全文获取类型
收费全文 | 1231篇 |
免费 | 253篇 |
国内免费 | 183篇 |
专业分类
测绘学 | 142篇 |
大气科学 | 312篇 |
地球物理 | 585篇 |
地质学 | 247篇 |
海洋学 | 209篇 |
天文学 | 36篇 |
综合类 | 52篇 |
自然地理 | 84篇 |
出版年
2024年 | 7篇 |
2023年 | 13篇 |
2022年 | 29篇 |
2021年 | 43篇 |
2020年 | 63篇 |
2019年 | 50篇 |
2018年 | 35篇 |
2017年 | 51篇 |
2016年 | 56篇 |
2015年 | 44篇 |
2014年 | 73篇 |
2013年 | 50篇 |
2012年 | 69篇 |
2011年 | 78篇 |
2010年 | 56篇 |
2009年 | 84篇 |
2008年 | 96篇 |
2007年 | 100篇 |
2006年 | 86篇 |
2005年 | 64篇 |
2004年 | 69篇 |
2003年 | 49篇 |
2002年 | 58篇 |
2001年 | 36篇 |
2000年 | 34篇 |
1999年 | 34篇 |
1998年 | 28篇 |
1997年 | 35篇 |
1996年 | 27篇 |
1995年 | 27篇 |
1994年 | 27篇 |
1993年 | 18篇 |
1992年 | 19篇 |
1991年 | 13篇 |
1990年 | 14篇 |
1989年 | 10篇 |
1988年 | 3篇 |
1987年 | 1篇 |
1986年 | 2篇 |
1985年 | 4篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 1篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1954年 | 4篇 |
排序方式: 共有1667条查询结果,搜索用时 15 毫秒
991.
Unification of New Zealand’s local vertical datums: iterative gravimetric quasigeoid computations 总被引:2,自引:2,他引:0
New Zealand uses 13 separate local vertical datums (LVDs) based on geodetic levelling from 12 different tide-gauges. We describe
their unification using a regional gravimetric quasigeoid model and GPS-levelling data on each LVD. A novel application of
iterative quasigeoid computation is used, where the LVD offsets computed from earlier models are used to apply additional
gravity reductions from each LVD to that model. The solution converges after only three iterations yielding LVD offsets ranging
from 0.24 to 0.58 m with an average standard deviation of ±0.08 m. The so-computed LVD offsets agree, within expected data
errors, with geodetically levelled height differences at common benchmarks between adjacent LVDs. This shows that iterated
quasigeoid models have a role in vertical datum unification. 相似文献
992.
993.
994.
Fredrika Norrbin Hans Chr. Eilertsen Maria Degerlund 《Deep Sea Research Part II: Topical Studies in Oceanography》2009,56(21-22):1945
During a circumnavigation of the Svalbard archipelago in May 2006, simultaneous marine environmental (meteorology, heat flux, ocean turbulence, irradiance) and biological (phytoplankton and zooplankton biomass/species) data were sampled at selected stations. The zooplankton data were supplemented by high-resolution, high-speed VPR sampling down to 100 m depth at most stations. We were able to sample different phases of the phytoplankton spring bloom in Arctic as well as in Atlantic waters, and the stations represented different situations with respect to irradiance, turbulence and water-column stability. Phytoplankton growth and depth distribution were physically controlled, while zooplankton distributions were affected by biological parameters and turbulence. Development of the zooplankton followed the phytoplankton bloom phase, which was progressing in a direction from west to east in the waters north of Svalbard, and southwards in the Barents Sea. Our results also showed that the zooplankton did not avoid Phaeocystis pouchetii colonies, which have earlier been described as toxic. Despite an early retreat of the ice this year there was no apparent mismatch between the phytoplankton bloom and the dominant mesozooplankton, Calanus spp. 相似文献
995.
Derivation of unified wave overtopping formulas for seawalls with smooth,impermeable surfaces based on selected CLASH datasets 总被引:1,自引:0,他引:1
A set of unified formulas for prediction of the mean rate of wave overtopping at coastal structures with smooth, impermeable surfaces have been derived through the analysis of the selected CLASH datasets. The mean wave overtopping rate is expressed as the function of the significant wave height at the structural toe and the relative freeboard. The formulas are applicable for both vertical walls and inclined seawalls with smooth transition between them. The formulas are simple but cover the full range of water depth from the shoreline to deep water. The effects of the toe depth and the seabed slope on wave overtopping rate are duly incorporated in the formulas. Prediction performance of the new formulas is better than the EurOtop formulas for both vertical walls and inclined seawalls. 相似文献
996.
Within the CLASH project, wave overtopping at the vertical seawall at Samphire Hoe was measured by HR Wallingford (HRW), and compared laboratory tests in 2 & 3 dimensions carried out at the University of Edinburgh and HRW. At Samphire Hoe, overtopping volumes were captured in three volumetric tanks capable of measuring wave-by-wave and total overtopping volumes. The three tanks were placed progressively farther back from the seawall edge so that the spatial distribution of the overtopping discharges could be determined. The field measurement equipment was successfully deployed on three occasions, and measured overtopping discharges ranged from that barely considered to be hazardous to the public to over q = 3.0 l/s/m. The 2d testing at Edinburgh was modelled at a scale of 1:40, and the 3d model at HRW was modelled at 1:20. For both sets of laboratory tests, a range of conditions, representative of the storm wave conditions and water levels, was reproduced in addition to a set of parametric conditions. The storm conditions allowed a direct comparison between the field and laboratory measurements, and the parametric conditions were used to test the generic overtopping behaviour of the structure. For both sets of laboratory tests, mean overtopping discharges and the spatial distribution were measured separately. Analysis of the distribution data relates the proportion of the discharge that has landed as a function of (x / Lo); where x is the distance behind the crest, and Lo is the offshore wavelength. Analysis of the field, 2d & 3d laboratory data, and empirical prediction methods have not identified any scale effects for overtopping discharges at vertical and near-vertical seawalls. 相似文献
997.
