Study On The Influence Of Urbanization On Summer High Temperature In Yangtze River Delta Region

The Yangtze River Delta(YRD)is one of the regions with the most developed economy and the highest degree of urban agglomeration in China.With the continuous improvement of urbanization level,the temperature in this area has risen remarkably in recent years,and heat wave events occur frequently in summer.Researching the impact of urbanization on temperature,especially in summer,is of great significance to human health and social sustainable development,as well as related departments to prevent natural disasters and improve urban environment.This study analyzes the urban spatial expansion process of YRD from 1992 to 2013 through DMSP/OLS night lighting products,also combines with statistical data to divide 127 meteorological stations into urban or rural stations by dichotomy method.Then compare the temperature and summer heat wave events between urban and rural stations.Finally,combined with NCAR/NCEP-1 reanalysis data to quantitative analysis of the contribution of urbanization and land use/cover type change to summer high temperature by OMR.The main conclusions are as follows:(1)The YRD region experienced a rapid urban expansion process in 1992-2013.From 1992 to 2001,the urbanization process was slow,and the development of each city was dotted,spreading around the county and the city.From 2001 to 2010,the speed of urbanization had obviously accelerated.In the southern part of Jiangsu Province,Shanghai,and northern part of Zhejiang Province,contiguous development trends began to appear,four major urban agglomerations formed in Shanghai,Suxichang,Nanzhenyang and Hangzhou Bay,and stabilized in 2010.Urbanization slowed down in2010-2013.(2)From 1971 to 2017,the annual average temperature,annual average maximum temperature,and annual average minimum temperature of the Yangtze River Delta showed similar fluctuation characteristics,all of which showed a trend ofwarming.The trend of temperature increase began to increase significantly in the mid-1980 s.The climatic tendency rate of three temperatures is from high to low: the annual average minimum temperature is 0.42?/10 a,the annual average maximum temperature is 0.39?/10 a,and the annual average temperature is 0.39 ?/10 a.The three kinds of temperature elements fluctuated greatly in summer,and their temperature rising trend began to increase significantly in the late 1990 s.The climatic tendency rate of summer average minimum temperature climate tendency rate is the largest,0.31?/10 a,summer average maximum temperature and summer average temperature climate tendency rate are 0.29?/10 a and 0.26?/10 a.The area with large temperature increase rate is concentrated in the four major urban agglomerations.Follow the direction of Yangzhou-Nanjing-Changzhou-Suzhou-Shanghai-Hangzhou-Ningbo,like a zigzag shape.And most urban stations have a higher rate of warming than their surrounding rural stations.(3)Before the 1990 s,the number of high-temperature days in most rural stations was greater than that of urban stations.Since the 1990 s,the number of high-temperature days in urban stations has gradually higher than that in rural stations.After 2003,the difference in the number of high-temperature days between urban and rural stations increased,and the number of urban stations was significantly higher than that of rural stations.The growth rate of high-temperature days in urban stations was greater than that in rural stations.In the 21 st century,with the acceleration of urbanization in the YRD,the improvement of urbanization level,the proportion of population and the rate of built-up areas increased,the frequency of high-temperature heat waves,strong high-temperature heat waves,and extreme high-temperature heat waves has increased significantly,and the intensity has been significantly enhanced.(4)Sites and reanalysis data anomalies began to appear positively in the 1990 s,while OMR began to show positive continuity values in the 21 st century,with lags relative to changes in sites and reanalysis data.The climate tendency rate of OMR shows spatial differences.The climate tendency rate of OMR about the three kinds of temperature is larger in the southern part of Jiangsu Province than that in the north,and the value in northern Zhejiang is larger than that in southern Zhejiang.The large posotive value center is basically located in the four major urban agglomerations,the largest of which is in Shanghai.The contribution rates of annual OMR is not much different,both are close to 40%.But the the summer OMR's contribution rates are quite different,among which the summer average maximum temperature OMR contribution rate are as high as 54.61%,followed by the summer average temperature of 39.39%,and the summer average minimum temperature is only 26.92%.(5)In the summer of 2013,urban heat island(UHI)in Shanghai,Nanjing and Hangzhou showed obviously diurnal variations.The intensity of UHI is higher during the day and weaken at night,and the intensity during heat waves is higher than that during non-heat wave days.There differences between the intensity of UHI in June,July and August,the strongest is in July,followed by August and June.The intensity of UHI is highest in Hangzhou,and in Shanghai it is higher than that in Nanjing during daytime,lower at night.