Abstract: Three mecoscale convective systems (MCS) occurred successively in the west of Heilongjiang province on July 11-13, 2011, which accompanied short-time heavy precipitation, hail, gale, lightning and thunderstorm and so on. Based on the observational data, the circulation feature of MCS and its physical variables were analyzed in order to discuss the evolution of MCS under the low vortex system. The results indicate that the surface convergence line and dry line are key factors in severe convective trigger mechanism. Shear line and convergence line in different altitudes make MCS developing and moving to the areas where there is instability energy. Physical variables such as pseudo-equivalent potential temperature, K index, SI index, water vapor with upper dry and lower wet, temperature advection with upper cold and lower warm flow have important indicative significances for evolution of MCS. Squall line occurs in the front boundary of MCS where cloud cluster has major intensity gradient and short-time severe convection occurs frequently.

Key words: Mecoscale convective system (MCS), Surface convergence line, Low vortex, Squall line