全文获取类型
收费全文 | 8139篇 |
免费 | 1855篇 |
国内免费 | 2552篇 |
专业分类
测绘学 | 855篇 |
大气科学 | 1663篇 |
地球物理 | 2190篇 |
地质学 | 4511篇 |
海洋学 | 1143篇 |
天文学 | 285篇 |
综合类 | 871篇 |
自然地理 | 1028篇 |
出版年
2024年 | 51篇 |
2023年 | 176篇 |
2022年 | 501篇 |
2021年 | 602篇 |
2020年 | 470篇 |
2019年 | 615篇 |
2018年 | 600篇 |
2017年 | 557篇 |
2016年 | 620篇 |
2015年 | 542篇 |
2014年 | 626篇 |
2013年 | 584篇 |
2012年 | 623篇 |
2011年 | 585篇 |
2010年 | 505篇 |
2009年 | 489篇 |
2008年 | 439篇 |
2007年 | 387篇 |
2006年 | 340篇 |
2005年 | 262篇 |
2004年 | 206篇 |
2003年 | 244篇 |
2002年 | 199篇 |
2001年 | 199篇 |
2000年 | 213篇 |
1999年 | 227篇 |
1998年 | 223篇 |
1997年 | 202篇 |
1996年 | 189篇 |
1995年 | 194篇 |
1994年 | 163篇 |
1993年 | 153篇 |
1992年 | 109篇 |
1991年 | 71篇 |
1990年 | 58篇 |
1989年 | 56篇 |
1988年 | 54篇 |
1987年 | 52篇 |
1986年 | 34篇 |
1985年 | 18篇 |
1984年 | 20篇 |
1983年 | 9篇 |
1982年 | 16篇 |
1981年 | 11篇 |
1980年 | 15篇 |
1978年 | 5篇 |
1977年 | 7篇 |
1976年 | 5篇 |
1973年 | 4篇 |
1958年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 78 毫秒
991.
3维数字地球快速缓冲区分析算法 总被引:1,自引:0,他引:1
提出一种应用在3维数字地球中的通过图形处理器(GPU)快速实现矢量数据缓冲区分析的算法。使用一张4通道的纹理图作为容器将地理实体的矢量数据传入GPU,利用GPU的高效并行特性,将目标缓冲区纹理中的每个像素所对应的矢量坐标与原实体进行距离量算,在一次渲染中得到缓冲区纹理,最后提取出缓冲区纹理的边界。选择中国的流域和湖泊矢量数据,将本文算法与两种传统的CPU算法进行了缓冲区分析计算、测试和对比。结果显示,本文算法相对于传统矢量算法效率提高了9—16倍,相对于传统栅格算法效率提高11—20倍。实验证明,该算法计算简单,效果明显,特别是随着数据量增大,缓冲区计算速度显著优于传统算法,并能有效解决传统矢量法缓冲区分析中的数据自相交问题。 相似文献
992.
R. S. Gao K. H. Rosenlof D. W. Fahey P. O. Wennberg E. J. Hintsa T. F. Hanisco 《Journal of Atmospheric Chemistry》2014,71(1):55-64
In situ measurements of [OH], [HO2] (square brackets denote species concentrations), and other chemical species were made in the tropical upper troposphere (TUT). [OH] showed a robust correlation with solar zenith angle. Beyond this dependence, however, [OH] did not correlate to its primary source, the product of [O3] and [H2O] ([O3]?[H2O]), or its sink [NOy]. This suggests that [OH] is heavily buffered in the TUT. One important exception to this result is found in regions with very low [O3], [NO], and [NOy]. Under these conditions, [OH] is highly suppressed, pointing to the critical role of NO in sustaining OH in the TUT and the possibility of low [OH] over the western Pacific warm pool due to strong marine convections bringing NO-poor air to the TUT. In contrast to [OH], [HOx] ([OH] + [HO2]) correlated reasonably well with [O3]?[H2O]/[NOy], suggesting that [O3]?[H2O] and [NOy] are the significant source and sink, respectively, of [HOx]. 相似文献
993.
Improvement of the semi-Lagrangian advection scheme in the GRAPES model: Theoretical analysis and idealized tests 总被引:2,自引:0,他引:2
ABSTRACT The Global/Regional Assimilation and PrEdiction System (GRAPES) is the newgeneration numerical weather predic- tion (NWP) system developed by the China Meteorological Administration. It is a fully compressible non-hydrostatical global/regional unified model that uses a traditional semi-Lagrangian advection scheme with cubic Lagrangian interpola tion (referred to as the SL_CL scheme). The SL_CL scheme has been used in many operational NWP models, but there are still some deficiencies, such as the damping effects due to the interpolation and the relatively low accuracy. Based on Reich's semi-Lagrangian advection scheme (referred to as the R2007 scheme), the Re_R2007 scheme that uses the low- and high-order B-spline function for interpolation at the departure point, is developed in this paper. One- and two-dimensional idealized tests in the rectangular coordinate system with uniform grid cells were conducted to compare the Re..R2007 scheme and the SL_CL scheme. The numerical results showed that: (1) the damping effects were remarkably reduced with the Re_R2007 scheme; and (2) the normalized errors of the Re_R2007 scheme were about 7.5 and 3 times smaller than those of the SL_CL scheme in one- and two-dimensional tests, respectively, indicating the higher accuracy of the Re..R2007 scheme. Furthermore, two solid-body rotation tests were conducted in the latitude-longitude spherical coordinate system with non uniform grid cells, which also verified the Re_R2007 scheme's advantages. Finally, in comparison with other global advection schemes, the Re_R2007 scheme was competitive in terms of accuracy and flow independence. An encouraging possibility for the application of the Re_R2007 scheme to the GRAPES model is provided. 相似文献
994.
