Study on Northeast China Torandoes: The Environmental Characteristics

Northeast region of China is one of the favorable regions for tornado. In order to improve the understanding of environment that favorable for tornado within Northeast region of China, the environmental elements of 13 tornadoes of this region which occur in the past decade are investigated , and the physical processes that dominant the change of environmental condition are investigated. The study base on 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)Tornado occurred around the northeast cold vortex, directly affecting by the sub-scale short-wave trough at the south part of cold vortex, and often in the trough or behind the forward tilt trough. 2) Compared with tornado occur at summer Yangtze-Huaihe river basin and South China, the lapse rate is greater for Northeast China tornado, the temperature difference between 700hPa and 500hPa is 20-22 ℃, The temperature difference between 850hPa and 500hPa is 30-33 ℃. 3)the low level is often not significantly moist and the moist depth is thin, the surface dew point as low as 13 ℃, wet layer thickness less often 1.5km, the dew point at 850hPa often below 8 ℃. 4) Low-level jet and middle troposphere jet is strong, 850-925hPa wind speed is 16-20m/s, 500hPa wind speed is 20-25m/s, and middle level Jet is usually accompanied by dry downdraft. Therefore, the low and deep vertical wind shear are strong, low-level wind shear is about 12.0×10-3s^-1, deep vertical wind shear is greater than 4.0×10-3s^-1. Convective storms which produce tornadoes usually triggered by boundary convergence line, the temperature difference near the convergence line is not obvious, but the dew point difference is distinct. That’s to say, northeast China tornado mainly triggered by strong dry line and its associated convergence. Tornadoes usually occur around evening, and the environmental conditions at the morning usually can’t indicate tornadoes, the environmental parameters change significantly a few hours before tornado occur. 500hPa and low-level level strong jet is crucial to the environmental conditions of Northest China tornado. With the strengthening of 500hPa North-west air flow, it transport the high lapse rate layer to east，and overlap to low level moisture area. The low level jet transport warm moisture air, that humidity the atmosphere significantly and increase the temperature lapse rate. The tornado positions are usually in the dry area at morning with the 08:00 sounding data, where away from low level moist tongue about 100km at the North west side, whereas tornadoes mostly occur near the moist tongue around evening as a result of moist-air advection northwestward and middle level high lapse rate air advection Eastward from Da Xing’ an Ling Mountain