Analysis of the two tornado events during a process of regional torrential rain

A detailed analysis of the two tornado processes on 17 and 19 July 2010 in Henan was made by using a new generation of Doppler radar data, the conventional observations data, the NECP reanalysis data and other information. The results were as follows: (1)the weather background of the tornado occurrence: the two tornadoes occurred during a process of regional torrential and severe torrential rains in Henan, which were impacted by the southwestern flow in the subtropical high edge, the strong divergence areas north of the Tibetan high ridge and on the right rear of the entrance areas of the upper jet stream with the vortex, shear line and jet stream existing in the middle and lower levels.Tornadoes occurred in the southeast quadrant of mesoβcyclones on the ground, about 50 km from the cyclone center. (2) The environment field characteristics of the tornadoes covered: the convective available potential energy greater than 1000 J/kg, unstable atmospheric stratification, the K index greater than 36℃, the SWEAT index of the occurrence of strong tornadoes being about 400, the vertical wind shear for 0-1.5 km being up to 15 m/s, and a very low lifting condensation level that was from 0 to 300 m. (3) The analyzed results of the radar echo and characteristical parameters: two tornadoes both occurred in the βmeso scale spiral rainbands on the southeast side of the vortex with the intensity of the echo band of about 50 dBz and the top from 9 to 12 km, and the tornado was produced by a mini supercell that was in the center of the echo, with a significantly weak echo upward flow zone in the vertical profile and the interface zone between the rear flank downdraft and updraft is a key area which is prone to tornadoes. The γmesoscale cyclone series experinced a evolution process related to three dimensional correlated shear, mesocyclone, tornado vortex signature as shown in the velocity profiles. The mesocyclone occurred earlier about 0.5-1 h than the tornado, which made sense to estimating and warning tornado. The analyses of the mesocyclones (M) and the parameters of tornado vortex signature (TVS) show that, the height of mesocyclone and the bottom of TVS both were lower than 1 km with the height of the bottom of TVS being the same as or slightly lower than that of mesoclyclone: The height of the bottom of the F2 level tornado was < 0.5 km, the height of the top of TVS was generally from 2 to 4 km, the height of the top of mesocyclone was generally from 2 to 3 km; and from the maximum shear value, the maximum shear value of mesocyclone was generally (1.0-4.0)×10-2s-1, the maximum shear value of TVS was generally (2.0-5.0)×10-2s-1, the average height of the maximum shear appearred from 0.8 to 0.9 km, and the maximum shear height of the F2 level tornado was generally 0.5 km. On the comparison of the F1 and F2 tornadoes, the height of the characteristical bottom and top of the F2 tornado were both lower than that of the F1 tornado, and the maximum shear value of the F2 tornado was double than the F1 with the large shear in the lower level able likely to cause severe tornado disasters. According to the characteristics of the mesocyclone and the TVS, the thinking of warning the tornado in the regional torrential rainfall is illustrated. Finally, the reason of the tornadoes occurred during the regional torrential rainfall was discussed.