Study of Northeast China torandoes:The environmental characteristics

The Northeast region of China is one of the favorable regions for tornadoes. In order to improve the understanding of the environment favorable for tornado within the Northeast region of China, the environmental elements of the 13 tornadoes of this region which occurred in the past decade are investigated, and the physical processes that dominate the change of the environmental condition are investigated. The study is based on the conventional data, Automatic weather station (AWS) data, satellite data and numerical model analysis data. The environmental characteristics of Northeast China tornadoes are as follows. (1) Tornadoes occurred around the northeast cold vortex, directly affected by the sub-scale short-wave trough at the south part of cold vortex, and often occurred in the trough or behind the forward tilt trough. (2) Compared with tornadoes occurred at summer over the Yangtze-Huaihe river basin and South China, the lapse rate is greater for Northeast China tornadoes, with the temperature difference between 700 hPa and 500 hPa being 20-22 ℃, and the difference between 850 hPa and 500 hPa being 30-33 ℃. (3) The low level moisture is often not significant with the moist depth thin, the surface dew point as low as 13 ℃, the moisture layer often less than 1.5 km, and the dew point at 850 hPa often below 8 ℃. And (4) the low-level are and middle troposphere jet are strong, 850-925 hPa wind speed is 16-20 m/s, 500 hPa wind speed is 20-25 m/s, and the middle level jet is usually accompanied by dry downdraft. Therefore, the low and deep vertical wind shear are strong, the low-level wind shear is about 12.0×10-3s-1, and the deep vertical wind shear is greater than 4.0×10-3s-1. Convective storms which produce tornadoes are usually triggered by the boundary convergence line, in which the temperature difference near the convergence line is not obvious, but the dew point difference is distinct. That's to say, northeast China tornadoes are mainly triggered by strong dryline and its associated convergence. Tornadoes usually occur around evening, and the environmental conditions at the morning usually are not favorable for tornadoes, with the environmental parameters changing significantly a few hours before tornadoes occur. The 500 hPa jet and the low-level strong jet are crucial environmental conditions to Northeast China tornadoes. With the strengthening of 500 hPa northwestly air flow, it transports the high lapse rate layer to its east,and overlaps to the low level moisture area. The low level jet transports warm moisture air, which humidifies the atmosphere significantly and enhances the temperature lapse rate. The tornado positions are usually in the dry section at morning as shown by the 08:00 sounding data, which are away from low level moist tongue about 100 km at its northwestern side, whereas tornadoes mostly occur near the moist tongue around evening as a result of the moist-air advection northwest and the middle level high lapse rate air advection eastward from the Da-xing'an-ling Mountain.