| In recent years,global warming and frequent extreme abnormal weather have seriously threatened human living environment and public health,especially urban thermal environment problems represented by heat island effect.The heat island effect has not only brought about the intensified use of energy,but also caused serious environmental deterioration,hindering the construction of urban ecological civilization and sustainable development.Urban built environment is not only the carrier of residents' production and life,but also an important factor affecting urban heat island effect.Therefore,analyzing the spatial distribution and change characteristics of heat island effect,identifying the spatial characteristics and patterns of urban built environment,clarifying the relationship and influence between them,and putting forward the control strategies to mitigate the urban heat island effect can not only enrich and improve the relevant theories of healthy cities,but also provide decision support for quantitative land spatial planning,optimization and management.On the basis of reviewing the relevant literatures at home and abroad,this paper summarizes the shortcomings of existing research in emphasizing "local" rather than "whole",emphasizing "structure" rather than "function",emphasizing "connection" rather than "cause and effect" and emphasizing "theory" rather than "practice".In this paper,589 control detailed planning management units in the main urban area of Wuhan are taken as research objects.Landsat 8 image data,urban land use data,surface coverage data,road network data,population distribution data and POI data are taken as data sources.The following research contents are put forward and relevant conclusions are drawn.(1)Previous studies only focus on the spatial distribution and change analysis of heat island effect with single dimension or scale,but few studies focus on the spatial distribution and change of multi-dimension and multi-scale urban heat island effect.From the perspective of multi-dimension and multi-scale,this paper analyses the spatial distribution and change characteristics of heat island effect in the main urban area of Wuhan from five dimensions: quadrant dimension,loop dimension,regulatory detailed planning management unit dimension,urban land use type dimension and surface coverage type dimension,and urban road loop dimension and circle layer loop dimension.The situation and law of urban heat island provide a new way of thinking.The study found that:Firstly,based on atmospheric correction method,the urban surface temperature in the main urban area of Wuhan showed obvious spatial heterogeneity in 2009 and2013.The high temperature areas mainly concentrated in the northeast and southwest directions of the study area,the junction of the Yangtze River and the Hanjiang River and the coastal areas of the two rivers.The spatial pattern of "internal and external double high","multi-axis extension in high temperature zone","local high temperature agglomeration" and "local low temperature agglomeration" has basically been formed.The low temperature area mainly distributes in Yangtze River,Hanjiang River and various lakes.Secondly,from 2009 to 2013,the total heat island area in the main urban area of Wuhan increased by 40.83 square kilometers,accounting for 7.08% of the total area in the main urban area of Wuhan.Among them,in 2009,the main urban heat island area of Wuhan accounted for 75.04% of the total area of the study area;in 2013,the main urban heat island area of Wuhan accounted for 82.12% of the total area of the study area.Thirdly,the area and density of each intensity heat island area in 2009 and 2013 differ greatly in each dimension,and the change of area and density of each intensity heat island area from 2009 to 2013 also show obvious differences in each dimension.The area and density of extreme heat island areas need to be focused on NE-E quadrant,multi-circle layers outside the third ring line,multi-circle layers near the road ring line,industrial land and housing construction areas,cold island area and SW-W quadrant,outside the third ring line,city center and circle layers near the third ring line,green space square land and water area.From 2009 to 2013,we need to focus on the increase of the area of extreme heat island in SW-W quadrant,outside the Third Ring Line,residential land and housing construction area;the increase of the density of extreme heat island area in NE-E quadrant,outside the Third Ring Line,commercial land and buildings;the decrease of the area of cold island area in SW-W quadrant,inside the Third Ring Line,residential land and artificial dump ground;and the density of cold island area in SW-W quadrant.Reduction of land use and arable land outside the Third Ring Road,green squares.(2)Previous studies tend to pay more attention to the impact of one aspect of urban built-up environment on the heat island effect,while insufficient attention is paid to the impact of multi-factor built-up environment on the heat island effect.In this paper,an index system for measuring the spatial characteristics of urban built-up environment is constructed based on multiple factors.Based on previous studies,more attention has been paid to the impact of urban entity factors on the heat island effect,while the research on the impact of urban spatial function on the heat island effect is insufficient.In this paper,when choosing indicators of land use,we focus on the selection of urban functional land as the operational object.At the same time,based on previous studies,more attention has been paid to the relationship between urban built-up environmental indicators and heat island effect,while less attention has been paid to the common and individual characteristics of urban built-up environment.This paper identifies the main factors of the spatial characteristics of urban built environment,and typeifies the spatial pattern of urban built environment.The study found that:Firstly,an index system of urban built-up environment measurement is constructed from "land use-road network-urban morphology-green squares and waters".In terms of land use,10 indicators were selected from the three levels