Joint PDF Modelling of Turbulent Flow and Dispersion in an Urban Street Canyon

The joint probability density function (PDF) of turbulent velocity and concentration of a passive scalar in an urban street canyon is computed using a newly developed particle-in-cell Monte Carlo method. Compared to moment closures, the PDF methodology provides the full one-point one-time PDF of the underlying fields containing all higher moments and correlations. The small-scale mixing of the scalar released from a concentrated source at the street level is modelled by the interaction by exchange with the conditional mean (IECM) model, with a micro-mixing time scale designed for geometrically complex settings. The boundary layer along no-slip walls (building sides and tops) is fully resolved using an elliptic relaxation technique, which captures the high anisotropy and inhomogeneity of the Reynolds stress tensor in these regions. A less computationally intensive technique based on wall functions to represent the boundary layers and its effect on the solution are also explored. The calculated statistics are compared to experimental data and large-eddy simulation. The present work can be considered as the first example of computation of the full joint PDF of velocity and a transported passive scalar in an urban setting. The methodology proves successful in providing high level statistical information on the turbulence and pollutant concentration fields in complex urban scenarios.     

未来极端降水对气候平均变暖敏感性的蒙特卡罗模拟试验

利用Weibull分布拟合逐日降水的原始分布模式,并基于统计降尺度和蒙特卡罗随机模拟方法,对中国东部区域各站逐日极端降水量在未来气候变暖条件下的响应特征进行统计数值试验.结果表明,在全球变暖背景下,区域平均温度的改变即可导致区域极端降水概率分布特征的变动.从两个典型代表区域的预估结果中可见,长江中下游南部平均降水量对平均温度升高有正响应,模拟得到的区域极端降水概率分布曲线有明显的向右平移,导致大量级的极端降水的再现期缩短即概率增大.山东及渤海湾区域平均降水量对平均温度升高有负响应,模拟得到的区域极端降水概率密度分布尺度参数变小更明显,即方差增大,表现为左右两侧概率密度增加,同样导致大量级的极端降水再现期缩短即概率增大.本文仅考察了气候均值改变条件下,未来区域气候极端值的概率预估的可行性方案.对于未来气候方差的变化并未作试验,但理论上已经证明,未来气候极端值的概率对于气候方差变化的敏感性可能更大.由于目前尚未整卵出考察方差变化的较为完整的实际观测资料,该问题还有待进一步深入研究.     

华南地区汛期极端降水的概率分布特征

利用1960-2005年华南地区71个测站的逐日降水量资料和NCEP/NCAR南半球月平均海平面气压场再分析资料,采用Le Page榆验、广义极值分布等统计诊断方法,研究了华南地区近46 a前汛期(4-6月)和后汛期(7-9月)极端降水的时空变化及概率分布特征.并讨论了南半球澳大利亚高压和马斯克林高压强度指数与华南汛期暴雨日数间年代际变化的关系.结果表明:(1)1992年华南地区降水发生了由减少趋势到增多趋势的突变,降水趋势发生突变后前汛期极端降水量和日极端降水强度有所下降,而后汛期则是显著增强.(2)华南汛期年平均日最大降水量、50 a一遇日最大降水量极值和暴雨日数的空间分布特征相似,即前汛期的空间分布自南向北呈现"低-高-低"的分布趋势,后汛期呈现由沿海到内陆的"高-低"的分布趋势.(3)1992年发生突变后,前、后汛期年平均日最大降水量和年平均暴雨日数显著增加和减少的空间分布基本一致.(4)就年代际变化而言,南半球澳大利亚高压和马斯克林高压的强度变化是华南汛期降水异常的重要气候背景,即当两高压处在同时增强时期时,华南前汛期极端降水处于偏少阶段,后汛期则处于偏多阶段.     

降水气候强迫下非均匀地表区域平均径流的一种参数化方案

将任一中尺度区域的平均瞬间径流率考虑为区域平均降水量和地表土壤层水分渗透垦的余项.根据降水量在地理空间上分布的实测资料拟合其空间概率密度函数(PDF),并结合土壤入渗物理过程的数学描述及其经验公式,精确估计出地表土壤渗透率及其空间分布,由此建立区域地表径流率的统计-动力学估计方案.换言之,区域内地表产流率可视为区域平均降水量与区域平均的土壤下渗量之差值,而区域内土壤的平均下渗量又町分为非饱和区和饱和区两部分的下渗量来分别计算.就陆面水分循环的物理过程而言,地表入渗现象是在一定的下垫面特性基础上,由一定的水分供应源而形成的.根据大气降水向地表层输送水分的物理过程,在满足植被表层覆盖需水(截流水)和地表层土壤人渗水基础上,多余的降水量才会形成地表径流.凶此,推求地表产流率的主要关键在于地表土壤层需水量.为此奉文根据土壤水分通量方程推导出水分入渗公式.又从描述土壤水分和降水的空间PDF出发,推导出非均匀土壤含水量及降水气候强迫所形成的次网格尺度区域平均径流率计算公式.利用长江三角洲地区1996年降水量和土壤特性等实测资料建立区域平均地表径流率的估计公式,并对其影响凶素进行敏感性试验.结果表明,该方法与用Mosaic方法计算的区域径流率(或产流率)结果十分接近.由此可见,该文提出的降水气候强迫下非均匀地表区域平均径流的这种统计-动力参数化方案,具有相当的可靠性与可行性.     

