湿热地区高密度街区绿化热环境调节作用研究

Thermal Environment in a High-density Neighborhood with Different Greenings in a Hot-humid Climate Area

城市绿化对于城市热岛效应具有调节作用,但高密度老旧街区却往往缺乏用地从而难以提升绿量。以湿热地区广州市某高密度老旧街区为例,进行夏季典型高温日的日间热环境实测,评估街区与现存绿化措施的热环境影响作用与效果。实测结果显示,街区室外空间尺度较小,SVF均低于0.4,对太阳热辐射与热环境具有较为明显的调节作用,平均温度降幅为2.75~3.86℃,黑球温度降幅为5.75~9.86℃,平均辐射温度降幅为5.82~10.16℃,通用热环境气候指数(UTCI)降幅为3.1~4.68℃。街区闭塞环境、局部开放空间过密的树木冠层不利于街区内部自然通风的调节作用,实测时段外部平均风速接近2.0 m/s,街区内部风速均低于0.5 m/s。对比街区内部不同绿化要素,小叶榕叶面积指数(LAI)为2.4~3.1,小叶榄仁树阵LAI值为1.0~1.2,爬藤植物凉廊LAI值为1.6。结果显示爬藤凉廊对于日间太阳总辐射阻隔幅度达97%,且其所在测点热环境各指标均为最低值,对热环境的调节效果较好。在减碳增绿背景下,爬藤凉廊具有良好的种植适应性与热环境改善潜力,对于老旧街区的景观绿化更新提质具有参考价值。

Urban greening could provide optimizations on urban climate in the trend of urban heat island,however,the historical high-density area in cities is always lack of land resource to improve the tree planting.This study evaluated the thermal environment in a high-density neighborhood in Guangzhou,China as a case,where located in a hot and humid area,and compared the different effects of different urban space and greening methods via field measurements in a typical hot summer day.Field measurements of thermal indices were taken with HOBO data loggers,HD32.3 with probes,and TBQ-2 Series solar irradiation probe.Sky view factor(SVF)was measured with the Sigma EX-DC 4.5mm fisheye lens.The leaf area index(LAI)of the tree canopies was measured with LI-COR LAI-2200C.Results reveal that the scale of outdoor space in the dense neighborhood is quite small,the width of the street was about 2~4 m and the SVF was recorded lower than 0.4.The narrow space limits the solar irradiation(Ga)on the street surfaces in daytime.The thermal environment in the neighborhood was optimized.The average air temperature(Ta)was reduced by 2.75~3.86℃,globe temperature(Tg)was reduced by 5.75~9.86℃,the mean radiant temperature(MRT)was calculated and reduced by 5.82~10.16℃,and the universal thermal climate index(UTCI)was reduced by 3.1~4.68℃.A correlation analysis was given and revealed that the R2 of the SVF,UTCI,and Tg as well as Ta is higher than 0.8,implying that the scale of neighborhood affected the thermal environment obviously.However,the dense environment and tree crown in some small open space also limit the air flow in the neighborhood.The average wind velocity(Va)was measured close to 2.0 m/s in the contrast open space but lower than 0.5 m/s in the neighborhood.Canopies of the trees in the neighborhood were also measured and compared.The LAI of the Ficus macrocarpa,the Terminalia neotaliala,and a pergola with climbing plants was 2.4~3.1,1.0~1.2,and 1.6.The results reveal that the green pergola reduced the Ga up to 97%and the thermal indices in which performed the lowest level in the measurements.It implied that the green pergola may be an effective greening strategy in a high-density neighborhood because its land and soil requirement is relatively low but the effect on thermal comfort is as good as the other greenings.This study took some field measurements and revealed the characters and reasons of the thermal environment in a high-density neighborhood in hot and humid area.The greening strategies were also compared in the analysis and the green pergola was recommended as one of the landscape design and planting methods in the high-density urban area.