A preliminary estimate of changing calcrete carbon storage on land since the Last Glacial Maximum     

J. M. Adams  W. M. Post 《Global and Planetary Change》1999,20(4):162

The glacial-to-interglacial shift in land carbon storage is important in understanding the global carbon cycle and history of the climate system. While organic carbon storage on land appears to have been much less than present during the cold, dry glacial maximum, calcrete (soil carbonate) carbon storage would have been greater. Here we attempt a global estimation of this change; we use published figures for present soil carbonate by biome to estimate changing global soil carbonate storage, on the basis of reconstruction of vegetation areas for four timeslices since the Last Glacial Maximum. It appears that there would most likely have been around a 30–45% decrease in calcrete carbon on land accompanying the transition between glacial and interglacial conditions. This represents a change of about 500–400 GtC (outer error limits are estimated at 750–200 GtC) . In order to be weathered into dissolved bicarbonate, this would take up an additional 500–400 GtC (750–200 GtC) in CO₂ from ocean/atmosphere sources. An equivalent amount to the carbonate leaving the caliche reservoir on land may have accumulated in coral reefs and other calcareous marine sediments during the Holocene, liberating an equimolar quantity of CO₂ back into the ocean-atmosphere system as the bicarbonate ion breaks up.  相似文献   

European Circulation Patterns and Meteorological Situation in Estonia     

S. Keevallik  P. Post  J. Tuulik 《Theoretical and Applied Climatology》1999,63(1-2):117-127

Summary  An attempt has been made to relate a large-scale airflow classification to the local weather at two stations in Estonia. To describe the features of the general circulation over Europe, the large-scale circulation patterns (Grosswetterlagen) of the German Weather Service have been chosen. Daily precipitation and temperature deviations from the monthly mean in P?rnu and Tartu for 1961–1993 have been used. It can be said that the weather in Estonia shows a good relationship with the general circulation types (Zirkulationsformen) that are defined by means of the Grosswetterlagen. Zonal circulation brings to Estonia wet weather that in winter is warmer than average and in summer cooler than average. Mixed circulation is associated with warmer winters and average summers. Meridional circulation brings cold winters and variable weather in summer. To explain the large dispersion of values of meteorological elements within any one circulation type, a more detailed analysis of the relationships between circulation patterns and meteorological situation in Estonia is needed. Received June 17, 1998 Revised January 13, 1999  相似文献   

上地幔各向异性分析中地壳的影响   总被引：7，自引：2，他引：7       下载免费PDF全文

阮爱国  王椿镛  李清河  臧宏 《地球物理学报》2004,47(3):441-448

应用伪谱法研究了地壳各向异性对上地幔各向异性分析的影响. 用几种不同情况下的两层各向异性模型,模拟了全波场,着重分析了不同深度观测点上的剪切波分裂特征. 结果表明,当地壳为各向同性或其各向异性主轴与上地幔重合或相互垂直时,在地壳观测到的快、慢剪切波的偏振方向保持了上地幔中的偏振方向;重合时,快、慢波走时延迟增大;垂直时,走时延迟减小. 当地壳的各向异性主轴与上地幔成一般夹角时,其粒子运动轨迹变得十分复杂,不能直观地识别上地幔各向异性中的快波方向,对走时延迟也有一定的影响. 将常用的两种分析方法应用于对模拟结果的反演,发现当地壳各向异性与上地幔各向异性方向为一般夹角时,反演结果会出现较大的误差或误判. 进而提出解决这一问题的方法,一是综合多种方法的结果;二是由拟合残差或相关系数等值线作进一步的确定.  相似文献   

Response of braiding channel morphodynamics to peak discharge changes in the Upper Yellow River          下载免费PDF全文

Filip Schuurman  Wanquan Ta  Sander Post  Marius Sokolewicz  Marcela Busnelli  Maarten Kleinhans 《地球表面变化过程与地形》2018,43(8):1648-1662

The braiding intensity and dynamics in large braiding rivers are well known to depend on peak discharges, but the response in braiding and channel–floodplain transformations to changes in discharge regime are poorly known. This modelling study addresses the morphodynamic effects of increasing annual peak discharges in braiding rivers. The study site is a braiding reach of the Upper Yellow River. We estimated the effects on the larger‐scale channel pattern, and on the smaller‐scale bars, channel branches and floodplains. Furthermore, we determined the sensitivity of the channel pattern to model input parameters. The results showed that the dominant effect of a higher peak discharge is the development of chute channels on the floodplains, formed by connecting head‐cut channels and avulsive channels. Widening of the main channel by bank erosion was found to be less dominant. In addition, sedimentation on the bars and floodplains increased with increasing peak discharge. The model results also showed that the modelled channel pattern is especially sensitive to parametrization of the bed slope effect, whereas the effect of median grain size was found to be relatively small. Copyright © 2018 John Wiley & Sons, Ltd.  相似文献   

A Spatially Periodic Solute Boundary for MT3DMS and PHT3D          下载免费PDF全文

Tariq Laattoe  Vincent E. A. Post  Adrian D. Werner 《Ground water》2017,55(3):419-427

