全文获取类型
收费全文 | 4917篇 |
免费 | 820篇 |
国内免费 | 1033篇 |
专业分类
测绘学 | 1652篇 |
大气科学 | 1090篇 |
地球物理 | 885篇 |
地质学 | 999篇 |
海洋学 | 872篇 |
天文学 | 279篇 |
综合类 | 520篇 |
自然地理 | 473篇 |
出版年
2024年 | 19篇 |
2023年 | 74篇 |
2022年 | 159篇 |
2021年 | 248篇 |
2020年 | 248篇 |
2019年 | 277篇 |
2018年 | 180篇 |
2017年 | 311篇 |
2016年 | 282篇 |
2015年 | 276篇 |
2014年 | 318篇 |
2013年 | 414篇 |
2012年 | 369篇 |
2011年 | 340篇 |
2010年 | 266篇 |
2009年 | 282篇 |
2008年 | 333篇 |
2007年 | 347篇 |
2006年 | 288篇 |
2005年 | 247篇 |
2004年 | 259篇 |
2003年 | 233篇 |
2002年 | 152篇 |
2001年 | 138篇 |
2000年 | 119篇 |
1999年 | 78篇 |
1998年 | 91篇 |
1997年 | 77篇 |
1996年 | 68篇 |
1995年 | 48篇 |
1994年 | 51篇 |
1993年 | 41篇 |
1992年 | 32篇 |
1991年 | 22篇 |
1990年 | 19篇 |
1989年 | 18篇 |
1988年 | 12篇 |
1987年 | 12篇 |
1986年 | 10篇 |
1985年 | 1篇 |
1984年 | 3篇 |
1982年 | 1篇 |
1980年 | 2篇 |
1978年 | 2篇 |
1954年 | 3篇 |
排序方式: 共有6770条查询结果,搜索用时 15 毫秒
31.
Estuary/ocean exchange and tidal mixing in a Gulf of Maine Estuary: A Lagrangian modeling study 总被引:2,自引:0,他引:2
Ata Bilgili Jeffrey A. Proehl Daniel R. Lynch Keston W. Smith M. Robinson Swift 《Estuarine, Coastal and Shelf Science》2005,65(4):607-624
A Lagrangian particle method embedded within a 2-D finite element code, is used to study the transport and ocean–estuary exchange processes in the well-mixed Great Bay Estuarine System in New Hampshire, USA. The 2-D finite element model, driven by residual, semi-diurnal and diurnal tidal constituents, includes the effects of wetting and drying of estuarine mud flats through the use of a porous medium transport module. The particle method includes tidal advection, plus a random walk model in the horizontal that simulates sub-grid scale turbulent transport processes. Our approach involves instantaneous, massive [O(500,000)] particle releases that enable the quantification of ocean–estuary and inter-bay exchanges in a Markovian framework. The effects of the release time, spring–neap cycle, riverine discharge and diffusion strength on the intra-estuary and estuary–ocean exchange are also investigated.The results show a rather dynamic interaction between the ocean and the estuary with a fraction of the exiting particles being caught up in the Gulf of Maine Coastal Current and swept away. Three somewhat different estimates of estuarine residence time are calculated to provide complementary views of estuary flushing. Maps of residence time versus release location uncover a strong spatial dependency of residence time within the estuary that has very important ramifications for local water quality. Simulations with and without the turbulent random walk show that the combined effect of advective shear and turbulent diffusion is very effective at spreading particles throughout the estuary relatively quickly, even at low (1 m2/s) diffusivity. The results presented here show that a first-order Markov Chain approach has applicability and a high potential for improving our understanding of the mixing processes in estuaries. 相似文献
32.
This paper addresses an important problem in ship control application—the robust stabilization of underactuated ships on a linear course with comfort. Specifically, we develop a multivariable controller to stabilize ocean surface ships without a sway actuator on a linear course and to reduce roll and pitch simultaneously. The controller adapts to unknown parameters of the ship and constant environmental disturbances induced by wave, ocean current and wind. It is also robust to time-varying environmental disturbances, time-varying change in ship parameters and other motions of the ship such as surge and heave. The roll and pitch can be made arbitrarily small while the heading angle and sway are kept to be in reasonably small bounds. The controller development is based on Lyapunov’s direct method and backstepping technique. A Lipschitz continuous projection algorithm is used to update the estimate of the unknown parameters to avoid the parameters’ drift due to time-varying environmental disturbances. Simulations on a full-scale catamaran illustrate the effectiveness of our proposed controller. 相似文献
33.
