| Land use / land cover change(LUCC)is one of the main driving force of global change,and urbanization is one of the most prominent hot spots concerning the spatio-temporal variation of LUCC in China.The land use change caused by urban expansion during the high-speed urbanization period has great influence on the physical structure and characteristics of the underlying surface,and the accompanying regional thermal environment change has become an important part of the regional ecosystem and climate change research.The urbanization speed of Henan Province is higher than the national average,especially the Zhengzhou-Kaifeng metropolitan area,as the core growth pole of Zhongyuan Economic Zone,is undergoing major changes in urbanization,so its thermal environmental effects and drive mechanism is becoming an urgent concern.Therefore,further research on thermal environment in metropolitan areas has important practical significance to disentangle the formation mechanism of thermal environment effect and provide theoretical reference for urban planning,construction of ecological city,and livable city.In this work,taking Zhengzhou-Kaifeng metropolitan area as an example,in consideration of global change and urbanization,this study specifically addressed the spatio-temporal variations of urban thermal environment and its influence mechanism,and analyzed the thermal environment of Zhengzhou City,Kaifeng City and Zhongmou County from multiple perspectives using the research paradigm of landscape ecology and ideas of "pattern-process-mechanism-simulation".The main research methods and conclusions are as follows:(1)The land cover changed rapidly and the surface temperature was significantly different.Based on the RS Image data in 1988,2000,2009 and 2014,the land cover classification and surface temperature retrieval were carried out with the support of ENVI and ArcGIS to deal with the visible and thermal infrared bands.The results indicated that land-cover change was characterized by the increase of construction land and the decrease of agricultural land during 26 years.The land dynamic model showed that the expansion rate of built-up area in Zhengzhou was much faster than that in Kaifeng and Zhongmu,while the built-up areas of three cities were gradually approaching.The high-speed urbanization was accompanied by an increase in construction land and bare land,leading to increased land surface temperature.A comparison of land surface average temperature of three cities revealed that Zhengzhou City was slightly lower than Kaifeng City and Zhongmu County in the first two periods,while an opposite trend in the latter two periods.The average surface temperature of different land use types is from high to low: urban construction land > bare land > other construction land > forest land > cultivated land> unutilized land > water body.(2)The urban heat island was obviously changed,and it is closely related to urban extension.The surface temperature retrieval achieved by "mean-standard deviation" classification and background-removing normalization indicated that: First,changes in heat island from 1988 to 2014 was featured by "weak heat island-strong heat island-polar heat island-scattered heat island" in Zhengzhou-Kaifeng metropolitan area.Second,heat island index showed that the maximum value of the four periods appeared in the central area of Zhengzhou,and the average intensities of the heat island in the built-up area were 3~7 ?.Third,landscape change of heat island is highly consistent with the extended boundary of the built-up area.(3)There was a significant difference in the land surface temperature among typical landscape,hence the impact scope.Buffer analysis showed that water landscape,park landscape,riverbank landscape and high-rise building landscape had significant cooling effect with certain scale.Taking 0.1?,the adjacent buffer temperature difference as impact standard,the three kinds of landscape with cooling effect were: Water body landscape(300m)> Park landscape(240m)> Riverside landscape(180m).Due to the shadow and internal green structure,the cooling effect of high-rise building landscape was obvious,and its influential distance was up to 300 meters.A 10% increase in urban road area ratio will result in 1.2 ? increase in the land surface temperature.The impact scope of bare land,square,plastic field were 240 meters,180 meters,240 meters respectively.The degree to which landscape affecting the thermal environment depended on the nature of the landscape itself and surroundings.(4)The relationship between urban landscape pattern and thermal environment: Specifically,the scale indexes(PLAND,LPI,ED,PD)of urban landscape types(water body,riverbank landscape,park,high-rise building)with cooling effect were significantly negative correlation with LST of each type.Conversely,there were significant positive correlations between the LST of Landscape types with warming effect(road,block,development land,railway land)and their indexes(PLAND,LPI).In addition,the DIVISION index of the four kinds of landscapes with cooling effect was highly positively correlated with LST.Rather,the DIVISION and SPLIT indices of the three kinds of landscapes(except for the road)with warming effect displayed a remarkable negative relationship with LST.Overall,under the condition of scale control,the integrated distribution of landscape with cooling effect,the scattered distribution of landscape with warming effect and reduced connectivity of landscape with warming effect will make contribution to effectively alleviating the formation of urban heat island.