城市化影响热浪气候变化的归因研究

With global warming and fast urbanization in eastern China during the last few decades, climate extremes such as heat waves have tended to intensify with increasing threats to regional sustainable development. This project is to answer: how much the fast urbanization in recent decades has contributed to the observed intensifying trend of heat waves in eastern China. Most of studies of attribution of climate change were focused on identifying contribution of global effect of human activities (e.g., the enhanced atmospheric greenhouse effect) to observed large-scale climate change (e.g., global warming). Urbanization leads to climate warming at local scales, thus posing some confounding effect to the enhancing atmospheric greenhouse effect in local climate observations. It is a critical issue for an attribution study to identify confounding factors. This project is to (1) analyzing the patterns of climate changes especially of daily mean, maximum and minimum surface air temperatures and the probability distributions of temperature extremes associated with heat waves in China during the last few decades, when there happened a fast process of urbanization especially in eastern China, based on recently developed homogenized climate observations, in order to identify possible effects of the global and local factors; (2) conducting a set of regional climate simulations by using the WRF-UCM nested modeling system driven by different scenarios of urbanization as observed in the past decades and different boundary conditions induced from observations and a couple of CCSM4 global climate simulations driven by anthropogenic radiative forcings representing the present and pre-industrial level, respectively; and (3) attributing the observed climate change associated with heat waves in eastern China to urbanization and global warming, based on the results of (1) and (2). The project will for the first time provide a quantitative assessment of the contribution of urbanization to the regional climate trend of heat waves in eastern China, compared with that of global warming. The attribution study of climate change and climate extremes will help to promote quantitatitve studies of climate effects of urbanization in China, as well as to improve knowledge of impacts of global warming in China.

随着近代全球变暖及中国东部城市化发展，热浪对该区域可持续发展威胁日增。项目旨在解答：城市化是否及在何种程度上促进了近几十年观测到的区域热浪气候趋强之势？以往归因研究大都为辨别全球性人为强迫（如大气温室效应增强）对全球变暖等大尺度气候变化的贡献。城市化气候效应在当地与大气温室效应增强具有"混淆"性，而辨别混淆因素正是归因研究的关键困难所在。本项目基于最新发展的我国区域均一化气候观测资料，从多个气温指标及其极值分布演变的角度刻画热浪气候变化；进而通过多组WRF-UCM区域嵌套气候模拟及CCSM4全球耦合气候模拟大气温室效应增强的情景，进行针对性的对比分析，从中辨别全球变暖和城市化对近几十年中国东部热浪气候趋强的贡献。目前国内极端天气气候变化的归因研究极少，项目具有填补空白的示范意义，将促进城市化气候效应的定量化研究，并为理解过去、预估未来热浪气候演变提供新认识。

全球变暖背景下高温热浪事件趋频，城市因人口、资源聚集且叠加自身气候效应而面临更大的热浪灾害风险。研究城市化之于热浪气候变化的影响，可为城市化适应气候变化及可持续发展充实科学基础。项目旨在定量评估中国城市化对极端气温事件的影响并揭示相应物理机制。按计划开展如下研究。..（a）城市化气候效应的观测统计归因分析 - 基于均一化的中国气温观测序列集，结合遥感城市面积百分比变化及改进的OMR等方法，重新评估了中国城市化对观测气温升高趋势的贡献（区域平均而言约4%）；指出城市化效应在一些极端事件中更值得关注。就2013年夏华东极端热浪的统计归因分析表明，类似事件在工业化前的平均气候状态下几无可能发生；经历了近百年全球变暖的背景下，其发生概率达百年一遇；考虑叠加大洋多年代际振荡达最暖位相的情况，该概率达42年一遇；城市化则进一步促进之达30-40年一遇。..（b）城市化影响热浪的机制模拟及气候变化背景归因 - 运用高分辨区域气候模拟揭示了城市热岛和热浪的正反馈机制。就2013年夏季华东热浪而言，城市日均气温额外增1.5℃，夜间增温更甚；热浪加强城市热岛，反之亦然，热浪期间异常的地面气象过程是导致两者正反馈的关键。基于观测和CMIP5多模式多成员气候模拟，利用最优指纹法分析发现，近50年中国东部典型极端冷暖事件频数变化的主要因素可归于大气温室气体浓度增加。..（c）相关基础研究 - 发展了一套均一化的中国百年气温序列集，为研究区域长期气候变化及其中的城市化效应提供了新的资料基础。发展了针对逐日气象序列的均一化新方法。结合观测和模拟分析揭示了大规模城市化不利于北京城市及其下风向地区降水。指出OMR方法的缺陷导致以往研究大多高估了气温日较差（DTR）变化中的城市化效应。项目后期拓展出更多相关研究话题，如城市化进程中的霾气候预测问题、热浪所致生产率受损的世界格局及未来情景等。.. 发表论文13篇，包括9篇SCI，Yan et al（2016）入ACCR期刊“Most Cited”之列。培养博士5名，其中王君晋升副研。