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991.
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Climatic time series from historical documents and instrumental records from China showed temporal and regional patterns in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
992.
Wusi Yue Marc B. Parlange Charles Meneveau Weihong Zhu René van Hout Joseph Katz 《Boundary-Layer Meteorology》2007,124(2):183-203
Turbulent flow in a corn canopy is simulated using large-eddy simulation (LES) with a Lagrangian dynamic Smagorinsky model.
A new numerical representation of plant canopies is presented that resolves approximately the local structure of plants and
takes into account their spatial arrangement. As a validation, computational results are compared with experimental data from
recent field particle image velocimetry (PIV) measurements and two previous experimental campaigns. Numerical simulation using
the traditional modelling method to represent the canopy (field-scale approach) is also conducted as a comparison to the plant-scale
approach. The combination of temporal PIV data, LES and spatial PIV data allows us to couple a wide range of relevant turbulence
scales. There is good agreement between experimental data and numerical predictions using the plant-scale approach in terms
of various turbulence statistics. Within the canopy, the plant-scale approach also allows the capture of more details than
the field-scale approach, including instantaneous gusts that penetrate deep inside the canopy. 相似文献
993.
Particle image velocimetry (PIV) data obtained in a wind-tunnel model of a canopy boundary layer is used to examine the characteristics
of mean flow and turbulence. The vector spacing varies between 1.7 and 2.5 times the Kolmogorov scales. Conditional sampling
based on quadrants, i.e. based on the signs of velocity fluctuations, reveals fundamental differences in flow structure, especially
between sweep and ejection events, which dominate the flow. During sweeps, the downward flow generates a narrow, highly turbulent,
shear layer containing multiple small-scale vortices just below canopy height. During ejections, the upward flow expands this
shear layer and the associated small-scale flow structures to a broad region located above the canopy. Consequently, during
sweeps the turbulent kinetic energy (TKE), Reynolds stresses, as well as production and dissipation rates, have distinct narrow
peaks just below canopy height, whereas during ejections these variables have broad maxima well above the canopy. Three methods
to estimate the dissipation rate are compared, including spectral fits, measured subgrid-scale (SGS) energy fluxes at different
scales, and direct measurements of slightly underresolved instantaneous velocity gradients. The SGS energy flux is 40–60%
of the gradient-based (direct) estimates for filter sizes inside the inertial range, while decreasing with scale, as expected,
within the dissipation range. The spectral fits are within 5–30% of the direct estimates. The spectral fits exceed the direct
estimates near canopy height, but are lower well above and below canopy height. The dissipation rate below canopy height increases
with velocity magnitude, i.e. it has the highest values during sweep and quadrant 1 events, and is significantly lower during
ejection and quadrant 3 events. Well above the canopy, ejections are the most dissipative. Turbulent transport during sweep
events acts as a source below the narrow shear layer within the canopy and as a sink above it. Transport during ejection events
is a source only well above the canopy. The residual term in the TKE transport equation, representing mostly the effect of
pressure–velocity correlations, is substantial only within the canopy, and is dominated by sweeps. 相似文献
994.
Four comparative experiments and some supplementary experiments were conducted to examine the role of meridional wind stress anomalies and heat flux variability in ENSO simulations by using a high-resolution Ocean General Circulation Model （OGCM）. The results indicate that changes in the direction and magnitude of meridional wind stress anomalies have little influence on ENSO simulations until meridional wind stress anomalies are unrealistically enlarged by a factor of 5.0. However, evidence of an impact on ENSO simulations due to heat flux variability was found. The simulated Nino-3 index without the effect of heat flux anomalies tended to be around 1.0° lower than the observed, as well as the control run, during the peak months of ENSO events. 相似文献
995.
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998.
本文基于城市交通日益恶化现状和3S集成的迅速发展,表述了Spatio-temporal GIS的定义、时空路径、人类活动的四种时空关系及其坐标系统模型的建立;并根据GPS空间定位、空间导航的原理和模型,将Spatio-temporal GIS和GPS的功能优化集成,阐述了其对城市交通信息的收集、分析、显示等功能;最后论述了Spatio-temporal GIS和GPS的集成对城市交通预警、报警、协调等管制作用。 相似文献
999.
为研究1920年海原地震中石碑塬地区液化滑移灾害的形成机制、滑移特征及滑移距离，对石碑塬液化滑移区进行钻孔勘探、取样以及探槽开挖。分别对钻孔及所取试样进行剪切波速测试及颗粒分析、室内动三轴试验，得出石碑塬液化滑移区地层分布情况、震时液化土体层位以及不同地震烈度条件下砂质黄土层的液化情况。综合分析钻孔勘探、颗粒分析、动三轴试验结果，揭示出海原地震中石碑塬黄土地层液化滑移灾害的形成机制：砂质黄土层液化后在自重应力以及地震力的共同作用下"托浮"第一古土壤层以及上部"粉尘化"的非饱和黄土层沿缓斜坡运动，并利用液化滑移地层滑距公式对滑移距离进行估算，得到结果为223.35 m，与实际情况较为相符。研究结果可为黄土地层液化滑移灾害的预防提供一定的借鉴意义。 相似文献
1000.