Study Of Global Urban Heat Island Under Different Urban Size And Background Climate

In the past decades,the world has experienced a rapid and sustained process of urbanization.Urban heat island(UHI)is one of the negative effects of urbanization on the ecological environment.UHI is a phenomenon that the temperature of urban areas is higher than that of the surrounding suburbs.It will lead to the increase of heat-related mortality and the decline of air quality,which has a profound impact on the life and health of urban residents.In addition,UHI is also a key factor leading to global warming and many unpredictable climate changes.The study of UHI is of great significance to understand global climate change and human well-being.The existing research on UHI at global scale has two limitations:one is that it only pays attention to the phenomenon of UHI in large urban and ignores small and medium-sized urban.And lacks the research on unbiased global urban samples.Second,in the study of long time series,cities are usually regarded as static invariants,while the dynamic expansion of cities during this period is ignored,which leads to inaccurate calculation of UHI intensity.Based on the global urban boundary data and MODIS surface temperature data,we calculated the intensity of daytime/nighttime UHI of more than 7,000 with different size(large,medium and small)in the world from 2000 to 2015.We also explored the temporal and spatial distribution and variation of UHI in these cities.Then,the relationships between urban size/background climate and the intensity of UHI were studied,and the UHI in typical sub climate regions was discussed in detail.Finally,the study further analyzed the impact of urban expansion on the intensity of UHI.The main results are as follows:(1)In terms of interannual change,the global average intensity of UHI shows a slow growth trend in the 15 years from 2000 to 2015,with daytime/nighttime UHI intensity increasing by 10.2%and 8.8%respectively.As for seasonal variation,the global UHI shows a seasonal pattern of high in summer and low in winter.The difference of the intensity between the highest and lowest months in daytime is 0.72?,while the difference at night is only0.23?.In terms of diurnal pattern,daytime UHI intensity is higher than nighttime in summer,but the situation is opposite in winter.As for spatial distribution,the cities with the highest daytime UHI intensity in summer are mainly distributed in Brazil,Japan and Canada,with the highest value reaching 6.11?.Cities with low UHI intensity are located in the Mediterranean Sea and the Persian Gulf coast.In some areas,the temperature in the suburbs is even higher than that in the urban area,with the lowest value up to-2.95?.(2)At the global scale,the relationship between urban size and UHI intensity in historical years is similar to Sigmoid function,with an average fitting R~2of 0.89.However,there is some uncertainty in this relationship for large cities.The shape of the relationship function curve is also different for cities located in different climate types.The curve is high and steep in tropical cities,followed by continental cities,low and flat in temperate cities.The background climate also affects the intensity of UHI,and the larger the city is,the stronger the influence of climate is,therefore the influence of climate on the UHI intensity of large cities is stronger than small cities.The analysis of the typical sub-climate regions shows that the intensity of the daytime UHI is alleviated by the drought climate,while the intensity of the nighttime UHI is enhanced.The daytime UHI intensity is more susceptible to climate than nighttime.(3)The influence of urban expansion on the intensity of UHI is further analyzed.It is found that urban expansion has little effect on the intensity of the UHI before the urban expansion area reaches 100%-200%of the original area.However,if the proportion of urban expansion exceed the threshold,UHI intensity will more likely to show a significant upward trend.Finally,a controlled experiment on the static/dynamic urban boundary shows that the UHI intensity calculated using the static boundary in this experiment is 0.36?,while the dynamic boundary is 0.47?,which means that ignoring urban expansion will underestimate the global UHI intensity calculated from 2000 to 2015 by 23.4%.Which reveals the importance of using dynamic expansion urban boundary in the study of UHI.The study of global UHI provides important information reference for urban managers to design strategies to mitigate UHI in order to achieve urban sustainable development.