Hourly Variation Of Surface Urban Heat Island Over The Yangtze River Delta Urban Agglomeration

Urban heat island effect is a typical feature of the urban environment,which has a great impact on natural and residential environment.Urban heat island is usually divided into canopy and surface heat island.In recent years,with the accumulation of satellite thermal infrared data,the multi temporal scale variation of surface urban heat island has been gradually rising.But the temporal and spatial resolution of the remote sensing images usually contradicts each other.The higher spatial resolution of polar orbiting satellites is accompanied by the lower temporal resolution,so that the satellite-acquired data are difficult to meet the need of researches on hourly variation of urban heat island.Though geostationary meteorological satellite data can be applied to hourly variation study,it is hard to adapt to the urban scale because of its coarse spatial resolution.Therefore,the hourly regimes of SUHI over current typical urban agglomerations in typical seasons remain unclear.Using MODIS imageries as main data source and combining with four parameters model of diurnal variation over land surface temperature(i.e.,INNA08),we firstly examined the hourly variations of the spatial pattern and intensity of the SUHIs for the Yangtze River Delta urban agglomeration in both the summer and winter.Then we discussed the annual variation of the spatial pattern,intensity,strong area of the SUHIs and △DTR in according to hourly variation.The results demonstrated that,in the summer,as a whole,all the cities in the day have a heat island effect.Because of the cooling effect of urban vegetation or water body under strong solar insolation,’Cold spots’ also occur within some cities from around 08:00 to 21:00,and these ’cold spots’ disappear since 21:00.For this season,the hourly variations of the SUHI intensity are similar to those of the LSTs:they riserapidly in the morning,reach the maximum at around 12:00 to 14:00,then gradually decrease and continue until the sunrise of the next day.In the winter,surface urban cool islands(SUCIs)were observed in most cities,but these SUCIs mostly disappear during the afternoon to the early evening(around 14:00-17:00),then all the cities bounce back to exhibit heat islands.Within this season,the hourly variations of the SUHI intensity differ from those of the LST.There also exist large differences of the hourly variations of SUHI intensity between using the rural area and forest as the non-urban background for estimation of the intensity,with the former reaching its maximum(around 18:00)significantly later than the latter reaching its maximum(around 13:30).For annual variations in the spatial pattern and intensity of the SUHIs in 2003,2008 and 2013,’cold spots’ disappear completely around 21:00 and 17:00 in summer and winter,respectively.The characteristics of hourly variation in different years are mostly same with each other.For annual variations of strong heat island area,during the summer,hourly variations from 2003 to 2013 are similar.The trend is obviously increasing year by year in daytime,but remains stable at night.Unlike summer,in the afternoon and evening,the strong heat island area is markedly increasing year by year during the winter,remains steady.In addition,the hourly variation of the strong heat island area in winter is opposite to that in summer.For annual variations of ADTR,during the summer,the urban △DTR over the Yangtze River Delta shows a downward trend over the Yangtze River Delta from 2003 to 2013(urban DTR mildly declines,suburban DTR mildly rises).In addition,surface △DTR and canopy △DTR all show an obvious downward trend year after year,urban DTR declines,but suburban DTR rises.Surface urban DTR is larger than surface suburban DTR(especially in summer).However,canopy urban DTR is smaller than canopy suburban DTR.During the winter,△DTR is much smaller,and shows a downward trend over the Yangtze River Delta region from 2003 to 2013(urban DTR faintly declines,while suburban DTR slightly rises).In conclusion,we discuss hourly variation of surface urban heat island over the Yangtze River Delta urban agglomeration by combining MODIS and four parameter DTC model which is simple and effective.This method can be expanded on hourly variation of surface urban heat island over small or medium cities,which is expected to provide technical support for improving the natural environment and human settlements.