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991.
Donald H. Lenschow Mingyu Zhou Xubin Zeng Lianshou Chen Xiangde Xu 《Boundary-Layer Meteorology》2000,97(2):331-357
During the Dynamics and Chemistry of the MarineStratocumulus (DYCOMS) experiment in July–August 1985, the NCAR Electra aircraft flew a series of flight legs just at the top of the marinestratocumulus cloud decks that cap the mixed layer off the coast of southernCalifornia. Because of the corrugated structure of the cloud-top, the aircraft, which was flown at a nearly constant level and adjusted only to maintain its altitude at the average cloud-top height, was alternately within and above the clouds – roughly half the time in each domain. These legs were used to examine the structure of the cloud-top by compositing the segments on either side of the cloud/clear-air interface, which was identified by the transitions of liquid water measured by the Forward Scattering Spectrometer Probe (either increasing or decreasing) through a threshold of 0.04 × 10-3 kg m-3.An equivalent vertical distance (EVD) from the cloud-top was obtained from the horizontal flight legs by estimating the average slope of the cloud-top from the cloud-top radiation temperature. The results show that a near discontinuity occurs in variables across cloud top over an EVD of 0.3 m, but that above this, the air has already been modified by boundary-layer air. Thus, cloud-top is not the limit of mixing of boundary-layer air. This mixing may extend to tens of metres or more. The bulk Richardson number in the vicinity of cloud-top increases from near zero within the cloud to about 1.2 at an EVD of 3–6 m above cloud. Fluctuations of the three velocity components within cloud are nearly equal; above cloud the vertical component structure function is about half the horizontal components. The scalar structure functions are about an order of magnitude higher above cloud than in cloud. The structure parameters of temperature and humidity measured just below cloud-top agree reasonably well with predicted values based on a previously-developed model for the clear convective boundary layer. Above cloud, the scalar structure parameters are much larger, but their interpretation is questionable, since this region does notcontain isotropic turbulence. 相似文献
992.
Robert G. Ridgeway Jr. Donald C. Thornton Alan R. Bandy 《Journal of Atmospheric Chemistry》1992,14(1-4):53-60
A method has been developed for determining trace quantities of dimethyl sulfoxide (DMSO) in aqueous solutions using isotope dilution gas chromatography/mass spectrometry. The method consisted of first reducing DMSO to dimethyl sulfide (DMS), followed by purge and trap preconcentration of DMS. The concentration of DMSO was determined from the signal ratio obtained for the parent ions of DMS and d6-DMS, and the known aqueous concentration of d6-DMSO. The precision of this method was determined to be ± 2.9 %, based on replicate DMSO determinations of a test solution prepared by adding isotopically unenriched (h6-DMSO) to distilled water. The detection limit was 0.010 nmoles/L for the sampling conditions used in this study. This method was applied to the determination of DMSO concentrations in both rain and sea water. The concentration of DMSO in rain was found to be in the range of 2–4 nmoles/L for samples collected at an inland location (Phila, PA) and 8–11 nmoles/L for samples collected at a coastal location (Lewes, DE). Determination of DMSO in sea water required special sample preparation steps to eliminate positive interferences. A depth profile of DMS, DMSO, and dimethyl sulfoniopropionate (DMSP) concentrations is reported for sea water collected off the coast of Ocean City, Md, in August, 1990 相似文献
993.
