| Since the beginning of the industrial revolution,human activities,such as fossil fuel burning and land use change,have significantly increased the concentrations of greenhouse gases(GHG),such as carbon dioxide(CO2),methane(CH4),ozone(O3),nitrous oxide(N2O),and chlorofluorocarbons.Global warming has become an undisputed fact due to greenhouse effect.Urban with the most intense land use/land cover change has gathered half the world's population,consumed 60-85%of world energy and emitted 76-78%of the global carbon.Therefore,urban and urbanization are increasingly linked to global climate change.Urban heat island(UHI),as the most prominent characteristics of the impacts of human activities on urban climate and urban environment,has an important effect on urban eco-environment,living and production,and physical and mental health of the residents.Morever,urban warming especially summer heat wave caused by UHI exerts great influence on many aspects of the global economy,such as energy and water consumption,transportation and social economy.China has experienced a rapid development of social economy and urbanization in the past 40 years since reform and opening up in 1978."National new urbanization plan(2014-2020 years)" and "13th Five-Year National Development Plan" proposed the new urbanization development and the harmonious livable city construction policies,to build a people-oriented,harmonious and pleasant city with the gradual improvement of the ecological environment and air quality,and the safety of drinking water.Therefore.the study of UHI characteristics and formation mechanisms in the past 40 years has important theoretical and practical values for the reasonable planning,the livable city construction and city sustainable development.In addition,the knowledge of UHI is helpful to know urban thermal environment,predict city weather,and study global climate change.This study focuses on the Yangtze River Delta Urban Agglomeration(YRDUA)and the typical city Shanghai to elucidate the history of urbanization development,and the driving mechanisms of regional climatological conditions and urban internal structure changes on UHI from different spatial(YRDUA and Shanghai)and temporal scales(interannual,yearly,seasonal,monthly and daily).The aims are to:1)provide a scientific basis for the reasonable planning of YRDUA,landscape design and future climate warming study;2)support the studies of the response and adaptation of regional climate change especially the management of urban heat waves.Urban scale and quantity,urbanization rate,the industry structure,the composition of the three industry employees,and the area of urban construction land in Shanghai,Jiangsu,Zhejiang and Anhui provinces showed that YRDUA experienced a rapid urbanization in the past 40 years.At present,YRDUA has gradually grown into one of six largest urban agglomerations in the world.To summarize,the development of urbanization of YRDUA exhibited the following five characteristics.1)The number of cities and towns is increasing,showing the high speed growth and high density agglomeration characteristics.2)The urban hierarchy has been improved and a complete urban agglomeration system has been formed.3)Urban land area is expanded and the height of urban building rapidly extends to the high space.4)The investment in urban construction increases and the level of municipal facilities are substantially improved.5)Urbanization promotes the harmonious development of population and economy,and the scale of urban economy continues to rise.We presented evidence of a significant UHI effect on climate warming based on an analysis of the impacts of the urbanization rate,urban population,and land use changes on the warming rate of the daily average,minimal(nighttime)and maximal(daytime)air temperature in the YRDUA using 41 meteorological stations observation data from different grade cities.The effect of the UHI on climate warming showed a large spatial variability with the warming rates from 0.108 to 0.483?/decade from 1957 to 2013.Significantly positive correlations were found between the key indices of urbanization including urbanization rate,population,built-up area and warming rate of average air temperature,which can explain more than 80%variability of warming rate.Overall,there were siganificant regional differences in the UHI effect,and the relative contribution rate of UHI to warming rate ranged from 37.1%to 78.3%for different grade cities revealing the mechanism of urbanization on the urban heat island effect.Considerating the accessibility of the meteorological and urban structure data,we further focused our study on the the typical city of YRDUA,Shanghai.Based on the hourly observations of air temperature and other related meteorological factors in 2017,and the long-term daily observations of air temperature and other related meteorological factors from 1961 to 2015,we characterized the hourly and long-term trends of UHI intensity(UHII).The yearly-averaged daily UHII of Shanghai showed almost the same tendency with those of four seasons.Two peak values appeared in the midnight and before sunrise,while a valley was observed in the afternoon.Seasonally,the daily variation in UHII in winter was the most obvious,followed by that in spring and autumn,with these two seasons almost being equally developed,and that in summer was the least.Long-term trend showed three obvious phrase,the slow developing stage between 1960 and 1978,the rapid increasing stage of 1979-2003 and the alleviation phase between from 2004 to 2014.After 1978,Shanghai's maximum,average and minimum temperature UHII were all higher than those before reform and opening-up.The maximum temperature UHII showed the greatest change in all the monthly,seasonally and yearly variation.We analyzed the correlations between the key meteorological conditions(wind speed,relative humidity,cloud cover,precipitation etc.),urbanization indices(urban scale,structure and distribution of land use,population and population density,urban construction density and height,energy consumption,etc.)and UHII in Shanghai.The results showed that that sunny,dry,clam or breezy conditions were most conducive to the formation of UH? while UH? was relatively small under cloudy,strong wind and high relative humidity conditions.The threshold of 3.5m/s of wind speed,100%of total cloud cover and 90-100%of relative humidity were found for UHI formation.However,even under the most unfavorable meteorological conditions,the UHI still can develop,indicating the sigificant influence of anthropogenic heat release on UHI.Urbanization has changed the urban thermal environment to promote UHI formation by changing the city's underlying surface,topography,anthropogenic heat emission and the greening coverage rate.The number of high-rise buildings(more than 8 layers),gross production,population density,the green coverage rate and the number of social vehicles,were the main driving factors of nighttime UHII in Shanghai.We further investigated the effects of massive human migration on UHI during the Chinese New Year(CNY)holiday for the period of 2000-2015 in Shanghai.The UHI effects expressed as daily maximum,mean and minimum air temperature differences between urban and nearby rural stations averaged over the period 2000-2015 during the CNY week,were 0.35?,0.90? and 1.53?,respectively;and these values were 0.16?(31.4%),0.11?(10.9%)and 0.15?(8.9%)lower than those during the background period(2-4 weeks before and 2-4 weeks after the CNY week),respectively.Our findings highlighted the important role of massive human migration on urban climate based on a case study in Shanghai. |
PDF Full Text Request