城市绿地热效应全年变化特征及其与背景气象因子的关系

Annual temporal pattern and underlying meteorological factors of the thermal effects of urban green spaces

城市绿地能够调节热气候,是城市实现高质量可持续发展的重要空间载体。以往研究着重探讨城市绿地夏季降温效应的空间特征,较少分析绿地热效应的全年变化规律与潜在影响因子。在南京城区选择7个代表性城市公园,采用热气候定点观测方法获取公园大气温度全年观测数据,并与城市中心区参照点进行对比,分析公园热效应的全年动态变化规律及背景气象因子的影响。研究发现,由于植被覆盖率、冠层郁闭度及海拔不同,不同公园热效应的季节、昼夜规律存在显著差异,主要表现为3种形式:全年降温型,全年昼升夜降型,以及夏季全天降温、春秋冬季昼升夜降型。与城市中心区相比,公园夏季最高可降低气温7.7℃,冬季最高可增温3℃。大气温度、相对湿度,风速与太阳辐射4个背景气象因子能解释23.5%—77.4%的公园热效应变异。夏冬两季日间热效应主要受太阳辐射和气温的影响,夜间热效应主要受风速影响:太阳辐射每升高100W/m^(2),夏季日间降温强度减少0.3℃,冬季日间增温强度增加0.2—0.5℃;气温每升高1℃,夏季日间降温强度提高0.1—0.14℃,冬季日间增温强度提高0.05—0.08℃;风速每增加1m/s,夏季夜间降温强度减少0.05—0.78℃,冬季夜间降温强度减少0.4—1.1℃。研究结果有助于深入认识城市绿地热效应的复杂形成机制,并为夏热冬冷地区城市自然空间保护与气候适应性设计提供科学参考。

Urban green spaces(UGSs)can help cities achieve high-quality and sustainable development by regulating local climate through the thermal effects.A large number of studies have investigated the spatial pattern of the summer cooling effects of UGSs,while few studies explored the temporal pattern and underlying factors of UGS thermal effects on an annual basis.This study selected seven representative urban parks in Nanjing City to investigate their yearlong thermal-effect dynamics and underlying meteorological factors.Air temperature sensors were mounted on pole-like structures at around 2.5m above the ground at both the center and boundary of the parks.To assess the thermal effects,fixed measurements were also taken at two urban reference sites in the central business district that has a compact high-rise morphology.The results indicated significant differences in the seasonal and daily dynamics of thermal effects between the seven parks due to their differences in vegetation coverage,canopy closure and altitude.Three temporal patterns of thermal effects were discovered,including cooling throughout the year,daytime warming and nighttime cooling,and daylong cooling in summer and daytime warming nighttime cooling in other seasons.Compared with the urban center,the park could lower the air temperature by up to 7.7℃in summer and increase the temperature by 3℃in winter.However,the impacts of urban parks on urban climate were not always beneficial,since they can display a warming effect in summer daytime and a cooling effect in winter nighttime.The four background meteorological factors,including air temperature,relative humidity,wind speed and solar radiation,could explain 23.5%—77.4%of the variations in the thermal effects.Air temperature and solar radiation were the primary factors for daytime thermal effect,while wind speed mainly affected the nighttime thermal effect.For every 100W/m^(2) increase in solar radiation,the summer daytime cooling effect could decrease by 0.3℃,and the winter daytime warming effect could increase by 0.2—0.5℃.For every 1℃increase in air temperature,the summer daytime cooling effect could increase by 0.1—0.14℃,and the winter daytime warming effect could increase by 0.05—0.08℃.For every 1m/s increase in wind speed,the summer nighttime cooling effect could reduce by 0.05—0.78℃,and the winter nighttime cooling effect could decrease by 0.4—1.1℃.The above results revealed the complex formation mechanism of the thermal effects of urban green spaces,and might provide scientific references for natural conservation and climate adaptative design of urban parks in hot summer and cold winter areas.