Assessing probability of surface manifestation of liquefaction at a given site in a given exposure time using CPTU

This paper is a follow-up to a previous paper on the subject of liquefaction potential index (LPI), a parameter that is often used to characterize the potential for surface manifestation of liquefaction at a given site subjected to a given shaking level (represented by a pair of peak ground surface acceleration a_max and moment magnitude M_w). In the previous paper by Juang and his coworkers, the LPI was re-calibrated for a piezocone penetration test (CPTU) model, and a simplified model based on LPI was created for computing the conditional probability of surface manifestation of liquefaction (P_G). In this paper, the model for this conditional probability P_G is extended into a complete framework for assessing the probability of surface manifestation of liquefaction in a given exposure time at a given site subjected to all possible ground motions at all seismic hazard levels. This new framework is formulated and demonstrated with an example site in 10 different seismic regions in the United States.     

Neural network-based model for assessing failure potential of highway slopes in the Alishan, Taiwan Area: Pre- and post-earthquake investigation

This paper is aimed at creating an empirical model for assessing failure potential of highway slopes, with a special attention to the failure characteristics of the highway slopes in the Alishan, Taiwan area prior to, and post, the 1999 Chi-Chi, Taiwan earthquake. The basis of the study is a large database of 955 slope records from four highways in the Alishan area. Artificial neural network (ANN) is utilized to “learn” from this database. The developed ANN model is then used to study the effect of the Chi-Chi earthquake on the slope failure characteristics in the Alishan area. Significant changes in the degrees of influence of several factors (variables) are found and possible reasons for such changes are discussed. The novelty of this paper lies in the fact that the developed ANN models are used as a tool to investigate the slope failure characteristics before and after the Chi-Chi earthquake.     

Mapping and assessing crop diversity in the irrigated Fergana Valley,Uzbekistan

Crop diversity (e.g. the number of agricultural crop types and the level of evenness in area distribution) in the agricultural systems of arid Central Asia has recently been increased mainly to achieve food security of the rural population, however, not throughout the irrigation system. Site-specific factors that promote or hamper crop diversification after the dissolvent of the Soviet Union have hardly been assessed yet. While tapping the potential of remote sensing, the objective was to map and explain spatial patterns of current crop diversity by the example of the irrigated agricultural landscapes of the Fergana Valley, Uzbekistan. Multi-temporal Landsat and RapidEye satellite data formed the basis for creating annual and multi-annual crop maps for 2010–2012 while using supervised classifications. Applying the Simpson index of diversity (SID) to circular buffers with radii of 1.5 and 5 km elucidated the spatial distribution of crop diversity at both the local and landscape spatial scales. A variable importance analysis, rooted in the conditional forest algorithm, investigated potential environmental and socio-economic drivers of the spatial patterns of crop diversity. Overall accuracy of the annual crop maps ranged from 0.84 to 0.86 whilst the SID varied between 0.1 and 0.85. The findings confirmed the existence of areas under monocultures as well as of crop diverse patches. Higher crop diversity occurred in the more distal parts of the irrigation system and sparsely settled areas, especially due to orchards. In contrast, in water-secure and densely settled areas, cotton-wheat rotations dominated due to the state interventions in crop cultivation. Distances to irrigation infrastructure, settlements and the road network influenced crop diversity the most. Spatial explicit information on crop diversity per se has the potential to support policymaking and spatial planning towards crop diversification. Driver analysis as exemplified at the study region in Uzbekistan can help reaching the declared policy to increase crop diversity throughout the country and even beyond.     

The statistics of particle velocities and accelerations under waves with directional spread

The statistics of the horizontal component of the water particle velocity vector under random waves are considered. A spread in the directions of travel of the component wave trains does not affect the rms value of the modulus r but it affects the shape of its probability distribution in such a way that the pr3bability of extreme values is reduced. For small and moderate directional spreads the modulus for a given probability of exceedance is reduced by a factor which tends to F_s as the probability of exceedance tends to zero, where F_s is the spread factor (which is typically 0.9 in temperate storms). However, the convergence on this asymptote is not complete for probabilities of engineering importance, particularly for wide directional spreads. The standard deviation of the probability distribution of the direction of the vector is reduced roughly in inverse proportion to r. The same conclusions apply to particle accelerations.     

Significant load and green water on deck of offshore units/vessels

In this paper the probability of green water occurrence is investigated by taking into account the threshold of the vertical relative motion exceeding the freeboard. The number of wetting of the unit/vessel is predicted using probabilistic method. This paper compares the significant load, due to shipping of green water with the works of other researchers, and the loads are found to be close with the results presented in this paper. There is no direct relation between the velocities in the waves and the water velocity over the deck. The water velocities around the bow are heavily distributed by the presence of the bow. This scenario makes the flow very complex for schematic modelling.     

从234Th的固/液分配看海洋胶体的作用

1994—1995年期间,利用β计数法实测了南沙群岛海域、南海东北部海域、厦门湾塔角附近海域和九龙江河口区共计116份海水样品中溶解态与颗粒态234Th的放射性比活度。结果表明,颗粒态234Th占总34Th的份额大小顺序为：九龙江河口区＞厦门海塔角附近海域＞南沙海域。南海东北部海域。234Th的条件分配系数Kd介于1．1×104—20×106dm3／kg之间,平均为2．2×105dm3／kdKd与总悬浮颗粒物含量（7SAN呈负相关关系：lg（Kd）＝－0．59·lg（TSM）＋5．67,这一“颗粒物浓度效应”可归因于海洋胶体物质的存在。由上述关系获得4个研究海区胶体浓度与悬浮颗粒物浓度的函数关系：Cc＝f（TSM0．59）。