共查询到20条相似文献，搜索用时 31 毫秒
1.
古气候模拟对研究古气候变化,检验模式模拟与现代气候状况完全不同的古气候的能力,以及未来气候预测等都具有重要的科学和社会意义.国际古气候对比项目(PMIP,Paleoclimate Modeling Intercomparison Project)开展十多年来,取得了许多成果,我国科学家在古气候模拟方面也做了相当的研究.本文通过介绍古气候模拟的方法,评述PMIP试验和其研究成果以及中国科学家所做的研究工作,以使人们更多地了解古气候模拟研究工作的重要意义. 相似文献
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Bedload, the transport of sediment remaining in contact with the stream bed, has mainly been studied from the perspective of the correlation between fluid driving forces and the responding sediment flux. Yet grain–grain interactions are important and bedload should also be considered as a granular phenomenon. We review progress made recently in the study of granular flows, especially on segregation and rheology, that better illuminates the nature of bedload. Granular flows may exhibit gas‐like or fluid‐like flow, or quasi‐solid deformation. All three conditions might be duplicated in bedload. Understanding of intense bedload transport occurring continuously in a layer several grains deep – typical of sand beds – might greatly benefit from results in granular physics, as illustrated by grain‐inspired bedload results. However, processes restricted to the surface of the bed, when particles move intermittently and the bed becomes structured, while characteristic in gravel‐bed channels, are not well addressed in granular physics. Mutual study of these phenomena may benefit both physics and fluvial geomorphology. We intend, therefore, to contribute to an enhanced dialogue between granular physics and bedload science communities. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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Crustal neon: a striking uniformity 总被引：1，自引：0，他引：1
By combining data from a diverse suite of crustal fluid samples representing a broad geographical distribution, we have identified a well-defined nucleogenic (crustal) neon component. The neon is produced from (α, n) and (n, α) nuclear interactions involving nuclei of O, Mg, and F [1]. In the limiting case of 20Ne/22Ne = 0, the composition is: 21Ne/22Ne = 0.47 ± 0.01 and 21Ne/4He = (0.46 ± 0.08) × 10−7. A crustal O/F ratio of 110 (atomic) calculated from the 21Ne/22Ne ratio is 4–10 times less than the average crustal O/F ratio. The discrepancy can be accounted for by an enhanced O/F ratio within the 10–40 μm range of the U-Th-generated α-particles. 相似文献
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Low flow hydrology: a review 总被引：67，自引：0，他引：67
V. U. Smakhtin 《Journal of Hydrology》2001,240(3-4):147-186
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Redefining active volcanoes: a discussion 总被引：2，自引：0，他引：2
Alexandru Szakács 《Bulletin of Volcanology》1994,56(5):321-325
The analysis of the current definitions of active volcanoes indicates that they are empirical, conventional, inaccurate, nongeological, and arbitrarily constraining. Redefinition is therefore needed. One possible approach is to refine the current empirical definitions. A statistically reasonable and practical redefinition using a geologically based time convention-Holocene or 10000 years-is suggested. A set of time conditions according to volcano typology-i.e. 1000; 10000 and 100000 years for high-frequency basaltic shields, andesitic-dacitic composite volcanoes and low-frequency large silicic calderas, respectively-as further refinement of the empirical definition is also envisaged. Devising a phenomenological definition as a theoretical approach is another possibility, but in practice extant diagnostic means are still unsatisfactory to discriminate accurately between dormant and extinct volcanoes. As a consequence of the redefinition, a classification of volcanoes according to their eruptive status is proposed. Redefinition of active volcanoes might increase accuracy in the usage of basic terms in volcanology and influence volcanic hazard assessment and risk mitigation projects. 相似文献
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Giuseppe Lucido 《Physics of the Earth and Planetary Interiors》1985,39(2):134-140
The analogies between clustering processes in magmas and clustering ones in galaxies are pursued. By using reasonable assumptions about the properties of the fluids in magmas and in the Earth's mantle, it is shown that: (1) in both processes instabilities are formed by density fluctuations; (2) in both phenomena the process of clusters is of the hierarchical kind; (3) magmas and galaxies both exhibit a simultaneous superposition of mass, momentum and energy transports; (4) coalescence occurring in magmas can be considered the corresponding phenomenon to the close encounters in the galactic groups; and (5) vorticity of the Taylor Proudman type in magmatic systems can be regarded as the corresponding phenomenon to cannibalism in the galaxies. It is suggested that there is a single transport mechanism that may be universally applicable. 相似文献
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Drought characterization: a probabilistic approach 总被引：4，自引：0，他引：4
A. K. Mishra V. P. Singh V. R. Desai 《Stochastic Environmental Research and Risk Assessment (SERRA)》2009,23(1):41-55
