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诱发四川冕宁“6.26”山洪灾害的突发性暴雨特征及其形成机制

Characteristics and Formation Mechanism of the Sudden Rainstorm Inducing the “6.26” Mountain Torrent Disaster in Mianning, Sichuan Province

  • 摘要: 基于多源观测、再分析和对流可分辨模式预报资料,运用物理量诊断、标准化异常、相似过程比较等方法,分析了2020年6月26日四川冕宁突发性暴雨过程的特征和形成机制。结果表明:(1)该过程是一次伴有多条带状γ中尺度对流系统、“列车效应”产生极端小时雨量的局地突发性暴雨过程,其对流回波质心较低,对流云团具有中尺度对流复合体云团特征;(2)冕宁北部的对流冷池出流与较强的谷地偏南气流相遇形成的辐合抬升构成了对流的触发机制;(3)川西南低空偏南气流具有阶段性增强特征并提供了持续的暖湿空气输送,其在过程初期与下山冷池的相互作用及后期与盆地西部南下冷空气的汇合,使对流反复在冕宁站西侧和南侧初生,并在下游形成“列车效应”;(4)对比历史相似过程,环境大气的对流有效位能等物理量具有更显著的异常和异常持续性;(5)川西南北部的高海拔地形对延缓冷空气进入安宁河谷和维持河谷内的不稳定层结有显著作用,并且该区域地形强迫抬升形成了河谷上游地区潜在的对流触发条件。最后给出了此次暴雨过程形成机制的概念模型。

     

    Abstract: Based on multisource observations, reanalysis, and convection-resolving model forecast data, the characteristics and formation mechanism of the sudden rainstorm process on 26 June 2020 in Mianning, Sichuan Province, were analyzed by employing physical quantity diagnosis, standardized anomaly analysis, and comparison with similar processes. The results were as follows: (1) The process was a local sudden rainstorm characterized by several banded meso-γ convective systems and extreme hourly precipitation generated by the “train effect”. The convective cloud clusters had the features of a mesoscale convective complex with a low center of convective echoes. (2) Surface convergence and uplift, formed by the outflow of a convective cold pool in the northern part of Mianning and a strong southerly wind in the valley, triggered convection. (3) The southerly low-level flow in Southwest Sichuan exhibited phased enhancement and provided continuous warm and moist air transportation. Its interaction with the downhill cold pool early in the process and the confluence with the southward cold air from the western basin later in the process repeatedly triggered convective cells on the west and south sides of Mianning station, causing the “train effect” in downstream areas. (4) Physical quantities, such as the convective effective potential energy of the environmental atmosphere, had more significant anomalies and persistence of anomalies compared with a similar process in the past. (5) The high-altitude terrain in the northern part of Southwest Sichuan has a substantial effect on delaying the entry of cold air into the Anning River valley and maintaining unstable stratification in the valley. The forcing uplift of the terrain in this area formed potential convection-triggering conditions in the river valley upstream. Finally, the conceptual model of the formation mechanism of this rainstorm process was presented.

     

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