1
This paper presents preliminary results from a study considering the parameterisation of coarse-grid 2D flood models to take
into account sub-grid scale flow patterns occurring in the urban area. A simulation of a severe flood in an urbanized coastal
floodplain is first run using a fine grid that can resolve the flow around and between buildings. Next, the same model is
run again using the same underlying topography, although stripped from any buildings, and a set of 7 values of the roughness
parameter (Manning’s n), all larger than (or equal to) the value used in the original run. A further set of simulations is carried out using a five-fold
increased grid cell size. It is found that while it may be possible to model the overall effects of the buildings using strongly
increased roughness parameter values, using a coarse grid otherwise has implications related to the loss of information about
the site topography that results in flood flow routes being incorrectly modelled. 相似文献
7.
Vikas C. Baranwal Antje Franke Ralph-Uwe Börner Klaus Spitzer 《Journal of Applied Geophysics》2011,75(2):363-372
We present a 2-D inversion code incorporating a damped least-squares and a minimum-model approach for plane wave electromagnetic (EM) methods using an adaptive unstructured grid finite element forward operator. Unstructured triangular grids permit efficient discretization of arbitrary 2-D model geometries and, hence, allow for modeling arbitrary topography. The inversion model is parameterized on a coarse parameter grid which constitutes a subset of the forward modeling grid. The mapping from parameter to forward modeling grid is obtained by adaptive mesh refinement. Sensitivities are determined by solving a modified sensitivity equation system arising from the derivative of the finite element equations with respect to the model parameters. Firstly, we demonstrate that surface topography may induce significant effects on the EM response and in the inversion result, and that it cannot be ignored when the scale length of topographic variations is in the order of magnitude of the skin depth. Secondly, the dependency of the inversion on the starting model is discussed for VLF and VLF-R data. Thirdly, we demonstrate the inversion of a synthetic data set obtained from a model with topography. Finally, the inversion approach is applied to field data collected in a region with undulating topography. 相似文献
8.
Within this paper we present a simplified analytical model to provide insight into the key performance measures of a generic disposal system for high level waste within a geological disposal facility. The model assumes a low solubility waste matrix within a corrosion resistant disposal container surrounded by a low permeability buffer. Radionuclides migrate from the disposal area through a porous geosphere to the biosphere and give a radiological dose to a receptor. The system of equations describing the migration is transformed into Laplace space and an approximation used to determine peak values for the radionuclide mass transfer rate entering the biosphere. Results from the model are compared with those from more detailed numerical models for key radionuclides in the UK high level waste inventory. Such an insight model can provide a valuable second line of argument to assist in confirming the results of more detailed models and build confidence in the safety case for a geological disposal facility. 相似文献
9.
A. H. KLEYN 《Geophysical Prospecting》1977,25(1):125-140
After the sampling of a reflection time contour map, i.e. after times and time gradients at the grid points of a square sampling grid have been determined, its conversion into true depth contours can be performed by normal incidence ray tracing. At each grid point the spatial orientation of the ray is uniquely defined by a corresponding time gradient vector, whereas its continuation into the subsurface is controlled by Snell's law. For arbitrarily orientated velocity interfaces the 3 – D ray tracing problem can systematically be solved with the aid of vector algebra, by expressing Snell's law as an equation of vector cross products. This allows to set up a computer algorithm for migration of contour maps. Reliable sampling of reflection time contour maps in the presence of faults is essential for the realization of a practical map migration system. A possible solution of the relevant sampling problem requires a special map editing and digitization procedure. Lateral migration shifts cause a translation and distortion of the original sampling grid. On the transformed grid the true positions of faults can be related to their apparent ones on the reflection time contour map. Errors in the time domain correlations or an incorrect velocity distribution or a combination of both these effects may cause migration failures due to total reflection and time deficiencies, or give rise to an anomalous distortion of grid cells, the latter signifying a violation of the maximum convexity condition. Emphasis is placed upon the significance of map migration as an interpretive tool for solving time to depth conversion problems in the presence of severely faulted or salt intruded overburdens. 相似文献
10.
