Abstract: Based on the medium and small flood data from 1981-2012 in the upper reaches of Yangtze River and the NECP/NCAR reanalyzed data, the primary weather types of medium and small flood are summarized. They are zonal type, meridional type, easterly flow type and shear line between Tibetan high and subtropical high. The occurrence mechanism of flood-causing and the characters of medium and small flood have been researched by using the diagnostic analysis. The results show that the circulation of the zonal type is relatively flat in mid-high latitudes. Low trough in the northern XinJiang leads cold air flow to low-mid latitudes, and often accompanying with southwest jet at 850 hPa and 700 hPa, but the ground front position is slightly north. This type often has stronger precipitation processes, which are more severer and have long durations. And the medium and small flood presents double peaks or multiple peaks. The average peak discharge, the increasing discharge is the strongest. And the maintenance time of the flood process is the longest, about 14 days. The circulation of meridional type has deeper trough in Baikal lake and Northeast China, and has warm and humidity shear line or vortex development. The divergence in the exit area of the jet and cold advection is the most outstanding. The rainbelt of this type often appears from southwest to northeast. The medium and small floods present single peaks. The average peak discharge and the increasing discharge is medium. And the maintenance time of the flood process is the shortest, about 12 days and rising flow is faster. The high frequency of surface rainfall often occurs in the northern Jinshajiang River, Mingtuojiang River, Jialingjiang River and Yibin-Yichang watershed. The strong precipitation could occur in the whole watershed. Subtropical High over the Western Pacific and tropical cyclone meet toward to the upper reaches of Yangtze River with water vapor of the easterly flow type. The cold advection does not change significantly before and after the heavy rain. Influenced by topographic forcing uplift, the spatial distribution of heavy rain is frequently quasi-stationary, appeared as gobbets and sudden character. The medium and small flood of this type presents single peak. The average peak discharge and the increasing discharge is minimum and the rising flow is fastest. The strong precipitation occurs in the Mingtuojiang River and Jialingjiang River watershed. The circulation of shear line between Tibetan high and subtropical high is obvious saddle field. The power and water vapor conditions are weaker. This intensity of the precipitation is weaker, less stable, but accumulated precipitation is larger. The flood is given priority single peak, though bimodal peak occasionally occurred. The average peak discharge, the increasing discharge and the flood process is medium. The strong precipitation occurs in the Mingtuojiang River, Jialingjiang River and Yibin-Chongqing watershed.

Key words: the upper reaches of Yangtze River, flood-causing storm, synoptic type, characters of medium and small flood