(5)Thermal environment analysis at local scale: The thermal environment of different land types was analyzed through the combination of temperature data from airborne thermal imaging equipment and Google Images.For residential area,the bigger the horizontal projection proportion of building was,the higher the average surface temperature became.The land surface temperatures of squares and roads were not only determined by surface materials,but also the disturbance from surrounding land types.The cooling effect of forest land and grassland was related to artificial maintenance and canopy coverage.The surface temperature of contaminated and eutrophic water was significantly higher than that of clean water.Moreover,the roof color of buildings also had an important effect on its surface temperature.(6)The correlation between NDVI,NDMI and LST: This study also analyzed the correlation between NDVI,NDMI and LST calculated with the data from Landsat 8 at three levels(whole area,subarea and pixel)and three sampling intervals(150m,300 m and 450m)based on the GEO-Da software.Some conclusions had been drawn: Firstly,there was stronger linear negative correlation between LST and NDMI,and profile analysis displayed that NDMI had a mirror-image relationship with LST,whereas the correlation between LST and NDVI was much weaker.Secondly,buffer analysis showed that with the increase of the distance from downtown and land use types,the correlation between NDMI and LST also increased gradually.Thirdly,the spatial correlation analysis of Moran index indicated that the negative correlation between LST and NDVI,NDMI was significant,but NDMI presented much stronger spatial negative correlation with LST because of the huge difference of water body in the two index.Consequently,compared with NDVI,NDMI was better for quantitative analysis of LST.(7)Correlation between socioeconomic statistical indexes and urban thermal environment was carried out based on meteorological observation data and statistical data for 31 years.The heating rate of annual mean air temperature of Zhengzhou City was fastest,reaching 0.576?/10 a,but the rates of Kaifeng city and Zhongmu county were slightly slower.In autumn and winter,the seasonal temperature and land surface temperature fluctuated frequently with no clear trend,while in spring and summer there was an obvious upward trend.The correlation between the six statistical indicators reflecting the natural attributes and the Zhengzhou-Zhongmu temperature difference was greater than that of the six indicators representing the socioeconomic attributes,suggesting that the effect of the physical characteristics of the urban canopy structure on the temperature change was stronger than that of economic indicators.The obvious "inverted U" relationship between temperature change and social economic development level in Zhengzhou indicated the urban heat island intensity has been alleviated in recent years.(8)The WRF/UCM well simulated the instantaneous regional thermal environment,and the results were in good agreement with the land surface temperature derived from MODIS.The simulation showed that the surface heat island intensity was weaker in the daytime than night.The expansion of urban construction land should be responsible for the change in heat-island shape and the increase in heat-island area.The heat island intensity of air temperature was lower than that of the surface temperature.In the daytime,the relationship between the spatial distribution of temperature and the underlying surface type was not obvious,but obvious in the evening.From Zhengzhou City to Kaifeng City,the intensity of heat island was gradually reduced,which showed the corresponding changes with urban scale.The simulation also exhibited that the industrial belt will contribute greatly to the warming of the Zhengzhou-Kaifeng metropolitan area,and daily surface temperature and air temperature will increase by 0.48? and 0.19? respectively.Generally,this research analyzes the thermal environment of Zhengzhou-Kaifeng metropolitan area from three different aspects: spatio-temporal variation,influence mechanism and numerical simulation,and also provides some corresponding measures to mitigate urban thermal environment.However,restricted by the complexity of urban thermal environment and present research conditions,this study also raises two main changes in terms of scale issue and model simulation.The first concerns the spatial and temporal scaling,because present study doesn't involve this matter despite the analysis of thermal environment variation in different levels with multi-source data.The second is related to the combination of BEP-UCM,instead of WRF/UCM with urban planning so as to accurately simulate the urban thermal environment and provide more reasonable reference for ecological and livable urban construction. |
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