平台在海上受风、浪、流、冰等水平载荷作用产生倾斜 ,而土壤粘接力、摩擦力和土抗力是平台的抗拔和抗倾力 ,桶基平台应满足抗拔、抗倾稳定的要求 ,以保证平台的整体稳定性。讨论了桶基平台受水平力作用时 ,桶形基础的破坏模式及其计算方法 ,阐述了抗倾稳定计算方法 相似文献
34.
Noriaki Kimura 《Journal of Oceanography》2007,63(4):685-694
This paper examines the mechanism controlling the short time-scale variation of sea ice cover over the Southern Ocean. Sea
ice concentration and ice velocity datasets derived from images of the Defense Meteorological Satellite Program (DMSP) Special
Sensor Microwave Imager (SSM/I) are employed to reveal this mechanism. The contribution of both dynamic and thermodynamic
processes to the change in ice edge location is examined by comparing the meridional velocity of ice edge displacement and
sea ice drift. In the winter expansion phase, the thermodynamic process of new ice production off the ice edge plays an important
role in daily advances of ice cover, whereas daily retreats are mostly due to southward ice drift. On the other hand, both
advance and retreat of ice edges in the spring contraction phase are mostly caused by the dynamic process of the ice drift.
Based on the above mechanism and the linear relation between the degree of ice production at the ice edge and northward wind
speed, the seasonal advance of ice cover can be roughly reproduced using the meridional velocity of ice drift at the ice edge. 相似文献
35.
36.
37.
Temperature data at different layers of the past 45 years were studied and we found adiploe mode in the thermocline layer (DMT): anomalously cold sea temperature off the coast of Sumatra and warm sea temperature in the western Indian Ocean. First, we analyzed the temperature and the temperature anomaly (TA) along the equatorial Indian Ocean in different layers. This shows that stronger cold and warm TA signals appeared at subsurface than at the surface in the tropical Indian O-cean. This result shows that there may be a strong dipole mode pattern in the subsurface tropical Indian Ocean. Secondly we used Empirical Orthogonal Functions (EOF) to analyze the TA at thermocline layer. The first EOF pattern was a dipole mode pattern. Finally we analyzed the correlations between DMT and surface tropical dipole mode (SDM), DMT and Nino 3 SSTA, etc. and these correlations are strong. 相似文献
38.
39.
J. Perbos P. Escudier F. Parisot G. Zaouche P. Vincent Y. Menard F. Manon G. Kunstmann D. Royer L. -L. Fu 《Marine Geodesy》2003,26(3):147-157
On 7 December 2001, Jason-1 was successfully launched by a Boeing Delta II rocket from the Vandenberg Air Force Base, California. The Jason-1 satellite will maintain the high accuracy altimeter service provided since 1992 by TOPEX/Poseidon (T/P), ensuring the continuity in observing and monitoring the Ocean Dynamics (intraseasonal to interannual changes, mean sea level, tides, etc.). Despite one-fourth the mass and power, the Jason-1 system has been designed to have basically the same performance as T/P, measuring sea surface topography at a centimetric level. This new CNES/NASA mission also provides near real-time data for sea state and ocean forecast. The first two months of the Jason-1 mission have been dedicated to the assessment of the overall system. The goals of this assessment phase were:
1. To assess the behavior of the spacecraft at the platform and payload levels (Jason-1 being the first program to call on the PROTEUS versatile multimission platform for Low and Medium Earth Orbit Missions developed in partnership between Alcatel Space and CNES);
2. To verify that platform performance requirements are met with respect to Jason-1 requirements;
3. To verify that payload instruments performance requirements evaluated at instrument level are met;
4. To assess the performance of the Jason-1 Ground System.
This article will display the main outputs of the assessment of the system. It will demonstrate that all the elements of the onboard and ground systems are within the specifications. Provision of data to the Jason-1 Science Working Team started at the end of March 2002. This is the goal of a six-month phase after closure of the initial assessment phase to derive the error budget of the system in terms of altimetry user products. 相似文献
1. To assess the behavior of the spacecraft at the platform and payload levels (Jason-1 being the first program to call on the PROTEUS versatile multimission platform for Low and Medium Earth Orbit Missions developed in partnership between Alcatel Space and CNES);
2. To verify that platform performance requirements are met with respect to Jason-1 requirements;
3. To verify that payload instruments performance requirements evaluated at instrument level are met;
4. To assess the performance of the Jason-1 Ground System.
This article will display the main outputs of the assessment of the system. It will demonstrate that all the elements of the onboard and ground systems are within the specifications. Provision of data to the Jason-1 Science Working Team started at the end of March 2002. This is the goal of a six-month phase after closure of the initial assessment phase to derive the error budget of the system in terms of altimetry user products. 相似文献
40.