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931.
932.
Both observational and numerical studies suggest that the Eurasian winter snow cover has a strong influence on the subsequent
summer monsoon in Asia. An updated version of the ARPEGE climate model of Météo-France, including a simple but physically-based
snow parameterization, is used to test the impact of an increased snow mass prescribed at the beginning of March on the simulated
summer monsoon circulation and rainfall. The large-scale features of the Asian monsoon are reproduced in a realistic way in
the control integration, which is a necessary premise of such a sensitivity test. In the heavy snow cover experiment, the
anomalous persistence of the winter snow pack delays the springtime continental heating. This weakens the thermal low over
northern India and Persia as well as the southwesterly winds over the monsoon area. There is also a significant decrease in
the rainfall over western India and Bengal-Burma, which usually represent the centers of maximum precipitation. Radiative,
turbulence transfer and hydrological processes seem to be involved in the snow-monsoon relationship. The changes in the monsoon
precipitation are strongly related to changes in the atmospheric circulation and are not reinforced by a local evaporation/convection
feedback in our experiment.
Received: 17 May 1995 / Accepted: 27 November 1995 相似文献
933.
Ralf Döscher Klaus Wyser H. E. Markus Meier Minwei Qian René Redler 《Climate Dynamics》2010,34(7-8):1157-1176
The relative importance of regional processes inside the Arctic climate system and the large scale atmospheric circulation for Arctic interannual climate variability has been estimated with the help of a regional Arctic coupled ocean-ice-atmosphere model. The study focuses on sea ice and surface climate during the 1980s and 1990s. Simulations agree reasonably well with observations. Correlations between the winter North Atlantic Oscillation index and the summer Arctic sea ice thickness and summer sea ice extent are found. Spread of sea ice extent within an ensemble of model runs can be associated with a surface pressure gradient between the Nordic Seas and the Kara Sea. Trends in the sea ice thickness field are widely significant and can formally be attributed to large scale forcing outside the Arctic model domain. Concerning predictability, results indicate that the variability generated by the external forcing is more important in most regions than the internally generated variability. However, both are in the same order of magnitude. Local areas such as the Northern Greenland coast together with Fram Straits and parts of the Greenland Sea show a strong importance of internally generated variability, which is associated with wind direction variability due to interaction with atmospheric dynamics on the Greenland ice sheet. High predictability of sea ice extent is supported by north-easterly winds from the Arctic Ocean to Scandinavia. 相似文献
934.
U Cubasch G C Hegerl A Hellbach H Höck U Mikolajewicz B D Santer R Voss 《Climate Dynamics》1995,11(2):71-84
Due to restrictions in the available computing resources and a lack of suitable observational data, transient climate change experiments with global coupled ocean-atmosphere models have been started from an initial state at equilibrium with the present day forcing. The historical development of greenhouse gas forcing from the onset of industrialization until the present has therefore been neglected. Studies with simplified models have shown that this cold start error leads to a serious underestimation of the anthropogenic global warming. In the present study, a 150-year integration has been carried out with a global coupled ocean-atmosphere model starting from the greenhouse gas concentration observed in 1935, i.e., at an early time of industrialization. The model was forced with observed greenhouse gas concentrations up to 1985, and with the equivalent C02 concentrations stipulated in Scenario A (Business as Usual) of the Intergovernmental Panel on Climate Change from 1985 to 2085. The early starting date alleviates some of the cold start problems. The global mean near surface temperature change in 2085 is about 0.3 K (ca. 10%) higher in the early industrialization experiment than in an integration with the same model and identical Scenario A greenhouse gas forcing, but with a start date in 1985. Comparisons between the experiments with early and late start dates show considerable differences in the amplitude of the regional climate change patterns, particularly for sea level. The early industrialization experiment can be used to obtain a first estimate of the detection time for a greenhouse-gas-induced near-surface temperature signal. Detection time estimates are obtained using globally and zonally averaged data from the experiment and a long control run, as well as principal component time series describing the evolution of the dominant signal and noise modes. The latter approach yields the earliest detection time (in the decade 1990–2000) for the time-evolving near-surface temperature signal. For global-mean temperatures or for temperatures averaged between 45°N and 45°S, the signal detection times are in the decades 2015–2025 and 2005–2015, respectively. The reduction of the cold start error in the early industrialization experiment makes it possible to separate the near-surface temperature signal from the noise about one decade earlier than in the experiment starting in 1985. We stress that these detection times are only valid in the context of the coupled model's internally-generated natural variability, which possibly underestimates low frequency fluctuations and does not incorporate the variance associated with changes in external forcing factors, such as anthropogenic sulfate aerosols, solar variability or volcanic dust. 相似文献
935.