Mathilde Bøttger Sørensen Peter H. Voss Jens Havskov Søren Gregersen Kuvvet Atakan 《Journal of Seismology》2011,15(4):599-611
The seismically active Skagerrak region in the border area between Denmark and Norway has traditionally been associated with
uncertain earthquake locations due to the limited station coverage in the region. A new seismic station in southern Norway
and a recent update of the earthquake database of the Danish National Network have led to a much more complete and homogeneous
data coverage of the Skagerrak area, giving the possibility of improved earthquake locations in the region. In this study,
we relocate earthquakes in the Skagerrak area to obtain a more exact picture of the seismicity and investigate well-recorded
events to determine the depth distribution. Hypocenter depths are found to be generally in the range 11–25 km. Furthermore,
new composite focal mechanisms are determined for clusters of events with similar waveforms. Results indicate that the Skagerrak
seismicity is associated with shallow, crustal faults oriented in the NS direction south of the Sorgenfrei–Tornquist Zone
(STZ) as well as with the STZ itself. Mainly reverse faulting mechanisms along NE–SW oriented faults indicate maximum horizontal
compression in the NW–SE direction. This is in agreement with World Stress Map generalizations, most likely associated with
ridge push forces from the mid-Atlantic ridge, though modified probably by local crustal weaknesses. 相似文献
11.
D. Z. Fu Y. P. Li G. H. Huang 《Stochastic Environmental Research and Risk Assessment (SERRA)》2012,26(3):375-391
In this study, a fuzzy-Markov-chain-based stochastic dynamic programming (FM-SDP) method is developed for tackling uncertainties
expressed as fuzzy sets and distributions with fuzzy probability (DFPs) in reservoir operation. The concept of DFPs used in
Markov chain is presented as an extended form for expressing uncertainties including both stochastic and fuzzy characteristics.
A fuzzy dominance index analysis approach is proposed for solving multiple fuzzy sets and DPFs in the proposed FM-SDP model.
Solutions under a set of α-cut levels and fuzzy dominance indices can be generated by solving a series of deterministic submodels.
The developed method is applied to a case study of a reservoir operation system. Solutions from FM-SDP provide a range of
desired water-release policies under various system conditions for reservoir operation decision makers, reflecting dynamic
and dual uncertain features of water availability simultaneously. The results indicate that the FM-SDP method could be applicable
to practical problems for decision makers to obtain insight regarding the tradeoffs between economic and system reliability
criteria. Willingness to obtain a lower benefit may guarantee meeting system-constraint demands; conversely, a desire to acquire
a higher benefit could run into a higher risk of violating system constraints. 相似文献
12.
We assess the performance of an eddy-recognizing numerical ocean model in simulating the pattern and variability of the hydrography
in the Skagerrak/northern North Sea area. The model we use is a version of the widely used Princeton ocean model employing
a terrain-following vertical coordinate. Results from a series of five multi-year simulations of the mesoscale response are
described. The simulations differ in their representation of the lateral freshwater supply to the model ocean of which the
first is a reference simulation. The next four are variations in which the river discharges and/or the Baltic outflow are
given more realistic representations. For validation, we have used in situ hydrographic data. A novelty is that we use the
concepts of freshwater height and potential energy anomaly as objective validation tools. We find that, in general, the model
faithfully reproduces many of the observed hydrographic features including their mean patterns and their variance. Not surprisingly,
we find that the Baltic outflow is by far the most significant freshwater source in terms of its influence on the hydrography
in the area, a result corroborating earlier findings. The best validation is obtained when all freshwater supply is made as
realistic as possible, in particular the Baltic outflow. We also find that the large scale cyclonic circulation and the location
of fronts are robust characteristics of the Skagerrak/northern North Sea circulation given the impact changes in the freshwater
input has on the hydrography. Finally, we find that a further exploration of the impact of the lateral open boundary forcing,
e.g., the input of Atlantic water, is needed. 相似文献
13.
