Abstract: Based on the conventionally observational data and the NCEP/NCAR reanalysis data, the main weather systems (a shear line at 850 hPa) of snowstorm on February 12-13, 2009 and their evolution were analyzed. The results show that the strong snowfall center is located in the southeast side of shear line in the middle and western Liaoning province and near warm front of the top of Jianghuai cyclone. A shear line is related to a positive vortex area. The positive vortex belt is integrated and developed and moved to east, which influences the development and movement of shear lines. The positive vortex center at 850 hPa appears in advance of the snowstorm, which is significant for prediction. The dynamical mechanism of the generation and development of positive vortex belt and its center are influenced by the total vortex sources. Compared with vortex, vortex variability could reflect accurately and well ahead  the physical process of generation and development of snowstorm shear line. The vertical transportation of vortex and divergence effect of absolute vortex contribute significantly to the positive vortex variability, and the divergence effect of absolute vortex is the primary forcing source of the positive vortex variability. The decreasing positive vortex variability is mainly from twisting factor, which suppresses the development of snow.

Key words: Shear line, Vortex, Vortex variability, Divergence, Advection