利用探空、地面加密自动气象站、卫星云图、天气雷达和ECMWF (ERA-interim) 0.25°×0.25°再分析资料,从大气环流、云图和雷达回波特征、锋生及倾斜涡度发展等方面对2013年6月20日发生在甘肃陇东地区的大暴雨天气进行了诊断分析。结果表明:高原东北侧低空切变线、副热带高压及冷空气相互作用下,干冷空气和暖湿空气在中低空交汇产生强锋生,使切变线上垂直涡度、辐合快速发展,触发不稳定能量释放产生强对流,导致大暴雨发生。暴雨出现在TBB<-52℃的强对流云带中,雨强随着冷空气的向南侵入和渗透,由北向南在陇东地区逐步增强,进入四川后开始减弱。雷达强回波区高度<5 km,在0℃层以下具有低质心、暖云高效降水回波的特征。降水回波为南北带状,自南向北移动,不断影响同一地区,具有明显的"列车效应"。锋生强度与切变线强度及暴雨强度的增强、维持和减弱变化基本一致。分析锋生函数表明,在暴雨增强阶段,散度项锋生和形变项锋生起主要作用,形变项锋生作用更显著,倾斜项锋生作用相对较小;在暴雨持续阶段,散度项锋生和形变项锋生作用旗鼓相当,倾斜项起锋消作用。通过全型垂直涡度方程建立垂直涡度变化与锋生的直接联系,从而诊断锋生对天气系统强度的作用。锋生使得湿等熵面倾斜,在锋区前侧产生显著的正垂直涡度变化,切变线得到快速发展。
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