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991.
Ronaldo Rogrio de Freitas Mouro 《Astronomische Nachrichten》1977,298(4):227-232
The physical and orbital elements of the binary star ADS 1538 = Σ 168 (α1900 = 01h50m.7, δ1900 = +01°21'; α2000 = 01h55m·8, δ2000 = +01°51') are determined. The absolute visual magnitudes of the two components are 4m.04 resp. 4m.17, the masses 1.34 M⊙ resp. 1.29 M⊙. 相似文献
992.
993.
Summary Rainfall anomaly patterns are obtained for the city of Barcelona from a statistical and a spectral point of view. The time
series consists of monthly rainfall amounts recorded over 128 years without interruption. Monthly positive and negative anomalies,
obtained as the difference between monthly amounts and monthly threshold values, are used for both types of analyses. The
threshold levels are derived form the deciles of theoretical monthly rainfall distributions, which have been previously modelled
by the gamma distribution. Positive and negative anomalies of the monthly rain amounts are investigated for these threshold
levels. The statistical analysis is applied to each decile considered, yielding empirical exponential laws that can be used
to forecast the cumulative number of episodes of consecutive months with either positive or negative anomalies equalling or
exceeding a fixed length. A set of linear laws, relating the expected rainfall amount cumulated during an episode of a fixed
length, is also deduced. It is worthy of mention that, independently of the decile considered, all the exponential and linear
laws have satisfactory regression coefficients. At the same time, it has also been possible to establish the evolution of
the coefficients of these laws with respect to the different deciles considered. The exponential laws for episodes of positive
and negative anomalies are the starting point, together with two hypotheses, to model probabilities of repeated long episodes
over an arbitrary number of years and their return periods in terms of the Poisson distribution model. Moreover, power spectra
are derived for anomalies relative to the 50% decile at monthly and seasonal scale. The spectral estimates obtained are then
compared with theoretical spectra deduced from possible Markovian or random behaviour of the time series of anomalies. Finally,
the significant spectral peaks are discussed and compared with other significant spectral components deduced for some areas
of the Mediterranean domain.
Received November 11, 1999 Revised February 28, 2000 相似文献
994.
A Matrix Approach Coupled with Monte Carlo Techniques for Solving the Net Radiative Balance of the Urban Block 总被引:2,自引:0,他引:2
Marta J. N. Oliveira Panão Helder J. P. Gonçalves Paulo M. C. Ferrão 《Boundary-Layer Meteorology》2007,122(1):217-241
A new method is developed for solving the shortwave and longwave net radiative balance of a three-dimensional urban structure,
represented by parallelepiped blocks uniformly distributed in each direction. The method is based on a novel approach to determine
the shape factors among surfaces, which are estimated by Monte Carlo techniques due to the complex geometry associated with
the three-dimensional urban structure. Then, a set of linear equations is solved to quantify the radiative balance, in order
to obtain their exact solution, considering all the inter-reflections among surfaces. The comparison between the new and the
ray-tracing tracking methods resulted in a Pearson correlation coefficient of 0.996. However, by integrating the linear equations’
exact solution with Monte Carlo techniques, the new method reduces by a factor of 36 the central processing unit (CPU) time
used to perform the calculations of the ray-tracing tracking method. The use of the model for a sensitivity study allows us
to verify the effective absorptance and emittance increases with the canyon aspect ratio of the urban layout. An urban structure
formed by square cross-sectional blocks absorbs more solar radiation than an urban structure formed by rectangular cross-sectional
blocks. The approximation of a specific geometry for an equivalent bi-dimensional infinite street can be applied for rectangular
cross-sectional blocks, where the width is 11 times or more greater than the depth dimension. 相似文献
995.
Summary ?One of the most important features in analysing the climatology of any region is to study the precipitation and its periodicity
of different harmonics in order to study the behavior of the observed data. In this study the amplitude of frequencies, phase
angle and basic statistical parameters are calculated in order to depict spatial characteristics of precipitation over Jordan.
Precipitation records of 17 stations were chosen according to climatic regions of Jordan. The first and second harmonic analyses
explain more than 90% of the precipitation variation in Jordan effectively. The amplitudes of the first and second harmonic
were calculated in order to describe the climatic regions in the country. The maximum amplitudes were found in the northern
mountainous region. The phase angle representing the time of maximum rainfall is also used in the form of a contour chart.
It is found that Jordan has its main rainfall season in winter with maximum around January. The coefficient of variation shows
the high variability of rainfall of the country.
Received February 4, 2002; revised August 1, 2002; accepted August 6, 2002 相似文献
996.
Soil occupation and atmospheric variations over Sobradinho Lake area. Part two: a regional modeling study 总被引:1,自引:0,他引:1
M. F. Correia M. A. F. da Silva Dias M. R. da Silva Aragão 《Meteorology and Atmospheric Physics》2006,94(1-4):115-128
Summary The impact of the changes on soil cover and land use brought about by the construction of the Sobradinho Dam in the semi-arid
region of the S?o Francisco River Hydrographic Basin is analyzed by means of a numerical model RAMS. Disregarding the influence
of a large scale flow, a set of factors were responsible for the creation of a rather complex circulation system that includes
mountain-valley winds, lake breeze (LB) and non-conventional circulation all induced by the surface non-homogeneous aspect.