The assumption of spatial repetition is commonly made when producing bedform scale models of the hyporheic zone. Two popular solute transport codes, MT3DMS and PHT3D, do not currently provide the necessary boundary condition required to simulate spatial periodicity in hyporheic zone transport problems. In this study, we develop a spatially periodic boundary (SPB) for solutes that is compatible with a SPB that was previously developed for MODFLOW to simulate the flow component of spatially periodic problems. The approach is ideal for simulating groundwater flow and transport patterns under repeating surface features, such as ripples or dunes on the bottom of a lake or stream. The appropriate block‐centered finite‐difference approach to implement the boundary is presented and the necessary source code modifications are discussed. The performance of the solute SPB, operating in conjunction with the groundwater flow SPB, is explored through comparison of a multi‐bedform hyporheic‐zone model with a single bedform variant. The new boundary conditions perform well in situations where both dispersive effects and lateral seepage flux in the underflow regime beneath the hyporheic zone are minimal.  相似文献   

A multi-objective approach to the design of low thrust space trajectories using optimal control     

Michael Dellnitz  Sina Ober-Blöbaum  Marcus Post  Oliver Schütze  Bianca Thiere 《Celestial Mechanics and Dynamical Astronomy》2009,105(1-3):33-59

In this article, we introduce a novel three-step approach for solving optimal control problems in space mission design. We demonstrate its potential by the example task of sending a group of spacecraft to a specific Earth L ₂ halo orbit. In each of the three steps we make use of recently developed optimization methods and the result of one step serves as input data for the subsequent one. Firstly, we perform a global and multi-objective optimization on a restricted class of control functions. The solutions of this problem are (Pareto-)optimal with respect to ΔV and flight time. Based on the solution set, a compromise trajectory can be chosen suited to the mission goals. In the second step, this selected trajectory serves as initial guess for a direct local optimization. We construct a trajectory using a more flexible control law and, hence, the obtained solutions are improved with respect to control effort. Finally, we consider the improved result as a reference trajectory for a formation flight task and compute trajectories for several spacecraft such that these arrive at the halo orbit in a prescribed relative configuration. The strong points of our three-step approach are that the challenging design of good initial guesses is handled numerically by the global optimization tool and afterwards, the last two steps only have to be performed for one reference trajectory.  相似文献   

Quality analysis on 3D building models reconstructed from airborne laser scanning data     

Sander Oude Elberink  George Vosselman 《ISPRS Journal of Photogrammetry and Remote Sensing》2011,66(2):157-165

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Geochemical and Environmental Evaluation of Um Shashoba Gold Mine Area,Southeastern Desert,Egypt     

Mohamed Abdallah Gad DARWISH 《《地质学报》英文版》2011,85(4):881-898

A suite of elements(Ag,Au,Ba,Bi,Cd,Co,Cr,Cu,Ga,Hf,Hg,Mn,Mo,Ni,Pb,Rb,Sb,Se, Sr,Te and Zn),total organic carbon(TOC)and pH were analyzed in stream sediment and tailing samples from Um Shashoba area,in order to evaluate Au placer and the sediments being impacted by old mining activities.Analytical results were examined using statistical,graphical and mapping methods.In spite of the results revealing that Au and most of the elements in sediments were in general significantly lower than those in tailing,Au wa...  相似文献   

Modeling of twin-turbine systems with vertical axis tidal current turbine: Part II—torque fluctuation     

Ye Li  Sander M. Calisal 《Ocean Engineering》2011,38(4):550-558

We recently showed the advantage of using a numerical system to extract energy from tidal currents by developing a new twin-turbine model (Li and Calisal, 2010a). Encouraged by this result, we decided to use this model to study another important characteristic of the turbine system, torque fluctuation. This effort is summarized in this paper. The torque fluctuation is expected to reduce the fatigue life of tidal current turbines, though potentially it also may deteriorate the power quality of tidal current turbines. In this paper, after reviewing the twin-turbine model, we use it to predict the torque fluctuation of the system with the same configurations as we used to study the power output in Li and Calisal (2010a). Specifically, we investigate the torque fluctuation of twin-turbine systems with various turbine parameters (e.g., relative distance between two turbines and incoming flow angle) and operational condition (e.g., tip speed ratio). The results suggest that the torque of an optimally configured twin-turbine system fluctuates much less than that of the corresponding stand-alone turbine, under the same operating conditions. We then extensively compare the hydrodynamic interaction’s impact on the torque fluctuation and the power output of the system. We conclude that the hydrodynamic interactions pose more constructive impacts on the torque fluctuation than on the power output. The findings indicate that the optimally configured counter-rotating system should be a side-by-side system, and that the optimally configured co-rotating system should have the downstream turbine partially in the wake of the upstream turbine depending on the detailed configuration of the turbines. Furthermore, one must balance the optimal torque fluctuation against the optimal power output.  相似文献   

Living austerity urbanism: space–time expansion and deepening socio-spatial inequalities for disadvantaged urban youth in Ireland     

Sander van Lanen 《Urban geography》2017,38(10):1603-1613

After the 2008 financial crisis, Ireland implemented a severe austerity program which drastically reshaped the opportunities and constraints experienced by youth living in disadvantaged urban neighborhoods. Rising unemployment, reduced social welfare, and funding cuts for support organizations limited the opportunities of urban life for disadvantaged youth. This article uses the experience of austerity urbanism of young adults from Ballymun (Dublin) and Knocknaheeny (Cork), both among the most disadvantaged neighborhoods of their cities, to argue that austerity, through time-space expansion, removes services, facilities and opportunities from deprived urban neighborhoods, thus reinforcing and intensifying socio-spatial inequalities. In an effort to bring State finances under control and to revitalize the economy the whole urban fabric, and the urban population, is managed for the purpose of economic recovery. Urban life becomes restricted as disadvantaged urban youth becomes socially and spatially excluded from vital urban opportunities and amenities.  相似文献