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41.
42.
公路和城市街渠机动车大气污染物扩散模式发展综述 总被引:2,自引:0,他引:2
随着城市机动车数量的增加,机动车尾气污染已经成为城市污染物的重要来源。研究机动车尾气扩散规律,可为公路建设,车流量控制,街道大气污染的监测、评价与防治提供科学依据。对公路机动车污染物扩散模型的发展进行了回顾,详细论述了高斯模式、数值模式、统计模式等模式的发展历程及其目前存在的问题,并比较了几种典型模式的性能优劣及其各种条件下的适用性。随后对城市街渠峡谷机动车污染物扩散模型进行专述,指出了街渠峡谷模式研究的难点在于街渠流场模拟,介绍了国外最新街渠流场研究方法。最后提出了当前机动车大气污染物扩散模型存在的主要困难,展望了其解决途径和发展的方向。 相似文献
43.
合肥市夏季热岛特征研究 总被引:8,自引:1,他引:8
根据2002牟夏季高温期间合肥市城市小气候考察的资料,分析了合肥市夏季城市热岛特征以及热岛强度的历史变化。结果表明:1)合肥市夏季热岛强度的日变化与冬季明显不同,夏季晴天一天中热岛强度只出现一个峰值,其基本特征与Oke提出的理想状态下的城市热岛强度日变化的模式曲线非常相似,而冬季与高纬地区的加拿大卡尔加里城市的热岛强度日变化特征接近。这反映了冬、夏两季人类活动、能源消耗量的不同;2)随着城市范围的扩大和城市绿化工程的实施,合肥市热岛面积、分布形状有了一定的改变,但主要分布特征和强度基本没有变化。 相似文献
44.
45.
Lin Y. Hu 《Mathematical Geology》2002,34(8):953-963
Gradual deformation is a parameterization method that reduces considerably the unknown parameter space of stochastic models. This method can be used in an iterative optimization procedure for constraining stochastic simulations to data that are complex, nonanalytical functions of the simulated variables. This method is based on the fact that linear combinations of multi-Gaussian random functions remain multi-Gaussian random functions. During the past few years, we developed the gradual deformation method by combining independent realizations. This paper investigates another alternative: the combination of dependent realizations. One of our motivations for combining dependent realizations was to improve the numerical stability of the gradual deformation method. Because of limitations both in the size of simulation grids and in the precision of simulation algorithms, numerical realizations of a stochastic model are never perfectly independent. It was shown that the accumulation of very small dependence between realizations might result in significant structural drift from the initial stochastic model. From the combination of random functions whose covariance and cross-covariance are proportional to each other, we derived a new formulation of the gradual deformation method that can explicitly take into account the numerical dependence between realizations. This new formulation allows us to reduce the structural deterioration during the iterative optimization. The problem of combining dependent realizations also arises when deforming conditional realizations of a stochastic model. As opposed to the combination of independent realizations, combining conditional realizations avoids the additional conditioning step during the optimization process. However, this procedure is limited to global deformations with fixed structural parameters. 相似文献
46.
In this paper it is argued that the re-establishment of a metropolitan government in the Greater Copenhagen Area in 2000 (Greater
Copenhagen Authority) is not to be seen as a return to the kind of regional, coordinative authority (Greater Copenhagen Council)
that was abolished in the Thatcherite climate of the 1980s. Throughout the 1980s and 1990s new forms of urban governance,
entrepreneurialism and a higher national priority to the capital region were introduced, which changed the conditions under
which the new metropolitan government is operating. This `contextualization' of the analysis of the two generations of metropolitan
government points to the fact that typologies of metropolitan government, such as the one put forward by Sharpe (1995), often
lacks analytical cogency.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
47.
Traditional models of urban development are no longer adequate to describe current metropolitan transformations. These are
now at the centre of a debate concerning management and administration. In Italy, delays in resolving problems of urban and
metropolitan government, despite the legal framework provided by Law 142/90, have weighed heavily on the larger urban areas
of the country: Rome, Naples, Milan, which have not been able to tackle the issue of metropolitan government. Recent legislation,
while not providing a pre-defined institutional solution, allows separate administrative districts to collectively establish
metropolitan institutions of `variable geometry'. The Milan urban area is not one city, but a system of mutually-dependent
cities, linked to each other and the rest of the world by a transport network still requiring much investment. The vitality
of its economic structure (especially its small firms) is held back by seriously inadequate infrastructure and low external
economic efficiency. The provincial capital may boast `historic centrality' but the most interesting potential for development
is to be found on the periphery and in the administrative districts immediately surrounding it, in the recovery of derelict
industrial areas and dormitory towns established in the 1950s and 1960s, especially to the north. Recovery of derelict areas,
green areas, and better transport links within the urban area and with the outside world are the key elements in the reorganization
of `Greater Milan'. In this situation of rapid transformation the most appropriate political strategies involve negotiated
planning.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
48.
简要地介绍了开展大城市地震活断层探测的意义及世界各国开展活断层探测的基本情况。城市及邻近地区地震活断层引起的直下型地震 ,将会给大城市造成巨大的地震灾害。准确地探测出大城市的活断层分布 ,采取有效措施 ,可大大降低地震时的损失。文中重点介绍了开展地震活断层探测的地球物理方法的类型和主要特点 ,在区域探查与初步鉴定阶段、详细探测与精确定位阶段、孕震构造探测阶段可能涉及到的各种地球物理探测方法、适用范围及所能解决的主要问题 相似文献
49.
城市用地与人口的异速增长和相关经验研究 总被引:16,自引:5,他引:16
由于城市土地利用变化涉及的因素多,使获取动态研究研究所需的资料十分困难,所以,用少数几个主要因素定量地表达其变化就显得十分重要。从前人的成果,即以人口表示的城市位序-规模法则和建成区面积表示的位序-规模法则出发,绽绎出城市的用地规模和人口数量呈异速增长。这意味着,如果把整个城市看成是一个生命有机体,那么作为反映城市特征的城市用地规模和城市人口这两个重要变量,就是城市这个有机体的两个器官,他们的增长率是成比例的。还通过这个关系建立了城市建成区面积与市场人口和经济发展水平的数学模式。对我国部分城市的经验研究在一定程度上分别证明了这两经验关系。 相似文献
50.