Analysis on Elevated Thunderstorm in Shaanxi in Early Winter of 2016
Based on conventional observation data, Xi’an and Ankang Doppler weather radar observation data and ECMWF numerical model data (0.25°×0.25°), an elevated thunderstorm process in Shaanxi in early winter of 2016 is analyzed. The results show that the thunderstorm area in the center and south of Shaanxi Province was located in the area of 350-500 km after surface cold front. Deep cold air cushion existed under 3 km in thunderstorm area. At the same time, there was temperature inversion in middlelow level and absolute stability of atmospheric stratification in low level. All these make clear that the thunderstorm was an elevated one. Through analyzing moist potential vorticity, saturated pseudoequivalent potential temperature, pseudoequivalent potential temperature and absolute vorticity, we found that the unstable mechanism is different in different regions.Unstable mechanism for Xi’an Region is conditional symmetric instability while unstable mechanism for Ankang Region is conditional instability. In conditional symmetric instability area there are a number of parallel snowfall echoes, parallel with vertical wind shear vector from 0 to 6 km. In conditional instability area there are small scale echoes. The strong vertical wind shear indicates that the atmospheric baroclinic is strong. Warm moist air flows between middlehigh layers enhance atmospheric baroclinicity, thus producing conditional symmetric instability which results in slantwise updraft. Southerly airflow in the lowmiddle level convey warm advections and water vapor which makes the atmosphere moister and warmer, making the atmosphere moister and warmer, but highlevel temperature advection is weak and the atmosphere is relativerly dry. So potential instability is formed. When the front surface uplift warm moist air makes it saturated, potential instability changes into conditional instability, resulting in vertical convection. Thus, there is a good correspondence between the instability and the ascending motion and the echo height.