Distinct quasi-biweekly features of the subtropical East Asian monsoon during early and late summers 总被引:2,自引:0,他引:2
Jing Yang Qing Bao Bin Wang Dao-Yi Gong Haozhe He Miao-Ni Gao 《Climate Dynamics》2014,42(5-6):1469-1486
Using Global Precipitation Climatology Project daily rainfall and ERA interim reanalysis data, we investigate the distinct characteristic of quasi-biweekly variation (QBV: 12–20 days) over East Asia (EA) during early (June 10–July 20) and late (July 21–August 31) summer. The QBV maximum variance is found over the core region of EA (30°–40°N, 110°–130°E), which includes eastern China (lower reaches of the Yellow, Huaihe, and Yangtze rivers) and the Korean Peninsula. At both its peak wet and dry phases, QBV over the core region has a baroclinic structure, but with different spatial distributions, different lower-level prevalent wind anomalies, and different upper-level major circulation anomalies in the two subseasons. Meanwhile, the two subseasons have different propagating tracks prior to reaching the peak phase, and different precursors associated with the local genesis of QBV. Furthermore, during the transition from the peak dry to peak wet phase of QBV, the major monsoon circulations have different behaviors that tropical monsoon trough extends eastward in early summer but retreats westward in late summer and the South Asia high (SAH) and western North Pacific (WNP) subtropical high move toward (away from) each other in early (late) summer. The abrupt change of mean state in mid to late July, which includes the northward migration of westerly jet, SAH and WNP, and the weakening and broken of westerly jet, is considered the root cause of the change in behavior of QBV. Finally, we indicate that the tropical monsoon trough and midlatitude westerly jet are possible sources of QBV over subtropical EA in both subseasons and provide useful guidance for 2–3 week predictions over EA. 相似文献
995.
RDSS短报文是北斗卫星导航系统(以下简称北斗系统)的特色服务,实现了用户报文通信和位置报告。北斗系统早期的位置报告基于双星RDSS观测值,其精度受限于数字高程数据的精度。从BDS-2开始,北斗系统发展了融合RNSS和RDSS体制的位置报告方法,其基于北斗RDSS体制,将用户接收机的观测数据向中心站进行回传,并在中心站实现精密位置的解算和报告,提升了服务覆盖范围和定位精度。本文介绍了北斗精密定位报告系统采用的中心站处理算法,并采用分布于中国不同区域的50个北斗观测站数据对系统性能进行评估。分析了RDSS入站频度、用户站与监测站的基线长度等因素对用户定位的影响。结果表明,基于北斗分区综合改正数的精密单点定位的精度最优,其在2 min响应时间内的平面和高程精度(RMS)分别为0.51 m和0.94 m。北斗精密定位报告精度与基线长度相关性较小,在仅采用BDS-2数据的情况下,定位精度下降可达1倍以上,而在相同时长情况下,提高RDSS入站申请频度对定位性能不产生影响。 相似文献
996.
997.
998.
Re-Os同位素在沉积地层精确定年及古环境反演中的应用进展 总被引:6,自引:0,他引:6
Re-Os同位素亲有机质的性质,使得富有机质沉积岩在沉积的过程中能够吸附富集海水中的Re、Os,沉积岩的沉积压实过程也是其中Re-Os同位素体系封闭计时的过程,沉积岩Re-Os同位素等时线年龄代表地层沉积时代,Os同位素初始比值187Os/188Os反映沉积时海水的Os同位素比值。Re-Os同位素体系在富有机质沉积岩中的成功应用,能够直接确定地层沉积时代,从而对地层界线进行直接厘定,并且能够对一些沉积矿床形成时代、冰川事件发生时代进行厘定和限制。通过沉积岩Re-Os同位素特征,可以对古环境进行反演,有助于了解全球大气海洋的演化,气候的变化,对研究生物灭绝等重大地质事件发生的时限和机制以及金属矿床的成矿物质来源具有重要意义。文章阐述了Re-Os同位素在富有机质沉积岩中应用的原理,并列举了Re-Os同位素在沉积地层精确定年及古环境反演中应用实例,说明了富有机质沉积岩Re-Os同位素分析精度及其影响因素,以使Re-Os同位素体系作为一种强有力的工具,解决更多的沉积地层时代问题以及更好地对古环境进行反演。 相似文献
999.
冀北承德盆地中生代侏罗纪地层序列较齐全,但作为关键层位之一的九龙山组一直缺乏高精度年代学的约束。笔者首次对九龙山组凝灰岩夹层进行了LA-ICP-MS锆石U-Pb高精度定年,获得凝灰岩中岩浆锆石206Pb/238U加权平均年龄164.1±1.3 Ma。综合研究及对比表明,九龙山组的时限为158~164Ma,属中侏罗世晚期至晚侏罗世早期,对应于国际地层表的牛津期至卡洛维期,其沉积时间相对短暂。在此基础上,更新了研究区侏罗系年代地层柱状图,该认识对于深入研究燕山地区中生代侏罗纪地层格架及构造岩浆演化过程具有重要意义。 相似文献
1000.