Characteristics of urban heat island effect in Lhasa City

This paper analyzes the Urban Heat Island(UHI) effect in Lhasa City of Tibet using meteorological observations,the Normalized Difference Vegetation Index(NDVI),Land Surface Temperature(LST) data obtained from Earth Observing System/Moderate resolution Imaging Spectroradiometer(EOS/MODIS) information,and correlation and composite analyses.The results show:(1) Areas with high temperature are primarily located in the center of the city or nearby counties,while low temperature areas are in the suburbs of counties.The area with high temperature has expanded in recent years and some high-temperature centers have even migrated to certain other regions.(2) The UHI intensity tends to be stronger both in annual and seasonal variations,especially in winter.Also,LST is somewhat positively related to mean air temperature.(3) A negative correlation exists between the changes of LST and NDVI with the increase of vegetation from urban to rural areas in different seasons.(4) The UHI intensity is nega-tively correlated with precipitation while positively correlated with wind speed,and the relation between the UHI intensity and evaporation varies with the seasons,namely,the intensity is positively correlated with summer evaporation but negatively corre-lated with winter evaporation.(5) UHI intensity might be enhanced by intensified urbanization,wherein built-up areas expand,there is increased heat from human activity,and there is more artificial heat input to the atmosphere.