Using the alternative renewable process and run theory, this study investigates the distribution of drought interval time,
mean drought interarrival time, joint probability density function and transition probabilities of drought events in the Kansabati
River basin in India. The standardized precipitation index series is employed in the investigation. The time interval of SPI
is found to have a significant effect of the probabilistic characteristics of drought. 相似文献
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Mars: a geologically active planet 总被引：1，自引：0，他引：1
Karl L Mitchell and Lionel Wilson summarize the case for martian geological activity in recent times. 相似文献
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Cosmoclimatology: a new theory emerges 总被引：1，自引：0，他引：1
Henrik Svensmark 《Astronomy& Geophysics》2007,48(1):1.18-1.24
Henrik Svensmark draws attention to an overlooked mechanism of climate change: clouds seeded by cosmic rays. 相似文献
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Two-way embedding algorithms: a review 总被引：3，自引：2，他引：1
Local mesh refinement features have now been added to a number of numerical ocean models. In its crudest form, a high-resolution grid is embedded (or nested) in a coarse-resolution grid, which covers the entire domain, and the two grids interact. The aim of this paper is to review existing two-way grid embedding algorithms. The basic algorithms and specificities related to ocean modelling are first described. Then, we address several important issues: conservation properties, design of interpolation/restriction operators, and noise control techniques. 相似文献
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Susan E. Hough 《Bulletin of Earthquake Engineering》2014,12(1):135-155
Pioneering work by Nicolas Ambraseys and many collaborators demonstrates both the tremendous value of macroseismic data and the perils of its uncritical assessment. In numerous publications he shows that neglect of original sources and/or failure to appreciate the context of historical accounts, as well as use of unreliable indicators such as landsliding to determine intensities, commonly leads to inflated magnitude estimates for historical earthquakes. The U.S. Geological Survey “Did You Feel It?” (DYFI) system, which now collects and systematically interprets thousands of first-hand reports from felt earthquakes, provides the opportunity to explore further the biases associated with traditional intensity distributions determined from written (media or archival) accounts. I briefly summarize and further develop the results of Hough (2013), who shows that traditional intensity distributions imply more dramatic damage patterns than are documented by more spatially rich DYFI data, even when intensities are assigned according to the conservative practices established by Ambraseys’ work. I further consider the separate intensity–attenuation relations that have been developed to characterize intensities for historical and modern earthquakes in California, using traditionally assigned intensities and DYFI intensities, respectively. The results support the conclusion that traditionally assigned intensity values tend to be inflated by a fundamental bias towards reporting of dramatic rather than representative effects. I introduce an empirical correction-factor approach to correct for these biases. This allows the growing wealth of well-calibrated DYFI data to be used as calibration events in the analysis of historical earthquakes. 相似文献
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Quantifying water vulnerability: a multi-dimensional approach 总被引：2，自引：0，他引：2
Caroline A. Sullivan 《Stochastic Environmental Research and Risk Assessment (SERRA)》2011,25(4):627-640
In today’s uncertain world, vulnerability of water supplies is of increasing concern. A number of factors influence this,
ranging from physical conditions through to human management capacities. Across the Orange River Basin in southern Africa,
these threats arise from overpopulation and farming pressure, with agrochemical and industrial runoff as well as harsh weather
conditions giving rise to severe problems of erosion and land degradation. Under conditions of climate change, these threats
are exacerbated, as temperature rises and water resources become more erratic. Since water is both an essential instrument
of livelihood support and a crucial factor of production, there is a need to develop more effective mechanisms to identify
those areas where its scarcity or poor management can bring about a slowdown in the development process. This urgency is heightened
by the international commitment to the Millennium Development Goals (MDGs), supposedly to be reached by 2015. In addition
to the MDGs, governments are also committed to the development of basin management plans for Integrated Water Resources Management (IWRM). This means that, in order to try to allocate water in an equitable and efficient way, better understanding is needed
of all of the complexities of managing water across heterogeneous basins. It is now recognized that effective water management