Particulate organic carbon (POC) is vertically transported to the oceanic interior by aggregates and their ballasts, mainly CaCO3 and biogenic opal, with a smaller role for lithogenic aerosols through the mesopelagic zone. Diel migrating zooplankton communities effect vertical transport and remineralization of POC in the upper layers of the ocean. Below 1.5 km, the presence of zooplankton is reduced and thus the aggregates travel mainly by gravitational transport. We normalized the fluxes of POC, CaCO3, and biogenic opal from data published on samples collected at 134 globally distributed, bottom-tethered, time-series sediment trap (TS-trap) stations to annual mole fluxes at the mesopelagic/bathypelagic boundary (m/b) at 2 km and defined them as Fm/bCorg, Fm/bCinorg, and Fm/bSibio. Using this global data set, we investigated (1) the geographic contrasts of POC export at m/b and (2) the supply rate of ∑CO2 to the world mesopelagic water column. Fm/bCorg varies from 25 (Pacific Warm Pool) to 605 (divergent Arabian Sea) mmolC m−2 yr−1; Fm/bCinorg varies from >8 (high latitude Polar Oceans) or 15 (Pacific Warm Pool) to 459 (divergent Arabian Sea) mmolC m−2 yr−1; and Fm/bSibio, the most spatially/temporally variable flux, ranges from 6 (North Atlantic Drift) to 1118 (Pacific Subarctic Gyre) mmolSi m−2 yr−1. The oceanic region exhibiting the highest POC flux over a significantly large region is the area of the North Pacific Boreal Gyres where the average Fm/bCorg = 213, Fm/bCinorg = 126, and Fm/bSibio = 578 mmol m−2 yr−1. Fm/bCorg and Fm/bCinorg are particularly high in large upwelling margins, including the divergent Arabian Sea and off Cape Verde. One of the data sets showing the lowest flux over a significant region/basin is Fm/bCorg = 39, Fm/bCinorg = 69, and Fm/bSibio = 22 mmol m−2 yr−1 in the North Pacific subtropical/tropical gyres; Pan-Atlantic average fluxes are similar except Fm/bSibio fluxes are even lower. Where Corg/Cinorg and Sibio/Cinorg are <1 defines the “Carbonate Ocean”, and where these ratios are ?1 defines the “Silica Ocean”. The Carbonate Ocean occupies about 80% of the present world pelagic ocean between the two major oceanographic fronts, the North Pacific Polar Front and the Antarctic Polar Front, and the Silica Ocean is found on the polar sides of these fronts. The total global annual fluxes of Fm/bCorg, Fm/bCinorg, and Fm/bSibio at m/b calculated by parameterizations of the export flux data from 134 stations are surprisingly similar; 36.2, 33.8, and 34.6 teramol yr−1 (120, 112, and 114 mmol m−2 yr−1), respectively, resulting in a near uniform binary ratio between the above three elements of about one. The global ternary % ratios estimated from 152 TS-trap samples of the three elements are 35:32:33. From our global Fm/bCorg and a published model estimate of the global export production, we estimate the regeneration rate of CO2 through the mesopelagic zone by the biological pump is 441 teramolC yr−1. Based on our global Fm/bCinorg and recently estimated global primary production of PIC, 36-86 teramolC yr−1 of PIC is assumed to be dissolved within the upper 2 km of the water column. 相似文献
998.
Realistic representation of sea ice in ocean models involves the use of a non-linear free-surface, a real freshwater flux and observance of requisite conservation laws. We show here that these properties can be achieved in practice through use of a rescaled vertical coordinate “z*” in z-coordinate models that allows one to follow undulations in the free-surface under sea ice loading. In particular, the adoption of “z*” avoids the difficult issue of vanishing levels under thick ice.Details of the implementation within MITgcm are provided. A high resolution global ocean sea ice simulation illustrates the robustness of the z* formulation and reveals a source of oceanic variability associated with sea ice dynamics and ice-loading effects. The use of the z* coordinate allows one to achieve perfect conservation of fresh water, heat and salt, as shown in extended integration of coupled ocean sea ice atmospheric model. 相似文献
999.
1000.
赵岳仁 《成都信息工程学院学报》1993,(3)
盛夏季节,宜宾雷暴的形成主要与对流层内高温高湿、风及其在对流层的中上部垂直切变小的特点相关联。西宁雷暴的形成主要与上层为深厚的干冷、下层为浅薄的暖湿、对流层的中上层风及其垂直切变大的特点相关联。分析又得到以下结论:西宁雷暴的形成又与高空冷空气加强南移、副热带急流南侧反气旋式切变得到加强和高空出现辐散相关联。 相似文献