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991.
992.
993.
In recent years field experiments have been undertaken in the lower atmosphere to perform a priori tests of subgrid-scale
(SGS) models for large-eddy simulations (LES). The experimental arrangements and data collected have facilitated studies of
variables such as the filtered strain rate, SGS stress and dissipation, and the eddy viscosity coefficient. However, the experimental
set-ups did not permit analysis of the divergence of the SGS stress (the SGS force vector), which is the term that enters
directly in the LES momentum balance equations. Data from a field experiment (SGS2002) in the west desert of Utah, allows
the calculation of the SGS force due to the unique 4 × 4 sonic anemometer array. The vector alignment of the SGS force is
investigated under a range of atmospheric stabilities. The eddy viscosity model is likely aligned with the measured SGS force
under near-neutral and unstable conditions, while its performance is unsatisfactory under stable conditions. 相似文献
994.
Andrey Sogachev 《Boundary-Layer Meteorology》2009,130(3):423-435
The note presents a rational approach to modelling the source/sink due to vegetation or buoyancy effects that appear in the
turbulent kinetic energy, E, equation and a supplementary equation for a length-scale determining variable, φ, when two-equation closure is applied to canopy and atmospheric boundary-layer flows. The approach implements only standard
model coefficients C
φ1 and C
φ2 in the production and destruction terms of the φ equation, respectively. Numerical tests illustrate the practical applicability of the method, where, for example, simulations
with the E–ω model (where is the specific dissipation and is the dissipation rate of E) properly reproduce both the surface-layer wind profile estimated from the Monin-Obukhov similarity theory and the mixing-height
evolution observed above forested terrain in Southern Finland. 相似文献
995.
The intermittent structure of turbulence within the canopy sublayer (CSL) is sensitive to the presence of foliage and to the
atmospheric stability regime. How much of this intermittency originates from amplitude variability or clustering properties
remains a vexing research problem for CSL flows. Using a five-level set of measurements collected within a dense hardwood
canopy, the clustering properties of CSL turbulence and their dependence on atmospheric stability are explored using the telegraphic
approximation (TA). The binary structure of the TA removes any amplitude variability from turbulent excursions but retains
their zero-crossing behaviour, and thereby isolating the role of clustering in intermittency. A relationship between the spectral
exponents of the actual and the TA series is derived across a wide range of atmospheric stability regimes and for several
flow variables. This relationship is shown to be consistent with a relationship derived for long-memory and monofractal processes
such as fractional Brownian motion (fBm). Moreover, it is demonstrated that for the longitudinal and vertical velocity components,
the vegetation does not appreciably alter fine-scale clustering but atmospheric stability does. Stable atmospheric stability
conditions is characterized by more fine scale clustering when compared to other atmospheric stability regimes. For scalars,
fine-scale clustering above the canopy is similar to its velocity counterpart but is significantly increased inside the canopy,
especially under stable stratification. Using simplified scaling analysis, it is demonstrated that clustering is much more
connected to space than to time within the CSL. When comparing intermittency for flow variables and their TA series, it is
shown that for velocity, amplitude variations modulate intermittency for all stability regimes. However, amplitude variations
play only a minor role in scalar intermittency. Within the crown region of the canopy, a ‘double regime’ emerges in the inter-pulse
duration probability distributions not observed in classical turbulence studies away from boundaries. The double regime is
characterized by a power-law distribution for shorter inter-pulse periods and a log-normal distribution for large inter-pulse
periods. The co-existence of these two regimes is shown to be consistent with near-field/far-field scaling arguments. In the
near-field, short inter-pulse periods are controlled by the source strength, while in the far-field long inter-pulse periods
are less affected by the precise source strength details and more affected by the transport properties of the background turbulence. 相似文献
996.
Using the conditional average formulation, we suggest a new explanation for why the stress in the atmospheric surface layer is often observed to
vary with height. In essence, because turbulence series are always correlated for small lags, the steady-state equations of
motion with negligible viscous terms that traditionally require vertical fluxes to be constant with height accordingly now
require the vertical fluxes to vary with height. This result has implications for interpreting and validating Monin–Obukhov
similarity theory. 相似文献
997.
Philip S. Anderson 《Boundary-Layer Meteorology》2009,131(3):345-362
The empirical dependence of turbulence Prandtl number (Pr) on gradient Richardson number (Ri) is presented, derived so as to avoid the effects of self-correlation from common variables. Linear power relationships between
the underlying variables that constitute both Pr and Ri are derived empirically from flux and profile observations. Pr and Ri are then reconstructed from these power laws, to indicate their interdependence whilst avoiding self-correlation. Data are
selected according to the stability range prior to regression, and the process is iterated from neutral to higher stability
until error analysis indicates the method is no longer valid. A Butterworth function is fitted to the resulting Pr
−1(Ri) regression to give an empirical summary of the analysis. The form suggests that asymptotically Pr
−1 decreases as Ri
3/2. Scatter in the data increases above Ri ~ 1, however, indicating additional constraints to Pr are not captured by Ri alone in this high stability regime. The Butterworth function is analytic for all Ri > 0, and may be included in suitable boundary-layer parameterisation schemes where the turbulent diffusivity for heat is
derived from the turbulent diffusivity for momentum. 相似文献
998.