Small‐scale variations in surface moisture content were measured on a fine‐grained beach using a Delta‐T Theta probe. The resulting data set was used to examine the implications of small‐scale variability for estimating aeolian transport potential. Surface moisture measurements were collected on a 40 cm × 40 cm grid at 10 cm intervals, providing a total of 25 measurements for each grid data set. A total of 44 grid data sets were obtained from a representative set of beach sub‐environments. Measured moisture contents ranged from about 0% (dry) to 25% (saturated), by weight. The moisture content range within a grid data set was found to vary from less than 1% to almost 15%. The magnitude of within‐grid variability varied consistently with the mean moisture content of the grid sets, following an approximately normal distribution. Both very wet and very dry grid data sets exhibited little internal variability in moisture content, while intermediate moisture contents were associated with higher levels of variability. Thus, at intermediate moisture contents it was apparent that some portions of the beach surface could be dry enough to allow aeolian transport (i.e. moisture content is below the critical threshold), while adjacent portions are too wet for transport to occur. To examine the implications of this finding, cumulative distribution functions were calculated to model the relative proportions of beach surface area expected to be above or below specified threshold moisture levels (4%, 7%, and 14%). It was found that the implicit inclusion of small‐scale variability in surface moisture levels typically resulted in changes of less than 1% in the beach area available for transport, suggesting that this parameter can be ignored at larger spatial scales. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
14.
Joseph G. Alfieri Dev Niyogi Margaret A. LeMone Fei Chen Souleymane Fall 《Pure and Applied Geophysics》2007,164(8-9):1789-1809
With increasing computational resources, environmental models are run at finer grid spacing to resolve the land surface characteristics. The land use/land cover (LULC) data sets input into land surface models are used to assign various default parameters from a look-up tables. The objective of this study is to assess the potential uncertainty in the LULC data and to present a reclassification method for improving the accuracy of LULC data sets. The study focuses on the Southern Great Plains and specifically the Walnut River Watershed in southeastern Kansas, USA. The uncertainty analysis is conducted using two data sets: The National Land Cover Dataset 1992 (NLCD 92) and the Gap Analysis Program (GAP) data set, and a reclassification logic tree. A comparison of these data sets showed that they do not agree for approximately 27% of the watershed. Moreover, an accuracy assessment of these two data sets indicated that neither had an overall accuracy as high as 80%. Using the relationships between land-surface characteristics and LULC, a reclassification of the watershed was conducted using a logical model. This model iteratively reclassified the uncertain pixels according to their surface characteristics. The model utilized normalized difference vegetation index (NDVI) measurements during April and July 2003, elevation, and slope. The reclassification yielded a revised LULC dataset that was substantially improved. The overall accuracy of the revised data set was nearly 93%. The study results suggest: (i) as models adopt finer grid spacings, the uncertainty in the LULC data will become significant; (ii) assimilating NDVI into the land-surface models can reduce the uncertainty due to LULC assignment; (iii) the standard LULC data sets must be used with caution when the focus is on local scale; and (iv) reclassification is a valuable means of improving the accuracy of LULC data sets prior to applying them to local issues or phenomena. 相似文献
15.
《Journal of Atmospheric and Solar》1999,61(5):385-397
First results are presented from a 3-D, time dependent, high resolution, nested grid model that has been developed to study mesoscale processes in the global, coupled thermosphere–ionosphere system. This new Thermosphere–Ionosphere Nested Grid (TING) model, which is an extension of the National Center for Atmospheric Researchs thermosphere–ionosphere general circulation model (NCAR–TIGCM), runs on a UNIX workstation. The TING model simultaneously calculates global (coarse resolution) and local (high resolution) distributions of neutral and plasma winds, temperature and composition. It is comprised of two coupled codes—a global TIGCM and an adjustable nested grid code which uses the same solvers as the TIGCM, but has higher spatial and temporal resolution. The size, location and level of nesting of the high resolution grid(s) are adjustable to suit the specific application. The coupling between the coarse (TIGCM) grid and the nested interior grids is via a one-way interaction scheme. In this scheme, the TIGCM output influences the nested grid model by providing initial conditions and temporally evolving boundary conditions, but the outputs from the nested grid are not permitted to influence the TIGCM. Diurnally-reproducible results of the TING model are presented for solar-maximum, winter solstice, geomagnetically-quiet conditions. The TING model successfully simulates well-known thermosphere–ionosphere features that are smeared or not modeled at the spatial resolutions used in standard TIGCMs. These include the sub-auroral electron density trough, the polar cap hole and the polar cap tongue of ionization. 相似文献
16.
Numerical studies of flow over a sill: sensitivity of the non-hydrostatic effects to the grid size 总被引：1，自引：1，他引：0
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. 相似文献
17.
