全新世气候系统的突变及周期性   总被引：6，自引：0，他引：6  

鲜锋  周卫健  余华贵 《海洋地质与第四纪地质》2006,26(5):109-115

高纬冰心、海洋沉积物及陆相古气候沉积记录揭示了全新世冰后期(最近1万年以来)存在系列气候突变事件及百年—千年尺度的气候周期韵律,证实了在冰期-间冰期旋回大尺度气候背景下全球气候存在较大不稳定特征这一基本事实。全新世作为与人类文明衔接的最新地质时段,各国科学家针对全新世气候系统变化特征及驱动机制的研究不断取得新的进展,这些成果将为未来气候演化趋势预测提供重要的历史相似性。总结了近年来关于全新世气候系统的突变及周期性研究的一些成果,并进行了概略的评述和展望。  相似文献   

Empirical transformations from U.T. to E.T. for the period 1800–1988 (Astronomisches Rechen-Institut — Mitteilungen Serie B)     

L. D. Schmadel  G. Zech 《Astronomische Nachrichten》1988,309(3):219-221

Approximation formulae describing the long-term changes in the rotation of the Earth are evaluated. The time scale reduction from Universal to Ephemeris Time can be accomplished by a 12th degree polynomial with a mean error of less than one second for the entire time span 1800–1988. The maximum residuals of least squares fits of the 19th and 20th century data, respectively, can be limited to this accuracy by even simpler expressions.  相似文献   

Objective assessment of maturity in the Patagonian squid Loligo gahi (Cephalopoda: Loliginidae) from Falkland Islands waters     

Hatfield  E. M. C.; Murray  A. W. A. 《ICES Journal of Marine Science》1999,56(5):746-756

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Two-Point Correlation Analysis Of Neutrally Stratified Flow Within And Above A Forest From Large-Eddy Simulation   总被引：5，自引：5，他引：0  

Hong-Bing Su  Roger H. Shaw  Kyaw Tha Paw U 《Boundary-Layer Meteorology》2000,94(3):423-460

Two-point space-time correlations ofvelocities, a passive scalar and static pressure arecalculated using the resolvable flow fields computedby large-eddy simulation (LES) of neutrally stratifiedflow within and above a sparse forest. Zero-time-lagspatial auto-correlation contours in thestreamwise-vertical cross-section for longitudinal andlateral velocities and for a scalar are tilted fromthe vertical in the downstream direction, as istypical in near-wall sheared flow. On the other hand,auto-correlations of vertical velocity and of staticpressure are vertically coherent. Zero-time-lagspatial auto-correlations in the spanwise-verticalcross-section show no distinct tilt, and those forboth longitudinal and vertical velocities demonstratedistinct negative side lobes in the middle forest andabove, while longitudinal velocity in the subcrowntrunk space is laterally in-phase. Static pressureperturbations appear to be spatially coherent in thespanwise direction at all heights, especially insidethe forest. Near the forest floor, longitudinalvelocity is found to be in-phase with static pressureperturbation and to be closely linked to theinstantaneous streamwise pressure gradient, supportinga previous proposal that longitudinal velocity in thisregion is dominantly modulated by the pressurepatterns associated with the coherent sweep/ejectionevents. Near treetop height, a lack of linkage betweenthe pressure gradient and the local time derivative ofthe longitudinal velocity supports the hypothesis ofadvection dominating turbulent flow.The major phase characteristics of the two-pointcorrelations essentially remained the same from fourLES runs with different domain size and/or gridresolution. A larger LES domain yielded betteragreement with field observations in a real forest onboth the magnitudes of the correlations and thesingle-point integral time scales. A finer gridresolution in the LES led to a faster rate of decreaseof correlation with increasing separation in space ortime, as did the higher frequency fluctuations in theturbulent records from field measurements. Convectivevelocities estimated from the lagged two-pointauto-correlations of the calculated flow fields werecompared with similar calculations from wind-tunnelstudies. At the canopy top, estimates from thecorrelation analyses agree with the translationvelocity estimated from instantaneous snapshots of ascalar microfront using both LES and field data. Thistranslation velocity is somewhat higher than the localmean wind speed. Convective velocities estimated fromlagged correlations increase with height above thecanopy. It is suggested that an appropriate filteringprocedure may be necessary to reduce the effects ofsmall-scale random turbulence, as was reported in astudy over an orchard canopy. The mean longitudinalvelocity near the treetops is found to be moreappropriate than the local mean longitudinal velocityat each height to link single-point integral timescales with directly calculated spatial integralstreamwise length scales.  相似文献   

