陕西蓝田致灾大暴雨的环境场特征及触发机制分析

Analysis of Environmental Field Characteristics and Trigger Mechanism ofDisaster-Induced Heavy Rain in Lantian of Shaanxi

利用3部相控阵雷达、FY-4A高分辨率卫星资料、ERA5逐小时再分析资料和常规气象观测资料,对2021年8月19日发生在西安蓝田的突发极端大暴雨天气的环境场特征及触发机制进行分析.结果表明:“8·19”暴雨主要有两个强降水时段,期间西安均处在副热带高压边缘,高温高湿且位于假相当位温“上低下高”的位势不稳定区,为强降水的发生发展提供了有利的环境场条件;然而两降水时段的触发机制却存在明显不同,分散性对流降水阶段,底层大气接近超绝热状态,极易触发对流,地面上雷暴高压的冷池出流造成蓝田南部山区的扰动温度梯度增大,动力不稳定增强,触发新生对流,造成蓝田出现局地短时强降水;持续降水发生时,近地层有明显的冷锋锋生,锋前触发的多个中γ尺度雷暴单体发展合并,与秦岭山区触发的沿着山体向东传播的对流云团不断合并加强产生“列车效应”,导致蓝田出现持续强降水.

Using three phased-array radars,FY-4A high-resolution satellite data,ERA5 reanalysis data and conventional meteorological observation data,the environmental field characteristics and triggering mechanism of the short-term heavy rain in Lantian of Xi’an on August 19,2021 are analyzed.Results show that the“8·19”rainstorm mainly has two periods of heavy precipitation,during which Xi’an is on the edge of subtropical high with high temperature and high humidity,and is located in the potential unstable area of false equivalent temperature,which provides favorable environmental field conditions for the occurrence and development of heavy precipitation.However,the trigger mechanism of the two main precipitation periods is obviously different.In the dispersed convective precipitation stage,the underlying atmosphere is close to the superadiabatic state,which is conducive to the triggering of convection,and the cold pool outflow of the thunderstorm high pressure on the ground causes the disturbance temperature gradient and dynamic instability of the southern mountain area of Lantian to increase,constantly triggering new convection,resulting in local short-term heavy precipitation in Lantian.When the continuous heavy precipitation occurs,there is obvious cold front frontogenesis near the ground,and multipleγmesoscale thunderstorms triggered by the front develop and merge,and the convective clouds propagating eastward along the mountains triggered by the Qinling Mountains continue to merge and strengthen to produce“train effect”,which results in the continuous heavy rain in Lantian.