基于陕西WARMS模式、FY2F卫星云图、多普勒天气雷达和地面加密观测等资料，分析总结了2015年8月3日西安致灾大暴雨过程(以下简称“8.3”大暴雨)的环境条件与中尺度特征。结果表明：该过程强度大、突发性强、降水落区集中，中低层快速东移南压的冷式切变线和地面冷锋是其主要影响系统，地面切变辐合偏弱、整层偏南水汽输送及其辐合不明显是大暴雨持续时间短、范围小的重要原因；地面冷锋后部偏北风遇秦岭北麓地形作用形成初始对流，高层北路冷空气侵入导致不稳定能量增大，二者共同作用触发对流与能量强烈释放，形成β中尺度对流系统，产生大暴雨；低层辐合、高层辐散和垂直上升运动中心偏强而无次级环流，造成暴雨范围小、持续时间短；暴雨区主要位于对流云团云顶亮温(TBB)梯度大值区，与3 h显著正变压中心梯度大值区和切变线交汇点南侧对应；雷达强回波区呈垂直塔状，质心低，属热带海洋型降水回波。在不稳定层结尤其是低层超绝热状态下，加强雷达资料分析研判，跟踪紧邻山地杂波的、孤立的、中心像素点反射率超过60 dBz的小尺度对流单体发展，可提前发布秦岭北麓暴雨预警。%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 east-wards and southwards in the mid-and low-level and a surface cold front. The surface shear is relatively weak. Vertically integrated water va-por 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 convec-tion and intensive energy release, which in turn inducesβ-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 un-stable stratification, especially when low layer is super adiabatic, through analysis and judgment of radar data and especially small-scale con-vection 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.
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