我国南方持续性冰冻雨雪灾害前兆信号和预测机理研究

The regional persistent freezing rain and snow disaster events are catastrophic to people’s lives, local economy, and national safety. However, researches on these events are very limited, particularly, formation mechanisms of the persistent freezing rain and snow disaster are only explored in terms of case study on 2008 winter. In fact, climate anomalies similar to 2008 persistent freezing rain and snow disaster are not unique in the history. The scientific questions are whether there is a precursory signal of such climate anomaly events? Can it be predicted? And how to predict? In 18 strong freezing rain and snow years from 1951 to 2009, The principle investigator (PI) of the current proposal, has found higher than normal index of Siberian High (Southern Branch Trough) which is indicative to an ‘abnormal combination’ (AC) of the cold and warm air masses over Mongolia-Yangtze river basin of China. A highly significant correlation (16/18) was observed between the persistent freezing rain and snow disaster and the 'AC' high index. This project intends to use this AC as the breakthrough point and the following research will be carried out by uses of statistical diagnosis and numerical experiments:1) Patterns of AC of the circulation system will be analyzed for all years with the persistent freezing rain and snow disaster, the predominant modes will be determined and the dynamic structures potentially responsible for persistent freezing rain and snow disaster will be revealed;2)The seasonal precursory signal for the major modes of AC will be identified, and the main pathway and physical mechanism from precursor signals to the persistent freezing rain and snow events will be explored;3) The intraseasonal variations of the major modes of AC will be analyzed and the relationship with persistent freezing rain and snow disaster will be focused. Through research efforts described above, the new insights and ideas on the seasonal and intraseasonal predictability of persistent freezing rain and snow disaster in southern China will postulated and a practice procedure for forecasting these events will be put forward.

区域性持续冰冻雨雪事件致灾性极强，给民众生产生活与国民经济带来了严重影响。至今有关其形成机理的研究主要是针对2008年个例展开，但这种气候异常在历史上并非绝无仅有。那么，这类事件是否有前兆？能否预测？如何预测？我们预研究发现在1951-2009年18个强冰冻雨雪灾害年，以蒙古高压(南支槽)为标志的冷(暖)空气的“异常组合”季平均高指数与其密切相关(16/18)。本项目拟以这种“异常组合”为切入点，通过统计诊断和数值试验相结合的方法开展如下研究：1)理清历史上冰冻雨雪灾害年环流系统“异常组合”的主要形态，揭示主要形态的关键“异常组合”及其造成冰冻雨雪的动力学机制；2)寻求关键“异常组合”的跨季节前兆信号，揭示其影响机制；3)分析冰冻雨雪年关键“异常组合”的季节内变化过程及前兆信号，揭示其与冰冻雨雪天气过程的联系。以期提出冰冻雨雪气候事件的跨季节预测与冰雪天气过程的季节内预测相结合的新思路。

区域性持续冰冻雨雪事件致灾性极强，给民众生产生活与国民经济带来了严重影响。本项目确定了1951-2017年冬季我国南方37次强持续性冰冻雨雪事件，对其时空特征、区划、环流特征、形成机制及前兆信号进行了综合研究，结果表明：1）南方持续性冰冻雨雪事件存在显著的2-3年周期性变化，1985年前后发生了突变，虽然近年来强度呈减弱趋势，但仍然发生了多次强持续性冰冻雨雪事件；在南方中西部发生频次高、持续日数长，而中东部强度更强并呈上升趋势；2）强持续性冰冻雨雪事件分为华中型、华南型、西南型三类；事件爆发日北方高空均呈现欧亚大陆北高南低的位势高度异常，低空蒙古高压显著偏强、中心南进，该配置有利于北方冷空气向南输送；南方南支槽显著加深，水汽向北输送活跃。三者不同在于蒙古高压强度和影响范围，西南型最强、范围最大、南伸显著，华南型次之；华中型、华南型水汽输送由南支槽和副热带高压共同影响，西南型事件水汽则仅由南支槽调控。3）蒙古高压和南支槽协同加强是造成西南型冰冻雨雪过程的直接原因，由二者建立的关键异常组合指数能较好地表征西南地区多数强降温降水过程。关键异常组合指数的增强与超前15日的冰岛低压加深及北大西洋-欧洲关键区高度场增强相关。前期上游的长波扰动能量向下游频散，北支波列引起下游乌拉尔山-贝加尔湖高压脊加强、东亚大槽加深，使得冷空气在西伯利亚区域堆积，导致蒙古高压增强；南支波列输送至阿拉伯半岛时波能辐合，急流加速，增强了东亚急流入口区的辐散上升，导致中低层南支槽加强。因此，格陵兰负高度扰动及北大西洋-欧洲正高度扰动可作为西南区域强持续性冰冻雨雪事件爆发的延伸期前兆信号。4）前期10月太平洋、印度洋关键区海温与北极海冰覆盖面积耦合指数与南方冬季冰冻雨雪事件关系最密切，该指数可作为南方冬季冰冻雨雪事件的跨季节前兆信号。以上结果，可为南方持续性冰冻雨雪事件的跨季节及延伸期预报提供借鉴。