of "land use quantity,land use pattern and land mixed use";in terms of road network,9indicators were selected from the two levels of "road density and network centrality";in terms of urban form,three indicators were selected from the three levels of "building density,volume ratio and sky openness";In terms of water area,six indicators are selected from the two levels of "the number of green squares and water areas and the pattern of green squares and water areas".The spatial characteristics of urban built-up environment are also measured.Secondly,the principal component analysis method is used to identify 13 main factors of the spatial characteristics of the built environment in the leading cities.The contribution rate on schedule is the location factor of the road center,the urban spatial form factor,the water area factor,the industrial land area factor,the green square area factor,the commercial land area factor,the residential land area factor and the mixed land use factor.Main road density factor,water area shape factor,secondary road density factor,green square land shape factor and residential land shape factor.Thirdly,according to the data characteristics of the dominant factors of urban built-up environment spatial characteristics,the types of urban built-up environment pattern are divided into 10 categories by using self-organizing mapping neural network.Type I urban built environment is a moderately mixed sheltered area with high density residence as its central location;Type II urban built environment is a medium density residential area with high mixing degree and high openness;Type III urban built environment is a high mixing degree and high open low density residential area with irregular square shape;Type IV urban built environment is a medium mixing degree and high openness with irregular residential land shape.Green space square area;Type V urban built environment is a large-area industrial zone with medium mixing density and high openness;Type VI urban built environment is a medium mixing degree and high open area;Type VII urban built environment is a green space square area with medium mixing degree and high openness and dense roads;Type VII urban built environment is a large-area green space square area with medium mixing degree and high openness;Type VII urban built environment is a medium mixing degree and high open degree;Type VIII urban built environment is a medium-sized green space square area.Large area of water area with high mixing degree and open degree,and large area of Commerce and water area with medium mixing degree and high opening degree.(3)Based on previous studies,more attention has been paid to the object of study,while neglecting the impact of the surrounding environment.From three aspects of social factors,economic factors and location factors,this paper chooses indicators as external environmental factors,and puts them into the model at the same time to analyze the surface temperature.At the same time,based on previous studies,the correlation analysis of land surface temperature and quantitative statistical indicators is mostly used,but the causal relationship between urban built environment and heat island effect is not clear enough.In this paper,regression analysis is used instead of correlation analysis to compare the effect of urban built environment indicators on heat island effect under the simultaneous action of multiple factors,and multi-regression model is used to compare and select the best model to screen the indicators that have significant effect on heat island effect.The study found that:Firstly,correlation analysis is used to test the external environmental factors and urban built-up environmental factors which are significantly related to the average surface temperature.In addition to the average shape index of industrial land,the average shape index of residential land,the average patch size of residential land and the degree of land use mixing at the land use level,there are four indicators that are not related to the average surface temperature,and the other indicators are significantly related to the average surface temperature.Secondly,OLS model is used to analyze the influence of control index and urban built-up environment index on average surface temperature.The effects of commercial land area,industrial land area,residential land area,average patch size of industrial land,road area,bus station density,building density,volume ratio and sky openness on average surface temperature are analyzed.Degree plays a positive role.Distance from industrial land,average patch size of commercial land,density of secondary trunk roads,proportion of green square land area,proportion of water area and average shape index of green square land are negative effects.Among them,sky openness,linear centrality,volume ratio and water area proportion have the greatest impact on the average surface temperature,while the average shape index of green square land,the average patch size of commercial land,the average patch size of industrial land and the proportion of industrial land area have the smallest impact on the average surface temperature.Thirdly,considering the spatial autocorrelation of land surface temperature,the spatial error model is used to interpret that the distance between the two control indicators and industrial land and road has a significant negative effect on the average land surface temperature.The built-up environmental indicators that have significant positive effects on the average surface temperature are the proportion of commercial land,industrial land,residential land,average patch size of industrial land,proportion of road area,density of bus stations,building density,volume ratio,average patch size of green square land and sky openness,which have significant negative effects on the average surface temperature.The built environment indexes include the average patch size of commercial land,the density of sub-main roads,the proportion of green square land area,the proportion of water area and the average shape index of green square land.The most important indicators for the average surface temperature are sky openness,proportion of land area in water area,building density and volume ratio.The smallest indicators for the average surface temperature are the average patch size of commercial land,the average patch size of green square land,the average shape index of green square land and the average patch size of industrial land.Fourthly,comparing the OLS model with the SEM model,we find that the parameters and coefficients of the SEM model are better than those of the OLS model.Compared with the SEM model,OLS model has overestimated,underestimated,misjudged and failed to identify the external and built environmental indicators that affect the heat island effect.