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821.
We present a method to detect and locate dielectric objects buried under a rough surface. The method is based on the determination of appearing surface impedance of the half-space, where the dielectric objects are located. The equivalent surface impedance is obtained directly from the impedance boundary condition, which requires the knowledge of the electric field and its normal derivative on the surface. These field values are obtained by measuring the far-field data and using single-layer-potential representation of the scattered field. Using the equivalent surface impedance, one can detect and locate the buried objects. The efficiency and efficacy of the method are tested via numerical simulations 相似文献
822.
A. V. Suresh Babu V. Venkateshwar Rao I. V. Muralikrishna 《Journal of the Indian Society of Remote Sensing》2007,35(1):81-91
Water Utilisation Index (WUI) defined as area irrigated per unit volume is a measure of water delivery performance and constitutes
one of the important spatial performance indicators of an irrigation system. WUI also forms basis for evaluating the adequacy
of seasonal irrigation supplies in an irrigation system (inverse of WUI is delta, i.e. depth of water supplied to a given
irrigation unit). In the present study WUI and adequacy indicators were used in benchmarking the performance of Nagarjunasagar
Left Canal Command (NSLC) in Andhra Pradesh. Optimised temporal satellite data of rabi season during the years 1990–91 and
1998–99 was used in deriving irrigated crop areas adopting hierarchical classification approach. Paddy is the predominant
crop grown and cotton, chillies, sugarcane etc. are the other crops grown in the study area. Equivalent wet area (paddy crop
area) was estimated using the operationally used project specific conversion factors. WUI was estimated at disaggregated level
viz., distributary, irrigation block, irrigation zone level using the canal discharge data. At project level, WUI estimated
to be 65 ha/MCM and 92 ha/MCM during rabi season of 1990–91 and 1998–99 years respectively. A comparison of total irrigated
area and discharges corresponding to both the years indicate that irrigation service is extensive and sub optimal during 1998–99
and it is intensive and optimal in 1990–91. It was also observed that WUI is lesser in blocks of with higher Culturable Command
Area (CCA) compared to the blocks of lower CCA. All the disaggregated units were ranked into various groups of different levels
of water distribution performance. The study demonstrates the utility of WUI as spatial performance indicator and thus useful
for benchmarking studies of irrigation command areas. The WUI together with satellite data derived spatial irrigation intensity,
crop productivity constitutes important benchmarking indices in irrigation command areas. 相似文献
823.
Summary We investigated the ratio of photosynthetically active photon flux (Q
p
) to global solar radiation (R
s
) at three sites along different altitudes in Naeba Mountain, Japan at various temporal scales based on 3 years measurement
data (1999–2001). The lowest values of the ratio ever reported were found for all sites on both an hourly and a daily scale.
A similar slight diurnal pattern was found for all sites based on the monthly mean hourly values of the ratio. However, different
sites exhibited different seasonal courses. Statistically significant altitude dependencies were found for the hourly Q
p
/R
s
under both clear and cloudy weather conditions (t-test, P < 0.001). For clear weather conditions, the hourly Q
p
/R
s
exhibited an increasing trend with altitude at an average rate of 3.6% per km. The increasing rate was more noticeable below
900 m (8.8% per km) than above (0.7% per km). The inverse trend was found for hourly Q
p
/R
s
with the altitude under cloudy weather conditions. The hourly Q
p
/R
s
decreased from 550 m to 1500 m at a rate of 1.8% per km. Again, a major decrease occurred below 900 m, which had the rate
of 4.2% per km, compared with 0.2% per km over 900 m. Although the same tendencies were noted for daily Q
p
/R
s
, under clear sky conditions, they were not as statistically significant as the hourly counterpart (t-test, P < 0.021). The
increasing rate of Q
p
/R
s
at this scale under clear weather conditions was near that of the hourly rate, but below the 900 m rate was reduced to near
half of the hourly rate (4.7% per km). And the rate over 900 m increased to 2.7% per km. On the other hand, statistically
significant altitude effect was noted for the daily under cloudy weather conditions (t-test, P < 0.002). A rapidly decreasing
rate was found for it along the altitude gradient. The reason was due to the large decreasing rate below 900 m (12.4% per
km). But again similar to the hourly Q
p
/R
s
, a very small decreasing rate of daily Q
p
/R
s
was found over 900 m under cloudy weather conditions. These results suggest the necessity of considering the altitude dependency
of Q
p
/R
s
in future studies. 相似文献
824.
