The effect of mountainous topography on moisture exchange between the “surface” and the free atmosphere   总被引：1，自引：0，他引：1  

Andreas P. Weigel  Fotini K. Chow  Mathias W. Rotach 《Boundary-Layer Meteorology》2007,125(2):227-244

Typical numerical weather and climate prediction models apply parameterizations to describe the subgrid-scale exchange of moisture, heat and momentum between the surface and the free atmosphere. To a large degree, the underlying assumptions are based on empirical knowledge obtained from measurements in the atmospheric boundary layer over flat and homogeneous topography. It is, however, still unclear what happens if the topography is complex and steep. Not only is the applicability of classical turbulence schemes questionable in principle over such terrain, but mountains additionally induce vertical fluxes on the meso-γ scale. Examples are thermally or mechanically driven valley winds, which are neither resolved nor parameterized by climate models but nevertheless contribute to vertical exchange. Attempts to quantify these processes and to evaluate their impact on climate simulations have so far been scarce. Here, results from a case study in the Riviera Valley in southern Switzerland are presented. In previous work, measurements from the MAP-Riviera field campaign have been used to evaluate and configure a high-resolution large-eddy simulation code (ARPS). This model is here applied with a horizontal grid spacing of 350 m to detect and quantify the relevant exchange processes between the valley atmosphere (i.e. the ground “surface” in a coarse model) and the free atmosphere aloft. As an example, vertical export of moisture is evaluated for three fair-weather summer days. The simulations show that moisture exchange with the free atmosphere is indeed no longer governed by turbulent motions alone. Other mechanisms become important, such as mass export due to topographic narrowing or the interaction of thermally driven cross-valley circulations. Under certain atmospheric conditions, these topographical-related mechanisms exceed the “classical” turbulent contributions a coarse model would see by several times. The study shows that conventional subgrid-scale parameterizations can indeed be far off from reality if applied over complex topography, and that large-eddy simulations could provide a helpful tool for their improvement.  相似文献   

我国低钛月海型模拟月壤初始物质选择的地球化学依据   总被引：1，自引：0，他引：1  

刘春茹  王世杰  冯俊明  李雄耀  付绍洪  欧阳自远  郑永春 《矿物岩石》2007,27(3):28-33

以Apollo14月壤样品和美国JSC-1模拟月壤的地球化学特征为基础,结合我国低钛玄武岩火山的分布,对我国不同地区新生代玄武岩的化学成分、年龄、储量等方面进行对比分析结果表明,滇西北金沙江地区和吉林辉南红旗林场-四海地区的玄武质火山喷发物的化学成分与Apollo14月壤样品和美国JSC-1模拟月壤相似,比较适合用于我国低钛月海玄武岩模拟月壤研制的初始物质。野外地质调查发现,吉林省辉南县金川镇红旗林场—四海一带产出的玄武质火山渣为距今1600 a的该玄武质火山岩的喷发物,其储量大、质地纯、粒度均匀,而且比滇西北金沙江地区最近一期的玄武质火山渣新鲜,最适合作为我国低钛月海玄武岩模拟月壤研制的初始物质。  相似文献   

Modeling of gold mass transfer during the listwanitization and rodingitization using the example of the Ust’-Dep ophiolite complex in the upper Amur territory     

L. P. Plyusnina  G. G. Likhoidov  V. P. Molchanov  Zh. A. Shcheka 《Russian Journal of Pacific Geology》2007,1(5):464-472

Gold mass transfer with chloride and carbonate-chloride solutions was examined at the 300 and 400°C isotherms and P _tot = 1 kbar by means of experimental modeling and theoretical simulations. CO₂ was confirmed to suppress Au solubility in fluids. The low Au solubility (mAu < 10^?8) determined in the experiments explains the mechanism of its precipitation when serpentinites and listwanites interact with acidic mineralized solutions. Listwanitization, which was genetically related with the emplacement of orogenic granitoids, was determined to have overprinted serpentinites and rodingites and strongly affected Au transport in the oregeochemical system. The characteristics of the metasomatic processes in the Ust’-Dep ophiolites and the gold concentration in the rocks produced by these processes confirm this conclusion.  相似文献   

Effects of mesoscale sea-surface temperature fronts on the marine atmospheric boundary layer   总被引：1，自引：0，他引：1  

