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711.
以1978—2007年四川省宜宾探空站200 km范围内共45例降雹事件与非降雹事件作为时间样本,将利用T-lnp探空资料计算表征热力、动力、水汽条件、温度、高度等物理参数作为预报因子,采用因子组合分析方法对3244个预报因子进行筛选,找出影响宜宾探空站附近降雹的预报因子,并计算得到它们的阈值及组合关系,建立了冰雹预报指标判别式。两个主要因子为400 hPa饱和湿静力温度与850 hPa湿静力温度之差 (Tσ400*-Tσ850) 和400 hPa与地面垂直气压梯度之差 (Gz400-Gzsurface),两个条件因子为700 hPa水汽压与饱和水汽压之差 (e700-es700) 和700 hPa露点温度与饱和湿静力温度之差 (Td700-Tσ700*)。将2008年全年732例时次的T-lnp探空资料代入判别式进行试报,探测率为84%,空报率为67.7%,成功指数为30.4%,试报结果表明:基于探空资料因子组合分析方法得到的冰雹预报指标判别方法可行,具有一定准确性。 相似文献
712.
将 1 960~ 1 991年的月平均 FSU风应力资料分解为旋转部分和辐散辐合部分 ,分别用以强迫模式海洋。所用的模式为一个 2层热带太平洋区域海洋模式。结果表明 ,就季节变化而言 ,不论是用旋转分量还是散合分量强迫海洋 ,都不能产生合理的冷舌 ,哪怕将旋转或散合分量放大一倍或缩小二分之一 ,也不能使冷舌的强度和分布得到合理的改善。若采用气候平均的含有季节变化的风应力 (未对旋转和辐散分量进行分离 ) ,则可产生与实际相符的海表温度分布。在此基础上 ,分别叠加旋转和辐散分量的年际异常部分 ,通过对海洋的强迫 ,可产生海表温度异常。在年际异常旋转分量的强迫下 ,可产生较强的 SSTA振荡且具有明显的 ENSO周期 ;而在辐散辐合年际异常风应力的强迫下 ,则产生较弱的 SSTA,且振荡频率较高 ,ENSO周期不很明显。这些结果说明 ,风应力的涡旋和辐散辐合分量在海温季节变化和年际变化的形成中具有不同的作用 ,即合理的冷舌分布需要风应力旋转分量和散合分量同时作用于海洋方可产生 ,而仅有异常风应力的旋转强迫就可产生合理的 EL Nino/ La Nina现象。同时 ,风应力的辐散辐合分量在海洋平均状态的形成过程中是重要的 ,但在 EN-SO过程中就对海洋的作用而言则不如旋转分量重要。 相似文献
713.
Ian P. Castro 《Boundary-Layer Meteorology》2017,164(3):337-351
Using analyses of data from extant direct numerical simulations and large-eddy simulations of boundary-layer and channel flows over and within urban-type canopies, sectional drag forces, Reynolds and dispersive shear stresses are examined for a range of roughness densities. Using the spatially-averaged mean velocity profiles these quantities allow deduction of the canopy mixing length and sectional drag coefficient. It is shown that the common assumptions about the behaviour of these quantities, needed to produce an analytical model for the canopy velocity profile, are usually invalid, in contrast to what is found in typical vegetative (e.g. forest) canopies. The consequence is that an exponential shape of the spatially-averaged mean velocity profile within the canopy cannot normally be expected, as indeed the data demonstrate. Nonetheless, recent canopy models that allow prediction of the roughness length appropriate for the inertial layer’s logarithmic profile above the canopy do not seem to depend crucially on their (invalid) assumption of an exponential profile within the canopy. 相似文献
714.
河南省近年来遥感监测的森林火灾时空分布规律分析 总被引:2,自引:0,他引:2
森林火灾作为一种自然灾害,其发生原因不仅来自于自然因素,从众多的火灾调查中发现,更多地来自于人类活动因素,其发生的时空分布特点和规律,受自然和人类活动共同影响.对2003-2008年春、冬季(11月-次年4月)河南省森林防火期内遥感监测并已查明的森林火灾进行统计分析,结果发现:河南省森林火灾近年来有逐年增多的趋势;冬、春之交的3月为森林火灾的高发月份;从火灾的日变化规律来看,12-15时为一天中森林火灾的高发时段;从火灾发生的空间分布来看,伏牛山南麓发生森林火灾的频率较高. 相似文献
715.
