夏季极端高温对快速城市化的响应及其缓解研究

Extreme heat events are occurring more frequently under the background of global warming and effects of rapid urbanization on this phenomenon cannot be ignored on the local scale. However, less quantitative studies exist in this research field at present. This study intends to select the Beijing-Tianjin-Hebei region as the study area, which has experienced rapid urbanization, and focuses on the scientific questions that how the urbanization affects the extreme summer heat. First, the variation trends of the extreme summer heat during the process of urbanization are analyzed base on observation data of meteorological stations. Second, the role of land underlying surface change on extreme summer heat events caused by urbanization are evaluated through WRF numerical modelling by replacing original land underlying surface data with high accuracy land cover proportion data and remote sensing based biophysical parameters products. Then biogeophysical mechanisms are explored from the perspective of surface radiant budget and energy balance, and land-atmosphere energy exchange. Finally, this study develops strategies to mitigate the extreme summer heat based on the research results, such as increasing the proportion of green space and giving priority in developing small towns. This study will provide scientific basis for planning urban development, adapting cities to climate change, and coping with frequent extreme climate events.

全球变暖背景下极端高温事件越加频发，快速城市化对局域极端高温的影响不可忽视，目前这方面的定量研究较少。本研究拟选择经历快速城市化的京津冀地区为研究区，围绕城市化对夏季极端高温影响这一前沿的科学问题开展研究。首先，基于气象站点观测数据分析城市化过程中夏季极端高温的变化趋势；其次，将高精度土地覆盖百分比数据和遥感地表物理参数产品嵌入WRF模式，模拟城市化导致的下垫面改变在夏季极端高温案例事件中的作用，并从地表辐射能量平衡、陆气能量交换的角度探讨其生物地球物理机制；最后，寻求优化城市布局（如增加绿地比例、小城镇优先发展）缓解夏季极端高温的措施。本研究将为城市发展规划、气候适应型城市建设和极端气候事件应对等提供一定的科学依据。

本项目基于均一化气温数据从观测事实角度分析北京市不同类型气象站点极端高温变化趋势；将中科院高精度土地覆盖百分比数据嵌入WRF区域气候模式，优化模式原有地表物理参数，从模拟角度定量刻画京津冀地区快速城市化对局域/区域气温的影响；并基于模型模拟结果厘定局域地表温度/气温变化响应城市化的敏感性，从陆气能量交换角度出发拆分各地表辐射能量分量对城市化温度效应的贡献率；此外，探讨中国地表热岛时空演变规律及决定因素。主要结论如下：（1）北京市城市站点极端高温日数和强度明显高于乡村、近郊及山地站点，城市热岛对极端高温日数和强度有明显的增幅作用。（2）京津冀地区各个省市建设用地面积均在持续增长，但增长程度不一，主要集中在地形平坦的城镇外围；建设用地占用其他土地类型后，地表反照率、叶面积指数呈下降趋势；夏季，当京津冀地区城市用地百分比每增加10%，局域气温（地表温度）将升高0.212℃（0.464℃），即京津冀区域城市扩展导致下垫面类型发生改变后通过生物地球物理过程导致局地气温（地表温度）升温幅度可超过2℃（4℃），城市化引起的局地增温幅度在夏季极端高温中不可忽视。（3）通过归因分析，城市扩展导致的潜热减少是京津冀地区城市化引起局地增温的最主要原因，表明增加绿地面积比例将增加蒸散发（潜热），进而有助于减缓热岛强度和缓解夏季极端高温。（4）地表城市热岛强度时空变化的空间集聚和分异性显著，且昼夜机制不同，应加强不同城市群的对比研究。