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261.
An inter-comparison of regional climate models for Europe: model performance in present-day climate 总被引:3,自引:0,他引:3
Daniela Jacob Lars Bärring Ole Bøssing Christensen Jens Hesselbjerg Christensen Manuel de Castro Michel Déqué Filippo Giorgi Stefan Hagemann Martin Hirschi Richard Jones Erik Kjellström Geert Lenderink Burkhardt Rockel Enrique Sánchez Christoph Schär Sonia I. Seneviratne Samuel Somot Aad van Ulden Bart van den Hurk 《Climatic change》2007,81(1):31-52
The analysis of possible regional climate changes over Europe as simulated by 10 regional climate models within the context of PRUDENCE requires a careful investigation of possible systematic biases in the models. The purpose of this paper is to identify how the main model systematic biases vary across the different models. Two fundamental aspects of model validation are addressed here: the ability to simulate (1) the long-term (30 or 40 years) mean climate and (2) the inter-annual variability. The analysis concentrates on near-surface air temperature and precipitation over land and focuses mainly on winter and summer. In general, there is a warm bias with respect to the CRU data set in these extreme seasons and a tendency to cold biases in the transition seasons. In winter the typical spread (standard deviation) between the models is 1 K. During summer there is generally a better agreement between observed and simulated values of inter-annual variability although there is a relatively clear signal that the modeled temperature variability is larger than suggested by observations, while precipitation variability is closer to observations. The areas with warm (cold) bias in winter generally exhibit wet (dry) biases, whereas the relationship is the reverse during summer (though much less clear, coupling warm (cold) biases with dry (wet) ones). When comparing the RCMs with their driving GCM, they generally reproduce the large-scale circulation of the GCM though in some cases there are substantial differences between regional biases in surface temperature and precipitation. 相似文献
262.
Evaluating the response of climate to greenhouse gas forcing is a major objective of the climate community, and the use of large ensemble of simulations is considered as a significant step toward that goal. The present paper thus discusses a new methodology based on neural network to mix ensemble of climate model simulations. Our analysis consists of one simulation of seven Atmosphere–Ocean Global Climate Models, which participated in the IPCC Project and provided at least one simulation for the twentieth century (20c3m) and one simulation for each of three SRES scenarios: A2, A1B and B1. Our statistical method based on neural networks and Bayesian statistics computes a transfer function between models and observations. Such a transfer function was then used to project future conditions and to derive what we would call the optimal ensemble combination for twenty-first century climate change projections. Our approach is therefore based on one statement and one hypothesis. The statement is that an optimal ensemble projection should be built by giving larger weights to models, which have more skill in representing present climate conditions. The hypothesis is that our method based on neural network is actually weighting the models that way. While the statement is actually an open question, which answer may vary according to the region or climate signal under study, our results demonstrate that the neural network approach indeed allows to weighting models according to their skills. As such, our method is an improvement of existing Bayesian methods developed to mix ensembles of simulations. However, the general low skill of climate models in simulating precipitation mean climatology implies that the final projection maps (whatever the method used to compute them) may significantly change in the future as models improve. Therefore, the projection results for late twenty-first century conditions are presented as possible projections based on the “state-of-the-art” of present climate modeling. First, various criteria were computed making it possible to evaluate the models’ skills in simulating late twentieth century precipitation over continental areas as well as their divergence in projecting climate change conditions. Despite the relatively poor skill of most of the climate models in simulating present-day large scale precipitation patterns, we identified two types of models: the climate models with moderate-to-normal (i.e., close to observations) precipitation amplitudes over the Amazonian basin; and the climate models with a low precipitation in that region and too high a precipitation on the equatorial Pacific coast. Under SRES A2 greenhouse gas forcing, the neural network simulates an increase in precipitation over the La Plata basin coherent with the mean model ensemble projection. Over the Amazonian basin, a decrease in precipitation is projected. However, the models strongly diverge, and the neural network was found to give more weight to models, which better simulate present-day climate conditions. In the southern tip of the continent, the models poorly simulate present-day climate. However, they display a fairly good convergence when simulating climate change response with a weak increase south of 45°S and a decrease in Chile between 30 and 45°S. Other scenarios (A1B and B1) strongly resemble the SRES A2 trends but with weaker amplitudes. 相似文献
263.