C.P. Anil Kumar C. Panneerselvam K.U. Nair H. Johnson Jeyakumar C. Selvaraj S. Gurubaran C. Venugopal 《Atmospheric Research》2009,91(2-4):201-205
In this paper we are concerned with the variation of the atmospheric electric field and the air–earth current due to the excessive power generated by the solar wind-magnetosphere dynamo during geomagnetic storms, recorded at Maitri in Antarctica during 2004. A major part of the power generated by the solar wind-magnetosphere dynamo is used in the formation of the ring current and the rest is utilized for Joule heating and auroral particle precipitation. The method adopted by Frank-Kamenetsky et al. [Frank-Kamenetsky A.V., Troshichev O.A., Burns G.B., Papitashvili V.O., 2001. Variations of atmospheric electric field in the near-pole region related to the interplanetary magnetic field. J. Geophys. Res. 106, A1, 179–190.] was utilized to delineate the variations due to the signatures of tropical thunderstorm activity from the geoelectric data; while statistical methods used in our earlier studies were used to delimit variation due to the constant buffeting of the solar wind. We find that the solar wind-magnetosphere energy coupling function (ε) to be well correlated with the atmospheric electric parameters during the onset of geomagnetic disturbances. However the correlation breaks down during minor storms and sub-storm events. 相似文献
999.
Clment Fesquet Philippe Drobinski Christian Barthlott Thomas Dubos 《Atmospheric Research》2009,94(2):254-269
This study investigates the impact of terrain heterogeneity on local turbulence measurements using 18 months of turbulence data taken on a 30 m tower at the SIRTA mixed land-use observatory under varying stability conditions and fetch configurations. These measurements show that turbulence variables such as the turbulent kinetic energy or momentum fluxes are strongly dependent on the upstream complexity of the terrain (presence of trees or buildings, open field). However, using a detection technique based on wavelet transforms which permits the isolation of the large-scale coherent structures from small-scale background fluctuations, the study shows that, for all stability conditions, whatever the upstream complexity of the terrain, the coherent structures display universal properties which are independent of the terrain nature: the frequency of occurrence, time duration of the coherent structures, the time separation between coherent structures and the relative contribution of the coherent structures to the total fluxes (momentum and heat) appear to be independent of the upstream roughness. This is an important result since coherent structures are known to transport a large portion of the total energy. This study extends to all stability conditions a numerical study by Fesquet et al. [Fesquet, C., Dupont, S., Drobinski, P., Barthlott, C., Dubos, T., 2008. Impact of terrain heterogeneities on coherent structures properties: experimental and numerical approaches. In: 18th Symposium on Boundary Layers and Turbulence. No. 11B.1. Stockholm, Sweden., Fesquet, C., Dupont, S., Drobinski, P., Dubos, T., Barthlott, C., in press. Impact of terrain heterogeneity on coherent structure properties: numerical approach. Bound.-Layer Meteorol.] conducted in neutral conditions which shows that a reason for such behavior is that the production of local active turbulence in an internal boundary layer associated with coherent structure originating from the outer layer and impinging onto the ground is not sensitive to the nature of the terrain. 相似文献
1000.
将区域海气耦合模式RegCM3-POM和区域气候模式RegCM3 40年(1963-2002年)的模拟结果与NCEP/NCAR再分析资料进行对比,检验区域海气耦合模式对中国华北地区夏季大气水汽含量和水汽输送特征的模拟能力,比较耦合模式与单独区域气候模式的差异.结果表明,区域海气耦合模式RegCM3-POM的模拟性能相对于单独区域气候模式RegCM3,大气水汽输送特征的模拟能力有了较大的改进.分析显示两种模式都能够较好地再现东哑地区气候平均夏季大气水汽储量浅红和水汽输送的空间分布特征,而耦合模式对大气水汽输送的模拟更为合理.在对流层中低层更接近观测;耦合模式对中国华北地区夏季平均大气水汽输送通量在垂直方向卜的分布型及水平4个边界水汽输送收支的模拟,相对于单独大气模式有了一定的改进;耦合模式对伴随华北地区夏季早涝的大气水汽异常输送也具有较好的模拟能力,其模拟的水汽输送异常的来源与观测基本一致,尤其是在20°N以北地区,耦合模式结果相对于单独区域气候模式有了很大的改进.但同时耦合模式在低纬度海洋上对气候平均夏季大气水汽含量模拟的偏差比区域气候模式显著;与观测相比,耦合模式对来自孟加拉湾地区的大气水汽输送模拟偏弱,而对西太平洋副热带高压西侧水汽输送模拟偏强,与华北夏季旱涝相联系的水汽输送异常的模拟在低纬度海洋上也存在明显偏差. 相似文献