笛卡尔坐标系中的经典程函方程在静校正、叠前偏移、走时反演、地震定位、层析成像等很多地球物理工作中都有应用,然而用其计算起伏地表的地震波走时却比较困难.本文通过把曲线坐标系中的矩形网格映射到笛卡尔坐标系的贴体网格,推导出曲线坐标中的程函方程,而后,用Lax-Friedrichs快速扫描算法求解曲线坐标系的程函方程.研究表明本文方法能有效处理地表起伏的情况,得到准确稳定的计算结果.由于地表起伏,导致与之拟合的贴体网格在空间上的展布呈各向异性,且这种各向异性的强弱对坐标变换法求解地震初至波的走时具有重要影响.本文研究表明,随着贴体网格的各向异性增强,用坐标变换法求解地表起伏区域的走时计算误差增大,且计算效率降低,这在实际应用具有指导意义. 相似文献
18.
数值差分格式及格点设置对土壤温度模拟结果的影响 总被引：2，自引：2，他引：0
土壤温度是反映气候系统和生态系统能量循环的重要地球物理学参量,土壤温度的模拟精度直接影响着气候系统模式以及陆面物理过程模式的模拟结果.为了提高模式对土壤温度的模拟能力,本文利用土壤热扩散方程的傅里叶解析解定量研究了差分方案、格点设置以及时间步长对土壤温度模拟结果的影响;提出了一种优化的格点设置方案,并利用巴丹吉林沙漠观测数据检验了该方案的性能.研究结果表明:三种差分方案中,显式方案的模拟误差最小,Crank-Nicolson方案其次,隐式方案的模拟误差最大;每一种格点设置方案均存在一个使模拟结果误差最小的最优化时间步长;常用格点设置方案的最优化时间步长为5358 s,最小标准差为0.156 K,优化方案的最优化时间步长为1694 s,最小标准差为0.0465 K;取时间步长为1800 s时,采用常用格点设置方案,巴丹吉林沙漠10 cm深度土壤温度模拟结果的标准差为1.61 K,而采用优化方案,模拟结果的标准差降至0.21 K,改进效果明显. 相似文献
19.
空间格网数据相比于矢量数据具有运算速度快、处理简单的特点,适合地震灾害损失震前预测或震后快速评估。但地震损失评估涉及地震危险性及人口、房屋建筑及其地震易损性等不同类型数据在全国范围内的千米格网分布,数据量大,数据变化时形成新的格网数据的工作量较大,使用常规震害预测算法会影响评估效率。依据地震损失评估原理,采取前置确定性损失评估策略和算法优化,结合GIS功能设计并编程实现了具有风险评估相关数据千米格网化处理、地震损失预测与震后快速评估等核心功能的软件系统。利用该系统进行了2016～2025年中国大陆千米格网地震损失预测,结果表明评估效率显著提高,该系统为我国新一代地震重点监视防御区的确定提供了实用化的震害损失预测工具,同时,在地震损失快速评估中亦得到较好应用。 相似文献
20.
A parameter-estimation technique based on existing hydrological, geophysical, and geological data was developed to approximate transmissivity values for use in a ground-water flow model of the Animas Valley, southwest New Mexico. Complete Bouguer gravity anomaly maps together with seismic-refraction profiles, geologic maps, geologic, geophysical, and drillers' logs, water levels, and pumping-test data provide insight into the transmissivity of bolson deposits throughout the basin. The transmissivity distribution was primarily based on reported pumping and specific-capacity tests in conjunction with complete Bouguer gravity anomaly maps and well log data. Reported transmissivity values were characterized by gravity values and well log data. In grid blocks lacking pumping and specific-capacity tests, transmissivity values were assigned based on the relationship of gravity values and well log data within the grid block to gravity values and well log data within other grid blocks for which transmissivity values are available. A two-dimensional, finite-difference, ground-water flow computer code was used to evaluate the effectiveness of the parameter-estimation technique. Although the trial-and-error method of calibration was employed, the actual computer time necessary for model calibration was minimal. The conceptually straightforward approach for parameter estimation utilizing existing hydrological, geophysical, and geological data provides realistic parameter estimates. 相似文献