Research On The Evolution Characteristics And Regulation Mechanism Of Urban Thermal Environment And Comfort

The deterioration of the urban thermal environment seriously threatens the health of urban residents,it is an important predisposing factor for the occurrence of diseases related to the respiratory system,circulatory system,digestive system,and nervous system.In recent years,in the dual role of urban heat islands and climate warming,most cities are threatened by"high temperature heat waves"in summer.Therefore,the urban thermal environment problem has become the frontier and hotspot of domestic and foreign research,and how to improve the urban thermal environment has become an important and difficult problem that needs to be solved urgently.Based on the long-term series of daily meteorological data,this study clarified the characteristics of the spatiotemporal evolution of urban thermal environment and thermal comfort in China,analyzed the driving mechanism of urban thermal environment and comfort evolution during rapid urbanization,and reveal the comprehensive regulation mechanism of urban forest on urban thermal environment and comfort at the community and landscape scale.The main methods and results are as follows:The Mann-Kendall trend analysis and Sen's slope methods are used comprehensively to study the evolution characteristics of summer thermal environment factors and comfort indexes in 183 key cities during the period 1990-2016,results reveal the driving mechanism for the evolution of the urban thermal environment and comfort in the summer during rapid urbanization.The results show that in 108 cities,the summer thermal comfort index PET is greater than 29°C,and residents are exposed to moderate or strong heat stress.Residents of 14 cities have no heat stress in summer,and residents of 7 and 54 cities have slight cold stress and slight heat stress in summer,respectively.Most cities in China become warmer and drier in summer,the temperature rises by 0.027°C per year on average,and the relative humidity drops by 0.181%per year on average.The trends of wind speed have strong spatial heterogeneity,which decrease by 0.006m/s per year on average.After the combined influence of such thermal environmental factors,thermal comfort index PET and discomfortable days in summer increase by 0.028°C and 0.142 days per year on average,respectively.PET in 49 cities located in the Northeast Plain,Northwest China,and Southeast Coastal Area increased significantly,with an average increase of 0.098°C per year.Among these cities,discomfort periods of 28 cities have also prolonged significantly,with an average increase of 0.702 days per year.The effects of urbanization factors on thermal environment and comfort dynamics are significantly different under different climate background conditions.Cities with higher global solar radiation,lower relative humidity,and higher wind speeds are more susceptible to urbanization.Changchun is a typical city in the Northeast,this study further analyzes the deterioration characteristics and driving mechanism of urban thermal environment.During 1990-2016,thermal environment and thermal comfort difference between urban and suburban of Changchun have changed significantly around 2002.After 2002,the phenomenon of heat island and dry island in the urban area of Changchun has become increasingly obvious,and the wind speed in the urban area dropped significantly.The increase of artificial heat sources(such as transportation)is an important reason for the increase in temperature and humidity in urban areas.The change in land use patterns(such as the increase in the area of built-up areas and roads)is the main reason for the decrease in urban wind speed.Changes in urban thermal environmental factors have resulted in the increasement of PET and discomfortable days in summer,and makes urban residents to bear higher thermal stress and heat risks.In response to the deterioration of the urban thermal environment,this study revealed the mechanism by which urban forests regulate the urban thermal environment at the community scale.Leaf area index and canopy density are the key indicators to regulate the urban thermal environment and comfort,and there are obvious thresholds.In this study,when the leaf area index reached 3,the temperature and PET in the plot could be decreased by 1.7°C and 8.1°C,respectively.When the degree of canopy closure was 0.4,the temperature and PET in the plot could be decreased by 1.5°C and8.1?,respectively.It is suggested that the leaf area index reaches 3 and the canopy closure reaches 0.4 as the threshold for urban forests planning and designing.Above this threshold,the effects of cooling,humidifying and alleviating thermal stress are significant.This study further clarified the regulation function and mechanism of urban forest and building landscape patterns on urban thermal environment at the landscape scale.Urban forest area,urban forest COHESION,building area,building volume,and building COHESION in the square buffer zone with side lengths of 2000m,1200m,1200m,1900m,1400m and 1700m have strongest correlation with temperature,respectively.Based on the traverse measurement method,the effect of landscape patterns on the urban thermal environment and comfort and its heterogeneity under different urbanization intensities are further explored.Studies have shown that urban forests and building landscape patterns have great potential for improving the urban thermal environment,especially for highly urbanized communities.In the effective influence range,when the urban forest area is greater than 30 hectares(7.5%),the urban forest can play a stable cooling effect and improve thermal comfort,and the more urban forest patches physically connected,the stronger its regulating effect.The increase in building area and building volume will lead to an increase in temperature.When the building volume is less than 15×10~6m~3,its shading effect is the best,which can effectively reduce PET and improve thermal comfort level.The regulation effect of building COHESION has a diurnal difference.Increased building COHESION will lead to the increment of temperature and PET during the day,but it will lead to the decrement of temperature and PET at night.This study first clarified the spatiotemporal evolution characteristics and driving mechanism of urban thermal environment and thermal comfort on the national scale,systematically revealed the adjustment mechanism of urban forest on urban thermal environment at the community and landscape scales,provide scientific reference for reasonable urban planning policies,and play an important role in improving the urban thermal environment and thermal comfort.