Abstract:

 Based on the NCEP grid data, a hail weather process on August 21, 2011 in Shenyang region was simulated and its formation mechanism was analyzed using the automatic observation data collected at the airport, Doppler radar data and satellite image. The results show that the hail weather is caused by mesoscale vortex of cold vortex over northeastern China, so is mesoscale cloud cluster on the satellite cloud images. The 0 ℃ and -20 ℃ isothermal lines are located in 600 hPa and near 450 hPa, respectively, and the thickness between them is about 2 km. Before the hail occurs, the maximum ascending speed of obliquely ascending airflow is more than 20 m·s^-1 and water vapor accumulation region is located in the layer above 0 ℃, which ensures that the water accumulated area consists of super cooled water droplet. The upper dry cold air invades into the warm-wet air of middle-low level, which is important to convective occurrence and development. The MPV1 in the low level provides the convective instability mechanism for strong convection; the MPV2 in the middle level is for symmetric instability mechanism; two unstable mechanisms coexist in the high level and both contribute to the development of convection. The evolution of convective available potential energy accumulation and release with the time is indicative to hail. At 10:30, the echo intensity increases with the height on Doppler radar map. There is southwest low level jet on the velocity map. Both are favorable to the development of convective.

Key words:  Mesoscale convective cloud cluster, Thunderstorm, Low-level wind shear