Spatialtemporal Changes Of Urban Thermal Environment Pattern Based On Time Series

The unprecedented urbanization process has not only witnessed the prosperity of human society,but consequently given rise to a sucession of environmental side-effects.Particularly,Urban Heat Island(UHI)effect,as a typical urban climate problem,has raised great awareness of the public.Hangzhou is one of the most rapidly developing cities in China,and is also severely subjected to the UHI effect.Analyzing the spatio-temporal dynamic pattern of UHI is of critical significance in assessing the quality of ongoing urban development and urban planning layout,providing supporting knowledge for mitigating the effect for other cities,and most importantly optimizting the living environment of urban citizens.In this study,the downtown districts of Hangzhou were selected as study area.Spatio-temporal fusion method was used to product land surface temperature(LST)time series with both high spatial and temporal resolution.We also adopted other methods,including time series decomposition algorithm,Local Moran's I analysis,and gravity model,in order to qualitatively and quantitively analyze the spatio-temporal characters,hotspots,dynamic trajectory,and the influence among different UHI clusters.The main methodology and corresponding results are as follows:(1)We acquired land surface temperature time series with high temporal and spatial resolution every two months from 2003 to 2014,using ESTARFM spatiotemporal fusion algorithm and the fitting of missing pixels.The fusion accuracy of the algorithm was evaluated.Finally,the LST time series can be used to demonstrate the spatiotemporal evolution of LST reliably.(2)From 2003 to 2014,the yearly LST distribution has a certain degree of commonality.In the spring,summer,and early autumn(March to October),the high-temperature and sub-high temperature areas in the downtown districts of Hangzhou are relatively concentrated,mainly distributed in Gongshu District,Shangcheng District and Xiacheng District,which are in the northern part of downtown districts.There are also relatively fixed distributions in Jianggan District in the northeast of downtown districts and Binjiang District on the south bank of the Qiantang River.The high temperature agglomeration is most significant in the summer(July to August).And the low-temperature areas are mainly concentrated in Qiantang River,West Lake and Xixi wetlands,and play a cooling role.In late autumn and winter(November to February),due to the slower cooling effect of water bodies than land,Qiantang River and West Lake are no longer the low-temperature areas of downtown districts.The distribution of high temperature areas and sub-high temperature areas is scattered.Since 2003,the high-temperature areas and the sub-high temperature areas in Jianggan District have been steadily and gradually expanding,thanks to the development of Xiasha Higher Education Park and Qianjiang New City.(3)BFAST algorithm was used to decompose the time series of LST and the average temperature time series of construction land,green land,and water samples in Hangzhou's downtown districts from 2003 to 2014.The results show that both the maximum and minimum values of LST increase steadily over time.There is no obvious abrupt change,reflecting the steady rise of LST in the process of urbanization.Among the three indicators,the minimum LST experiences the most significant increase,and the increase of the mean LST is relatively steady compared to the other two.The LST of the three land types is showed as:construction land>green space>water body.All of them have increased over time.Compared with green space and water body,the LST of construction land shows more changes.Ecological landscapes such as urban green spaces and water bodies can make contribution to alleviate the heat island effect.(4)Urban hotspots were extracted based on high-temperature agglomeration.The results show that from 2003 to 2014,stable hotspots appeared in Gongshu District,Shangcheng District and Xiacheng District,and some of them are in the form of multi-centers.Hotspots in Jianggan District have gradually expanded since 2003 and tend to be stable.Binjiang District began to show obvious hotspots in 2005.The hotspot in the north of the downtown districts is regarded as the major hotspot.From the major hotspot's trajectory migration,the center of the major hotspot in 2003 was found to be concentrated at the junction of Gongshu District and Xiacheng District,and then moved to the east and maintained this trend.After 2007,the center moved mainly from Xiacheng District to Jianggan District.The hotspot intensity usually peaks in May to August each year.The peak of the yearly hotspot intensity shows a trend of steady decrease.In addition to the outliers in March-April 2005,it fell from 5.64? in 2003 to 3.43°C in 2014.The area of the major hotspot showed an increasing trend from 2003 to 2014,reflecting the expansion of the hotspot.The change trend of the attraction of the major hotspot to other hotspots is highly consistent with the hotspot intensity of the major hotspot,peaking in May to June or July to August each year.After 2003,with the gradual expansion of the hotspots,the attraction of the major hotspot has increased significantly and maintained at a certain level.After 2008,the major hotspot has been closer to the hotspots in Jianggan District,and the attraction of the major hotspot has increased.