Study on Hydrological and Rainfall Characteristics and Circulation Anomalies in Autumn Flood Season of HanJiang River in 2021

Based on NCEP/NCAR reanalysis data and conventional meteorological and hydrological observation data, the meteorological and hydrological characteristics as well as circulation animalies and possible causes of the autumn floods in HanJiang River in 2021 are analyzed.The results show that the precipitation in HaiJiang River in 2021 was more in the west and less in east. The accumulated rainfall in he upper reaches of HaiJiang River was 400-985 mm, and the accmulated areal rainfall ranked the first since 1961.During the autumn flood season, the Danjiangkou reservoir experienced the flood peak process of more than 10,000 m³•s^-1 for 7 times. In terms of atmospheric circulation, the low trough of Balkash Lake and Baikal region, the high ridge from the east of mainland China to the northwest Pacific, the low trough in the areas to the east of Japan Island, the subtropical high, the South Asian high anf the East Asia subtropical westerly jet were all abnormally stronger than those in the same period in history. The formation of abnormally low-level flow field and water vapor convergence center, combined with tje divergence and suction of high-level flow field,were the large-scale circution background favorable for the continuous and enhancement of precipition. When the inflow of DanJiangKou reservoir was close to 2,000 m³•s^-1, the 5 d accumulated area ranfll exceeding 60 mm inthe upper reaches of Hanjiang River was closely related to the appearance of flood peaks above 10,000 m³•s^-1. After the upstream inflow increased, the narrowing river corase of the middle and lower reaches of the Hanjang River was the main reason for the over-alarm-line floods in the middle and lower reaches of the Hanjiang River where the precipitation was not so heavy.