Investigation of electric fields inside and outside a magnetised cold plasma sphere     

李应乐  王明军  董群峰 《海洋学报(英文版)》2010,29(11)

Based on the scales transformation of electromagnetic theory, the analytical expressions of electric fields inside and outside a magnetised cold plasma sphere are presented by reforming the spherical electromagnetic parameter. The obtained results are in good agreement with that in the literature. The angle between the direction of inside field and that of outside field is derived. In S wave band, calculations for the effects induced by parameters of the inner field are established. Simulations show that the angle between incident field and the outside magnetic field influences the inner field remarkably. The inner field will increase as the electron density increases. The inner field varies with frequency nonlinearly. There is an angle between the inner field and the incident field, it changes nonlinearly with the frequency.  相似文献   

The Navier–Stokes-α equations revisited     

Paul H. Roberts  Andrew M. Soward 《地球物理与天体物理流体动力学》2013,107(4):303-316

The Navier–Stokes-α equation is a regularised form of the Euler equation that has been employed in representing the sub-grid scales in large-eddy simulations. Determined efforts have been made to place it on a secure deductive foundation. This requires two steps to be completed. The first is fundamental and consists of establishing from the equations governing the fluid flow, a relationship between two velocities called by Holm (Chaos, 2002a, 12, 518) the “filtered” and “unfiltered” velocities. The second consists of the relation between these two velocities. Until now, the preferred route to the first objective has been variational, by varying the action using Hamilton's principle. Soward and Roberts (J. Fluid Mech., 2008, 604, 297) followed that variational route and established the existence of an important but unwelcome term omitted by Holm in his derivation. It is shown here that the Soward and Roberts result may be derived from Euler's equation by a direct approach with considerably greater efficiency. Holm achieved the second objective by making a “Taylor hypothesis”, which we use here to evaluate the unwelcome term missing from his analysis of the first step. The resulting model equations differ from those of Holm's α model, and the attractive mean Kelvin's circulation theorem that follows from his α equations is no longer valid. For that reason, we call the term omitted by Holm unwelcome.  相似文献   

Discrimination of mangrove forests and characterization of adjoining land cover classes using temporal C-band Synthetic Aperture Radar data: A case study of Sundarbans     

《International Journal of Applied Earth Observation and Geoinformation》2013

C-band dual polarization (HH, HV) Synthetic Aperture Radar (SAR) data from Radarsat-2 were used to discriminate and characterize mangrove forests of the Sundarbans. Multi-temporal data acquired during winter and rainy seasons were analysed for the segregation of mangrove forest area. A decision rule based classification involving combination of three-date HH (range −11 to −2 dB) with single-date cross-polarization ratio (2–8) was applied on the datasets for discriminating mangrove forests from other land cover classes. Application of textural measures (entropy and angular second moment) in the aforesaid decision rule based classification produced three broad homogeneous mangrove classes. The area covered by the most homogeneous class increased from January to March and decreased from July to September, and correlated well to the change in the phenological status of the mangroves. Extent of homogeneous areas was more in the eastern region of the Sundarbans than that of the central and western side. Thus, the study revealed that textural measures combined with multi-temporal HH backscatter and single-date cross-polarization ratio in a decision rule classification could be satisfactorily used for characterization of the mangrove forests.  相似文献   

