Synoptic patterns and characteristics of medium and small flood events in the upper reach of the Yangtze River

Based on flow data from Yichang hydrological station from 1981 to 2012, firstly, we found that 128 medium and small flood events over the upper reach of the Yangtze River by a general survey. Secondly, using rainfall data from national weather stations and reanalysis data from NCEP, we classified these medium and small flood events into four patterns, namely, the zonal circulation-pattern, the meridional circulation-pattern, the between two high pressures-pattern, and the easterly flow-pattern. On the basis of background circulation and rainfall characteristics, the second pattern is again divided into the Bay Lake trough-type and the Northeast China trough-type. Lastly, we summarized the main influence systems, physical quantity feature and flood characteristic for the four patterns, and established corresponding weather models. Results showed that for the first pattern, low troughs in Northern Xinjiang are stronger than they are during the same period of normal years. For the second one, low troughs in Bay Lake and Northeast troughs in Northeast China are stronger than they are during the same period of normal years. For the third one, there are shear lines formed between the Tibetan high and the subtropical high. For the last one,most of them are influenced by typhoons. Flood-causing severe precipitation occurs in the region where there are divergence in the upper level,with high precipitable water vapor, positive vorticity and moisture flux convergence in the middle level, and warm advection in the midand low-level. Of all four patterns, both the diurnal flow extreme increment and the mean flow increment of event are the greatest for the first pattern, but the smallest for the last one.