Eric D. Skyllingstad  Dean Vickers  Larry Mahrt  Roger Samelson 《Boundary-Layer Meteorology》2007,123(2):219-237

A numerical modelling study is presented focusing on the effects of mesoscale sea-surface temperature (SST) variability on surface fluxes and the marine atmospheric boundary-layer structure. A basic scenario is examined having two regions of SST anomaly with alternating warm/cold or cold/warm water regions. Conditions upstream from the anomaly region have SST values equal to the ambient atmosphere temperature, creating an upstream neutrally stratified boundary layer. Downstream from the anomaly region the SST is also set to the ambient atmosphere value. When the warm anomaly is upstream from the cold anomaly, the downstream boundary layer exhibits a more complex structure because of convective forcing and mixed layer deepening upstream from the cold anomaly. An internal boundary layer forms over the cold anomaly in this case, generating two distinct layers over the downstream region. When the cold anomaly is upstream from the warm anomaly, mixing over the warm anomaly quickly destroys the shallow cold layer, yielding a more uniform downstream boundary-layer vertical structure compared with the warm-to- cold case. Analysis of the momentum budget indicates that turbulent momentum flux divergence dominates the velocity field tendency, with pressure forcing accounting for only about 20% of the changes in momentum. Parameterization of surface fluxes and boundary-layer structure at these scales would be very difficult because of their dependence on subgrid-scale SST spatial order. Simulations of similar flow over smaller scale fronts (<5 km) suggest that small-scale SST variability might be parameterized in mesoscale models by relating the effective heat flux to the strength of the SST variance.  相似文献   

Large-eddy simulation of plant canopy flows using plant-scale representation     

Wusi Yue  Marc B. Parlange  Charles Meneveau  Weihong Zhu  René van Hout  Joseph Katz 《Boundary-Layer Meteorology》2007,124(2):183-203

Turbulent flow in a corn canopy is simulated using large-eddy simulation (LES) with a Lagrangian dynamic Smagorinsky model. A new numerical representation of plant canopies is presented that resolves approximately the local structure of plants and takes into account their spatial arrangement. As a validation, computational results are compared with experimental data from recent field particle image velocimetry (PIV) measurements and two previous experimental campaigns. Numerical simulation using the traditional modelling method to represent the canopy (field-scale approach) is also conducted as a comparison to the plant-scale approach. The combination of temporal PIV data, LES and spatial PIV data allows us to couple a wide range of relevant turbulence scales. There is good agreement between experimental data and numerical predictions using the plant-scale approach in terms of various turbulence statistics. Within the canopy, the plant-scale approach also allows the capture of more details than the field-scale approach, including instantaneous gusts that penetrate deep inside the canopy.  相似文献   

Three-dimensional simulations of air flow and momentum transfer in partially harvested forests     

Terry L. Clark  Stephen J. Mitchell 《Boundary-Layer Meteorology》2007,125(3):505-524

In order to predict wind loading on trees (canopy height h) in partially harvested forests, it is necessary to characterize air flow and momentum transfer in progressively more complex patterns where groups of trees (or aggregates) are retained. In this study, we used large-eddy simulation to explore the effects of aggregate size, inter-aggregate spacing, and the ratio between the aggregate size and inter-aggregate spacing on air flow and momentum transfer. Forty-five grid points across an aggregate were needed to achieve an adequate level of turbulence. Using grid sizes of h/15 throughout was too viscous for the smaller aggregates. Vertical and horizontal flow deflection by the leading aggregates sheltered some of the downstream aggregates to varying degrees where turbulence increased for subsequent rows. The number of rows of protected aggregates decreased as aggregate dimensions and the space between aggregates increased. A theoretical treatment of time-dependent wind is presented for the lead aggregate and a simulation case is presented for the case of a gust of reduced wind passing through the aggregate pattern. The leading aggregate responded with decreasing moment for decreasing ambient wind speed as predicted by theory. However, downwind aggregates experienced substantial increases in bending moment. The overall results of the disruptive aspects of time dependence agrees with arguments regarding the role of irrotational (potential) flow to this problem. Our treatment of retention pattern design is only a first step and further research suggestions are presented.  相似文献   

Negative water vapour skewness and dry tongues in the convective boundary layer: observations and large-eddy simulation budget analysis     