P. Braconnot F. Hourdin S. Bony J. L. Dufresne J. Y. Grandpeix O. Marti 《Climate Dynamics》2007,29(5):501-520
The simulation of the mean seasonal cycle of sea surface temperature (SST) remains a challenge for coupled ocean–atmosphere
general circulation models (OAGCMs). Here we investigate how the numerical representation of clouds and convection affects
the simulation of the seasonal variations of tropical SST. For this purpose, we compare simulations performed with two versions
of the same OAGCM differing only by their convection and cloud schemes. Most of the atmospheric temperature and precipitation
differences between the two simulations reflect differences found in atmosphere-alone simulations. They affect the ocean interior
down to 1,000 m. Substantial differences are found between the two coupled simulations in the seasonal march of the Intertropical
Convergence Zone in the eastern part of the Pacific and Atlantic basins, where the equatorial upwelling develops. The results
confirm that the distribution of atmospheric convection between ocean and land during the American and African boreal summer
monsoons plays a key role in maintaining a cross equatorial flow and a strong windstress along the equator, and thereby the
equatorial upwelling. Feedbacks between convection, large-scale circulation, SST and clouds are highlighted from the differences
between the two simulations. In one case, these feedbacks maintain the ITCZ in a quite realistic position, whereas in the
other case the ITCZ is located too far south close to the equator. 相似文献
825.
An empirical orthogonal functions analysis of the onshore flow of moisture along the west coast of southern Africa using NCEP-DOE AMIP II Re-analyses suggests two dominant modes of variability that are linked to (a) variations in the circulation linked with the South Atlantic anticyclone (b) the intensity of the flow that penetrates from the tropical Atlantic. The second mode, referred as the Equatorial Westerly mode, contributes the most to moisture input from the Atlantic onto the subcontinent at tropical latitudes. Substantial correlations in austral summer between the Atlantic moisture flux in the tropics and rainfall over the upper lands surrounding the Congo basin suggest the potential role played by this zonal mode of water vapour transport. Composites for austral summer months when this Equatorial Westerly mode had a particularly strong expression, show an enhanced moisture input at tropical latitudes that feeds into the deep convection occurring over the Congo basin. Sustained meridional energy fluxes result in above normal rainfall east and south of the Congo belt. During years of reduced equatorial westerly moisture flux, a deficit of available humidity occurs in the southern tropics. A concomitant eastward shift of deep convection to the southwest Indian ocean and southeastern Africa, leads to below normal rainfall over the uplands surrounding the Congo basin. 相似文献
826.
Summary A series of numerical experiments on an f plane are conducted using the fifth-generation Pennsylvania State University-National Center for Atmospheric Research Mesoscale
Model, version 3 (MM5) to investigate how environmental vertical wind shear affects the motion, structure, and intensity of
a tropical cyclone. The results show that a tropical cyclone has a motion component perpendicular to the vertical shear vector,
first to the right of the shear and then to the left. An initially axisymmetric, upright tropical cyclone vortex develops
a downshear tilt and wavenumber-one asymmetry when embedded in environmental vertical wind shear.
In both small-moderate shears, a storm weakens slightly compared to that in a quiescent environment. The circulation centers
between 300 hPa and the surface varies from 20 km to over 80 km. The secondary circulation becomes quite asymmetric about
the surface cyclone center. As a result, convection on the upshear-right quadrant diminishes, limiting the upward heat transport
in the eyewall and thus lowering the warm core and leading to a weakening of the storm. In strong vertical shear (above 12 m s−1), the vertical tilt exceeds 160 km in 48 h of simulation and the secondary circulation on the upshear side is completely
destroyed with low-level outflow. The axisymmetric component of eyewall convection weakens remarkably and becomes much less
penetrative. As a result, the warm core becomes weak and appears at lower levels and the storm weakens rapidly accordingly.
