Characteriistics of Precipitation Concentration Degree and Precipitation Concentration Period during the Flood Season in Jinhua

Generally, Jinhua has a uniform temporal distribution of precipitation; however, with large interannual variability, there is a quasi-18-year precipitation concentration degree and concentration period. To investigate the nonuniform distribution characteristics of the flood season precipitation and trend, this research analyzes the temporal distribution characteristics and evolution trend of the flood season precipitation in Jinhua from 1971 to 2020. The research involves using daily precipitation data combined with ECMWF reanalysis data and employing improved precipitation concentration and precipitation concentration period indices. The results show that the initial day of the precipitation concentration period is June 13, which coincides with the initial day of the monsoon season. With regard to the sea surface temperature (SST) field, an anomalously high SST results in the east−central Pacific Ocean during the winter before the year of precipitation concentration anomaly; then, the SST difference in the east-central Pacific Ocean at the equator gradually turns into a significant negative anomaly, suggesting that years with anomalously concentrated precipitation may occur during the decaying stage of El Niño. The most notable anomalies in the situation field are the strong and westerly subtropical high pressure in the western Pacific and the anticyclones in the Philippines region. The anomalous anticyclones allow for high moisture transport from the southwesterly flow along the eastern coast of China during the early summer season. This simultaneous movement causes the subtropical-high pressure to increase and extend westward. The potential gradient between the increased subtropical-high pressure and westerly trough can increase the transport of warm and humid air from the southwest and slow the movement of the westerly trough ridge. The gradient then forms a stable circulation configuration, resulting in the emergence of sustained precipitation. After the increased subtropical-high pressure continues to extend westward and lift northward, the westerly wind belt system is blocked southward. The high pressure of the involved large sinking-warm-air flow leads to persistent high temperatures and drought heat weather, resulting in anomalous precipitation concentration.