G. Delgado A. Redaño J. Lorente R. Nieto L. Gimeno P. Ribera D. Barriopedro R. García-Herrera A. Serrano 《Meteorology and Atmospheric Physics》2007,96(1-2):141-157
Summary This paper reports a cloud cover analysis of cut-off low pressure systems (COL) using a pattern recognition method applied
to IR and VIS bispectral histograms. 35 COL occurrences were studied over five years (1994–1998). Five cloud types were identified
in COLs, of which high clouds (HCC) and deep convective clouds (DCC) were found to be the most relevant to characterize COL
systems, though not the most numerous.
Cloud cover in a COL is highly dependent on its stage of development, but a higher percentage of cloud cover is always present
in the frontal zone, attributable due to higher amounts of high and deep convective clouds. These general characteristics
are most marked during the first stage (when the amplitude of the geopotencial wave increases) and second stage (characterized
by the development of a cold upper level low), closed cyclonic circulation minimizing differences between rearward and frontal
zones during the third stage. The probability of heavy rains during this stage decreases considerably. The centres of mass
of high and deep convective clouds move towards the COL-axis centre during COL evolution. 相似文献
264.
Enrique García-Melendo 《Astrophysics and Space Science》2001,275(4):479-484
Photometric data extracted from more than 5000 CCD observations of theMilky Way field star NSV 01450, with galactic co-ordinates l =145°.9and b=4° .9, show that this object is a bright classical Cepheid witha period of 12.639 ± 0.020 days and an average V magnitude of 11.045± 0.016. The computed colour excess is E(B-V)= 0.553± 0.056, yielding a distance modulus m
V -MV of 15.537 ± 0.212.In addition, it was found that the star GSC 3730_0797 in the field ofNSV 01450 is also variable. 相似文献
265.
Robert R. Lane John W. Day Brian Marx Enrique Reves G. Paul Kemp 《Estuaries and Coasts》2002,25(1):30-42
The objective of this study was to examine the interaction between the Atchafalaya River and the Atchafalaya Delta estuarine complex. Measurements of suspended sediments, inorganic nutrients (NO3 ?, NH4 +, PO4 3?), chlorophylla (chla), and-salinity were taken monthly from December 1996 to January 1998. These data were compiled by season, and the Atchafalaya River plume data were also analyzed using the Generalized Additive Model technique. There were significant decreases in NO3 ? concentrations during summer, fall, and winter as river water passed through the estuary, that were attributable to chemical and biological processes rather than dilution with ambient water. In some regions there were higher chla concentrations during summer and fall compared to winter and spring, when river discharge and the introduction of inorganic nutrients were highest, suggesting biological processes were active during this study. The presence of NH4 +, as a percentage of available dissolved inorganic nitrogen, increased with distance from the Atchafalaya River, indicative of remineralization processes and NO3 ? reduction. Mean PO4 3? concentrations were often higher in the estuarine regions compared to the Atchafalaya River. During summer total suspended solid (TSS) concentrations increased with distance from the river mouth, suggesting a turbidity maximum. Highest chla concentrations were found in the bayous and shallow water bodies of the Terrebonne marshes, as were the lowest TSS concentrations. The low chla concentrations found in other areas of this study, despite high inorganic nutrient concentrations, suggest light limitation as the major control of phytoplankton growth. Salinity reached near seawater concentrations at the outer edge of the Atchafalaya River plume, but much lower salinities (<10 psu) were observed at all other regions. The Atchafalaya Delta estuarine complex buffers the impact of the Atchafalaya River on the Louisiana coastal shelf zone, with a 41% of 47% decrease in Atchafalaya River NO3 ? concentrations before reaching Gulf waters. 相似文献
266.
