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 were taken as the research object to study the characteristics of flood peak patterns and the evolution laws of corresponding weather systems, and a conceptual model of flood causing rainstorm weather was constructed. The results shown that: (1) during the autumn flood season, the single peak flood process in the upper reaches of the Han River was the most frequent, and the flood hydrograph had diversity; The bimodal type had a large flood volume, high peak, and rapid rise and fall of water, with a mostly pointed and thin shape; The multi peak flood had the largest volume and longest duration, with varying peak heights. (2) The duration of the unimodal flood process was short, and the daily cumulative precipitation fluctuates greatly, with rapid peak formation, all of which were sharp and thin; The duration of the bimodal process was generally no less than 11 days, the interval between rainstorm processes was short, the difference between the main peak and the secondary peak was small, and the peak value was more than 20000 m3 · s-1; The duration of the multi peak precipitation process was the longest, with continuous cloudy and rainy weather lasting for more than 20 days, and the main peak value was lower than that of the bimodal type. (3) From the perspective of large-scale circulation patterns, in mid to high latitudes, there were generally three more obvious pattern adjustments in the bimodal pattern, with a larger meridional degree of circulation; There were few adjustments to the multi peak circulation pattern, and the southern side of Baikal was mostly characterized by small trough and ridge activity; There was no significant adjustment in the unimodal pattern. In the middle and low latitudes, the western Pacific subtropical high with multiple peaks was most strongly moving westward, with no typhoon or tropical cyclone activity; The bimodal subtropical high oscillates frequently from east to west, often involving tropical cyclones; The unimodal pattern was often accompanied by the merging of continental and oceanic high-pressure systems, with little activity of typhoons or tropical cyclones. (4) The conceptual weather models of flood generating rainstorm in the upper reaches of the Hanjiang River during the autumn flood season can been divided into five categories: high trough jet forcing (A-I), high trough low vortex shear (A-III), subtropical high periphery jet forcing and (B-I), subtropical high periphery southerly flow weak forcing (B-II) and subtropical high interior low vortex shear (C-III). In the above model, bimodal floods are mostly dominated by type A, while multimodal floods are mostly dominated by type B, with both single peak types A and B appearing, and type C was only unique to single peak types. In addition, ground type I and type II were often combined with A-I, A-III, B-I, etc. When the three-layer weather system was well configured, the probability of flooding will been significantly increased.