Environment conditions and mesoscale characteristics of an extremely rainstorm event in Xi'an on 3 August 2015

Based on WARMS model, FY2F satellite cloud images, Doppler weather radar data and observations from intensive automatic weather station (AWS), we have analyzed the environment conditions and mesoscale characteristics of an extremely disastrous rainstorm event (hereinafter referred to as "8.3" event) occurred in Xi'an on 3 August 2015. The results indicate that the "8.3" event is characterized by high intensity, strong burst and concentrated precipitation zone, whose major influencing systems are a cold shear line that rapidly moved eastwards and southwards in the mid- and low-level and a surface cold front. The surface shear is relatively weak. Vertically integrated water vapor transportation from south and water vapor convergence are unobvious, which lead to short precipitation duration and small rain area during the "8.3" event. Northerly wind at the rear of the surface cold front encountering the northern Qinling Mountains triggers initial convection, and the cold air intruding from upper level by north route results in the increase of unstable energy. Such combined condition triggers convection and intensive energy release, which in turn induces β-mesoscale convective system and generate extreme rainstorm. Convergence in the low-level, divergence in the upper level, strong vertical upward movement center and no secondary circulation make precipitation duration short and rain area small during the "8.3" event. The rainstorm occurs near the area with great value of brightness temperature gradient, which corresponds with the positive 3-hourly pressure difference center and the south side of shear line intersection point. Strong radar echo is observed to show vertical tower-shape and low centroid height, which is a characteristic of the tropical marine precipitation echo. Under unstable stratification, especially when low layer is super adiabatic, through analysis and judgment of radar data and especially small-scale convection cells with reflectivity factor greater than 60 dBz which is adjacent to but separate from mountain clutter echo, we can issue the early warning of rainstorm occurred in the northern Qinling Mountains ahead of time.