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61.
Flux-Variance Method for Latent Heat and Carbon Dioxide Fluxes in Unstable Conditions 总被引:4,自引:0,他引:4
Xiaofeng Guo Hongsheng Zhang Xuhui Cai Ling Kang Tong Zhu Monique Y. Leclerc 《Boundary-Layer Meteorology》2009,131(3):363-384
Applied previously to momentum and heat fluxes, the present study extends the flux-variance method to latent heat and CO2 fluxes in unstable conditions. Scalar similarity is also examined among temperature (θ), water vapour (q), and CO2 (c). Temperature is adopted as the reference scalar, leading to two feasible strategies to estimate latent heat and CO2 fluxes: the first one relies on flux-variance similarity relations for scalars, while the second is based on the parameterization
of relative transport efficiency in terms of scalar correlation coefficient and a non-dimensional quantity. The relationship
between the θ-to-q transport efficiency (λ
θ
q
) and θ-q correlation coefficient (R
θ
q
) is used to describe the intermediate hydrological conditions. We also parameterize the θ-to-c transport efficiency (λ
θ
c
) as a function of the θ-c correlation coefficient (R
θ
c
) by introducing a new non-dimensional ratio (α). The flux-variance method is a viable technique for flux gap-filling, when turbulence measurements of wind velocity are
not available. It is worth noting that the extended method is not exempt from a correction for density effects when used for
estimating water or carbon exchange. 相似文献
62.
A Simple Method of Estimating Scalar Fluxes Over Forests 总被引:1,自引:0,他引:1
Nelson Luis Dias Jinkyu Hong Monique Y. Leclerc T. Andrew Black Z. Nesic P. Krishnan 《Boundary-Layer Meteorology》2009,132(3):401-414
A simple aerodynamic-variance method is proposed to fill gaps in continuous CO2 flux measurements in rainy conditions, when open-path analysers do not function. The method requires turbulent conditions
(friction velocity greater than 0.1 ms–1), and uses measurements of mean wind speed, and standard deviations of temperature and CO2 concentration fluctuations to complement, and at times replace, eddy-covariance measurements of friction velocity, sensible
heat flux and CO2 flux. Friction velocity is estimated from the mean wind speed with a flux-gradient relationship modified for the roughness
sublayer. Since normalised standard deviations do not follow Monin-Obukhov similarity theory in the roughness sublayer, a
simple classification scheme according to the scalar turbulence scale was used. This scheme is shown to produce sensible heat
and CO2 flux estimates that are well correlated with the measured values. 相似文献
63.
The turbulence field obtained using a large-eddy simulation model is used to simulate particle dispersion in the convective
boundary layer with both forward-in-time and backward-in-time modes. A Lagrangian stochastic model is used to treat subgrid-scale
turbulence. Results of forward dispersion match both laboratory experiments and previous numerical studies for different release
heights in the convective boundary layer. Results obtained from backward dispersion show obvious asymmetry when directly compared
to results from forward dispersion. However, a direct comparison of forward and backward dispersion has no apparent physical
meaning and might be misleading. Results of backward dispersion can be interpreted as three-dimensional or generalized concentration
footprints, which indicate that sources in the entire boundary layer, not only sources at the surface, may influence a concentration
measurement at a point. Footprints at four source heights in the convective boundary layer corresponding to four receptors
are derived using forward and backward dispersion methods. The agreement among footprints derived with forward and backward
methods illustrates the equivalence between both approaches. The paper shows explicitly that Lagrangian simulations can yield
identical footprints using forward and backward methods in horizontally homogeneous turbulence. 相似文献
64.
65.
66.
Seventy-five species and lower taxonomic units of rotifers have been identified in Lake Geneva (= lac Léman); seven species and one form are new for the lake. When using indicators of trophic conditions, the qualitative composition of the biocenosis is still dominated by oligo-mesotrophic lake indicators, mixed with eutrophic and oligotrophic ones. However the rotifer biocenosis has not yet reached a steady state. 相似文献
67.
Isabelle Larocque-Tobler Roberto Quinlan Monique M. Stewart Martin Grosjean 《Quaternary Science Reviews》2011,30(13-14):1770-1779
Chironomids (non-biting midges) can provide accurate climate reconstructions from the Late Glacial to the present. Until now, anoxic lakes have been avoided for temperature reconstructions since chironomid assemblages are sensitive to changes in oxygen concentrations in the hypolimnion. However, anoxic lakes may have varved sediments, providing the possibility for near-annual climate reconstructions. Here, we tested the applicability of two calibration methods to reconstruct mean July air temperatures from chironomid assemblages preserved in the sediments of the anoxic Seebergsee located in the northern Swiss Alps: a calibration-in-space approach and a calibration-in-time approach. The calibration-in-space approach (i.e. chironomid assemblages from surficial lake sediments (0–1 cm) calibrated against meteorological data) provided accurate inferences (i.e. similar temperature changes as measured at the closest meteorological station, and at regional stations) in the Seebergsee stratigraphy until anoxia increased in the lake. With the increase of anoxia, the chironomid-inferred temperatures were generally colder than measured temperatures. A calibration-in-time approach (i.e. calibration of chironomid assemblages in a time series against instrumental data from the closest meteorological station) provided accurate reconstructions (i.e. similar to the regional records) for the past 100 years, including the time period of inferred anoxia. However, its applicability should be further tested on longer temporal scales. 相似文献
68.
69.
Didier?LaporteEmail author Michael?J.?Toplis Monique?Seyler Jean-Luc?Devidal 《Contributions to Mineralogy and Petrology》2004,146(4):463-484
A new technique that allows extraction of liquids from peridotite at degrees of melting as low as 0.2 wt% is presented. Microfractures that formed in the graphite sample container at the beginning of the experiments were used as traps for the liquid phase. Glass-filled cracks (or 'microdikes') unaffected by quench crystallisation were produced in all experiments and were analysed using standard electron microprobe techniques. Reversal experiments demonstrated that, at moderate degrees of melting (4.4 and 6.5 wt%), the segregated liquid was in equilibrium with the neighbouring peridotite. At very low degrees of melting (0.3 wt%), the liquid in the microdikes failed to fully equilibrate with the peridotite after 5 days and the sandwich technique was used in combination with the microdike technique to approach more closely the equilibrium composition of near-solidus partial melts. The microdike technique was used to study melting of a depleted peridotite at 1 GPa and 1,220 to 1,360 °C.Editorial responsibility: T.L. Grove 相似文献
70.
We perform Monte Carlo simulations of synthetic EMSS cluster samples, to quantify the systematic errors and the statistical uncertainties on the estimate of Ω0 derived from fits to the cluster number density evolution and to the X-ray temperature distribution up to z =0.83 . We identify the scatter around the relation between cluster X-ray luminosity and temperature to be a source of systematic error, of the order of Δsyst Ω0 =0.09 , if not properly taken into account in the modelling. After correcting for this bias, our best Ω0 is 0.66. The uncertainties on the shape and normalization of the power spectrum of matter fluctuations imply relatively large uncertainties on this estimate of Ω0 , of the order of Δstat Ω0 =0.1 at the 1 σ level. On the other hand, the statistical uncertainties due to the finite size of the high-redshift sample are twice as small. Therefore, what is needed in order to improve the accuracy of Ω0 estimates based on cluster number density evolution is a more reliable measure of the local temperature function and a better understanding of the cluster observed properties both in the local Universe and at high redshift, that is the relation between cluster mass, temperature and luminosity. This requires detailed observations of X-ray selected cluster samples, in comparison with hydrodynamic simulations including refined physics. 相似文献