Hailstorms Characteristics in Doppler Radar Radial Velocity Fields

In severe hailstorms, hailstones always falls accompanied with some other weather disasters such as thunder, strong wind and rainstorm. Generally speaking, different kinds of strong convective weather bring damage and even social influences to different extents. Beijing is one of the areas that hail occurs frequently and suffers from previous damage every year. With the widely deployment of Doppler radars all over the country in recent years, more studies on the local convective weather systems including hails are gradually put into practice. It is well-known that the hail formation relates closely to various atmospheric physical conditions. The formation of those large hailstones to a great extent relies on the changes of wind structure which always has a storm scale. Among the 32 hailstorms occurred in 2001 and 2002 of Beijing, four kinds of weather phenomena are categorized according to their surface observation including hailfall together with wind and rain, hailfall with wind only, hailfall with rain only, and the others. The radar image characteristics and the Doppler radar radial velocity fields from these 32 hailstorms observed in Beijing are discussed in detail. Statistical analysis shows that the characteristics of "gale region" and "mesocyclone" can always be identified from the radar echo images. The "gale region" is an area which displays a maximum Doppler radar radial velocity along the wind direction in radar PPI (V) echoes. The radial velocity is more than 20 m/s. On the front edge of the wind, the velocity grads is so great that the wind direction changes abruptly. The "gale region" is a downwards airflow related to cold air, often stimulates squall line and falls into the category of β mesoscale synoptic system. Observed from radar PPI (V) echoes, the "mesocyclone" is a cyclonic convergence or rotating area in the Doppler radar radial velocity fields. It is easy to be identified from continuous animated radar echoes. Different air flows converge together and form such a special cyclone. With a small scale and short lifetime, the "mesocyclone" belongs to γ mesoscale synoptic system where weather disaster bursts out very easily. In terms of the statistical analysis, the "gale region" observed in the Doppler radar radial velocity fields usually appears with hailfall together with strong wind, while the "mesocyclone" generally indicates the occurrence of heavy rain besides the hailfall. Though the physical pro-cess of hail formation is very complicated and also there are many other scientific hypothesis on the mechanism of hail formation, the characteristics of "gale region" and "mesocyclone" observed in the Doppler radar radial velocity fields indicate the dynamic structure of the atmospheric stream field is in favor of the occurrence of those previous mentioned convective phenomena. And the study can contribute to the forecast of weather phenomena probably occurring in the detected hailstorm. Combined with other observations, it can improve the ability of the nowcasting for the severe convective disastrous weather too.