Analysis on the characteristics and extended range forecast of Meiyu anomaly in Shanghai in 2020

Using NCEP daily reanalysis and observation data, we analyzed the characteristics of the 2020 Meiyu in Shanghai and evaluated the effectiveness of the extended range forecast which is based on the atmospheric low-frequency characteristics. The results show that (1) the 2020 Meiyu in Shanghai was the most severe one since 2000. (2) The excessive precipitation during Meiyu is caused due to the influence by the cold phase of equatorial central-east Pacific and the anomalously warm SST of the tropical Indian Ocean in summer, the strong and stable anomalous anticyclone in the northwest Pacific Ocean and the abnormally strong water vapor transport from its western turn, and the frequent cold air transfer along the direction of wave-activity flux. (3) The precipitation events in the 2020 Meiyu show obvious quasi-biweekly oscillation characteristics, with all four strong precipitation events corresponding to the peaks of low-frequency precipitation. The extended range precipitation forecast method developed based on the low-frequency characteristics is more accurate in predicting the first strong precipitation event in the 2020 Meiyu season.