A Tornado in South China in May 2015

A severe tornado event near Baoan International Airport of Shenzhen, Guangdong Province on 11 May 2015 is investigated. Based on the routine upper-air, surface automatic weather station (AWS), Doppler radar, wind profile data and NCEP 6-hour analysis data (1°×1°), the environmental condition, structure and evolution are analyzed. The intensity of this tornado belongs to the F1 tornado intensity category. The tornado event occurs in front of 500 hPa trough, warm and moist area ahead of 850 hPa shear lines and the warm section before surface cold front. It is significant that the atmospheric convective instability is strengthened by the low temperature and humidity increasing, and the dry cold air behind 500 hPa trough moving eastward. The calculation of atmospheric convective parameters shows that there is powerful convective available potential energy (CAPE), strong low-level vertical wind shear and abundant water vapor in atmospheric environment before the tornado occurs. The analysis of Doppler radar products also indicates that the storm has a life span lasting about 1 hour, during which its echo top extends the height of nearly 5 km. The tornado initially comes from a quasi-linear convective system along the surface convergence line. The quasi-linear convective system moves slowly down and becomes a massive comma echo, finally develops into hooked echo, and the tornado is detected near the weak echo area. The echo of the strongest center value reaches 62 dBZ. The tornado locates at the edge of the strongest echo gradient region near the weak echo region, which indicates that the strong updraft contributes most to the tornado. The mesocyclone first appears in the middle cell of the storm, beginning at 3 km height and then developing upward and downward. The height of the strong core (no less than 50 dBZ) is below 5 km in the tornado event, making it a low centroid convective system. The mesocyclone always shows cyclonic rotation characteristics from originated to maturity in the radial velocity chart. When the rotational speed increases, the radius of the mesocyclone decreases and the largest vertical vorticity associate with the mesocyclone is 1.2×10^-2 s^-1. The distance separating the strongest inbound and outbound radial velocities (called velocity couplet) is reduced from 8 km to 6 km. The mesocyclone deepens gradually downwards, producing the tornado. When the tornado is underway, strong divergence occurs at the storm top above the tornado. Therefore, in operational work more attention should be paid to the sudden change of echo shape and rapidly developing cyclone vortex.