湿地对北京城市热岛效应的缓解作用及其时空特征

Urbanization is the result of the social-economic development that intensifies the utilization of land, energy and water resources. Changes from natural land covers into settlements also bring about the emergence of urban heat island (UHI). The higher temperature in urban comparing to the rural areas is problematic for metropolis, e.g. Beijing, during summer times. Trees can help mitigate the UHI by evaporating sap and shading the land surface beneath the trees. Water is also a good mitigation measure by absorbing the incoming solar energy. The cooling effect of surface water, however, is contradictive among previous studies. In the present study, we plan to estimate the cooling effect of wetlands in Beijing via technologies of remote sensing, geographic information systems (GIS) and on site measurements of the land surface and the ambient temperatures. The study will be carried out at both the site scale and the regional scale of the entire city, on issues of the spatial gradient of the temperatures away from the wetlands, forests and grasslands and the cooling effects of all the wetlands and green lands of the city. The result of the study will help to improve the capability of the urban planning.

城市化有利于提高土地、能源以及水资源的利用效率。但城市化建设造成了地表覆盖和土地利用的迅速改变，形成城市热岛。气候变暖以及城市规模的快速扩张所导致的热岛效应加剧对特大城市的居住环境来说是不得不严肃面对的关键环境问题。城区绿化造林可通过冠层对太阳辐射的截获和冠层蒸发的潜热吸收减缓地表升温，从而效缓解热岛效应导致的局域气温偏高。与以往关注城市绿地建设缓解热岛效应的研究不同，本研究拟采用包括遥感数据反演、GIS空间分析以及基于观测数据的统计分析等，对北京市湿地在缓解城市热岛方面的作用进行分析。主要关注湿地水分蒸发及其在城市中的温度梯度特征，对比湿地、林地和草地对城市热岛缓解效应的差别，量化评估湿地在城市尺度上对热岛的缓解作用，以利于提高城市规划科学性，从而有效缓解城市热岛效应，提高其宜居性。

城市热岛效应是人为影响气候最显著的特征之一。热岛引起地表温度的升高和水分改变，对城市生态系统的物质能量流动和城市生态系统结构与功能必将产生深远的影响。本项目采用了遥感数据反演、GIS空间分析以及基于观测数据的统计分析对城市热岛效应进行了评估。结果显示：北京市郊区农田的夜间大气和地表热岛效应具有显著的季节变化特征，其最大值出现在冬季。山区林地站点夜间热岛效应的季节变化可忽略不计。城区长波辐射降温的低有效性是夜间地表热岛效应的主要原因。除人为热源释放外，热岛效应更多的受城市设施的材料对能量日间储存夜间释放的影响。城市中的自然生态系统或自然—人工生态系统可通过改善城市下垫面的热属性，对热岛效应有一定的缓解作用。