Mechanism of a summer heavy rain in Songnen Plain,China

Based on the meteorological observation data and NCEP reanalysis data,the features and forming reasons of a heavy rain in summer of 2006 in Songnen plain were analyzed.The results indicate that the strong rain district is all located in the left side of northwesterly upper-level jet stream exportation and the rear of southwesterly upper-level jet stream.Westerly trough at 500 hPa combines with vortex at 850 hPa and they move to the east,and Mongolia frontal cyclone develops intensively,whose ground inverse trough approaches Northeast Plain.The low level jet stream appears after the most severe rainfall.The maximum values of K index,precipitable water(PW) and negative moist Richard number occur at Qiqihar weather station before the heavy rain,while those of A index,mean specific humidity and relative humidity at middle and high levels appear after the heavy rain.Water vapor comes from the East Sea of the periphery of super typhoon SaoMai to the south of Japan.Water vapor convergence caused by advection is strong at the low level before the heavy rain,while water vapor convergence at the upper level is mainly caused by advection and that at the low level is mostly gathered by wind convergence when the heavy rain is happening.