Craig Stroud Sasha Madronich Elliot Atlas Christopher Cantrell Alan Fried Brian Wert Brian Ridley Fred Eisele Lee Mauldin Richard Shetter Barry Lefer Frank Flocke Andy Weinheimer Mike Coffey Brian Heikes Robert Talbot Donald Blake 《Journal of Atmospheric Chemistry》2004,47(2):107-138
Local ozone production and loss rates for the arctic free troposphere (58–85° N, 1–6 km, February–May) during the TroposphericOzone Production about the Spring Equinox (TOPSE) campaign were calculated using a constrained photochemical box model. Estimates were made to assess the importance of local photochemical ozone production relative to transport in accounting for the springtime maximum in arctic free tropospheric ozone. Ozone production and loss rates from our diel steady-state box model constrained by median observations were first compared to two point box models, one run to instantaneous steady-state and the other run to diel steady-state. A consistent picture of local ozone photochemistry was derived by all three box models suggesting that differences between the approaches were not critical. Our model-derived ozone production rates increased by a factor of 28 in the 1–3 km layer and a factor of 7 in the 3–6 kmlayer between February and May. The arctic ozone budget required net import of ozone into the arctic free troposphere throughout the campaign; however, the transport term exceeded the photochemical production only in the lower free troposphere (1–3 km) between February and March. Gross ozone production rates were calculated to increase linearly with NOx mixing ratiosup to 300 pptv in February and for NOx mixing ratios up to 500 pptv in May. These NOx limits are an order of magnitude higher thanmedian NOx levels observed, illustrating the strong dependence ofgross ozone production rates on NOx mixing ratios for the majority of theobservations. The threshold NOx mixing ratio needed for netpositive ozone production was also calculated to increase from NOx 10pptv in February to 25 pptv in May, suggesting that the NOx levels needed to sustain net ozone production are lower in winter than spring. This lower NOx threshold explains how wintertime photochemical ozone production can impact the build-up of ozone over winter and early spring. There is also an altitude dependence as the threshold NOx neededto produce net ozone shifts to higher values at lower altitudes. This partly explains the calculation of net ozone destruction for the 1–3 km layerand net ozone production for the 3–6 km layer throughout the campaign. 相似文献
994.
Intermittent Turbulence Associated with a Density Current Passage in the Stable Boundary Layer 总被引:1,自引:0,他引:1
Jielun Sun Sean P. Burns Donald H. Lenschow Robert Banta Rob Newsom Richard Coulter Stephen Frasier Turker Ince Carmen Nappo Joan Cuxart William Blumen Xuhui Lee Xin-Zhang Hu 《Boundary-Layer Meteorology》2002,105(2):199-219
Using the unprecedented observational capabilities deployed duringthe Cooperative Atmosphere-Surface Exchange Study-99 (CASES-99),we found three distinct turbulence events on the night of 18October 1999, each of which was associated with differentphenomena: a density current, solitary waves, and downwardpropagating waves from a low-level jet. In this study, we focus onthe first event, the density current and its associatedintermittent turbulence. As the cold density current propagatedthrough the CASES-99 site, eddy motions in the upper part of thedensity current led to periodic overturning of the stratifiedflow, local thermal instability and a downward diffusion ofturbulent mixing. Propagation of the density current induced asecondary circulation. The descending motion following the head ofthe density current resulted in strong stratification, a sharpreduction in the turbulence, and a sudden increase in the windspeed. As the wind surge propagated toward the surface, shearinstability generated upward diffusion of turbulent mixing. Wedemonstrate in detail that the height and sequence of the localthermal and shear instabilities associated with the dynamics ofthe density current are responsible for the apparent intermittentturbulence. 相似文献
995.
A short-term predictive system for surface currents from a rapidly deployed coastal HF radar network
Donald Barrick Vicente Fernandez Maria I. Ferrer Chad Whelan Øyvind Breivik 《Ocean Dynamics》2012,62(5):725-740
In order to address the need for surface trajectory forecasts following deployment of coastal HF radar systems during emergency-response situations (e.g., search and rescue, oil spill), a short-term predictive system (STPS) based on only a few hours data background is presented. First, open-modal analysis (OMA) coefficients are fitted to 1-D surface currents from all available radar stations at each time interval. OMA has the effect of applying a spatial low-pass filter to the data, fills gaps, and can extend coverage to areas where radial vectors are available from a single radar only. Then, a set of temporal modes is fitted to the time series of OMA coefficients, typically over a short 12-h trailing period. These modes include tidal and inertial harmonics, as well as constant and linear trends. This temporal model is the STPS basis for producing up to a 12-h current vector forecast from which a trajectory forecast can be derived. We show results of this method applied to data gathered during the September 2010 rapid-response demonstration in northern Norway. Forecasted coefficients, currents, and trajectories are compared with the same measured quantities, and statistics of skill are assessed employing 16 24-h data sets. Forecasted and measured kinetic variances of the OMA coefficients typically agreed to within 10–15%. In one case where errors were larger, strong wind changes are suspected and examined as the cause. Sudden wind variability is not included properly within the STPS attack we presently employ and will be a subject for future improvement. 相似文献
996.
