【作者】 晏海；

【导师】 董丽；

【作者基本信息】 北京林业大学 ， 园林植物与观赏园艺， 2014， 博士

【摘要】 随着城市化的快速推进,城市热岛效应越来越明显。城市热岛现象造成都市高温化进而影响居住环境的安全性和舒适性,并加剧能源消耗及温室气体排放。已有研究表明,增加城市公园绿地是降低城市热岛效应最有效的方式之一。然而,公园绿地降温效应受到诸多自身及环境因子的影响,对其机制的研究还十分薄弱。本研究针对具有特定气候条件和城市形态的北京市,以北京奥林匹克公园和奥林匹克森林公园为例,研究了城市公园内不同树木群落、不同下垫面组成的空气温湿度差异、变化规律及其对公园微尺度热环境的影响,探索了大型公园绿地对周围城市区域环境的降温效应和影响因素,揭示了局地尺度上城市气温与植被覆盖关系的时空变化。主要结论如下：(1)在夏季,树木群落可显著降低空气温度和光照强度,提高相对湿度。与群落外空旷地的对照点相比,植物群落内的日均降温强度为1.6-2.5℃；增湿强度为2.9%～5.2%；遮光率为61.0%～96.9%。同时,不同树种群落的微气候因子也存在着显著差异。植物群落与对照点的日平均不舒适指数差异不显著,但与对照点相比,群落都能降低一定的不舒适指数,降低不舒适指数率为2.5%～4.3%。相关性分析表明,树木群落的冠层特征(叶面积指数、冠层盖度和天空可视因子)对群落内的微气候因子具有重要的调节作用,群落叶面积指数和冠层盖度越大,群落内的光照强度越低,则群落内的空气温度也越低。(2)在夏季,不同下垫面的气温存在明显的差异,尤其是午后这种差异最为明显,此时路面的气温最高,其次是草坪和水体,林地的气温最低；而到了晚上,不同下垫面之间的气温差异较小,此时路面的气温仍然最高,而草坪的气温变得最低。不同下垫面的湿度格局和温度呈相反的变化趋势。不管白天还是晚上,公园内不同下垫面的湿度均高于公园外城市环境的湿度,形成“公园湿岛”；但是,午后公园内路面和草坪的气温有可能比附近城市环境的气温更高,从而形成不舒适的热环境。在午后,林地的面积比率对公园局地小气候起着主导作用,而到了晚上,草坪的面积比率对公园局地小气候有着决定性的作用。(3)公园区域比周围城市环境拥有更低的空气温度和更高的相对湿度,尤其是在午夜时,公园的降温效应最为明显,最大降温强度达4.8℃,平均降温强度为2.8℃。观测也发现,随着距公园边界距离的增加,空气温度呈上升趋势,尤其在午夜这种气温上升趋势最为明显,距公园边界距离每增加O.1km气温上升0.20～0.31℃。这种现象表明公园对周围的城市环境也产生一定的降温效应,这种降温效应可以延伸到距公园边界1km以外的地方。公园的降温强度和降温范围受周围城市环境的影响,周围城市环境温度越高,公园降温强度越大；公园周围环境越开阔,公园降温距离越远。通过回归分析发现,空气湿度和气温呈显著的负相关,表明蒸散作用对于公园降温效应起着重要的调节作用。此外,测点周围的下垫面组成对测点气温也有重要的影响,植被覆盖率越高气温越低：而不透水表面比率越高则气温越高。(4)在局地尺度的城市区域内仍然存在明显的空气温度差异,城市气温及其差异的形成主要受不同范围的植被覆盖以及测点环境特征的影响。植被的状况对于局地气温的分布与温度差异大小有着重要的影响,除了冬季白天,植被在冬季夜晚、夏季白天和夏季夜晚都有显著的降温效应,降温强度呈现出冬季夜晚>夏季夜晚>夏季白天>冬季白天的趋势。城市气温与测点周围植被覆盖率间的相关程度随不同时相而变化,两者之间的相关性夏季比冬季密切,晚上比白天密切。城市气温与测点周围植被覆盖率间的相关程度随周围范围大小而有变化,在白天时,测点气温主要受测点周围20m半径范围内植被覆盖率的影响；而到了晚上,气温主要受周围150m半径范围内植被覆盖情况的影响。城市气温也受到城市冠层结构的影响,在白天时气温随着天空可视因子的增加而升高,而到了晚上则呈现出相反的趋势。此外,测点的地理位置也对其气温的大小有影响,随着距公园和水体距离的增加空气温度呈上升趋势。更多还原