This up-down weakening mechanism discussed in this study is different from those previously discussed. It emphasizes the penetrative
role of eyewall convection in transporting heat from the ocean to the mid-upper troposphere, maintaining the warm core structure
of the tropical cyclone. The vertical shear is found negative to eyewall penetrative convection. 相似文献
827.
Initialization and simulation of a landfalling typhoon using a variational bogus mapped data assimilation (BMDA) 总被引:2,自引:0,他引:2
Summary Recently, a new data assimilation method called “3-dimensional variational data assimilation of mapped observation (3DVM)”
has been developed by the authors. We have shown that the new method is very efficient and inexpensive compared with its counterpart
4-dimensional variational data assimilation (4DVar). The new method has been implemented into the Penn State/NCAR mesoscale
model MM5V1 (MM5_3DVM). In this study, we apply the new method to the bogus data assimilation (BDA) available in the original
MM5 with the 4DVar. By the new approach, a specified sea-level pressure (SLP) field (bogus data) is incorporated into MM5
through the 3DVM (for convenient, we call it variational bogus mapped data assimilation – BMDA) instead of the original 4DVar
data assimilation. To demonstrate the effectiveness of the new 3DVM method, initialization and simulation of a landfalling
typhoon – typhoon Dan (1999) over the western North Pacific with the new method are compared with that with its counterpart
4DVar in MM5. Results show that the initial structure and the simulated intensity and track are improved more significantly
using 3DVM than 4DVar. Sensitivity experiments also show that the simulated typhoon track and intensity are more sensitive
to the size of the assimilation window in the 4DVar than that in the 3DVM. Meanwhile, 3DVM takes much less computing cost
than its counterpart 4DVar for a given time window. 相似文献
828.
Mouradian Z. Soru-Escaut I. Hiei E. McALLISTER A. H. Shibasaki K. Ohyama M. Khan J. I. Uchida Y. 《Solar physics》1998,180(1-2):313-329
The 7 May 1992 filament disappearance in the low corona is analyzed. The cool and hot components of this event are studied, using H, soft X-ray and radio data. We first show the general effect of the disparition brusque (DB) on the life of the filament, which was a quiescent filament in the vicinity of an active region, and then give the history of the development of the 7 May event. The main stages of the event are: (i) the formation of hot arches spanning the cool filament; (ii) rise of the filament, with plasma ejection into the corona, in which we note some spreading of loops from the main body, with two distinct rising velocity phases of the H filament; (iii) formation of X-ray arches below the filament, the foot points of the arcades being two-ribbon H flare patches. The dynamics of H and X-rays features are given. 相似文献
829.
Abstract— Chondrules in the Bali-like CV chondrite Kaba and the Allende-like portion of the Mokoia breccia have been studied to explore the relationship between hydrous alteration to form phyllosilicates and anhydrous alteration resulting in secondary olivine zonation, replacement of enstatite by ferroan olivine and formation of feldspathoids (nepheline and sodalite). All Kaba chondrules experienced extensive hydrous alteration; whereas, anhydrous alteration was minor and resulted only in the olivine zonation. On the other hand, all of the Mokoia chondrules experienced both extensive anhydrous and hydrous alteration. Bronzite rims formed between relic enstatite grains and phyllosilicates in both Kaba and Mokoia during the hydrous alteration. Petrographic observations indicate that phyllosilicates in Mokoia postdate formation of the secondary ferroan olivine and feldspathoids. We conclude that anhydrous alteration in Kaba and Mokoia predated hydrous alteration and took place before accretion of chondrules into the CV parent asteroid. 相似文献
830.