Abstract: Using conventional and unconventional observational data with high temporal and spatial resolutions,the formation mechanisms of two tornado cases that occurred on May 31 and June 8,2015 in Tongyu County (referred to as "513" Tornado and "608" Tornado) were analyzed in detail.The results showed that the two tornados are both affected by the sub-scale short-wave trough or shear line in front of the Northeast Cold Vortex.The area of the large temperature difference between 700 hPa and 500 hPa moves eastwards due to the strong westerly jet at middle levels of the troposphere,and the area of high moisture obviously stretches northwards due to the low-level southwest jet.Tornados occur at the edge of a moist tongue and the vicinity of overlaying area of the multi-scale systems.The "531" Tornado has larger jet velocity,the temperature difference between 700 hPa and 500 hPa,and the 925 hPa dew point value and stronger intensity than those of the "608" Tornado,because the convergence and ascending motion are stronger due to the shear line and negative variable pressure at 850 hPa.Before the formation of the two tornado events,the atmosphere is characterized by the convective available potential energy (CAPE) larger than 1500 J·kg^-1,a temperature inversion layer at lower levels,the low lifting condensation level (LCL),and a shallow moist layer.The convergence shear line or cold front is the direct trigger mechanism of the two tornados,and the significant convergence of wind speed at both sides of the convenience line during the "531" Tornado event provides strong inflows to enhance a convective storm.Few clear physical characteristics occurred before the formation of the Tornado,such as the large pressure zone with the higher CAPE value,the lower height of LCL,but the thickness of the wet layer is very thin,the shear lines moving slowly to the east which are the direct facts triggering the formation of the tornados.The both sides of the shear line of "531" tonado have wind speed convergence which provides the strong inflow for the development of the convective storm.The "531" Tornado belongs to a supercell tornado under conditions of strong vertical shear of horizontal winds,while It has a clear hook structure,a weak echo area at lower levels and the draping characteristics of echos in the middle and high levels with the echo top height of higher than 11 km.The supper cell crosses the boundary of a shear line and locates at the side of cold air.The horizontal vorticity reached a maximum due to the effect of atmospheric baroclinicity.A strong meso-cyclone occurs at low levels,with its bottom extending to the height of less than 1 km,and exhibits obvious characteristics of tornado vortexes,which is favorable for the formation of tornados.the "608" Tornado is a non-supercell tornado and it is under the conditions of weak vertical wind shear.

Key words: Jet at middle and low levels, Supercell, Non-supercell, Draping structure