F. Couvreux  F. Guichard  V. Masson  J.-L. Redelsperger 《Boundary-Layer Meteorology》2007,123(2):269-294

This study focuses on the intrusion of dry air into the convective boundary layer (CBL) originating from the top of the CBL. Aircraft in-situ measurements from the IHOP_2002 field campaign indicate a prevalence of negative skewness of the water vapour distribution within the growing daytime CBL over land. This negative skewness is interpreted according to large-eddy simulations (LES) as the result of descending dry downdrafts originating from above the mixed layer. LES are used to determine the statistical properties of these intrusions: their size and thermodynamical characteristics. A conditional sampling analysis demonstrates their significance in the retrieval of moisture variances and fluxes. The rapid CBL growth explains why greater negative skewness is observed during the growing phase: the large amounts of dry air that are quickly incorporated into the CBL prevent a full homogenisation by turbulent mixing. The boundary-layer warming in this phase also plays a role in the acquisition of negative buoyancy for these dry tongues, and thus possibly explains their kinematics in the lower CBL. Budget analysis helps to identify the processes responsible for the negative skewness. This budget study underlines the main role of turbulent transport, which distributes the skewness produced at the top or the bottom of the CBL into the interior of the CBL. The dry tongues contribute significantly to this turbulent transport.  相似文献   

Galactic Dynamo and Spiral Arms - 3D MHD Simulations     

Elstner  D.  Lesch  H.  von Linden  Susane  Otmianowska-Mazur  Katarzyna  Urbanik  M. 《Studia Geophysica et Geodaetica》1998,42(3):373-381

In the present project we investigate the evolution of a three-dimensional (3D), large-scale galactic magnetic field under the influence of gas flows in spiral arms and in the presence of dynamo action. Our principal goal is to check how the dynamical evolution of gaseous spiral arms affects the global magnetic field structure and to what extent our models could explain the observed spiral patterns of polarization B-vectors in nearby galaxies. A two-step scheme is used: the N-body simulations of a two-component, self-gravitating disk provide the time-dependent velocity fields which are then used as the input to solve the mean-field dynamo equations. We found that the magnetic field is directly influenced by large-scale non-axisymmetric density wave flows yielding the magnetic field locally well-aligned with gaseous spiral arms in a manner similar to that discussed already by Otmianowska-Mazur et al. 1997. However, an additional field amplification, introduced by a non-zero -term in the dynamo equations, is required to cause a systematic increase of magnetic energy density against the diffusive losses. Our simulated magnetic fields are also used to construct the models of a high-frequency (Faraday rotation-free) polarized radio emission accounting for effects of projection and limited resolution, thus suitable for direct comparisons with observations.  相似文献   

Numerical simulation of fully saturated porous materials     

Boris Jeremić  Zhao Cheng  Mahdi Taiebat  Yannis Dafalias 《国际地质力学数值与分析法杂志》2008,32(13):1635-1660

Fully coupled, porous solid–fluid formulation, implementation and related modeling and simulation issues are presented in this work. To this end, coupled dynamic field equations with u?p?U formulation are used to simulate pore fluid and soil skeleton (elastic–plastic porous solid) responses. Present formulation allows, among other features, for water accelerations to be taken into account. This proves to be useful in modeling dynamic interaction of media of different stiffnesses (as in soil–foundation–structure interaction). Fluid compressibility is also explicitly taken into account, thus allowing excursions into modeling of limited cases of non‐saturated porous media. In addition to these features, present formulation and implementation models in a realistic way the physical damping, which dissipates energy. In particular, the velocity proportional damping is appropriately modeled and simulated by taking into account the interaction of pore fluid and solid skeleton. Similarly, the displacement proportional damping is physically modeled through elastic–plastic processes in soil skeleton. An advanced material model for sand is used in present work and is discussed at some length. Also explored in this paper are the verification and validation issues related to fully coupled modeling and simulations of porous media. Illustrative examples describing the dynamical behavior of porous media (saturated soils) are presented. The verified and validated methods and material models are used to predict the behavior of level and sloping grounds subjected to seismic shaking. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

Hydrogen adsorption and diffusion on amorphous solid water ice     

A. Al-Halabi  E. F. van Dishoeck † 《Monthly notices of the Royal Astronomical Society》2007,382(4):1648-1656

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