Abstract:Based on the NCAR/NCEP reanalysis data and conventional meteorological and hydrological observation data, 15 cases of numbered floods in the upper reaches of the Hanjiang River in the autumn flood season are taken as the research object to study the characteristics of flood peak patterns and the evolution laws of corresponding weather systems, and a weather conceptual model of flood-causing rainstorm is constructed. The results shown that during the autumn flood season, single peak flood processes occur most frequently in the upper reaches of the Hanjiang River, and the flood hydrograph has diversity. The bimodal pattern has a large flood volume, high peak, and rapid rise and fall of water level, with a mostly pointed and thin shape. The multi-peak flood has the largest volume and longest duration, with varying peak heights. The duration of the unimodal flood process is short, and the daily accumulated precipitation fluctuates greatly, with rapid peak formation, and all of them are sharp and thin. The duration of the bimodal process is generally no less than 11 d, the interval between rainstorm processes is short, the difference between the main peak and the secondary peak is small, and the peak value is more than 20 000 m3·s-1. The duration of the multi-peak precipitation process is the longest, and in the continuous cloudy and rainy weather can last for more than 20 d, but the main peak value is lower than that of the bimodal pattern. From the perspective of large-scale circulation patterns, in mid to high latitudes, there are generally three more obvious pattern adjustments in the bimodal pattern, with a larger meridional degree of circulation. There are fewer adjustments to the multi-peak circulation pattern, and the southern side of Lake Baikal is mostly characterized by small trough and ridge activities. There is no significant adjustment in the unimodal pattern. In the middle and low latitudes, the western Pacific subtropical high with multiple peaks moves most strongly westward, without typhoon or tropical cyclone activities. The bimodal subtropical high oscillates frequently from east to west, often involving tropical cyclones. The unimodal pattern is often accompanied by the merging of continental and oceanic high-pressure systems, with few activities of typhoons or tropical cyclones. The weather conceptual models of flood-causing rainstorm in the upper reaches of the Hanjiang River during the autumn flood season can be divided into five categories: high trough jet forcing (A-Ⅰ), high trough low vortex shear (A-Ⅲ), subtropical high periphery jet forcing (B-Ⅰ), subtropical high periphery southerly flow weak forcing (B-Ⅱ) and subtropical high interior low vortex shear (C-Ⅲ). Bimodal floods are mostly dominated by pattern A, while multimodal floods are mostly dominated by pattern B, with both single peak pattern A and pattern B appearing, and pattern C is only unique to single peak pattern. In addition, ground pattern Ⅰ and pattern Ⅱ are often combined with A-Ⅰ, A-Ⅲ, B-Ⅰ, etc. When the three-layer weather system is well configured, the probability of flooding will be significantly increased.