(4)Based on previous studies,more attention has been paid to the regulation direction of urban built environment indicators,but insufficient attention has been paid to the regulation size of indicators.This paper calculates the threshold values of the built environment indicators for the emergence of the strong heat island as the indicators for early warning.At the same time,based on previous studies,there is still a gap between the theoretical results and the specific operation practice.Based on the regulatory detailed planning management unit,this paper proposes countermeasures to mitigate the heat island effect for typical and special types of urban built-up environment.The study found that:Firstly,on the basis of the analysis and discussion in the previous paper,a guiding index system to alleviate the urban heat island effect is constructed by synthesizing external environmental indicators and establishing environmental indicators.The weakening indicators of heat island effect include distance from industrial land,distance from road,average patch size of commercial land,density of sub-main road,proportion of green square land area,proportion of water area and average shape index of green square land,and the strengthening indicators of heat island effect include proportion of commercial land area,proportion of industrial land area and proportion of residential land respectively.The average patch size of industrial land,the proportion of road area,the density of bus stations,building density,volume ratio,sky openness and the average patch size of green square land.Secondly,calculating the threshold of the probability of strong heat island in the guiding index system,according to the land use type index,pattern type index,density type index and shape type index,to provide index early warning and control reference for mitigating urban heat island effect.Specifically,the main ways to mitigate the urban heat island effect are through adjusting the regulatory detailed planning management unit of residential agglomeration to control the proportion of commercial land area not exceeding 10%,industrial land not exceeding 30%,green square land area exceeding 8%,and water area exceeding 3%.At the same time,some commercial land and industrial land mixed in residential areas are mixed.Industrial land is relocated to commercial and industrial agglomeration areas.The main way to alleviate the urban heat island effect is to adjust the average area of industrial land in the regulatory detailed planning management unit to no more than 25,and to lay out the area of green squares in small plots and large quantities.At the same time,attention should be paid to the design of irregular shapes.Density indicators are mainly for road-related operations,which can be carried out by widening the road green belt.The main way to mitigate the urban heat island effect is to strictly control the construction approval of regulatory detailed planning management units with building density less than 0.24 and volume ratio less than 1.2.For those with building density greater than0.24,volume ratio greater than 1.2 and sky openness between 0.83 and 0.87,the regulatory detailed planning management units can be gradually adjusted through the transformation of old cities.Thirdly,according to the surface temperature of different types of urban built environment,urban built environment is divided into typical urban built environment type and special urban built environment type.Suggestions on how to mitigate the urban heat island effect by differentiation are put forward.Typical types of urban built environment can mitigate urban heat island effect by gradually transforming old cities according to type I urban built environment;type II urban built environment should focus on increasing central green space,subsidiary green space and pre-residence green space in residential areas,and migrating commercial land and industrial sites mixed in residential areas;The built environment of type V cities is mainly concentrated industrial zones,which are suitable for centralized layout in the suburbs of cities on the macro level.The residential land mixed in them is moved out and the temperature is lowered by enlarging the spacing of factory buildings and increasing greening.;type VI cities should pay attention to controlling the construction index and adjusting the surface temperature by adding small green landscape with multiple levels and rich types through functional replacement.The built environment should ensure that the number of land used for green squares does not decrease;the built environment of type IX cities should ensure that the area of land used for water areas does not decrease.The surrounding construction of large areas of water bodies should pay attention to the building's height and set up corridors with the same direction of urban wind,so as to enhance the cooling function of water bodies.The special type of urban built environment can alleviate the urban heat island effect.According to III type of urban built environment,the greenbelt should be added between residential and industrial land.IV type of urban built environment can ensure that the area of green space square is not reduced,while keeping a close watch on the rapid growth of impervious surface and building density;VII type urban built environment and ? type urban built environment should build an examination and approval system so as to maintain the current situation for maintaining the status quo as the main objective. |
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