川东北地区, 极端暴雨, 中尺度对流系统（MCS）, 列车效应, 后向传播

An extreme rainstorm occurred in the northeastern region of the Sichuan Basin from 20:00 on 7 to 20:00 on 8 August 2021,

with the daily rainfall and the hourly rainfall during the night rain period breaking historical records. The meteorological observation

data, Doppler radar data and ERA5 reanalysis data were utilized to analyze the evolution and causes of this process. The results are as follows: 1) Before the process, the pseudo-equivalent potential temperature (θse) at 700 hPa deviated from the climate average by 2.5 standard deviations, and the specific humidity at 700, 850, and 925 hPa all deviated by more than 2 standard deviations. 2) The stable ground convergence zone and the low vortex shear system in the middle and lower layers continuously triggered new convection and constantly merged with the original convection system to form “train effect”, ensuring the maintenance and development of the original convection system; 3) The convergence of low-level warm-humid air and dry-cold air formed a stable θse front area, the meridional warm and humid updraft, together with the updraft of the zonal secondary closed circulation, provided stable energy and water vapor transportation for the convective system. The convergence wind field near 700 hPa, in combination with the blocking effect of the descending and northerly airflow in the meridional secondary circulation at the middle and lower levels, made the convective system present a quasi-stationary “backward propagation” feature, which was conducive to the formation of this extreme rainstorm.

northeast Sichuan, extreme rainstorm, Mesoscale Convective System （MCS）, train effect, backward propagation