Study on mitigation of heat island and air pollution with vegetation in urban area

城市面积的扩大往往会使夏季的热环境恶化，即炎热难耐的夏季。城市地区的高温通常是由城市表面类型的变化引起的，如从森林到铺装的道路和建筑物，但东京市中心等高度集中的地区除外，那里密集的能源使用产生的余热可能会直接升高气温。因此，通过在城市地区引入植被来缓解高温应该是一种很有前途的方法。植物还可以通过吸收污染物来减少空气污染。在这项研究中，我们建立了城市冠层模型，用于预测不同高度的植被和建筑物对温度、风和空气污染的影响。该模型将亚网格尺度的建筑物和植被对树冠内和树冠与上层之间的动量、热量、水分和空气污染物传输的影响进行了参数化，并考虑了建筑物和植被的叶面积指数(LAI)、植物在土壤中的水分有效性、建筑物的储热以及植被和建筑物对短辐射和长辐射的响应。通过改变网格单元内建筑物和植被的面积比对模型性能进行了一系列敏感性测试，得到了合理的结果，模型较好地再现了持续晴天植被引起的日最高气温的降低，表明即使在干燥的表层土壤条件下，植被也可以利用深层土壤中的水分进行蒸发。利用该模型对植被对大气污染物的吸收进行了评价。

Expanded urban area tends to make thermal environment worse in summer time, i.e.unbearably hot summer. The high temperature in urban area is usually caused by the change of urban surface type, e.g.from forest to paved road and buildings, except for extremely concentrated area such as the central Tokyo where the waste heat due to dense energy use may raise temperature directly. Thus mitigation of the high temperature through introduction of vegetation into urban area should be a promising way. The vegetation could also reduce air pollution by absorbing the pollutants. In this study we have developed urban canopy model for prediction of the effects of vegetation and buildings, with a variety of heights, on temperature, wind, and air pollution.The model parameterizes effects of subgrid-scale buildings and vegetation on the tansfer of momentum, heat, moisture, and air pollutants within canopy and between canopy and upper layrs, and accounts for LAI (Leaf Area Index) of buildings and vegetation, plant's availability of water in soil, heat storage of the buildings, and response of vegetation and buildings to short and long radiation. A series of sensitivity test was carried out on the model performance by changing area ratio of buildings and vegetation within a grid cell, and reasonable results were obtained on the model, e.g.the model well reproduced reduction of daily maximum temperature due to vegetation under continued sunny days, indicating the vegetation can utilize water in deep soil layr for evaporation even under dry surface soil condition. Absorption of air pollutant by vegetation was also evaluated using the model.