一种逐级合并OD流向时空联合聚类算法     

项秋亮  邬群勇  张良盼 《地球信息科学学报》2020,22(6):1394-1405

现有OD流向聚类多将O点和D点相分离或者将OD流向看作4维空间的数据点进行聚类处理,忽视了流向长度、方向、时间对流向聚类的影响。本文以流向作为研究对象,提出一种基于流向间相似性度的逐级合并OD流向时空联合聚类算法。首先在充分研究OD流向的空间信息和时间信息的基础上,构建合理的OD流向间时空相似性度量方法,对OD流向间的时空相似性进行量化;然后提出逐级合并OD流向聚类策略,优化类簇合并的顺序,以减少层次聚类的时间开销,实现OD流向的时空联合聚类。以成都市的滴滴出行OD数据和纽约市出租车数据为例对本文方法进行了验证,结果表明：① 本算法聚类获得的流向类簇不仅带有空间特征还具备时间特征;② 在不同参数下本方法可以得到不同时空尺度的聚类结果;③ 与现有较高水平的流向聚类算法相对比,本文方法的聚类效果更好。这体现在流向类簇内部的流向之间有着充分的相似性,以及本文方法不仅可以提取出显著的流向类簇,还可以提取出非热点区域之间的流向类簇。本算法顾及空间因素和时间因素,可以通过调整时空相似性度量方法中的时间参数和空间参数以实现不同时空尺度的流向聚类,这使得从不同时空角度研究城市居民出行模式成为可能。本文提出的OD流向时空联合聚类算法从联合时间信息和空间信息的角度获得对运动数据的新见解,有助于合理全面地研究居民的移动模式、区域之间的空间联系、已知出行结构的确定以及出行目的的探索,是后续一系列分析工作的基础。  相似文献   

The influence of land use patterns on water quality at multiple spatial scales in a river system          下载免费PDF全文

Guoqiang Wang  Yinglan A  Zongxue Xu  Shurong Zhang 《水文研究》2014,28(20):5259-5272

The influence of land use patterns on water quality in a river system is scale‐dependent. In this study, a four‐order hierarchical arrangement method was used to select water sampling sites and to delineate sub‐basins in the Daliao River Basin, China. The 20 sub‐basins were classified into four spatial scales that represented four different stream orders. Pearson correlation analysis was used to quantify relationships between land use composition and the river's physical‐chemical variables for all samples collected. This analysis showed that the presence of forest cover was associated with higher water quality at the scale of the whole basin. The scale effects of land use patterns on water quality were then examined using stepwise multiple regression analysis that compared different land use types with water quality variables. The results from this analysis showed that urban areas, as opposed to forest areas, became the most significant contributors of water pollutants when scale effects were considered. The influence of urban land cover on water pollution was significantly higher at larger scales. The lack of a significant regression correlation for the forest land use type at smaller scales revealed that forest located upstream of the Daliao River Basin did not provide a buffer for improved water quality. Further analysis showed that this result could be because of disproportionate spatial distributions for forest and urban land use types. The topographic characteristics of sub‐basins, such as average slope (S) and size (A), were determined to be secondary explanatory variables that affected land use impacts on stream water quality. Areas with steep slopes were associated with increased water oxygenation, whereas areas with flatter slopes were associated with higher concentrations of pollutants. These results are significant because they can provide a better understanding of the appropriate spatial scale required for effective river basin management in the future. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Variations in runoff,sediment load,and their relationship for a major sediment source area of the Jialing River basin,southern China     

Yiting Shao  Xingmin Mu  Yi He  Kai Chen 《水文研究》2021,35(7):e14297

Investigation of the variations in runoff, sediment load, and their dynamic relation is conducive to understanding hydrological regime changes and supporting channel regulation and fluvial management. This study is undertaken in the Xihanshui catchment, which is known for its high sediment-laden in the Jialing River of the Yangtze River basin, southern China, to evaluate the change characteristics of runoff, sediment load, and their relationship at multi-temporal scales from 1966 to 2016. The results showed that runoff changed significantly for more months, whereas the significant changes in monthly sediment load occurred from April to September. The contributions of runoff in summer and autumn and sediment load in summer to their annual value changes were greater. Annual runoff and sediment load in the Xihanshui catchment both exhibited significant decreasing trends (p < 0.05) with a significant mutation in 1993 (p < 0.05). The average annual runoff in the change period (1994–2016) decreased by 49.58% and annual sediment load displayed a substantial decline with a reduction of 77.77% in comparison with the reference period (1966–1993) due to climate change and intensive human activity. The power functions were satisfactory to describe annual and extreme monthly runoff–sediment relationships, whereas the monthly runoff–sediment relationship and extreme monthly sediment-runoff relationship were changeable. Spatially, annual runoff–sediment relationship alteration could be partly attributed to sediment load changes in the upstream area and runoff variations in the downstream region. Three quantitative methods revealed that the main driver for significant reductions of annual runoff and sediment load is the human activity dominated by soil and water conservation measures, while climate change only contributed 22.73%–38.99% (mean 32.07%) to the total runoff reduction and 3.39%–35.56% (mean 17.32%) to the total decrease in sediment load.  相似文献