Spatiotemporal Evolution Analysis Of Urban Thermal Environment In Hangzhou Based On Remote Sensing Over The Past Thirty-five Years

Under the background of global warming,natural landscapes have been rapidly replaced with artificial surface,leading to severe urban thermal environment problems.People's residential environment and physical condition have consequently been put in danger,meanwhile,the sustainable development of the region is facing severe challenges.It is of great significance to study the evolution and influencing factors of the spatiotemporal patterns of urban thermal environment.The research results could help local governments lay out urban construction land and execute planning and design on urban landscape rationally,thus improving the urban ecological environment and promoting the sustainable development of the city.As the political,economic,cultural,financial and transportation center of Zhejiang Province,Hangzhou is also one of the central cities of the Yangtze River Delta.With the rapid urbanization process,its thermal environment problems are increasingly prominent,which greatly restricts further development of its economy and society.In recent years,Hangzhou has become a new"stove"city in the Yangtze River Basin,ranking third in the country in terms of its hot degree.Therefore,consideration must be given to both the optimization of urban thermal environment and socioeconomic development in Hangzhou.Taking the urban districts of Hangzhou as the research object,this paper constructs 2D/3D urban morphology database and urban thermal environment database of Hangzhou,by using multi-source remote sensing data,meteorology and statistics data from 1984 to 2019.Based on the theory and method of spatial information technology,the characteristics of urban expansion and spatiotemporal evolution of urban thermal environment in Hangzhou during the past 35 years are analyzed.In addition,this paper explores the relationship between urban thermal environment and urban morphology at different spatiotemporal scales.The main research contents and conclusions are as follows:(1)From 1984 to 2019,the two-dimensional and three-dimensional expansion changes of Hangzhou are remarkable,and the area of construction land increases 417.47km~2 in the horizontal direction,showing a trend of expansion towards the East and South.The expansion intensity and expansion rate display a gradual upward and downward trend.The overall compactness of urban construction land in Hangzhou is low,while its fractal dimension is relatively high.From 2015 to2019,the building density dynamics in Hangzhou shows the characteristics of transition from low density to medium density,medium-high density and high density,it is dominated by the increase of low-rise buildings and medium-rise buildings,urban floor area ratio increases from 0.35 to 0.38.In the study area,low-rise buildings,medium-rise buildings,high-rise buildings and super high-rise buildings all increase by different ranges.In terms of urban skyline,low-rise buildings,medium-rise buildings and high-rise buildings have emerged in the East of study area.(2)From 1984 to 2019,the urban thermal environment dynamics in Hangzhou has shown a trend of"aggregation,diffusion and aggregation",and forms a polycentric pattern.In addition,the aggregation range and degree of aggregation of land surface temperature in Hangzhou decrease after initial increase.On the whole,the fragmentation of land surface thermal landscape in Hangzhou has changed from decreasing trend to increasing trend,the shape complexity of patch has increased,the degree of aggregation of pixels into a patch has changed from large to small,and the connectivity of the same patches is higher.The richness of land surface thermal landscape patches in Hangzhou has increased,and its distribution is more even.There is a significant difference in land surface thermal landscape index at all levels in Hangzhou.The PLAND of urban heat island increases first and then decreases,while,the PLAND of cold island shows an increasing trend as a whole.The AI of the transition area of cold island and heat island is relatively high,with good connectivity,which plays a leading role in the land surface thermal landscape of Hangzhou;The AI of the edge heat island and the edge cold island is relatively low,showing a relatively high fragmentation.There is a strong consistency between the transfer direction of the spatial centroid of urban heat island and the spatial centroid of urban construction land.(3)Under different observational scales,vegetation fraction,water fraction,building density,building height,floor area ratio and sky view factor have a strong correlation with the mean land surface temperature in four seasons over the years,and the correlation strength will fluctuate with the change of observational scale and season.The influence of vegetation fraction and water fraction on urban thermal environment in spring,autumn and winter is more significant.The influence of building density on urban thermal environment in summer is most significant.The influence of building height on urban thermal environment in four seasons is less significant.Compared with building height,the influence of floor area ratio and sky view factor on urban thermal environment in four seasons is more significant.Overall,the relationship between the selected urban morphology parameters and land surface temperature has a strong dependence on time(i.e.,seasonal and interannual),and a relatively weak dependence on space(i.e.,different observational scales).