Evolution Characteristics Of The Cloud-to-ground Flash During Two Squall Line Processes In East China

The triggering mechanism of the squall line system has great regional and environmental differences, and there are great differences in the evolution and spatial distribution of the cloud-to-ground (CG) lightning activity in different squall line systems. In this paper, the synoptic background and the characteristics of atmospheric stratification in the two squall line process are discussed by using the synoptic system configuration analysis chart and temperature logarithmic pressure chart. Based on the national lightning location data, the temporal and spatial distribution characteristics of the different polarity CG flash in the radar scanning region during two squall line processes are analyzed. Combined with radar echo data,the relationship between the CG flash activity and the evolution of the convective cell in the two squall line process is discussed, the distribution of the CG flash activity and radar echo is explored. The results are as follows:(1) The results show that the occurrence of the squall line depends on the unstable atmospheric environment, sustaining high humidity in lower layer and low humidity in high layer. The inversion layer of low layer prevents the vertical exchange of heat and water vapor on the ground and high altitude, which is beneficial to the accumulation of unstable energy. Jet in front of the upper trough brought plenty of moisture and heat. The middle and lower shear line, the main line and the mesoscale low pressure convergence system provide favorable conditions for the generation of strong convection. These weather conditions are conducive to the occurrence of hail, thunderstorms, strong rainfall and other severe convective weather in Jiangsu Province.(2) The negative cloud-to-ground (-CG) flash is predominant at the squall line process, the change frequency basically reflects the evolution of strong convection.The positive cloud-to-ground (+CG) flash and -CG flash changes in the mature stage of squall line is relatively consistent, there is a difference in its dissipation phase.Hail activity can make the frequency of the CG flash surge and fall. In the process of the evolution of the CG flash activity, there exists a regular variation of the period about 40?60 min, which is related to the merger and division of the multi monomer structure in the squall line system.(3) The spatial distribution of positive and negative ground lightning generally has two kinds, one is the 'dipole' type, namely -CG flashes are generally concentrated in the intense echo region with high reflectivity (45 dBZ?55 dBZ),while the occurrence of the +CG flash is more inclined to the echo region with reflectivity (30 dBZ?45 dBZ) at the periphery of strong convective centers. Another one showed the opposite polarity to the spatial distribution characteristic of the CG flash, namely +CG flashes are mainly distributed in the center of the strong convection and nearby (more than 40dBZ). The -CG flash requires a relatively weaker convective intensity than that of the positive one. When the convection is weakened, the distribution of +CG and -CG flashes is consistent.(4) CG flashes in the strong convective system are mainly focus on the area of echo top height, where breaks through the height of -20? layer. The height of rich electric ice particles, the larger contribution of supercooled water droplets and graupel,and the frequency of the CG flash is positively related to the total echo area(echo top height about 9km?10km).(5) CG flashes are concentrated in the area of 40dBZ echo top height, where breaks through the height of 0? layer, especially -10?. When the echo intensity is more than 55 dBZ, the internal CG flash of the monomer is less, and there is no -CG flash in the location of the +CG flash at the same time. In a strong convective system of the spatial distribution of the CG flash ('dipole' type), along with the weakening of convective systems, the -CG flash tends to occur in the strong echo center, while the+CG flash mainly distributes in the transition zone between the echo monomers and requires a relatively weaker convective intensity than that of the negative one. The other is vice versa. The strong convective system in opposite polarity to the spatial distribution of the CG flash, formed echo pendency at one side of reflectivity echo wall, this side is not prone to lightning, while the other side is more prone to lightning, and the CG flash near the front of the hail area is relatively concentrated.