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291.
An original implementation of a non-hydrostatic, free-surface algorithm based on a pressure correction method is proposed for ocean modelling. The free surface is implemented through an explicit scheme combined with a mode-spitting method but the depth-averaged velocity and the position of the free surface are updated at each non-hydrostatic iteration. The vertical momentum equation is also integrated up to the surface enabling a natural and accurate treatment of the surface layer. The consistent specification of the numerical schemes provides balanced transfers of potential and kinetic energy. This algorithm is well-suited for implementation as a non-hydrostatic kernel on originally hydrostatic free-surface ocean models such as Symphonie (http://poc.obs-mip.fr/pages/research_topics/modelling/symphonie/symphonie.htm) for which it has originally been developed.Energy balances associated with the propagation of short surface waves and solitary waves are presented for two dedicated well-documented configurations over closed domains. The buoyancy flux, the work rate of the pressure force together with the power of the advective terms are evaluated and discussed for the generation and the propagation of these two types of waves. The dissipation rate is in particular shown to be several orders of magnitude smaller than the work rates of the hydrostatic and non-hydrostatic pressure forces confirming the necessity for the exchanges of energy to be numerically balanced. The algorithm is subsequently applied to the complex generation of non-linear solitary internal waves by surface tides over Georges Bank, in the Gulf of Maine. The generation and the propagation of the observed non-linear and non-hydrostatic features in this region are correctly reproduced.  相似文献   
292.
A catastrophic rain event occurred in early June 2002 during a major drought over the Canadian Prairies. The storm brought record-breaking rainfall and major flooding to many locations in the region. Given the importance of this event, this study's overall objectives are to characterize and to understand the physical nature of the rainstorm better. The event was associated with a major extratropical cyclone which acted in concert with the Great Plains low-level jet to transport a tremendous amount of moisture into the eastern Prairies producing intense diurnal mesoscale convective systems over the region. At the same time, moisture was transported to the western Prairies by a strong easterly low-level jet which produced heavy and long-lived orographic precipitation near the foothills. Several working hypotheses were developed to explain the severity and longevity of the rainstorm; it was found that the Rockies played a central role in the organization and development of the system.

Atmospheric features that are critical to the development of an important class of extreme rain events in the Canadian Prairies were also identified. The severity of the June 2002 system is partly a result of the rare co-occurrence of these features during the period. Results from a preliminary analysis show that the atmospheric conditions associated with the extreme background drought enhanced the likelihood of the co-occurrence of these features during spring 2002, hence facilitating the development of the extreme rain event. In return, the tremendous precipitation from the storm alleviated the drought conditions in the southern Prairies.

R ésumé ?[Traduit par la rédaction] Un événement de pluie catastrophique s'est produit au début de juin 2002 pendant une sécheresse importante dans les Prairies canadiennes. La tempête a produit des chutes de pluie records et des inondations importantes à plusieurs endroits dans la région. Étant donné l'importance de cet événement, les objectifs généraux de la présente étude sont de mieux caractériser et de mieux comprendre la nature physique de la tempête de pluie. L'événement a été causé par une importante dépression extratropicale qui s'est associée au courant-jet à basse altitude des Grandes Plaines pour transporter une énorme quantité d'humidité dans l'est des Prairies et produire d'intenses systèmes convectifs de mésoéchelle diurne dans la région. En même temps, de l'humidité transportée dans l'ouest des Prairies par un fort courant-jet à basse altitude soufflant de l'est a produit d'intenses précipitations orographiques persistantes près des contreforts. Nous avons formulé plusieurs hypothèses de travail pour expliquer l'intensité et la persistance de la tempête de pluie; il ressort que les Rocheuses ont joué un rôle central dans l'organisation et le développement du système.

