Abstract:

From May 4 to 6, 2022, an extreme rainstorm event appeared in the Yili River Valley of Xinjiang, recording historical maximum precipitation values at many stations. Using hourly precipitation data of automatic ground stations and the NCEP/NCAR 1°×1° reanalysis data, the paper analyzed the water vapor characteristics and instability mechanisms of this extreme rainstorm event. The results are as follows: 1) Under the background of a mid-latitude shortwave at 500 hPa and low-level convergent shear circulation, extreme rainstorms may occur in the Yili River Valley. The westward-opening “horn mouth”terrain feature leads to topographic convergence and forced ascent, enhancing the dynamic trigger mechanism for local rainstorms. 2) Water vapor mainly originates from the Mediterranean, Red Sea and Caspian Sea, with two main transport pathways in the west and southwest. The intensity of water vapor transportation along the low-level westward pathway is stronger than that of the mid-to-upper-level southwestern pathway. The western boundary serves as the main water vapor input boundary, contributing approximately 85% of the total water vapor input. Strong water vapor convergence from the surface to 700 hPa facilitates rapid water vapor accumulation. 3) Before precipitation, convective instability in the lower troposphere accumulated unstable energy, playing a crucial role in the occurrence of the heavy rainfall. During precipitation, the lower troposphere was affected by convective instability, while the middle and upper troposphere were affected by conditional symmetric instability. The combined effects of these two instability mechanisms led to this extreme rainstorm event.

Key words: extreme rainstorm, water vapor transportation, water vapor budget, mechanisms of instability, the Yili River Valley

摘要：

2022年5月4—6日，新疆伊犁河谷出现极端暴雨天气，多站降水量突破历史极值。使用地面自动站逐时降水资料、美国国家环境预报中心/美国国家大气研究中心（National Centers for Environmental Prediction/National Center for Atmospheric Research，NCEP/NCAR）1°×1°再分析资料，分析此次极端暴雨事件的水汽特征和不稳定机制。结果表明：1）在500 hPa中纬度短波和低层辐合切变环流背景下，伊犁河谷可能发生极端暴雨天气，向西开口的“喇叭口”地形特征导致地形辐合和强迫抬升，增强了局地暴雨发生的动力触发机制。2）水汽主要来源于地中海、红海及里咸海地区，存在偏西和西南两条主要输送路径。低层偏西路径水汽输送强度大于中高层的西南路径，西边界为主要水汽输入边界，水汽输入贡献比约为85%，且从地面至700 hPa的强水汽辐合有利于水汽快速积聚。3）降水前对流层低层存在对流不稳定为暴雨天气积聚不稳定能量，对强降水的发生起重要作用；降水期间，对流层低层受对流不稳定影响，而对流层中高层受条件对称不稳定影响，这两种不稳定机制共同作用造成此次极端暴雨事件的发生。

关键词: 极端暴雨, 水汽输送, 水汽收支, 不稳定机制, 伊犁河谷