Evaluation Of Multi-scale Spatial Heterogeneity Of Urban Thermal Environment In South China

Urbanization drives human progress,but serious challenges and problems continue to emerge in the process of urbanization.Problems such as urban water pollution,air environment problems,climate change,and excessive ecological security risks continue to emerge.Among them,as urbanization continues to accelerate,urban population is growing rapidly,urban land is undergoing rapid urbanization,construction land is expanding rapidly,and the urban substrate is changing drastically.The change of urban substrate surface leads to the change of thermal properties of the city,which in turn leads to significant changes in the internal climate of the city and drastic changes in the urban thermal environment.When the urban thermal environment is seriously affected,it is harmful to the urban climate and the production.For example,it leads to the deterioration of urban air quality,the increase of urban energy consumption,and the threat to the health of urban residents.Therefore,the study of urban thermal environment is an important ecological research topic in the process of urbanization,and the study of spatial heterogeneity of urban thermal environment under the change of subsurface scenario can provide theoretical basis for mitigating the impact of thermal environment.However,existing studies lack multi-scale studies,and the studies are mostly focused on a single spatial scale;on the other hand,there is a lack of multidimensional analysis,and the studies focus on two-dimensional.In addition,urban expansion and urban morphology,as important driving forces of urban subsurface change,are key factors affecting the spatial heterogeneity of the urban thermal environment.However,existing studies describing urban areas and urban morphology lack simple and convenient quantitative expressions.Therefore,there is an urgent need for an alternative method of depicting built-up areas to carry out urban expansion.As well,there is an urgent need to develop an urban morphology determination method with universal applicability.In view of this,this paper investigates the spatial heterogeneity of urban thermal environment based on different spatial scales and different spatial dimensions by exploring the method of subsurface scene identification and studying the spatial heterogeneity of urban thermal environment under the change of subsurface scene at multiple spatial scales.The research content is as follows.Calculating the coupling dynamics of surface changes and urban thermal environment changes in urban cluster-scale regions by using coupling models.A new approach to calculate urban expansion based on the concept of natural cities to explore the impact of urban expansion on the spatial heterogeneity of the thermal environment at the urban scale.Using open data to construct various types of urban morphology and assess the impact of urban morphology on the urban thermal environment within small local scales of cities.As well as using the localized contour tree method to identify LST clusters and construct an urban functional grid in order to explore the characteristics of the thermal environment and landscape distribution within the LST cluster.The main findings of this paper are as follows:(1)This study obtained the land surface temperature(LST)data of China's three major urban agglomerations(CTM-UAs)in 2000,2005,2010 and 2015.Then this paper used overall coupling model and spatial coupling model to reveal the coupling relationship between the CL expansion and HTA expansion of CTM-UAs in different periods.The results showed that:(1)From 2000 to 2015,the trend of CL expansion and HTA expansion in CTM-UAs was synchronous;(2)The distance between the CL barycenter and HTA barycenter in CTM-UAs was shrinking,the spatial overlap was constantly strengthening,and the overall coupling was constantly strengthening.And the highest value of the overall coupling was reached in 2015;(3)The number of grid units of very strong coupling and strong coupling increased with fluctuation in Beijing-Tianjin-Tangshan Urban Agglomeration(BTT),that in Yangtze River Delta Urban Agglomeration(YRD)was increasing,and that of Guangdong-Hong Kong-Macao Urban Agglomeration(GHM)has gone through the process of increasing to decrease.(2)This research includes a quantitative research on urban expansion based on the natural city(NC)concept using open data and the footprint of the thermal environment(TE FP)model to reveal the influences in different time periods between2014 and 2018 in Guangzhou,China.And the TE FP is an indication used to evaluate whether certain area is affected by the thermal environment of the NC.The results demonstrated the following.(1)The high-temperature area in Guangzhou was2293.55 km² in 2014,2330.24 km² in 2016,and 2609.81 km² in 2018 and expanded by 1.60%(2014–2016),12.00%(2016–2018)and 13.79%(2014–2018),respectively.(2)The urban area increased from 182.10 km² in 2014 to 266.90 km² in 2018,which represented an increase by almost half,with an average annual expansion rate of10.03%.(3)In 2014,2016 and 2018,the area representing the TE FP consisted of the areas of five buffer zones,six buffer zones and eight buffer zones,respectively.This study provides a new approach to the study of urban expansion and a new perspective for exploring the influence of the urban expansion process on the spatial heterogeneity of the thermal environment.(3)Two building geometry indexes(BGIs),a two-dimensional Area Index(2DAI)and a three-dimensional Volume Index(3DVI)were proposed in this study.Furthermore,we constructed multilayer buffer zones around weather stations and used Spearman's rank correlation to assess the correlation between the 2DAI,3DVI and T_air to explore the buffer zone with the highest correlation coefficient(Optimal Buffer Zone,OBZ)that exists between BGIs and T_air.The results demonstrated the following.(1)The correlation coefficient between the 3DVI and T_air was higher than that of 2DAI,but this coefficient was affected by the season and month.(2)The OBZ between the BGIs and T_air was the 250 m buffer zone,but the OBZ between the 3DVI and T_air could only be concentrated during the high-temperature period.(3)The change mechanisms of the correlation coefficients and the OBZs between the BGIs and T_air were related to the upper limit of the 2DAI that exists in the buffer zone and the stronger thermal sensitivity of the 3DVI in a high-temperature environment.(4)The upper limits of the 2DAI and 3DVI in grids should be determined according to the actual situation of the BGIs in all grids of the study area.This study provides a new perspective from which to explore the correlation between buildings and T_air and identifies the optimal size of grid units that can be used in urban thermal environment planning units.(4)To understand the characteristics of LST clusters,we selected the central urban area of Guangzhou as the study area and used the localized contour tree method to identify LST clusters(high-and low-LST clusters).Moreover,we used thermal field value(TFV)data classified from LST data and urban functional grids(UFGs)determined from multi-source data to explore the characteristics of the thermal environment and landscape distribution within the LST cluster,respectively.We found that the characteristics of the high-and low-LST clusters significantly differed.Compared with that of the low-LST clusters,the spatial distribution of the high-LST clusters is denser.The number and total area of low-LST clusters are smaller than that of high-LST clusters,although the average area of low-LST clusters is larger.The spatial heterogeneity of the 10 types of UFGs within the high-and low-LST clusters is significant.The developed and ecological UFGs classified from 10 types of UFGs affect the formation of LST clusters.In addition,there are main UFGs that affect the mean TFV of the LST clusters.This study provides a new approach for the identification of LST clusters and a new perspective for exploring the spatial heterogeneity of urban thermal environments.This study extracts innovative research contents from different scales,spaces and times as a breakthrough to improve the urban thermal environment research system,which can deepen a more comprehensive understanding of the thermal environment research.The research content and results can complement the research system of urban ecology and provide a new idea for the research of urban thermal environment.In addition,the application of the results in urban planning and management will help improve urban air quality and ecological environment,and improve the physical and mental health and human living environment of urban residents.