大气低频振荡对四川盆地持续性强降水的影响

Effects of Low-frequency Oscillation on the Persistent Extreme Precipitation in Sichuan Basin

利用1981-2016年我国地面气象站降水资料和NCEP/NCAR冉分析资料,通过小波分析、合成分析、Butterworth滤波等方法 ,定义了适用于四川盆地的持续性强降水,对降水和大气15～30 d低频振荡特征进行详细分析,可为该地延伸期预报提供参考。结果表明:该地降水具有15～30 d和30～60 d低频振荡特征,其中以15～30 d振荡为主。降水期间各高度层和各纬度带低频系统具有垂直斜压性,在三维空间上相互配合,形成有利于降水产生的低频环流形势。低层和中层南北气流汇合于四川盆地形成辐合区,高层表现为北风南下。低层低纬度气旋在西太平洋生成并逐渐向西北移动至南海、华南,带来暖湿气流,中高纬度贝加尔湖东南侧生成气旋并向东移动至鄂霍次克海附近,加强北风输送。中层中高纬度欧亚大陆低压中心向东南方移动,降水时到达蒙古,并分裂小槽传播至下游,在日本海加强,高压中心紧随其后到达乌拉尔山。降水期间四川盆地上空高层为辐散区,有利于中低层辐合上升运动的维持。

Based on precipitation data of surface meteorological stations and NCEP/NCAR reanalysis data during1981 - 2016,the persistent extreme precipitation is defined for Sichuan Basin, and 15 - 30-day low-frequency oscillation characteristics of precipitation and atmosphere are analyzed in detail by means of wavelet analysis, synthetic analysis and Butterworth filtering, which provide theoretical basis and reference for extended period forecast.Results show that persistent extreme precipitations in Sichuan Basin are concentrated from June to September, which generally last for 3 days. The precipitation has low-frequency oscillation characteristics of 15 - 30 days and 30 - 60 days, and mainly 15 - 30-day oscillation. During the precipitation, the South Asia high and upper jet form a high-level divergence field, the subtropical high is strong and extends westward, the low pressure in the Lake Baikal and the Sea of Okhotsk extends southward, and the airflow in South China Sea and the airflow in the west side of subtropical high converge and pass northward. During the precipitation, low-frequency systems in each height layer and each latitude cooperate with each other in three-dimension space to form a low-frequency circulation which is favorable for precipitation. The vertical baroclinicity of the low-frequency system is conducive to the accumulation of unstable energy and provides energy conditions for precipitation.In the low-frequency flow field, during the precipitation period, the low-level and middle-level northsouth airflows merge into the basin to form a convergence area, and the upper layer appears as a northerly wind. The low-latitude cyclone in the lower layer is generated in the western Pacific Ocean and gradually moves northwestward to the South China Sea to bring warm and humid airflow. The southeast side of the mid-high-latitude Lake Baikal generates a cyclone which then moves eastward to the vicinity of the Okhotsk Sea to enhance the northerly wind transport. The mid-high-latitude Eurasia low-pressure center in the middle layer corresponds to the transverse trough in the polar region of the original field. With the transverse trough turning vertical, the center of the low-pressure moves to the southeast, arriving at Mongolia in precipitation,and splits the small trough to the downstream, strengthening in the Sea of Japan, followed by the high-pressure center to the Ural Mountains. During the precipitation period, the divergence of Sichuan Basin at high-level is positive, which is favorable for the continuous convergence and upward movement at lower and middle layers.