716.
The Beijing meteorological tower is located in an area of Beijing, China, which has developed from a suburban to an inner city setting over the past 30 years. The impacts of this urbanization process on both the vertical profile and diurnal cycles of air temperature are investigated using hourly data collected from a series of monitoring levels (up to 325 m high) on the Beijing meteorological tower since 1984. We find that the inter-decadal temperature has increased gradually, and that a more significant increase occurred during the 1980s and 1990s due to the effects of urbanization. A well-defined change in temperature stratification was also observed over this period. The height of the temperature inversion layer decreased from the 1980s to the 2000s. A well-defined nighttime temperature inversion developed below 50 m during the summer in the 1980s, but this near-surface inversion is not seen in data from the 1990s and 2000s. This change can be related to an increase in turbulent mixing caused by urban roughness and surface heat storage that disturbs the near-surface temperature inversion layer. In addition, the diurnal change in temperature in the city in summer shows a maximum increase from sunrise to the early afternoon, which is mainly caused by the nature variability and global warming in both the summer and winter. The urbanization mainly contributes to the temperature increase in the afternoon and nighttime. Moreover the urbanization dominates the increase in daily mean near-surface temperature. 相似文献
717.
718.
719.
Pietro Salizzoni Raphaël Van Liefferinge Patrick Mejean Lionel Soulhac Richard J. Perkins 《Boundary-Layer Meteorology》2010,135(3):455-467
The influence of surface roughness on the dispersion of a passive scalar in a rough wall turbulent boundary layer has been
studied using wind-tunnel experiments. The surface roughness was varied using different sizes of roughness elements, and different
spacings between the elements. Vertical profiles of average concentration were measured at different distances downwind of
the source, and the vertical spread of the plume was computed by fitting a double Gaussian profile to the data. An estimate
of the integral length scale is derived from the turbulence characteristics of the boundary layer and is then used to scale
the measured values of plume spread. This scaling reduces the variability in the data, confirming the validity of the model
for the Lagrangian integral time scale, but does not remove it entirely. The scaled plume spreading shows significant differences
from predictions of theoretical models both in the near and in the far field. In the region immediately downwind of the source
this is due to the influence of the wake of the injector for which we have developed a simple model. In the far field we explain
that the differences are mainly due to the absence of large-scale motions. Finally, further downwind of the source the scaled
values of plume spread fall into two distinct groups. It is suggested that the difference between the two groups may be related
to the lack of dynamical similarity between the boundary-layer flows for varying surface roughness or to biased estimates
of the plume spread. 相似文献
720.
Climatic regime shift and decadal anomalous events in China 总被引:15,自引:0,他引:15
Climatic time series from historical documents and instrumental records from China showed temporal and regional patterns in
the last two to three centuries, including two multidecadal oscillations at quasi-20-year and quasi-70-year timescales revealed
by signal analysis from wavelet transform. Climatic anomalous events on the decadal timescale were identified based on the
two oscillations when their positive (or negative) phases coincide with each other to amplify amplitude. The coldest event
occurred in the decade of 1965–1975 in eastern China, while the periods of 1920–1930, 1940–1950, and 1988–2000 appeared to
be warmer in most parts of China. For the precipitation series in northern China, the dry anomalous event was found in the
late 1920s, while the wet anomalous event occurred in the 1950s. A severe drought in 1927–1929 in northern China coincided
with the anomalous warm and dry decade, caused large-scale famine in nine provinces over northern China. Climatic anomalous
events with a warm-dry or cold-wet association in the physical climate system would potentially cause severe negative impacts
on natural ecosystem in the key vulnerable region over northern China. The spatial pattern of summer rainfall anomalies in
the eastern China monsoon region showed an opposite variations in phase between the Yellow River Valley (North China) and
the mid-low Yangtze River Valley as well as accompanied the shift of the northernmost monsoon boundary. Climatic regime shifts
for different time points in the last 200 years were identified. In North China, transitions from dry to wet periods occurred
around 1800, 1875, and 1940 while the transitions from wet to dry periods appeared around 1840, 1910, and the late 1970s.
The reversal transition in these time points can also be found in the lower Yangtze River. Climatic regime shifts in China
were linked to the interaction of mid- and low latitude atmospheric circulations (the westerly flow and the monsoon flow)
when they cross the Tibetan Plateau in East Asia. 相似文献