Summary A three-dimensional, nonhydrostatic numerical model with high spatial resolution, in which a simple energy closure scheme
is employed, has been developed to simulate the spray dispersion over complex terrain. The evaporation, condensation, and
dispersion of the spray and moisture are taken into account in model equations. The term of latent heat due to phase transformation
is considered in detail to account for its effects on the temperature field and airflow. As an application of the model, the
spray concentration and air relative humidity are calculated under neutral condition. The results indicate that under the
neutral condition, the spray is transported to about 0.6 km downwind from the source, and its effects on the air humidity
reach a further distance of 0.9 km downwind from the source.
Attention is given to the dependence of the results upon the various factors influencing the simulation, such as the intensity
of the source, the atmospheric stratification, and the dynamic factor of the terrain. Some numerical tests were carried out
to provide extra insight to the effects of these factors. It has been demonstrated that the simulation results such as relative
humidity and temperature are sensitive to these factors, especially to the thermal stratification. Under unstable conditions,
the effects of the spray source increase significantly, and the variation extent of the temperature, relative humidity and
flow field is larger than that under neutral condition. The effects of dynamic and thermal factors on the air flow field are
discussed through the comparison of the modeling results over complex terrain and flat terrain.
Received June 8, 1998 Revised April 17, 1999 相似文献
936.
A note on estimating the effect of a limited fetch on micrometeorological evaporation measurements 总被引:7,自引:0,他引:7
J. H. C. Gash 《Boundary-Layer Meteorology》1986,35(4):409-413
A formula for the effective fetch of micrometeorological evaporation measurements is derived by application of diffusion theory, using Calder's approximation of a uniform wind field and neutral atmospheric stability. This simplification allows estimation of the likely sampling error, which would result from an upwind step-change in evaporation rate, without change in roughness. 相似文献
937.
We present a comparison of the zonal mean meridional circulations derived from monthly in situ data (i.e. radiosondes and
ship reports) and from the NCEP/NCAR reanalysis product. To facilitate the interpretation of the results, a third estimate
of the mean meridional circulation is produced by subsampling the reanalysis at the locations where radiosonde and surface
ship data are available for the in situ calculation. This third estimate, known as the subsampled estimate, is compared to
the complete reanalysis estimate to assess biases in conventional, in situ estimates of the Hadley circulation associated
with the sparseness of the data sources (i.e., radiosonde network). The subsampled estimate is also compared to the in situ
estimate to assess the biases introduced into the reanalysis product by the numerical model, initialization process and/or
indirect data sources such as satellite retrievals. The comparisons suggest that a number of qualitative differences between
the in situ and reanalysis estimates are mainly associated with the sparse sampling and simplified interpolation schemes associated
with in situ estimates. These differences include: (1) a southern Hadley cell that consistently extends up to 200 hPa in the
reanalysis, whereas the bulk of the circulation for the in situ and subsampled estimates tends to be confined to the lower
half of the troposphere, (2) more well-defined and consistent poleward limits of the Hadley cells in the reanalysis compared
to the in-situ and subsampled estimates, and (3) considerably less variability in magnitude and latitudinal extent of the
Ferrel cells and southern polar cell exhibited in the reanalysis estimate compared to the in situ and subsampled estimates.