is much more dependent on effective governance than on hydrologic regimes. Ranging from traditional local customary norms
and practices dating back through generations to the latest state-of-the-art science-based international agreements, water
governance is a key to supporting the lives and livelihoods of local populations. Access to information is an essential feature
of any of these approaches, and harmonization of data on water issues is long overdue. This paper provides an outline of an
index-based methodology on which an assessment of water vulnerability can be made. In this approach, supply-driven vulnerability (from water systems) and the demand-driven vulnerability (from water users), are evaluated at the municipal scale. By combining these various dimensions together mathematically,
a Water Vulnerability Index (WVI) can be generated. 相似文献
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Practical VTI approximations: a systematic anatomy 总被引：3，自引：0，他引：3
Transverse isotropy (TI) with a vertical symmetry axis (VTI) often provides an appropriate earth model for prestack imaging of steep-dip reflection seismic data. Exact P-wave and SV-wave phase velocities in VTI media are described by complicated equations requiring four independent parameters. Estimating appropriate multiparameter earth models can be difficult and time-consuming, so it is often useful to replace the exact VTI equations with simpler approximations requiring fewer parameters. The accuracy limits of different previously published VTI approximations are not always clear, nor is it always obvious how these different approximations relate to each other. Here I present a systematic framework for deriving a variety of useful VTI approximations. I develop first a sequence of well-defined approximations to the exact P-wave and SV-wave phase velocities. In doing so, I show how the useful but physically questionable heuristic of setting shear velocities identically to zero can be replaced with a more precise and quantifiable approximation. The key here to deriving accurate approximations is to replace the stiffness a13 with an appropriate factorization in terms of velocity parameters. Two different specific parameter choices lead to the P-wave approximations of Alkhalifah (Geophysics 63 (1998) 623) and Schoenberg and de Hoop (Geophysics 65 (2000) 919), but there are actually an infinite number of reasonable parametrizations possible. Further approximations then lead to a variety of other useful phase velocity expressions, including those of Thomsen (Geophysics 51 (1986) 1954), Dellinger et al. (Journal of Seismic Exploration 2 (1993) 23), Harlan (Stanford Exploration Project Report 89 (1995) 145), and Stopin (Stopin, A., 2001. Comparison of v(θ) equations in TI medium. 9th International Workshop on Seismic Anisotropy). Each P-wave phase velocity approximation derived this way can be paired naturally with a corresponding SV-wave approximation. Each P-wave or SV-wave phase velocity approximation can then be converted into an equivalent dispersion relation in terms of horizontal and vertical slownesses. A simple heuristic substitution also allows each phase velocity approximation to be converted into an explicit group velocity approximation. From these, in turn, travel time or moveout approximations can also be derived. The group velocity and travel time approximations derived this way include ones previously used by Byun et al. (Geophysics 54 (1989) 1564), Dellinger et al. (Journal of Seismic Exploration 2 (1993) 23), Tsvankin and Thomsen (Geophysics 59 (1994) 1290), Harlan (89 (1995) 145), and Zhang and Uren (Zhang, F. and Uren, N., 2001. Approximate explicit ray velocity functions and travel times for P-waves in TI media. 71st Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, 106–109). 相似文献
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Electrical properties of rocks depend on composition (i.e. bulk properties of the constituents), micro structure (i.e. geometrical arrangement of the constituents) and interfacial effects. We consider here a rock as a three component system — grains, pores, and interfaces — in order to account for the observed behaviour. We review first the main results relative to DC. conductivity. Surface conductivity effects show up clearly in the case of shaly formations or at low salinities. Although Archies' law (1942) and Waxman and Smits model (1968) are widely used, a more physically based model is that of Johnson and Sen (1988). We review also the variable frequency conductivity (complex conductivity) data and models. The important effect in that case is the enhancement of the dielectric constant at low frequencies (Knight and Nur, 1987) which can be interpreted as a geometrical effect although electrochemical interactions may also play an important role at low frequencies, depending on the rock type. 相似文献
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《Astronomy& Geophysics》2008,49(1):1.09-1.12
Ian Crawford, Mahesh Anand, Andrew Ball and Katherine Joy report on a UK community meeting on lunar exploration, held at the Open University on 24 and 25 September 2007. 相似文献