The Environmental Characteristics and Trigger Mechanism of Severe Convection under the influence of Subtropical High in Shaanxi Guanzhong Area
Using conventional observation data, doppler radar data of Xi’an station, ground weather station observation data and ERA5 reanalysis data (0.25*0.25), the environmental conditions and triggering mechanism of four severe convection weathers in Guanzhong area under the influence of subtropical high from 2015 to 2018 were comparative analyzed. These four cases resulting in local geological disasters and urban flooding, however, the global model failed to forecast a short-term rainstorm. Results show that the severe convection in Guanzhong area under the control of subtropical high often coexists with the hot weather. When severe convection occurs, low-level atmospheric water vapor content is large, the atmospheric precipitable water can be more than 50 mm, relative to the systemic rainstorm occurred in the peripheral of the subtropical high, the water vapor transmission is relatively weak, the strength of the water vapor transmission determines the total rainfall in the severe convection process. Convergence center is located on the ground to 800 hPa, compared with the systemic rainstorm, the convergence center position is lower, and the main role of low-level convergence is to overcome convection suppression, release unstable energy. Favorable environmental conditions for the occurrence of severe convection including strong potential unstable layer, low LFC, medium intensity convective available potential energy and thick warm clouds. The trigger mechanism is mainly cold front and low-level wind convergence line, when there is a cold front in the upstream, the cooling effect of precipitation behind the cold front will increase the temperature gradient to enhance the cold front, cold air in the lower layer invades the atmosphere with high temperature and high humidity controlled by subtropical high, triggering severe convection occurs; The convection in warm regions is usually triggered by ground convergence line, convection induced cold pools can form gust front and trigger new convection. Qinling terrain has a significant impact on the movement and spread of the severe convection. The convective that triggers in north of Guanzhong area and move southward can easily cause heavy rain storm because of the rain belt blocked by Qinling. The new convective occurred along the terrain convergence line in the northern Qinling triggers new convective development in Guanzhong area, as the potential energy is converted to kinetic energy during the descending process, the convection system moves faster and the duration of heavy precipitation is shorter, it is easier to produce large-scale thunderstorm and gale weather.