| Urbanization changes landuse-landcover classes, the conversions changes the biogeochemical and biogeophysical processes of the land surface and energy balance processes between land surface and the atmosphere, so urban thermal environment is affected. The urban sustainable development is restricted by a series of environmental problem and ecological security arised from ruban thermal environment deterioration.Nanjing is one of big cities in China and is called "stove" in summer,its thermal environment is unoptimistic.In this study, we applied historical meteorological data, remote sensing data, and observation data to analyze and evaluate the temporal variation and spacial pattern of urban heat environment in Nanjing. We identified the influences of urbanization on thermal environment and the reasons that caused the urban heat effects. The main results show as follows:(1)The urbanization in Nanjing city and its suburb is so fast in last 20 years, especially in last 10 years. Impervious land surface has increased by an average of 1.37% a year since 2002. The vegetation cover area had decreased by an average of 0.88% a year from 1988 to 2013.(2)The statistical analysis based on historical meteorological data show that:The UHI intensity calculated by the three types of air temperature showed interannual variation, annual variation, and seasonal variation. The Urban Heat Island (UHI) effect is strong in Nanjing. The average UHI intensity is 0.40℃,0.41℃, and 0.43℃, respectively, calculated by daily mean air temperature, daily highest air temperature, and daily lowest air temperature. The UHI intensity is enhanced during 1980s and 1990s,and the UHI intensity calculated by daily lowest air temperature had increased most obviously. The UHI intensity showed pronounced seasonal variation with two peaks. The UHI intensity calculated by daily mean air temperature and daily lowest air temperature reached peak from March to May, and reached the second peak from September to October. The UHI intensity was lowest from November to January. However, the UHI intensity calculated by daily highest air temperature reached second peak from July to September, and was the lowest in November. In clear day without wind and fog, the UHI intensity was the strongest. The UHI intensity in surface layer at night was higher than that in daily, and was the lowest at noon in Nanjing. And the UHI intensity was higher in clear day than that in cloud day.(3)The temporal and spacial pattern of land surface temperature inversed by the remote sensing data was related to land cover/use and air temperature obtained from remote sensing. The standard deviation of land surface temperature was higher in spring and summer than that in autumn and winter. Average standard deviation method was applied to divide the land surface thermal field degree in Nanjing. The spacial pattern of land surface heat environment showed interannual and seasonal variation. The high temperature classes of land surface heat field almost located in the old urban area before 2002. But the high temperature classes of land surface heat field almost located in new urban area after 2002. The middle and high temperature classes of land surface heat field located in the current urban area from April to September. The land surface heat environment in the current urban area was almost the same to rural area from October to February. Moreover, the class of land surface heat field in the main urban area was lower than that in rural area when the daily mean air temperature was lower, which mean that the urban was’cold island’. The area of medium and the above temperature classes increased dynamically in last 25 years.which was caused by the increasing in secondary high temperature classes area.(4)The rapid development of urbanization has some effects on urban thermal environment.The frequency of the high temperature and heatwave day increased after 2000.and that of above 37℃ increased obviously,the highest temperature exceeded 40℃ during heatwave again.The temperature diurnal range in Nanjing site decreased much faster than in Jurong site. In Nanjing, the land surface heat island intensity showed interannual and annual variation in old urban area, industrial area and new urban area. The land surface heat island intensity in new urban area was lower before the rapid development of urbanization,then it increased quickly during rapid development of urbanization. The abnormal extreme high temperature classes of land surface heat field almost located in new urban area after 2002. The rapid development of urbanization changed the characteristics of heat landscape. In general, the area of higher classes heat landscape increase in recent 25 years.The rapid expansion of urban space disrupted the largest patch of the eight class heat landscape in old urban area before 2002,the heterogeneity of landscape increased, the degree of landscape fragmentation was bigger. The different types of patches mixed deeply each other in the middle period of rapid urbanization.At landscape scale, the morphological feature of high heat landscape classes was regular.(5) In Nanjing, the surface layer UHI effect correlated to meteorological factors, such as atmospheric pressure, vapor pressure, relative humidity, precipitation, wind speed, and total sunshine hour, and boundary layer structure. In warm season, the sequence of contribution for different factors was total sunshine hour, wind speed, atmospheric pressure, precipitation, relative humidity, and vapor pressure. In cold season, the sequence of contribution for different factors was total sunshine hour, vapor pressure, wind speed, relative humidity, precipitation, and atmospheric pressure. The boundary layer structure affect daily variation of UHI intensity. The inversion layer make the UHI form and become strong. The development of urbanization promoted UHI intensity. Land surface heat environment related into land use/cover, city function district and the building material (color). In this city, the land use/cover of eight and above classes abnormal extreme high temperature in four season was industrial area, independent industrial area, down town, large square, bare land, important traffic hub, and wood in hill ridge, which showed seasonal variation.Urban environment improvement should consider changing the spatial pattern of urban water area and green area, city ventilation, building pattern, urbanization processes, building material application, greenhouses gases emission, anthropogenic heat emission, industrial area pattern, and industrial heat emission. |
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