陕西后向传播雷暴统计特征与机理初步研究

A Preliminary Study on Statistical Characteristics and Mechanism of Backward-Propagating Thunderstorms in Shaanxi

通过2012-2016年陕西多普勒雷达产品和观测资料,制定了应用于陕西的后向传播雷暴普查标准,统计分析了5年内48个后向传播雷暴过程的时空分布特征等。结果表明,6-8月为出现雷暴后向传播的主要月份,出现时段集中在14:00-18:00(北京时),地区以陕北和关中北部为主,致灾天气以短时强降水为主;新生回波生命史在1~2 h,中心在10~30 min可发展至最强。按照新回波移动方向变化分为3种类型:沿平流方向前移型、持续向后传播型和稳定少动型,分别选取典型过程进行诊断分析表明:(1)沿平流方向前移型雷暴发生在高空冷涡与低空切变线形成不稳定环境中,地面辐合线与露点锋相互配合是新生雷暴的触发机制,对流运动沿辐合线和露点锋区与偏东平流反向发展造成雷暴后向传播,强环境风平流与弱垂直风切变导致雷暴新生后随平流前移;(2)持续向后传播型中雷暴传播与阵风锋移动有密切关联,地面冷池前沿阵风锋强迫地面辐合线附近暖湿气团抬升产生新雷暴,阵风锋在移动过程中与地面辐合线相交处即为雷暴传播方向;(3)稳定少动型雷暴发生在高空槽随高度前倾结构中,地面辐合线和显著露点锋区触发生成单体风暴,强垂直风切变加强了风暴维持时间,风暴运动具有向露点锋区西北侧不稳定层结发展的后向传播特征,自身传播与偏东平流运动近似抵消造成稳定少动。

Based on Doppler Radar products and observation data in Shaanxi from 2012 to 2016,the standard for the backward-propagating thunderstorm survey in Shaanxi was established.The temporal and spatial distribution characteristics of 48 backward-propagating thunderstorm processes in 5 years were statistically analyzed.It indicates that backward-propagating thunderstorms tend to emerge between 14:00 and 18:00(Beijing Time)from June to August in area of Shanbei and northern Guanzhong.The disasters are mainly short-term heavy rainfall.The new echo survives 1 to 2 hours,while its center can develop to be the strongest at 10 to 30 minutes.According to the moving direction,the new echo pattern can be categorized into three types:Forward-propagation along advection(type 1),continuous backward propagation(type 2)and low movement(type 3).Typical processes are selected for analysis.The results show that type 1 thunderstorm occurs in the unstable environment of upper-level cold vortex and lower-level shear line,which is triggered by coordination of ground convergence line and the dew point front.The convective movement develops along the convergence line and the dew-point front,and eastward advection develops reversely,which forces the thunderstorm propagates backwards.After regeneration,the strong environmental wind and the weak vertical wind shear cause the thunderstorm to move forward with the advection.The propagation of type 2 thunderstorm is closely related to the movement of gust front.The gust front in the front of cold pool on the ground forces warm moist air to ascend near the ground convergence line to generate a new thunderstorm.The thunderstorm propagates along the intersection of the moving gust front and the ground convergence line.Type 3 thunderstorm occurs where the upper-level trough leans forwards.In this structure,the ground convergence line and the significant dew point front trigger the generation of a storm cell,and the strong vertical wind shear strengthens its maintenance.The storm tends to move towards the unstable layer on the northwest side of the dew-point front,which shows characteristics of backward propagation.However,this self-propagation is offset by eastward advection,resulting in stable and low movement.