Abstract: In this paper,the environmental background,formation and evolution processes of a tornado-like storm and local heavy rainfall caused by a β mesoscale system on July 5,2014 in the central region of Heilongjiang province were analyzed based on conventional meteorological data,observations from regional automatic intensive weather stations,cloud images from the T639 model and FY-2 satellite,and data of new generation weather radar,as well as the NECP reanalysis data with 1°×1° resolution.The results show that this process is controlled by a cold vortex at higher altitudes and a low pressure at ground.The weak cold air at middle altitudes invades into strong warm air at low altitudes,and then forms the unstable condition.Low level jet (LLJ) and strong shear at low levels favor the formation of β mesoscale system.The strong vertical wind shears at low levels enhance strong convective conditions and lead to the formation and development of three supercells.The intensity of convective clouds increases rapidly during the primary development stage.Strong winds prevail in the early mature stage,and the supercells exist in the initial of mature convective clouds and develop up to the top of clouds.With the change of β mesoscale to α mesoscale,the clouds become weak and the severe weather generally ends.The formation and extinction of three supercells are clearly found according to the radar detection.The echo from cyclone and vault is also distinct and lasts for a long period of time.The characteristics of storm are shown from the vertical wind profiles and radial vertical velocity.The strong echoes of precipitation move to Langxiang region,and their intense and location vary little,which mainly results in a persisted heavy rainfall in this region.

Key words: β mesoscale system, Tornado-like storm, Strong wind, Heavy rainfall