Spatial-Temporal Changes Of Urban Heat Island Based On Downscaling Of Land Surface Temperature Of MODIS

In recent years,economy has grown rapidly and steadily,and the urbanization rate has been continuously rising.Along with the accumulation of population and changes in the industrial structure,the problem of urban ecological environment has become prominent gradually.The urban heat island is one of them.In this paper,five provinces are selected in northwestern,with 272 cities selected among them.Using the MODIS land surface temperature,vegetation index,and land cover product from 2002 to 2016,the Ts HARP algorithm is used to calculate land surface temperature by 250 m.Temperature difference between urban and suburban areas is used as an indicator to evaluate the urban heat island characteristics of 272 cities in the five provinces of the Northwest.Fistly,the paper compares the four regression functions by Ts HARP algorithm,and selects a one-quadratic equation to predict the land surface temperature with higher resolution,and the predicted result is better.Secondly,the quantitative indices of seven surface urban heat island intensity commonly used are compared.The temperature difference between urban and suburbs is selected finally,the difference of average surface temperature between the built-up area and the ring-shaped suburb buffered by 100% of the built-up area.The results of 272 cities' s SUHII in five northwestern provinces from 2002 to 2016 are obtained.The temporal characteristics are studied by analyzing annual,seasonal,monthly,and diurnal variations.For provincial cities are selected as major cities to analyze the spatial characteristics of urban heat island.The main conclusions are as follows:(1)Using Ts HARP algorithm to perform surface temperature downscaling of MODIS land surface temperature is conducive to the study of urban heat island in the five northwestern provinces.The relationship between NDVI and land surface temperature explained as the one-quadratic equation is suitable for the downscaling of land surface temperature in the northwest China.According to the quantitative indices of urban heat island,the suburban temperature difference is more suitable for measuring urban heat island in 272 cities in five provinces in the northwest than other indices.(2)Choosing four provincial capitals,Lanzhou,Xi'an,Xining and Urumqi to Study the spatial characteristics of urban heat island.In the view of the distribution,urban heat island in Lanzhou City mainly distributes in Chengguan District and Qilihe District;Urumqi City mainly distributes in Shayibake District and Tianshan District;Xi'an City mainly distributes in southeastern Lianhu District and southwest of Xincheng District.Beilin District and the north of Yanta District;Xining Urban Heat Island mainly distributs in Chengzhong District,Chengdong District and the east of Chengxi District.The area of urban heat island in Lanzhou increases most,with an average annual increase by 2.77km2,and the urban heat island in Xi'an show a decreasing trend.The area of urban heat island decreased by 0.11km2 each year,and the urban heat island in Urumqi and Xining grew close to 0.79km2 and 0.73km2 respectively.(3)From 2002 to 2016,272 cities' s SUHII in five provinces in northwest China fluctuates.Taking 2009 as the demarcation point,SUHII can be divided into two stages.SUHII from 2002 to 2009 is significantly higher than that from 2010 to 2016,and On the whole it is decreasing at a rate of 0.0063 K per year.Judging by the number of urban heat island,it is the highest in 2008 and the lowest in 2014;by the average SUHII,it is the highest in 2005 and the lowest in 2014.The urban heat island in the northwest during the 12 months is the strongest in July,with an average SUHII of 0.96 K.There are 217 cities with urban heat island;the weakest is in January,the mean SUHII is-0.43 K,and there are only 82 cities with heat island.However,the number of heat island is the lowest in December,and there are only 76 cities.(4)The diurnal variations of urban heat island in the northwestern region are as follows: SUHII is greater at day in the spring,summer,and autumn,and SUHII is greater than during the night in the winter.The seasonal changes of SUHII in the five northwestern provinces are: summer> autumn> spring> winter.