大气低频振荡对川西持续性强降水的影响

利用1981—2016年川西台站降水资料和NCEP/NCAR再分析资料，通过合成分析、Butterworth滤波等方法，分析了大气低频振荡对川西持续性强降水的影响，以期为该地强降水预报工作提供理论参考。结果表明：川西降水主要表现为15—30 d低频振荡特征。降水期间，低层低频东北风波列和西南风波列在川西相遇，辐合上升。前期在喀拉海和西北亚分别生成低频反气旋和气旋，两个系统向东南方向移动至贝加尔湖和日本海附近，为川西降水输送冷空气，印度半岛低频气旋转换为低频反气旋为川西输送阿拉伯海水汽。中层中高纬度东北风低频波列向南传播至雨区，有低频高压和低频低压分别与低层低频反气旋和气旋相对应，向东南方向传播从而影响川西地区。高层低频波传播也主要发生在中高纬度，降水前生成于贝加尔湖以西的低频反气旋和辐散中心先向东南移动，再向西南伸展，降水开始时控制川西地区。

Effects of low-frequency oscillation on the persistent extreme precipitation in Western Sichuan

Based on the precipitation data from the surface meteorological stations and the NCEP/NCAR (the National Centers for Environmental Prediction-the National Center for Atmospheric Research Reanalysis) data during 1981-2016, the effects of low-frequency oscillation on the persistent heavy precipitation in Western Sichuan were analyzed using the synthetic analysis and Butterworth filtering methods which provided a theoretical reference for the extended period forecast.The results show that the precipitation in Western Sichuan is mainly characterized by low-frequency oscillation in 15-30 d.During the precipitation period, the low-frequency northeast wave and southwest wave in the low level merge and form a convergence area over Western Sichuan.Before precipitation, a low-frequency anticyclone and a cyclone are generated over the Kara Sea and northwest Asia, respectively.These two systems move southeast to the vicinity of Lake Baikal and the Sea of Japan, respectively, which transport cold air for precipitation in Western Sichuan.The low-frequency cyclone over the Indian Peninsula converts into an anticyclone to transport water vapor from the Arabic Sea to Western Sichuan.On the middle level, the low-frequency northeast airflows from the mid-high latitude propagate southward to the rain area.The low-frequency high pressure and low-frequency low pressure correspond to the low-frequency anticyclone and cyclone in the low level, respectively, which propagates to the southeast and affects the Western Sichuan region.On the upper layer, the propagation of low-frequency waves mainly occur in the mid-high latitude.Before the precipitation, the low-frequency anticyclone and the divergence center formed in the west of Lake Baikal first move to the southeast and then extend to the southwest, which controls the beginning of precipitation over the Western Sichuan area.