ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
Environmental Characteristics and Trigger Mechanism of Severe Convection Under the Influence of Subtropical High in Guanzhong Area of Shaanxi Province
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Abstract:
Using conventional observation data, Doppler radar data of Xi’an Station, surface densely observed weather data and ERA5 reanalysis data (0.25°×0.25°), the environmental conditions and triggering mechanism of four severe convection weather processes in Guanzhong Area of Shaanxi Province, under the influence of subtropical high from 2015 to 2018 are comparatively analyzed. These four cases resulted in local geological disasters and urban flooding, but the global model failed to forecast the short-term rainstorms. 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 and the atmospheric precipitable water can be more than 50 mm. Relative to the systemic rainstorm that occurs in the peripheral of the subtropical high, the water vapor transmission in such weather is relatively weak. The strength of the water vapor transmission determines the total rainfall in the severe convection process. The convergence center is located at the height from the surface to 800 hPa. Compared with the systemic rainstorm, the convergence center position is lower. The main role of low-level convergence is to overcome convection suppression, and to release unstable energy. Favorable environmental conditions for the occurrence of severe convection including intense geopotential unstable layer, low level of free convection, medium intensity of convective available potential energy and thick warm clouds. The triggering mechanism is mainly cold front and low-level wind convergence line. When there is a cold front in the upwind, the cooling effect of precipitation behind the cold front increases the temperature gradient to enhance the cold front, and cold air in the lower layer invades the atmosphere with high temperature and high humidity controlled by subtropical high, triggering severe convection. The convection in warm sectors is usually triggered by ground convergence line. The convection induced cold pools can form gusty front and trigger new convection. The terrain of Qinling Mountains has a significant impact on the movement and spread of the severe convection. The convection that occurs in north of Guanzhong Area and moves southward can easily cause rainstorm because of the rain belt blocked by Qinling Mountains. The new convection along the terrain convergence line in the northern Qinling Mountains 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, which is easier to produce large-scale thunderstorm and gale weather.
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