Meiyu in the Yangtze River Delta and its extended-range forecast associated with intraseasonal evolution of atmospheric circulation

Based on daily precipitation data obtained from 62 national basic meteorological stations in the Yangtze River Delta (YRD) and the NCEP/NCAR reanalysis dataset from 1981 to 2020, the relationship of intraseasonal precipitation in Meiyu season with the evolution of atmospheric circulation over the YRD is studied. The extended-range precipitation forecast model based on atmospheric circulation evolution is then constructed by using improved Spatial-temporal Projection Method (STPM). 10—80 d intraseasonal oscillation is found in the daily variation of Meiyu precipitation, and the amplitude differs in spatial and temporal scales. The stronger intraseasonal oscillation denotes heavier precipitation. From 15 to 10 d lead, the tropical intraseasonal convections become active with meridional propagations. The anomalously strong convections over the YRD enhance. The intraseasonal anticyclone (cyclone) — cyclone (anticyclone) — anticyclone (cyclone) wave train is triggered in the lower troposphere (upper troposphere), allowing divergence (convergence) to gradually establish. The intraseasonal atmosphere response leads to a stable Western Pacific subtropical high and obvious north-south and east-west oscillations of the South Asian High. All these cooperative intraseasonal changes of atmosphere from different levels and latitudes accelerate the development of precipitation. Based on the dynamic intraseasonal evolution of atmospheric circulation, the extended-range pentadly precipitation forecast model for the Meiyu season is constructed by using STPM. The independent evaluation of historic forecasts shows that the model has useful forecast skills at lead time of 10—25 d in the YRD.