Nous avons aussi identifié les caractéristiques atmosphériques essentielles à la formation d'une catégorie importante d'événements de pluie extrêmes dans les Prairies canadiennes. L'intensité du système de juin 2002 est en partie le résultat de la rare présence simultanée de ces caractéristiques durant la période. Les résultats d'une analyse préliminaire montrent que les conditions atmosphériques liées à la sécheresse extrême qui régnait en toile de fond ont augmenté la probabilité d'une présence simultanée de ces caractéristiques au cours du printemps 2002, ce qui a favorisé la formation de l'événement de pluie extrême. En revanche, les précipitations diluviennes produites par la tempête ont atténué les conditions de sécheresse dans le sud des Prairies.  相似文献   
293.
The computation on a relatively short time of a quantity, related to the largest Lyapunov Characteristic Exponent, called Fast Lyapunov Indicator allows to discriminate between ordered and weak chaotic motion and also, under certain conditions, between resonant and non resonant regular orbits. The aim of this paper is to study numerically the relationship between the Fast Lyapunov Indicator values and the order of periodic orbits. Using the two-dimensional standard map as a model problem we have found that the Fast Lyapunov Indicator increases as the logarithm of the order of periodic orbits up to a given order. For higher order the Fast Lyapunov Indicator grows linearly with the order of the periodic orbits. We provide a simple model to explain the relationship that we have found between the values of the Fast Lyapunov Indicator, the order of the periodic orbits and also the minimum number of iterations needed to obtain the Fast Lyapunov Indicator values.  相似文献   
294.
In a previous work [Guzzo et al. DCDS B 5, 687–698 (2005)] we have provided numerical evidence of global diffusion occurring in slightly perturbed integrable Hamiltonian systems and symplectic maps. We have shown that even if a system is sufficiently close to be integrable, global diffusion occurs on a set with peculiar topology, the so-called Arnold web, and is qualitatively different from Chirikov diffusion, occurring in more perturbed systems. In the present work we study in more detail the chaotic behaviour of a set of 90 orbits which diffuse on the Arnold web. We find that the largest Lyapunov exponent does not seem to converge for the individual orbits while the mean Lyapunov exponent on the set of 90 orbits does converge. In other words, a kind of average mixing characterizes the diffusion. Moreover, the Local Lyapunov Characteristic Numbers (LLCNs), on individual orbits appear to reflect the different zones of the Arnold web revealed by the Fast Lyapunov Indicator. Finally, using the LLCNs we study the ergodicity of the chaotic part of the Arnold web.  相似文献   
295.
Built environments have played an increasingly important and complex role in contemporary cities in the face of the significant number of discretionary and non‐discretionary event‐based activities that take place indoors, and the great variety of three‐dimensional designs that indoor spaces have grown to assume. However, intelligent tools that can assist designers to achieve high accessibility, particularly for large and complex buildings, are still underdeveloped. This article presents an innovative custom computer‐based design tool, dubbed FloorplanAAU (floor plan accessibility auditing), that couples GIS data modeling, indoor traversable network accessibility analysis, and interactive three‐dimensional visualization for built environments, to interactively evaluate and audit floor plans for high accessibility. The proposed method contributes to the success of applying GIS and transportation geography concepts to establish computational tools toward intelligent design and assessment of built environments.  相似文献   
296.
297.
Measurements of the polarisation state of the atmosphere were performed at Tenerife in June–July 1997, in the framework of ACE‐2 (second Aerosol Characterization Experiment), by 2 ground‐based instruments: RefPol (a LOA prototype) which took measurements at 445, 665, 870, 1610 nm in the solar principal plane; and an automatic CIMEL (CE 318) sun/sky‐photometer which measured polarised radiation at 870 nm in the same observational geometry. Measurements acquired during the campaign, as well as AERONET (AErosol RObotic NETwork) measurements acquired at the sites of Cape Verde and M'Bour, are processed with an algorithm determining the polarised single‐scattering sky‐radiance due to aerosols, directly proportional to the aerosol polarised phase function (representing the probability to scatter polarised radiation in the direction of the scattering angle). A good correlation between the Ångström exponent α, representing the spectral dependence of the extinction measurements, and the polarised phase function is observed on each set of data. The uncertainty of retrievals at 445 nm makes the determination of the spectral dependence of polarisation inconclusive but does not prevent confirming the dependence of the aerosol polarised phase function on α, at all wavelengths. An Ångström exponent of 1 corresponds to a polarised phase function of around 0.1 (±0.04), at 870 nm and at a scattering angle of 60°. For α between 0 and 0.4, the average value of the polarised phase function is 0.05. The correlation shows that polarisation is more sensitive to small particles than to large particles. The discrepancy between retrievals and Mie calculations from an AERONET size distribution, inverted from Izaña measurements acquired during a dust event, suggests the presence of small particles, not detected by total sky‐radiance measurements.  相似文献   
298.
