The Effect Of Urban Green Areas On Urban Microclimate

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.