Quantifying The Effect Of Urbanization On Urban Heat Island And Its Climatological Analysis Using Satellite And Ground-based Data

Currently,approximately 54%of the world's population lives in urban areas,and this proportion is expected to increase to 66%by 2050.The rapid urbanization process results in the replacement of natural surfaces with anthropogenic surfaces such as pavements,buildings and other infrastructure,which have altered the radiative,thermal,and moisture properties of the urban environment.One of the environmental consequences of urbanization is the urban heat island(UHI)effect.UHI has caused significant impacts on environmental quality and human well-being in urban areas.The adverse effects are more pronounced in metropolitan areas,especially where there is an ever increasing population density and extensive anthropogenic activities.Therefore,the quantification of the effect of urban expansion on urban heat island intensity(UHII)and its climatological analysis have been of great importance to improve our understanding of the intra-urban and inter-urban temperature distribution,the vulnerability to heat risk of urban human populations,and human-environmental interactions,and the results can provide scientific insights for urban planning and environmental management in Shanghai.In this study,Shanghai City is selected as the study area.The study involves the extraction of the dynamic of urban construction land,the spatio-temporal dynamics and evolution of urban construction land expansion,the effect of urban expansion on urban heat island intensity and the climatological analysis of urban heat island based on high resolution gridded dataset of air temperature estimated from MODIS data as the main guideline to study the effect of urban expansion on urban heat island intensity in Shanghai using long-term series of Landsat data(MSS/TM/ETM+OLI)for the years 1974-2015,land surface temperature obtained from the MODIS during 2000-2013 and air temperatures for 131 meteorological stations.(1)The dynamic of urban expansion was extracted by the method integrating linear spectral mixture analysis and iterative self-organizing analysis(ISODATA)based on time series of satellite data(Landsat MS S/TM/ETM+OLI,H J-1B)from 1974 to 2015 in this study.Linear spectral mixture analysis was employed to generate high-albedo,low-albedo,blue roof,and vegetation.In addition,ISODATA method was employed to exclude soil,water,and shade in the fraction images of high albedo and low albedo.The impervious surface fraction was the sum of the blue roof fraction,bright and dark impervious surface fractions.The SPOT-5 and Google Earth images in 2005 were selected for validating the results.The result show that the RMSE for impervious surface fraction using LSMA combined with ISODATA is 0.121.The method was applied to obtain the dymanic of urban expansion during 1974-2015.The overall spatiotemporal characteristics of urban expansion from 1974 to 2015 was characterized at multiple scales using a GIS-based buffer gradient analysis and analyzed using urban expansion intensity index.The results show that the overall spatiotemporal changes in the urbanization gradient are largely influenced by the distance from the urban center,and there are significant differences in the intensity of construction land expansion in different periods and in different directions.From 1978 to 2015,the area of urban construction land in Shanghai gradually increased,especially after 2000.(2)The temporal variation of urban heat island was investigated based on air temperatures(month mean temperature,month mean maximum and minimum temperature)from meteorological stations from 1961 to 2013 in Shanghai.The UHII was quantified by the urban station(Xujiahui)and other suburban and rural stations including Baoshan,Jiading,Minhang,Pudong,Nanhui,Jinshan,Fengxian,Congming,Songjiang,and Qingpu.The result shows that UHII increased first and then decreased after reaching the maximum around the year 2004.According to the urban construction land information from 1974 to 2015 and station relocation histories,some suburban stations have been located in urban area due to urban expansion and resulted in an increase in air temperature.So the decline of the UHII after 2004 may be caused by station relocation or urban expansion in the suburban area.(3)In order to further investigate the causes of declines in UHII around the year 2004,we selected the island station of Shengsi as the rural reference station,which is almost not affected by urbanization.The UHIIs were calculated by air temperatures(annual mean temperature,annual mean minimum and maximum temperature)between the 11 meteological stations(Xujiahui and other 10 stations)and Shengsi station.The increasing rate of UHII based on minimum air temperature is larger than that based on mean and maximum air temperatures.The UHII curves displayed a general ascending trend for all Shanghai meteorological stations during 1961-2013.This result indicate the UHII in Xujiahui is expected to increase in the future,and the declines of UHII between Xujiahui and other stations around 2004 is affected by urban expansion in the suburban areas.Also,according to the change point in the UHII curve and station relocation histories,the UHII for Songjiang,Jinshan,and Chongming stations using Shengsi as rural reference all sharply increased after 2003 due to station relocation.(4)To evaluate the effect of urban expansion on urban heat island,the quantitative models based on UHII and built-up proportion within a range of buffer sizes of the Xujiahui station during 1974-2015 was established.The range ofR2 is 0.31-0.86,0.39-0.81 and 0.50-0.87 for UHII models based on mean,minimum and maximum air temperature,respectively.The results reveals a statistically significant correlation between UHII and the proportion of construction land in different buffer areas.To investigate the effect of suburban urbanization on UHII,the quantative model was established based on UHII and construction land proportion in the 5km buffer zone of the Minhang station after the UHII changing point.The result shows the UHII shows a negative correlation with construction land proportion in the 5 km buffer zone of Minhang station.The UHII based on Xujiahui and Minhang decrease with an increase in the proportion of construction land within a buffer area of the Minhang station.These phenomena that UHII is gradually reduced due to that fact that temperature of suburban station is increasing due to the rapid urbanization.Under the influence of urbanization,the suburbs are warmer than downtown in Shanghai.The result implied that the selection of rural or suburban meteorological station play an important role in the investigation of the effect of the urbanization on urban heat island.(5)Remote sensing techniques were used to produce monthly UHI climatology maps during the period 2000-2013,revealing the spatiotemporal characteristics of daytime and nighttime UHI during this period of rapid urbanization in Shanghai.Using stepwise linear regression,daytime and nighttime air temperatures at the four overpass times of Terra/Aqua were estimated based on time series of Terra/Aqua-MODIS LST and other auxiliary variables including enhanced vegetation index(EVI),normalized difference water index(NDWI),solar zenith angle(SZA)and distance to coast(DTC).The validation results indicate that the models produced an accuracy of 1.6-2.6 ?RMSE for the four overpass times of Terra/Aqua.The models based on Terra LST showed higher accuracy than those based on Aqua LST,and nighttime air temperature estimation had higher accuracy than daytime.The results from climatological analysis of urban heat island effect showed that the annual mean daytime UHII in Shanghai during 2000-2013 is 1.0 and 2.2? for Terra and Aqua overpass,respectively.The annual mean nighttime UHII is about 1.0? for both Terra and Aqua overpass.The seasonal analysis shows daytime UHII is strongest in summer and weakest in winter,while nighttime UHI is weakest in summer and strongest in autumn.The relationship between UHI intensity and NDVI varied depending on different seasons for both daytime and nighttime.The results suggest that green vegetation can mitigate UHI effects during summer and early autumn.The resultant UHI climatology maps provide a spatio-temporal quantification of UHI with emphasis on temperature gradients.This study has provided information of relevance to urban planners and environmental managers for assessing and monitoring urban thermal environments which are constantly being altered by natural and anthropogenic influences.