Abstract: The precipitation during main flood season was rankedin a manner that the first and last six orders were defined as rainy years and rainless years respectively. Based on the NECP reanalysis data with resolution of 2.5°× 2.5°, a series of physical quantities such as the average height field, temperature field, the wind field, specific humidity and vorticity during  flood season were analyzed. The structure and location as well as center intensity of synoptic system during flood season of rainy years and rainless years were compared according to synoptic theory and the conventional forecasting experience. The results indicate that compared with rainless years, the location of 568 isoheight at 500 hPa in rainy years is moved to the south about three latitudes, while high subtropical 588 isoheight is moved to the west about seven longitudes and to the north about one latitude. The lower convergence and upper divergence are the main features in a rainy year. The mean water vapor contents at 850 hPa and 700 hPa are higher in a rainy year than in a rainless year. The wind field near the Lake Baikal is a single convergent center in a typical rainy year at 850 hPa, while it is double convergent centers in a typical rainless year.

Key words: Flood season precipitation, Rainy and rainless years, NCEP data, Upper trough, High subtropical