Quantitative comparison shows that the subsampled estimate, relative to the reanalysis estimate, produces a stronger northern
Hadley cell (∼20%), a weaker southern Hadley cell (∼20–60%), and weaker Ferrel cells in both hemispheres. These differences
stem from poorly measured oceanic regions which necessitate significant interpolation over broad regions. Moreover, they help
to pinpoint specific shortcomings in the present and previous in situ estimates of the Hadley circulation. Comparisons between
the subsampled and in situ estimates suggest that the subsampled estimate produces a slightly stronger Hadley circulation
in both hemispheres, with the relative differences in some seasons as large as 20–30%. 6These differences suggest that the
mean meridional circulation associated with the NCEP/NCAR reanalysis is more energetic than observations suggest. Examination
of ENSO-related changes to the Hadley circulation suggest that the in situ and subsampled estimates significantly overestimate
the effects of ENSO on the Hadley circulation due to the reliance on sparsely distributed data. While all three estimates
capture the large-scale region of low-level equatorial convergence near the dateline that occurs during El Nino, the in situ
and subsampled estimates fail to effectively reproduce the large-scale areas of equatorial mass divergence to the west and
east of this convergence area, leading to an overestimate of the effects of ENSO on the zonal mean circulation.
Received: 16 September 1998 / Accepted: 22 April 1999 相似文献
938.
Undulation and anomaly estimation using Geos-3 altimeter data without precise satellite orbits 总被引:1,自引:0,他引:1
The paper describes results obtained from the processing of 53 Geos-3 arcs of altimeter data obtained during the first weeks
after the launch of the satellite in April, 1975. The measurement from the satellite to the ocean surface was used to obtain
an approximate geoid undulation which was contaminated by long wavelength errors caused primarily by altimeter bias and orbit
error. This long wavelength error was reduced by fitting with a low degree polynomial the raw undulation data to the undulations
implied by the GEM 7 potential coefficients, in an adjustment process that included conditions on tracks that cross. The root
mean square crossover discrepancy before this adjustment was ±12.4 meters while after the adjustment it was ±0.9 m. These
adjusted undulations were used to construct a geoid map in the Geos-3 calibration area using a least squares filter to remove
remaining noise in the undulations. Comparing these undulations to ones computed from potential coefficients and terrestrial
gravity data indicates a mean difference of 0.25 m and a root mean square difference of ±1.92 m.
The adjusted undulations were also used to estimate several 5o, 2o, and 1o anomalies using the method of least squares collocation. The resulting predictions agreed well with known values although
the 1o x 1o anomalies could not be considered as reliably determined. 相似文献
939.
Modelling the Spatial Distribution of DEM Error 总被引:7,自引:0,他引:7
Bruce H. Carlisle 《Transactions in GIS》2005,9(4):521-540
940.
鄱阳湖水龄季节性变化特征 总被引:2,自引:1,他引:2
基于环境水动力学模型EFDC源程序,建立了染色剂模型和水龄模型,在将模型与航测水文数据验证吻合的基础上,分别计算了鄱阳湖自然条件下春、夏、秋、冬季的水龄和倒灌前后鄱阳湖染色剂和水龄分布的变化,以及五河水系各分支河流水龄.分季节的水龄计算表明鄱阳湖水体交换受季节性来水影响明显.夏、秋季的水龄相对较小,在多数年份又受到长江水倒灌的影响导致水龄有所增大;冬、春季水龄较大,亦无长江水倒灌现象,相较于夏、秋季,水域面积明显减少.分支流的水龄计算表明,西南湖区的水体交换主要受到赣江的影响,西北湖区水体交换主要受到修水和赣江的影响,南部湖区主要受到抚河与信江的影响,东部湖区主要受到饶河的影响,湖心区和入江水道则受到五河水系的综合影响.同时水龄的研究表明拟建的鄱阳湖水利枢纽工程"调枯不调洪"的原则是合理的,为鄱阳湖水利枢纽工程论证提供了重要的参考依据. 相似文献