基于动力热力机理的我国江淮持续性极端降水事件归因研究

In recent decades, persistent extreme precipitation (PEP) event occurs frequently in the Yangtze-Huai River Valley (YHRV) in China, whether changes in the characteristics of such highly catastrophic extreme events are related to the human-induced climate warming is widely concerned by the government, the media and the society, which is also a hot topic at the forefront of climate change research. However, current attribution studies pay less attention to the persistence characteristics and show a tremendous lack of understandings in the human-induced dynamic and thermodynamic effects on PEP events. Accordingly, based on the evaluation of climate models used in this study for simulating precipitation and corresponding dominant circulation mode during a PEP event, the study aims to use the direct probabilistic event attribution and multi-step attribution methods to comprehensively reveal the role and contribution of human activities in the risk change of “frequency”,” intensity”,” duration” for PEP events in YHRV, and then clarify the confidence of the attribution conclusions. Furthermore, from the dynamic and thermodynamic perspective, the relative contributions of the changes in dynamics and in thermodynamics to the human-induced risk changes in the characteristics of PEP events would be assessed and the related uncertainty analyzed, in order to improve the capability and reliability of PEP event attribution in YHRV. Besides addressing the concerns from public and government precisely and timely, derived results are expected to provide decision-makers with a reliable scientific basis regarding risk changes in PEP event in YHRV to develop effective disaster prevention, mitigation and adaptation strategies.

近几十年来我国江淮流域持续性极端降水频发，因致灾性强，这类极端事件的特征变化是否与人类活动造成的气候变暖有关受到政府、媒体和百姓的广泛关注，也是气候变化研究的前沿热点问题。但目前的极端降水归因研究较少关注持续性这一致灾要素，且对人类活动引起的动力热力影响的认识十分欠缺。鉴于此，本项目拟在评估气候模式对江淮持续性极端降水及主导环流模态模拟能力的基础上，运用直接归因和多步归因法揭示并量化人类活动强迫对江淮持续性极端降水事件“频率”，“强度”及“持续时间”变化的影响及贡献，阐明归因结论信度。进一步从动力热力机理切入，区分并量化人类活动导致的江淮持续性极端降水事件各特征变化中动力热力因子的贡献，深化不确定性的认识，提高持续性极端降水事件归因能力与信度。研究成果有望准确、及时地回应公众及政府关切，并为决策者提供江淮持续性极端降水风险变化方面可靠的科学依据，以制定有效的防灾减灾和适应策略。

由于当前气候模式对我国东部季风区降水的变化趋势以及重大极端降水事件的模拟效果不佳，直接对降水进行归因存在很大不确定性，人类活动对我国东部季风区的降水变化是否有影响仍无定论。本项目另辟蹊径以海温及关键环流型等动力热力过程为桥梁开展了人类活动对江淮极端降水的归因分析。项目研究首先发现了动力热力过程对江淮流域的降水增多以及华北地区的降水减少均起到了正贡献，其中热力因子的贡献比动力因子大30%-40%，动力因子及其与热力因子的相互作用决定了“南涝北旱”分界线的位置。而人类活动导致的全球变暖引发的热力学影响是基本达成共识的，即变暖导致空气中水汽含量增多进而有利于降水。所以，项目进一步从动力过程入手识别了对“南涝北旱”降水趋势变化动力贡献最大的主导环流型，揭示了人类活动使得有（不）利于南涝北旱的环流型趋于增多（减少）, 确证了人类活动不仅影响热力因子，而且通过改变环流型发生频次这一动力因子，进而驱动我国东部夏季“南涝北旱”降水变化。此外，针对2020年6-7月江淮流域的超级梅雨事件，项目研究揭示了热带印度洋以及热带大西洋的海温异常通过影响环流对江淮流域极端降水起到促进作用，并进一步在海温长期变化趋势以及环流型发生概率的变化中检测出了人类活动信号，以“故事链”的形式证实了人类活动对江淮流域极端降水的促进作用。项目研制的基于动力热力过程的归因方法等研究成果为模式模拟效果不佳的极端事件的归因提供了突破口，可以更可靠的检测并论证人类活动在诸如极端降水这种局地性极强的事件中所起到的作用，为科研人员以及相关防灾减灾决策提供支撑。