Comparative Analysis Of The Thermal Environment Characteristics Of Chengdu-Chongqing Urban Agglomeration In Winter And Summer

Urban heat island refers to the phenomenon that urban temperature is obviously higher than the surrounding suburbs.With the development of global economy,large-scale urbanization has led to changes in the nature and structure of urban underlying surface.The rapid expansion of the urban population and the development of industrialization aggravate the heat emissionand affected the urban heat balance,thus forming the urban heat island.Urban heat island effect will also affect regional climate,which is an important environmental problem for urban sustainable development.This paper using MODIS surface temperature data,NDVI data,combining with DMSP/OLS night-time lamplight data,land use data and statistical data and applying RS and GIS technology,studies dynamically the thermal environment and vegetation coverage of Chengyu urban agglomeration from 2002 to 2017 of space-time view.A comparative analysis of the characteristics of winter and summer variations was carried out,and the effects of land use types and vegetation coverage on the thermal environment in the process of urbanization were discussed.The main conclusions are as follows:(1)Chengyu urban agglomeration has entered the rapid urbanization stage since 2000,and the level of urbanization has maintained steady growth.According to the change of DMSP/OLS night-time lamplight data,the city scope is expanding constantly with urban development.Through comparison,it is found that there is a characteristics of unbalanced regional urbanization level in the development process: the urbanization level of Chengdu and Chongqing is higher,while the urbanization development of Ya'an and Yibin is lagging behind.(2)From the interannual trend of change,the surface temperature of Chengyu urban agglomeration shows an obvious upward trend.In seasonal change,the surface temperature in summer increases most significantly,followed by spring,while the overall trend of decline in autumn and winter.The surface temperature in the winter and summer show the opposite obvious change tendency and the spatial distribution is consistent.From the distribution of heat field,the middle temperature zone is the main area in winter and summer,and the distribution of low temperature zone and sub-low temperature zone is relatively stable.In the summer of 2017,the range of high temperature area increased significantly,mainly distributed in the built-up area and urban expansion area.The change of summer heat field in Chengdyu urban agglomeration is closely related to the urbanization process.The high temperature area in the southwest of winter changed to the medium temperature area in 2017,which is consistent with the decreasing trend of surface temperature in winter.On the whole,the change of heat field in summer is smaller than that in winter.(3)In summer,the area of strong heat island in Chengyu urban agglomeration increases continuousl.With the development of urbanization,the area of heat island in each cityexpands gradually,and it is connected with the surrounding towns to form a large whole.However,the urban heat island effect is weak in winter,and the inter-annual change is not significant.The heat island of Chengyu urban agglomeration shows obvious seasonal difference,which is the strongest in summer,the second in spring,and the weakest in autumn and winter.Compared with 2002,the heat island intensity increased in summer and decreased in winter in 2017.(4)The vegetation coverage of Chengyu urban agglomeration shows an increasing trend as a whole,especially in winter.The vegetation coverage in summer was significantly higher than that in winter.The area of low vegetation coverage increased in summer 2017,which was 1.7 times larger than that in 2002.The vegetation coverage in urban expansion area decreases dramatically,which corresponds to the region with enhanced heat island effect,while the area far from the city shows higher vegetation coverage.(5)The evolution of land surface temperature and vegetation cover is closely related to the process of urbanization.Significant changes have taken place in the regions with rapid urbanization,which show that the vegetation coverage decreases and the surface temperature rises.On the contrary,the regions with low urbanization level change slightly.The vegetation coverage tends to be better in the urban center where the development tends to be gentle,and there is no obvious surface temperature phenomenon.The average surface temperature in four seasons is as follows: Town > cultivated land > water area > forest land > grassland.The heat contribution of different land use types to heat island is different.Town is the main heat contribution type of heat island,and forest land has significant cooling effect.There is a significant negative correlation between surface temperature and vegetation coverage,and the negative correlation between them is stronger in summer.According to the change trend,the cooling effect of vegetation in summer is better,and the surface temperature decreases significantly when the vegetation coverage exceeds 0.8.