秋行夏令:2021年10月初北方致灾性持续暴雨及水汽极端性分析

2021年10月3—6日，我国北方地区经历了历史罕见的持续性极端强降水过程，暴雨中心稳定维持在陕西中部、山西、京津冀、辽宁等地南部和山东北部，给上述地区造成了巨大的经济损失和严重的人员伤亡。基于台站观测降水、NCEP/NCAR和ERA5再分析资料诊断了本次降水过程的极端性。结果表明，本次暴雨过程无论是降水强度、持续时长还是经向水汽输送均表现出典型北方夏季暴雨和大气环流配置特征。上述五省二市区域平均的过程累计雨量强度远远超过秋季其他暴雨个例，即使在夏季也位列第二。本次过程的极端性与强降水中心稳定在上述地区密切相关。上述五省二市区域平均降水连续4日均超过15 mm，这在秋季历史上从未出现过。除过程的极端性强外，9月山西等地降水异常偏多对10月初秋涝也起到了叠加作用。本次秋涝对应的大气环流呈现出典型的北方夏季主雨季环流型，表现为西太平洋副热带高压（副高）偏西偏北，副高西侧的经向水汽输送异常强盛，同时10月4—6日北方地区发生一次强冷空气过程，冷暖气流交汇在上述地区。水汽收支计算表明，本次过程的经向水汽输送强度为秋季历史之最，甚至超过了盛夏时期北方大部分暴雨过程水汽输送强度。上述分析结果表明，即使在仲秋时节亦可产生有利于北方极端持续暴雨的环流形势和水汽输送，并导致秋涝发生。

Typical summer rainstorm occurred in mid-autumn:analysis of a disastrous continuous rainstorm and its extreme water vapor transport in northern China in early October 2021

In early October, 2021, northern China experienced an unprecedented autumn flood in history.From October 3 to 6, persistent extreme heavy rainstorm maintained stably in the regions centered along the middle-lower reaches of the Yellow River and Hai River (including the south of central Shaanxi-Shanxi-Beijing-Tianjin-Hebei-Liaoning and the north of Shandong).The flood caused huge economic losses and serious casualties.Based on the observed precipitation in China, the NCEP/NCAR and ERA5 reanalysis datasets, the extreme of this heavy rainfall case is diagnosed in this paper.Results show that the rainstorm process shows the characteristics of typical northern summer rainstorm and atmospheric circulation in terms of precipitation intensity, duration and meridional water vapor transport.The regional average 4-day accumulative rainfall intensity of the above five provinces and two cities is much higher than that of other rainstorms in autumn from 1961 to 2020, and ranks second even in summer.Its extreme intensity is closely related to the long-term persistency of heavy precipitation center in the above areas.The regional average precipitation of the above five provinces and two cities exceeds 15 mm for four consecutive days, which has never occurred in the history of autumn.In addition to the strong extremes of the process, the abnormally abundant precipitation in Shanxi and other places in September also superimposes the autumn flood in early October.The atmospheric circulation corresponding to this autumn flood presents a typical circulation pattern in the main rainy season in summer in northern China, which is manifested by a much stronger and more westward/northward western Pacific subtropical high, and the meridional water vapor transport in the west of the subtropical high is also extremely strong.At the same time, a strong cold air process invades northern China from October 4 to 6, and the cold dry and warm wet air flows converge in the above areas.The calculation of water vapor budget shows that the meridional water vapor transport intensity of this process is the highest in autumn history, and even exceeds that of most rainstorm processes in northern China in midsummer.Above results show that even in mid-autumn, the circulation pattern and water vapor transport conducive to extreme persistent rainstorm in northern China can also be triggered, resulting in severe autumn flood.