广义极值分布理论在重现期计算的应用

在气候统计学上,常用Weibull、Gumbel、Frechet统计分布函数对极端气候要素的分布进行拟合,广义极值分布理论综合了以上三种极值分布模型,在气候分析中得到了广泛应用。以南昌市年汛期日最大降水量为例,利用广义极值分布理论对其分布进行拟合,并对重现值及其置信区间进行计算,为气候要素极值的统计分析提供了一种新的手段。     

Road network selection for medium scales using an extended stroke-mesh combination algorithm

The road network is an essential feature class in topographic maps and databases. Road network selection for smaller scales forms a prerequisite for all other generalization operators and is thus a fundamental operation in the overall process of topographic map and database production. The objective of this paper was to develop an algorithm for automated road network selection from a large-scale (1:10,000) to a medium-scale database (1:50,000). The project was pursued in collaboration with swisstopo, the national mapping agency of Switzerland. Three algorithms (a stroke-based, a mesh-based, and a combined stroke-mesh algorithm) were implemented from the literature and analyzed using swisstopo’s large-scale TLM3D spatial database, with requirements set forth by expert cartographers. Initial experiments showed that the combination algorithm performed best, yet still it could not meet all requirements. Therefore, extensions to the basic stroke-mesh algorithm were developed, significantly improving the selection result with real-world, large test databases. Three extensions introduce modifications to the stroke-mesh combination algorithm. Furthermore, two extensions include external feature classes, ensuring accessibility of points of interest and appropriate network density representation in settlement areas, respectively. The results were evaluated by expert cartographers, who concluded that the proposed approach is ready to be deployed in production at swisstopo.     

A stroke-based method for automated generation of schematic network maps

This article deals with the graphic simplification of a network by schematization. A new method employing a stroke-based and progressive strategy is proposed to generate schematic network maps. This method treats a stroke (which is a long line with segments concatenated together) as a basic unit for the implementation. The procedure is as follows: (a) strokes are formed from line segments, (b) the strokes are re-orientated along grid lines and/or diagonals, and (c) two endpoints and all intersection points on (sub-)strokes are projected onto re-oriented straight lines, and (d) spatial inconsistency is detected and resolved. A methodology for each of these steps is described. This new method has been tested with a set of real-life road network data and evaluated by fractal analysis and empirical study. Experimental results show that this new method is more effective than segment-based methods and is able to produce graphics with great simplicity and clarity. Based on the results obtained, the stroke-based schematization with four primary directions is recommended.     

Sea level extremes in Marseille (NW Mediterranean) during 1885–2008

Sea level extremes and their temporal variability have been explored based on the hourly measurements at Marseille tide gauge for the period 1885–2008. A careful quality check has first been applied to the observations to ensure consistency of the record by eliminating outliers and datum shifts. Yearly percentiles have been used to investigate long-term trends of extremes revealing that secular variations in extremes are linked to mean sea level changes. The associated decadal changes show discrepancies between mean sea level trend and extreme fluctuations, due to the influence of the atmospheric forcing. A local regression model based on the generalized Pareto distribution has been applied to derive trends in return levels. The 50-years return levels reach values between 80 and 120 cm. The most significant changes in return levels are characterized by an increase since the 1970s.     

Influence of the Return Current on the EUV and X-Ray Flare Line Emissions

Electron beams accelerated during solar flares carry electric currents which should be neutralized by so-called return currents. Both the electron beam and return current modify the electron distribution function in the solar transition region and low corona. Thus, they influence the intensities of the spectral lines formed in these layers. Synthetic spectra for the solar flare atmosphere are computed from model conditions and the possibilities of diagnostics of the return current from the EUV and X-ray line spectra are discussed.     

Effect of oxygen on coral fanning by mutualistic fish

Coral branches partially block the flow of water through the colony, creating an inner zone of reduced water exchange where hypoxic conditions can develop during the night. Previous studies have shown that this oxygen deficiency is alleviated by the fanning behavior of mutualistic damselfish that spend the night between the coral branches, constantly moving their fins. Our findings show that fin‐stroking frequency during fanning by the damselfish Dascyllus marginatus is moderately plastic, with lower frequency under higher oxygen concentration, and vice versa. The inter‐play between oxygen concentration and fin motion maintains nearly steady oxygen concentration between the coral branches during the night.     

COMPUTATION OF RETURN FLOWS DUE TO NAVIGATION TRAFFICS IN RESTRICTED WATERWAYS

1 INTRODUCTION Movement of navigation traffic in restricted waterway can generate various hydrodynamic phenomena including return flow (RF), surge wave, drawdown and sediment re-suspension which are absent in open water conditions (Schofield and Martin, 1…     

降雨重现期及其用于滑坡概率分析的探讨

极端气候条件往往会诱发各种地质灾害,而降雨型滑坡的发生则与极值降雨关系密切。为了有效预防和控制汛期滑坡灾害的发生,定量评估滑坡灾害造成的人员与经济风险,讨论了汛期极值降雨条件下滑坡概率的分析方法。利用Gumbel极值分布理论,以三峡库区巴东县1990~2006年日降雨量为基础资料,采用统计分析方法,求取研究区在汛期(6月中旬至9月)最大一次连续降雨量、多日累积最大降雨量的极值及其分布曲线; 在此基础上,以研究区一个滑坡实例为对象,综合采用渗流模拟、稳定性分析和基于蒙特卡罗的滑坡概率分析方法,讨论了降雨极值及其重现期成果分别在降雨新生型滑坡和降雨复活型滑坡概率分析中的应用思路与方法。结果表明,随着重现期的增加,一次降雨过程的降雨量也增加; 随着降雨日数的增加,具有不同重现期降雨事件的累积降雨量均会增加,且重现期越长,累积降雨量值会越大; 降雨极值曲线分别应用于降雨新生型和降雨复活型滑坡概率分析的思路是可行并有效的。     

武汉市居民中暑与气象因子的统计特点研究

利用武汉市1994～2006年共13年1464个中暑病例资料和同期逐日气象资料,分别统计逐日中暑人数、日平均中暑人数与气象因子的线性、非线性相关系数,筛选出关键气象因子,建立中暑与多气象因子的非线性模型,制订中暑气象等级标准.结果表明:气温是中暑发生的最关键影响因子,不利气象因子3天或3天以上的累积效应才能导致中暑群发,中暑人数与气象因子呈现非线性关系.建立了日平均中暑人数与前3日平均气温、前3 日平均最小相对湿度的指数模型.将日平均中暑人数划分为5级,并应用该等级标准进行历史样本同代检验和独立样本预报检验,效果较好.     

一次多回击自然闪电的高速摄像观测

2006年8月1日在广东省从化市利用成像率为5000幅/s的高速摄像系统观测得到了一次包含有13个回击过程的自然负地闪, 其梯级先导的传播速度为106 m/s的量级; 一次企图先导的传播速度随高度的降低而减小; 一次直窜先导的传播速度随高度的降低而增加。有3次继后回击相对积分亮度的峰值大于首次回击相对积分亮度的峰值。研究发现:此次自然闪电的继后回击及紧跟其后的连续电流过程的发光总量与该次继后回击之前闪电通道的截止时间有关, 较大的发光总量对应于较长的截止时间, 较小的发光总量对应于较短的截止时间, 但两者没有固定的比例关系。