Objective Rapid development of urbanization has changed the nature and structure of underlying surface, which has affected the urban heat balance and led to the gradual spread of the urban heat island effect. Studying the spatial and temporal changes of urban underlying surface and its impact on urban heat islands is of great significance to alleviating urban thermal environment and reducing social and economic losses.
MethodIn this paper, the changes of regional normalized difference vegetation index (NDVI) were calculated and studied by Landsat remote sensing images from 2002 to 2017 in Chaoyang District of Beijing. Based on the atmospheric correction method, the land surface temperature (LST) was retrieved by Landsat thermal infrared band, and the obtained land surface temperature was further normalized. At the same time, using Google panchromatic satellite images from 2002 to 2017, based on DeepLabv3+ network, the underlying surface types in Chaoyang District were classified and its dynamic changes were analyzed by depth learning.
Result(1) The average land surface temperature in Chaoyang District showed rising trend in the past 15 years, and the urban heat island had intensified year by year. By 2017, there was no longer a single heat island center in the area, but it had become a complex mosaic structure and multi-center distribution. (2) The impervious surface area had decreased by 71.02 km2, a decrease of 20.98% in 15 years. The total water area decreased by 2.53 km2, a decrease of 24.12%; vegetation area increased by 73.54 km2, an increase of 56.57%; (3) The correlation analysis between the land surface temperature and the dynamic change of underlying surface type showed that the land surface temperature was positively correlated with the impervious surface area and negatively correlated with the vegetation area. (4) From the perspective of total amount, the total amount of vegetation and water area was negatively correlated with the urban heat island effect in Chaoyang District and increased significantly from 2012 to 2017, but the urban heat island effect did not decrease but increased instead.
ConclusionThe mitigation effect of vegetation and water area on urban heat island effect had gradually become limited under the premise of increasing land use intensity, building density and human activities. Under the current urban development model, the land surface temperature in Chaoyang District will continue to rise and the urban heat island will be further intensified.