When using γ-ray coded-mask cameras, one does not get a direct image as in classical optical cameras but the correlation of
the mask response with the source. Therefore the data must be mathematically treated in order to reconstruct the original
sky sources. Generally this reconstruction is based on linear methods, such as correlating the detector plane with a reconstruction
array, or non-linear ones such as iterative or maximization methods (i.e. the EM algorithm). The latter have a better performance
but they increase the computational complexity by taking a lot of time to reconstruct an image. Here we present a method for
speeding up such kind of algorithms by making use of a neural network with a back-propagation learning rule.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
267.
Jesús Solé Michael Cosca Zachary Sharp Pere Enrique 《International Journal of Earth Sciences》2002,91(5):865-881
The cooling history of Hercynian calc-alkalic, post-kinematic plutonic intrusions of the Montnegre massif (NE Spain) has been determined by 40Ar/39Ar analysis of two hornblendes, four biotites and eight K-feldspars (Kfs). The hornblendes have 40Ar/39Ar total fusion ages of 291Dž Ma and define the magmatic cooling of the basic and oldest structural intrusions. The biotites from the acid and intermediate rocks have 40Ar/39Ar plateau ages of 285Dž Ma, which date cooling of the intrusions through argon closure in biotite. The 40Ar/39Ar ages of the K-feldspars vary widely, ranging from 276-191 Ma. A correlation between K-feldspar 40Ar/39Ar total fusion age and several other features such as structural state, microstructures indicated by obliquity and, to some degree, optically visible perthites, is consistent with post-crystallisation partial argon loss in the K-feldspars. The 'D values of the biotites also correlate with age and chlorite contents, but this is not so for the '18O values of either feldspar or quartz. We infer that most microtextural changes occurred during cooling of the batholith, but a possibly Mesozoic, late disturbing hydrothermal event of weak intensity and with only minor fluid circulation must have occurred. This event provoked significant argon loss in the most structurally complex K-feldspars and is recorded in the hydrogen, but not oxygen, isotope data. 相似文献
268.
Enrique Merino 《Geochimica et cosmochimica acta》1979,43(9):1533-1542
Many water analyses contain more than the minimum number of analytical determinations needed to calculate the distribution of species at ~ 25°C. The speciation of a water that contains cations and ligands other than H+ and OH? can be calculated if just analytical determinations are available. In the frequent case where (or more) determinations are available, one can calculate from the speciation a value for the ()th variable. The comparison between this computed value and its analytical counterpart (not used in calculating the speciation) constitutes a test for internal consistency among the determinations that make up the whole analysis.Application of this test requires knowing the uncertainties of the calculated speciation and of the value computed for the ()th variable from the speciation. Such uncertainties have been ignored in previous algorithms, but have been determined herein by a Monte Carlo method of error propagation which is very well suited to systems of nonlinear algebraic equations.By comparing the titration alkalinity against the alkalinity calculated from the speciation, several published analyses for which the cation/anion balance is < 3% can be shown to be internally inconsistent. Such analyses may be unsuitable for geochemical calculations (water/rock interaction, estimation of reservoir temperatures, etc.).Several existing speciation algorithms disregard the possibility that the analytical data used as input are internally inconsistent. 相似文献
269.
270.
We present a kinetic theory for the development of lattice-preferred orientation in a uniaxiallystressed aggregate consisting of elastically uniaxial crystals. The fabric is brought about by grain-boundary migration alone; other fabric-producing mechanisms, such as plastic deformation, syntectonic recrystallization, and dissolution/reprecipitation, are specifically excluded from the theory. The formulation statistically averages the mass transfer between a typical crystal and its adjacent neighbors over all the possible crystallographic orientations of these neighbors and over all the possible orientations of the interfaces between the neighbors and the crystal of interest. The rate of mass exchange between two adjacent crystals is proportional (1) to the difference between chemical potentials and (2) to the surface area common to them. With these assumptions we find an equation for the massfraction density of crystals that have a given orientation (with respect to the stress) at any given time. The theory predicts that beyond a certain time a gap develops in the distribution of crystal orientations allowed: the only crystals that continue to exist are those for which the c-axis is within a certain, timedependent range about the stress. 相似文献