Christopher J. Patrick Donald E. Weller Robert J. Orth David J. Wilcox Michael P. Hannam 《Estuaries and Coasts》2018,41(1):85-100
Conserving and restoring submerged aquatic vegetation (SAV) are key management goals for estuaries worldwide because SAV integrates many aspects of water quality and provides a wide range of ecosystem services. Management strategies are typically focused on aggregated abundance of several SAV species, because species cannot be easily distinguished in remotely sensed data. Human land use and shoreline alteration have been shown to negatively impact SAV abundance, but the effects have varied with study, spatial scale, and location. The differences in reported effects may be partly due to the focus on abundance, which overlooks within-community and among-community dynamics that generate total SAV abundance. We analyzed long-term SAV aerial survey data (1984–2009) and ground observations of community composition (1984–2012) in subestuaries of Chesapeake Bay to integrate variations in abundance with differences in community composition. We identified five communities (mixed freshwater, milfoil-Zannichellia, mixed mesohaline, Zannichellia, and Ruppia-Zostera). Temporal variations in SAV abundance were more strongly related to community identity than to terrestrial stressors, and responses to stressors differed among communities and among species. In one fifth of the subestuaries, the community identity changed during the study, and the probability of such a change was positively related to the prevalence of riprapped shoreline in the subestuary. Mixed freshwater communities had the highest rates of recovery, and this may have been driven by Hydrilla verticillata, which was the single best predictor of SAV recovery rate. Additional species-specific and community-specific research will likely yield better understanding of the factors affecting community identity and SAV abundance, more accurate predictive models, and more effective management strategies. 相似文献
997.
Diann J. Prosser Thomas E. Jordan Jessica L. Nagel Rochelle D. Seitz Donald E. Weller Dennis F. Whigham 《Estuaries and Coasts》2018,41(1):2-18
The nearshore land-water interface is an important ecological zone that faces anthropogenic pressure from development in coastal regions throughout the world. Coastal waters and estuaries like Chesapeake Bay receive and process land discharges loaded with anthropogenic nutrients and other pollutants that cause eutrophication, hypoxia, and other damage to shallow-water ecosystems. In addition, shorelines are increasingly armored with bulkhead (seawall), riprap, and other structures to protect human infrastructure against the threats of sea-level rise, storm surge, and erosion. Armoring can further influence estuarine and nearshore marine ecosystem functions by degrading water quality, spreading invasive species, and destroying ecologically valuable habitat. These detrimental effects on ecosystem function have ramifications for ecologically and economically important flora and fauna. This special issue of Estuaries and Coasts explores the interacting effects of coastal land use and shoreline armoring on estuarine and coastal marine ecosystems. The majority of papers focus on the Chesapeake Bay region, USA, where 50 major tributaries and an extensive watershed (~ 167,000 km2), provide an ideal model to examine the impacts of human activities at scales ranging from the local shoreline to the entire watershed. The papers consider the influence of watershed land use and natural versus armored shorelines on ecosystem properties and processes as well as on key natural resources. 相似文献
998.
The mixing properties of aluminosilicate melts in the pseudobinary system NaAlSi3O8-KAlSi 3O8 have been determined by measuring the compositions of their saturated vapours by hightemperature Knudsen cell mass spectrometry. The melts mix close to ideally over most of the composition range with small positive deviations from ideality for K-rich compositions. These may be related to incipient partial ordering of melt constituents into leucite-like and SiO2-like structures above the feldspar liquidus. 相似文献
999.
大气中持续增长的CO_2是引起.温室效应.的主要原因.并给全球带来越来越严重的环境问题.消减CO_2是人类共同而临的生存挑战,也是技术难题.在全球碳循环过程中.有多种调节方法可以减少大气中CO_2含量.但目前只有地质储存被认为是可快速实施、见效明显的CO_2减排方式.CO_2流体-岩石相互作川是地质储存的核心科学问题,... 相似文献
1000.
In orogenic zones it is often considered appropriate to use a continuum rheology to model the deformation of the upper continental crust. In this paper we derive an applicable rheology utilizing fiber-bundle and continuum-damage models. We show that the results are identical and yield a power relation between stress σ and strain rate
of the form σ=
ρ−1. We constrain the applicable values of ρ utilizing Omori's law for the decay of aftershocks and conclude that ρ−1 is in the range of 5–15. With this strong nonlinear viscous rheology the behavior of the deforming upper crust approaches that of a perfect-plastic material. 相似文献