Abstract— It is now established that a large extraterrestrial object hit the Earth at the end of the Cretaceous period, about 65 Ma ago. We have investigated Re‐Os, Hf‐W, and Mn‐Cr isotope systems in sediments from the Cretaceous and the Paleogene in order to characterize the type of impactor. Within the Cretaceous‐Tertiary (K‐T) boundary layer, extraterrestrial material is mixed with terrestrial material, causing a dilution of the extraterrestrial isotope signature that is difficult to quantify. A phase essentially composed of Ni‐rich spinel, formed in the atmosphere mainly from melted projectile material, is likely to contain the extraterrestrial isotopic signature of the impactor. We show that the analysis of spinel is indeed the best approach to determine the initial isotope composition of the impactor, and that W and Cr isotopes confirm that the projectile was a carbonaceous chondrite.  相似文献   
299.
Understanding the processes at the origin of explosive events is crucial for volcanic hazard mitigation, especially during long-lasting eruptions at andesitic volcanoes. This work exposes the case of Tungurahua volcano, whose unrest occurred in 1999. Since this date, the eruptive activity was characterized by low-to moderate explosiveness, including phases with stronger canon-like explosions and regional ash fallout. However, in 2006, a sudden increase of the explosiveness led to pyroclastic flow-forming eruptions on July 14th (VEI 2) and August 16–17th (VEI 3). All magmas emitted from 1999 to 2005, as well as the samples from the 2006 eruptions, have homogeneous bulk-rock andesitic compositions (58–59 wt.% SiO2), and contain the same mineral assemblage consisting of pl + cpx + opx + mag ± ol. However, during the August 16–17th event, the erupted tephra comprise two types of magmas: a dominant, brown andesitic scoria; and scarce, light-grey pumice representing a subordinate, silica-rich juvenile component. For the andesitic magma, thermobarometric data point to magmatic temperatures ranging from 950 to 1015 °C and pressures in the range of 200 to 250 MPa, which corresponds to 7.5–9.5 km below the summit. Disequilibrium textures in plagioclase and pyroxene phenocrysts, particularly thin overgrowth rims, indicate the recharge of this magma body by mafic magma. Between 1999 and 2005, repeated injections from depth fed the intermittent eruptive activity observed while silica-rich melts were produced by in-situ crystallization in the peripheral parts of the reservoir. In April 2006, the recharge of a primitive magma produced strong convection and homogenisation in the reservoir, as well as pressure increase and higher magma ascent rate after seven years of only moderately explosive activity. This work emphasizes the importance of petrological studies in constraining the pre-eruptive magmatic conditions and processes, as a tool for understanding the fundamental causes of the changes in the eruptive dynamism, particularly the occurrence of paroxysmal phases in andesitic systems with open-vent behaviour.  相似文献   
300.
The study of the stochasticity of the asteroid belt requires the analysis of a large number of orbits. We detect the dynamical character of a set of 5 400 asteroids using the Fast Lyapunov Indicator, a method of analysis closely related to the computation of the Lyapunov Characteristic Exponents, but cheaper in computational time. For both regular and chaotic orbits we try to associate the motion to the underlying resonances network. For it we consider different methods of classification of rational numbers proposed by number theory, and we choose the one which seems to be strictly related to the dynamical behaviour of a system. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   
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