Analysis on the Three-Dimensional Structure and Formation Mechanism of a Severe Hailstorm with Single- and Dual-Doppler Radar Data

Using the data of radar reflectivity, radial velocity from a single Doppler radar and three-dimensional wind data retrieved from dual-Doppler radar, the formation, development, and maintenance mechanisms of a severe hailstorm were analyzed. The results showed that the hailstorm formed on the right side of an old disordered multi-cell storm and moved rightward, with moderate or higher vertical wind shear and large convective available potential energy (CAPE). In the old disordered multi-cell storm and at the initial stage of this hailstorm, negative radial velocity (toward the radar) was dominant. With the development of the hailstorm, positive radial velocity (outward the radar) appeared and increased rapidly, especially after the occurrence of small-size hail. When the two airmasses, showing positive and negative radial velocities, respectively, met near the strong echo, strong convergence formed and promoted the development and maintenance of the hailstorm. At the low level of the old multi-cell storm, the wind convergence was mainly caused by the weak wind direction change from northeast-southeast to northwest-southwest. The convergence in the severe hailstorm was formed by the northeast and southwest airmasses. A semicircular anticyclonic circulation formed at the lower level between the hailstorm and thunderstorms which were close to the hailstorm. Vertical wind field structure showed that the sinking divergent flow, which formed as a result of the combination of high-level divergent flow in the hailstorm and the high-level flow in the nearby thunderstorm cell, was the main reason for northerly inflow in the hailstorm. The interaction between high- and low-level wind in the hailstorm and the nearby thunderstorms was the cause that the hailstorm formed, developed, and maintained. 