【Abstract】 In the last decades, a great concentration of people around urban areas took place worldwide. The urbanization process, with its fast population increase, creates changes in the urban climate. A distinct feature of urban climate is the urban heat island (UHI) effect, which has significant negative effects on the buildings energy consumption, outdoor air quality, living environment, and habitability of cities. Therefore, there is a pressing need for urban researchers to evaluate strategies that may mitigate against further increases in temperatures in urban areas. Among all cooling measures, planting of vegetation in urban areas is one of the simplest and most effective strategies to mitigate the UHI effect. However, the cooling effect of urban green spaces are affected by park characteristics and their surrounding built areas. Understanding the causes of the cooling effect of urban green spaces is a first step in improving urban landscape design to ameliorate urban thermal environment. In the present study, the thermal performance of urban green spaces in Beijing were investigated through a field measurement campaign and statistical analysis. The main experimental results are as follows:(1) In hot summer, there existed significant differences in the air temperature, relative humidity and light intensity between tree communities and the control open site (CK). Compared with CK, the tree communities can decrease temperature by1.6～2.5℃, increase the relative humidity with2.9%-5.2%. Compared with the CK, all species communities are able to reduce the diurnal mean discomfort index (DI) in some degree. Correlation analysis between microclimate factors and the indices of the tree communities canopy structural characteristics show that canopy characteristics play important regulatory role in microclimate and thermal environment.(2) The average air temperature difference among different land cover types was large during the day and small during the night. At noon, the average air temperature differed significantly among four land cover types, whereas on night, there was no significant difference among different land cover types. The results of the linear regression indicated that during daytime, there was a good negative correlation between air temperature and percent trees cover; while at nighttime, the air temperature had significant negative correlation with the coverage of lawn area. It was shown that as the coverage of trees areas increased10%, the air temperature decreased by0.26℃during daytime, while as the coverage of lawn areas increased10%, the air temperature decreased by0.56℃during nighttime.(3) Results from field measurements in and around the park showed that urban park were on average cooler than their surroundings, especially during midnight, with a mean cooling intensity of 2.8℃. The results also indicated that the cooling effect of the urban green space was remarkable not only at park areas but also the surrounding built environments. Air temperatures gradually increased with increasing distance from the park boundary, for every O.lkm increase in distance will increase air temperature by0.20～0.31℃, and the cooling effect could reach up to lkm from the park boundary. Relative humidity was one of the key variables affecting local air temperature. The observed air temperature decreased as relative humidity increased during all the time. Land cover composition was another crucial parameter. The percentage vegetation cover was consistently negatively correlated with air temperature, whereas the percentage of impervious surface area was positively related.(4) Spatial temperature difference between the maximum and minimum observed air temperature at a local scale in urban area ranged from1.2to7.0℃, depending on season and time of the day. The magnitude and spatial characteristic of the air temperature variations depend strongly on the coverage of vegetation characterizing the immediate environment of the measurement sites. The air temperature had a significant negative correlation with the percentage vegetation cover, but the degree of correlation varied among different times and seasons. Moreover, the influence of vegetation on air temperature also varied with spatial extent scale. Site geometry was another crucial parameter because of its importance in determining the receipt and loss of radiation. The observed air temperature increased as sky view factor increased during daytime, while a contrary tendency was observed during nighttime. In addition, the air temperature increased with increasing distance from the park and water body boundary.更多还原