下垫面覆盖类型变化对城市热岛的影响——以北京市朝阳区为例

Effects of land surface type changes on urban heat island:a case study of Chaoyang District, Beijing

【目的】快速发展的城市化进程改变了下垫面的性质和结构,从而影响了城市的热量平衡,导致城市热岛效应蔓延。研究城市下垫面变化及其对城市热岛的影响,对缓解城市热环境、减少社会经济损失有重要意义。【方法】本文利用2002—2017年北京市朝阳区Landsat系列遥感影像计算研究区域归一化植被指数(NDVI)变化,并基于大气校正法利用Landsat热红外波段进行地表温度(LST)反演,进一步对所获地表温度进行归一化处理;同时利用2002—2017年北京市朝阳区谷歌全色卫星影像,基于DeepLabv3+网络利用深度学习对朝阳区下垫面分类并分析其动态变化。【结果】(1)朝阳区15年间平均地表温度呈上升趋势,城市热岛逐年加剧;至2017年,区内不再存在单一热岛中心,而转为复杂的镶嵌式结构、多中心分布。(2)15年间不透水面面积共减少71.02 km2,降幅20.98%;水域面积共减少2.53 km2,降幅24.12%;植被面积共增加73.54 km2,增幅56.57%。(3)对地表温度与下垫面类型动态变化相关性进行分析表明,地表温度与不透水面面积呈正相关,与植被面积呈负相关关系。(4)从总量来看,2012—2017年朝阳区与城市热岛效应成负相关关系的植被与水域面积总量增加明显,但城市热岛效应不降反增。【结论】植被和水域对城市热岛效应的缓解作用在用地强度、建筑密度和人类活动不断增长的前提下逐渐变得有限。在现有城市发展模式下,朝阳区地表温度将继续上升,城市热岛进一步加剧。

[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. [Method] In 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. By2017, 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 from2012 to 2017, but the urban heat island effect did not decrease but increased instead. [Conclusion] The 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.