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991.
Saurabh Singh Ravi Subrahmanyan N. Udaya Shankar Mayuri Sathyanarayana Rao B. S. Girish A. Raghunathan R. Somashekar K. S. Srivani 《Experimental Astronomy》2018,45(2):269-314
The global 21-cm signal from Cosmic Dawn (CD) and the Epoch of Reionization (EoR), at redshifts \(z \sim 6-30\), probes the nature of first sources of radiation as well as physics of the Inter-Galactic Medium (IGM). Given that the signal is predicted to be extremely weak, of wide fractional bandwidth, and lies in a frequency range that is dominated by Galactic and Extragalactic foregrounds as well as Radio Frequency Interference, detection of the signal is a daunting task. Critical to the experiment is the manner in which the sky signal is represented through the instrument. It is of utmost importance to design a system whose spectral bandpass and additive spurious signals can be well calibrated and any calibration residual does not mimic the signal. Shaped Antenna measurement of the background RAdio Spectrum (SARAS) is an ongoing experiment that aims to detect the global 21-cm signal. Here we present the design philosophy of the SARAS 2 system and discuss its performance and limitations based on laboratory and field measurements. Laboratory tests with the antenna replaced with a variety of terminations, including a network model for the antenna impedance, show that the gain calibration and modeling of internal additive signals leave no residuals with Fourier amplitudes exceeding 2 mK, or residual Gaussians of 25 MHz width with amplitudes exceeding 2 mK. Thus, even accounting for reflection and radiation efficiency losses in the antenna, the SARAS 2 system is capable of detection of complex 21-cm profiles at the level predicted by currently favoured models for thermal baryon evolution. 相似文献
992.
Summary A subset of world ocean monthly mean temperature climatology generated by Levitus and Boyer (1994), is utilised to describe the observed seasonal variability of the characteristics of the near-surface isothermal layer and thermocline for the entire tropical Indian Ocean (TIO). The most salient features of the observed annual cycle are described in terms of amplitude and phase of the annual and semi-annual frequencies employing Fourier analysis technique. On the annual mode, the near-surface isothermal layer depth (ILD), exhibits larger variability away from the equator with peak values in the northernmost Arabian Sea, the northernmost Bay of Bengal and the southern TIO, while on the semi-annual mode, it shows larger variability in the central Arabian Sea. The variability of the near-surface isothermal layer temperature (ILT), on the annual mode, is very weak in the warmpool region, and increases with latitude, while on the semi-annual mode, it shows larger variability in the northwestern Arabian Sea. The variability of 20°C isotherm topography (D20), on the annual mode, is weakest in the equatorial region and largest in the coastal regions of the Arabian Sea and the Bay of Bengal and in the southern T10, while on the semi-annual mode, it is prominent in the eastern and western equatorial regions. The thermocline gradient (TG) is very sharp below the warmpool region and diffuses meridionally. On the annual mode, it shows larger variability in the southern TIO, off Somalia and northernmost Arabian Sea, while on the semiannual mode, it shows larger variability in the southwestern Arabian Sea and eastern equatorial Indian Ocean. The relationship between near-surface isothermal layer and thermocline characteristics over an annual cycle are explored through correlation analysis. The correlation between ILD and ILT is strong over much of the basin with the exception of the equatorial and coastal upwelling/downwelling zones where internal ocean dynamics are important. In the southern TIO, entrainment of colder waters appears to be important in maintaining the annual cycle of ILT as strong correlation is noticed between ILT and TG. In the Indo-Pacific throughflow region and another region west of it, the annual Rossby waves appear to control D20, as correlations between D20 and other fields are strong in these regions. A similar strong correlation between D20 and ILD is also noticed in the southeastern Arabian Sea where mode-2 Rossby waves identified in numerical model solutions.With 5 Figures 相似文献
993.
994.
Summary. ?Cyclone track predictions in the Indian seas (Bay of Bengal and Arabian Sea) with a quasi-Lagrangian model (QLM) have been
attempted. QLM has a horizontal resolution of 40 km and 16 sigma levels in the vertical. It is integrated in a domain of about
4400 × 4400 km2. A new initialization procedure to provide initial fields for running the model has been designed. The initialization procedure
consists of updating the global model forecasts, used as first guess, provided by the National Center for Medium Range Weather
Forecasting (NCMRWF), New Delhi. A new version of IMD’s operational optimum interpolation scheme has been created to suit
the QLM grid structure. Lateral boundary conditions are computed from the extended forecasts of NCMRWF. The track forecasts
in each case show a reasonable skill of the forecast model in predicting the direction of movement within acceptable limits
of forecast errors, which are comparable to some of the best models operated by advanced NWP centers of the world. Even the
recurving storms are well predicted. Evolution of the vertical motion fields are also studied which reveal some interesting
features, which are described in detail in the text. The composited vertical motion fields are projected against observed
rainfall distribution, which show a good spatial correspondence.
Received August 9, 2001; revised March 12, 2002; accepted June 17, 2002
Published online: May 8, 2003 相似文献
995.
An analytical procedure to obtain the response of soil–structure interaction problems, time domain is described. The procedure makes use of large domain for descritization along with co-ordinate transformation using Lanczos vectors. The responses are obtained in time domain using an adaptive direct integration method. The scheme has the ability to estimate errors due to temporal discretization as well as co-ordinate transformation. The procedure has been applied to half-space problems and non-convex domains for validation of the scheme, and the scheme obeys causality condition in both the situations. The present method has all the advantages of time domain scheme which is local both in space and time with small computational effort. Copyright © 1999 John Wiley & Sons, Ltd. 相似文献
996.
2007年9-11月T213与ECMWF及日本模式中期预报性能检验 总被引:1,自引:0,他引:1
2007年9-11月冷空气频繁,气温变幅大,热带气旋活跃,给中期天气预报增加了一定的难度.为积累预报经验,提高中期预报准确率,对9-11月T213模式96小时预报产品进行了天气学检验,并与ECMWF、日本模式进行对比分析.结果表明,T213、ECMWF和日本模式对亚洲中高纬大气环流调整具有较好的中期预报性能,对转折性、灾害性天气的预报有较强指示意义.综合来看,ECMWF对天气系统和要素的预报误差最小,T213模式误差最大,日本模式温度预报性能与ECMWF相当.对0713号台风登陆前的位置和移速,ECMWF预报较为准确,T213模式对台风登陆前移速预报偏慢,对台风登陆后的位置预报偏差最小,日本模式对台风位置和强度预报较为失败. 相似文献
997.
998.
本文利用实例介绍峡谷河流水电站施工控制网的布设及作业方法。在带状及高差大地区注意本测区外业观测时间段,可以取得好的成果。 相似文献
999.
野外台站仪器安装结构不合理,不仅给台网的建设带不便,而且还影响台网的正常运行率,增加维护工作量,缩短设备的使用寿命。作者在文中提出了野外台站仪器结构模块化和采用密封箱式布局的具体方案,希望与从事地震遥测工作的同行探讨。 相似文献
1000.
K. Shankar Rao 《Boundary-Layer Meteorology》1999,90(3):541-549
Lagrangian stochastic models are well-suited for modeling dispersion in the stable boundary layer, especially in complex terrain. This note briefly describes the formulations and application of a Lagrangian stochastic model to predict dispersion of tracers released within nocturnal drainage flows. 相似文献