Results have demonstrated that the implementation of works of such magnitude brings about environmental changes in an area
that stretches far beyond the surroundings of the reservoir. The soil cover alterations due to the ever increasing development
of the area with the presence of irrigated crops in a sparsely vegetated region (caatinga) does affect land surface characteristics, occasioning for that matter the splitting of the available energy into latent
and sensible heat fluxes. LB behavior varies in accordance with atmospheric conditions and also in view of the type of vegetation
found in the lake surrounding areas. Hydro availability in root zones, even under adverse atmospheric conditions (high temperature
and low air humidity) brings up the high rates of evaporation and plant transpiration that contribute towards the increase
of humidity and the fall of temperature in lower atmospheric layers. 相似文献
997.
Domingo Muñoz-Esparza Branko Kosović Jeff Mirocha Jeroen van Beeck 《Boundary-Layer Meteorology》2014,153(3):409-440
With a focus towards developing multiscale capabilities in numerical weather prediction models, the specific problem of the transition from the mesoscale to the microscale is investigated. For that purpose, idealized one-way nested mesoscale to large-eddy simulation (LES) experiments were carried out using the Weather Research and Forecasting model framework. It is demonstrated that switching from one-dimensional turbulent diffusion in the mesoscale model to three-dimensional LES mixing does not necessarily result in an instantaneous development of turbulence in the LES domain. On the contrary, very large fetches are needed for the natural transition to turbulence to occur. The computational burden imposed by these long fetches necessitates the development of methods to accelerate the generation of turbulence on a nested LES domain forced by a smooth mesoscale inflow. To that end, four new methods based upon finite amplitude perturbations of the potential temperature field along the LES inflow boundaries are developed, and investigated under convective conditions. Each method accelerated the development of turbulence within the LES domain, with two of the methods resulting in a rapid generation of production and inertial range energy content associated to microscales that is consistent with non-nested simulations using periodic boundary conditions. The cell perturbation approach, the simplest and most efficient of the best performing methods, was investigated further under neutral and stable conditions. Successful results were obtained in all the regimes, where satisfactory agreement of mean velocity, variances and turbulent fluxes, as well as velocity and temperature spectra, was achieved with reference non-nested simulations. In contrast, the non-perturbed LES solution exhibited important energy deficits associated to a delayed establishment of fully-developed turbulence. The cell perturbation method has negligible computational cost, significantly accelerates the generation of realistic turbulence, and requires minimal parameter tuning, with the necessary information relatable to mean inflow conditions provided by the mesoscale solution. 相似文献
998.
Domingo Muñoz-Esparza Branko Kosović Clara García-Sánchez Jeroen van Beeck 《Boundary-Layer Meteorology》2014,151(3):453-478
The applicability of the one-way nesting technique for numerical simulations of the heterogeneous atmospheric boundary layer using the large-eddy simulation (LES) framework of the Weather Research and Forecasting model is investigated. The focus of this study is on LES of offshore convective boundary layers. Simulations were carried out using two subgrid-scale models (linear and non-linear) with two different closures [diagnostic and prognostic subgrid-scale turbulent kinetic energy (TKE) equations]. We found that the non-linear backscatter and anisotropy model with a prognostic subgrid-scale TKE equation is capable of providing similar results when performing one-way nested LES to a stand-alone domain having the same grid resolution but using periodic lateral boundary conditions. A good agreement is obtained in terms of velocity shear and turbulent fluxes, while velocity variances are overestimated. A streamwise fetch of 14 km is needed following each domain transition in order for the solution to reach quasi-stationary results and for the velocity spectra to generate proper energy content at high wavelengths, however, a pile-up of energy is observed at the low-wavelength portion of the spectrum on the first nested domain. The inclusion of a second nest with higher resolution allows the solution to reach effective grid spacing well within the Kolmogorov inertial subrange of turbulence and develop an appropriate energy cascade that eliminates most of the pile-up of energy at low wavelengths. Consequently, the overestimation of velocity variances is substantially reduced and a considerably better agreement with respect to the stand-alone domain results is achieved. 相似文献
999.
Summary A mesoscale convective system that affected Northeastern Spain on October 10, 1994, with rainfall amounts up to 400 mm, is simulated reasonably well by a nested 3-dimensional hydrostatic mesoscale model. Previous studies carried out in this region had already portrayed the main synoptic patterns that give rise to these devastating episodes. The present contribution takes a further step since it goes down to the mesoscale by means of a numerical model providing a more detailed representation not otherwise achieved by earlier analysis methods. Although the model was unable to forecast accurately the precipitation fields, it captured satisfactorily the framework in which the convective system originated and evolved.With 16 Figures 相似文献
1000.
Alexandre Bernardes Pezza Luke Andrew Garde José Augusto Paixão Veiga Ian Simmonds 《Climate Dynamics》2014,42(1-2):453-466
New aspects of the genesis and partial tropical transition of a rare hybrid subtropical cyclone on the eastern Australian coast are presented. The ‘Duck’ (March 2001) attracted more recent attention due to its underlying genesis mechanisms being remarkably similar to the first South Atlantic hurricane (March 2004). Here we put this cyclone in climate perspective, showing that it belongs to a class within the 1 % lowest frequency percentile in the Southern Hemisphere as a function of its thermal evolution. A large scale analysis reveals a combined influence from an existing tropical cyclone and a persistent mid-latitude block. A Lagrangian tracer showed that the upper level air parcels arriving at the cyclone’s center had been modified by the blocking. Lorenz energetics is used to identify connections with both tropical and extratropical processes, and reveal how these create the large scale environment conducive to the development of the vortex. The results reveal that the blocking exerted the most important influence, with a strong peak in barotropic generation of kinetic energy over a large area traversed by the air parcels just before genesis. A secondary peak also coincided with the first time the cyclone developed an upper level warm core, but with insufficient amplitude to allow for a full tropical transition. The applications of this technique are numerous and promising, particularly on the use of global climate models to infer changes in environmental parameters associated with severe storms. 相似文献