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21.
22.
The Effect of Stratification on the Aerodynamic Roughness Length and Displacement Height 总被引:1,自引:0,他引:1
S. S. Zilitinkevich I. Mammarella A. A. Baklanov S. M. Joffre 《Boundary-Layer Meteorology》2008,129(2):179-190
The roughness length, z
0u
, and displacement height, d
0u
, characterise the resistance exerted by the roughness elements on turbulent flows and provide a conventional boundary condition
for a wide range of turbulent-flow problems. Classical laboratory experiments and theories treat z
0u
and d
0u
as geometric parameters independent of the characteristics of the flow. In this paper, we demonstrate essential stability
dependences—stronger for the roughness length (especially in stable stratification) and weaker but still pronounced for the
displacement height. We develop a scaling-analysis model for these dependences and verify it against experimental data. 相似文献
23.
Nijs Jan Duijm 《Boundary-Layer Meteorology》1999,91(1):1-22
Some methods are evaluated and extended to estimate roughness length and zero plane displacement height for atmospheric flow over arrays of obstacles, typically buildings. It appears that the method proposed by Bottema, with an extension to account for low density obstacle arrays, performs best. Procedures are proposed to represent irregular obstacle arrangements by a representative regular array to which Bottema's method can be applied. It is shown that this can be done without loss of accuracy, in general, roughness length can be predicted within a factor of two in more than 74% of the cases (95% reliability estimate). The methods proposed by Lettau and Raupach have been included in the evaluation. Lettau's model, which only requires input on the frontal area density, predicts roughness length unbiassed for frontal area densities up to 0.3, but predictions will be within a factor of two in more than 59% of the cases only (95% reliability estimate). 相似文献
24.
In this experimental study,field observations and laboratory experiments have been carried out to assess the impacts of the vegetated channel walls and aspect ratio on flow velocity profiles,shear stress distribution and roughness coefficient of channel.Results show that the presence of vegetation cover on channel wall causes deviation of the Reynolds stress distribution from the linear one under uniform flow condition.It is also noticed that the Reynolds stress distribution is influenced by the aspect rati... 相似文献
25.
26.
The effects of scene heterogeneity on soil moisture retrieval from passive microwave data 总被引:1,自引:0,他引:1
The τ–ω model of microwave emission from soil and vegetation layers is widely used to estimate soil moisture content from passive microwave observations. Its application to prospective satellite-based observations aggregating several thousand square kilometres requires understanding of the effects of scene heterogeneity. The effects of heterogeneity in soil surface roughness, soil moisture, water area and vegetation density on the retrieval of soil moisture from simulated single- and multi-angle observing systems were tested. Uncertainty in water area proved the most serious problem for both systems, causing errors of a few percent in soil moisture retrieval. Single-angle retrieval was largely unaffected by the other factors studied here. Multiple-angle retrievals errors around one percent arose from heterogeneity in either soil roughness or soil moisture. Errors of a few percent were caused by vegetation heterogeneity. A simple extension of the model vegetation representation was shown to reduce this error substantially for scenes containing a range of vegetation types. 相似文献
27.
F. Petit 《地球表面变化过程与地形》1990,15(2):135-148
Shear stresses were evaluated at different sites on two rivers. The first (the Rulles) is characterized by a pebbly bedload and a meandering bed with riffles and pools. The second (the Rouge Eau) has mainly a sandy rippled bed where meandering is well developed but also flat gravelly sectors without meandering system. Shear stresses calculated from friction velocities (τ*) using a redefined y1 roughness height parameter were compared with total shear stresses calculated from the energy grade line and the hydraulic radius (τ), Divergence between these shear stresses seems to increase in the presence of bedforms and large-scale irregularities of the channel. The τ*/τ ratio is close to 0·5 in the gravelly sector of the Rouge Eau and reaches 0·65 in the riffles of the Rulles (generally located at the inflexion point of the meanders), while it is less than 0·3 in the pools of the same river (located in the loops) and only 0·2 in the sandy rippled sector of the Rouge Eau. Grain and bedform shear stresses were evaluated at these same sites by different methods. The grain shear stress (τ') represents on average 30 per cent of the total shear stress in the riffles of the Rulles and the gravelly sector of the Rouge Eau, but less than 15 per cent in the pools in the Rulles and the sandy sectors of the Rouge Eau. However, it emerges from experiments conducted with marked pebbles and in situ observations of erosion and transport of sandy and gravelly particles, that the grain shear stresses are underestimated and cannot explain the movements and modifications actually observed. Conversely, shear stresses calculated from friction velocities at the sites where erosion actually occurred (or failed to occur despite very high velocities) provide a better explanation of the observed movements. 相似文献
28.
通过对四种参数化方法的比较,认为风海雷诺数RB较风速、波龄和波陡能更好地描述拖曳系数CDN。利用分组平均法对CDN与RB关系进行处理,得到最佳的CDN参数化方案。利用COARE3算法测试了四种依赖海况的粗糙长度,与实测结果进行了比较,结果表明CDN与RB的关系式更真实地反映了海-气界面动量交换过程。 相似文献
29.
Water-flume experiments are conducted to study the structure of turbulent flow within and above a sparse model canopy consisting
of two rigid canopies of different heights. This difference in height specifies a two-dimensional step change from a rough
to a rougher surface, as opposed to a smooth-to-rough transition. Despite the fact that the flow is in transition from a rough
to a rougher surface, the thickness of the internal boundary layer scales as x
4/5, consistent with smooth-to-rough boundary layer adjustment studies, where x is the downstream distance from the step change. However, the analogy with smooth-to-rough transitions no longer holds when
the flow inside the canopy and near the canopy top is considered. Results show that the step change in surface roughness significantly
increases turbulence intensities and shear stress. In particular, there is an adjustment of the mean horizontal velocity and
shear stress as the flow passes over the rougher canopy, so that their vertical profiles adjust to give maximum values at
the top of this canopy. We also observe that the magnitude and shape of the inflection in the mean horizontal velocity profile
is significantly affected by the transition. The horizontal and vertical turbulence spectra compare well with Kolmogorov’s
theory, although a small deviation at high frequencies is observed in the horizontal spectrum within the canopy. Here, for
relatively low leaf area index, shear is found to be a more effective mechanism for momentum transfer through the canopy structure
than vortex shedding. 相似文献
30.
The mean flow and scalar concentration profiles within and above a tall canopy are well known to violate the standard boundary-layer
flux-gradient relationships. We present a theory for the scalar concentration profile that is comprised of a canopy exchange
model coupled to a modified surface-layer model. The coupling between the two components and the modifications to the surface-layer
profiles are formulated through the mixing-layer analogy for the flow at canopy top. This analogy provides an additional length
scale—the vorticity thickness—upon which the profiles depend and a set of criteria that allows a reduction in the empiricism
associated with earlier forms in the literature. Predictions of the mean scalar concentration profiles are shown to match
observations over a wide range of diabatic stabilities for both potential temperature and water vapour. 相似文献