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引发2022年7月15日甘肃东部强降水的两个中尺度对流系统触发维持机制的对比分析1

A comparative analysis of the characteristics, triggering and maintenance mechanisms of two mesoscale convective systems causing heavy rainfall in eastern Gansu on July 15, 2022

  • 摘要: 2022年7月15日,甘肃东部在2个中尺度对流系统(Mesoscale Convective System, MCS)影响下,先后导致庆阳市北部出现了突破历史极值的极端特大暴雨和东南部出现了局地大暴雨。本文根据影响时间的先后顺序分别将这两个MCS命名为MCS-A和MCS-B,采用多源观测资料和ERA5再分析数据,对比了其在环境场特征、结构形态、触发维持机制等方面的差异,并构建了甘肃东部两类MCS型暴雨的概念模型。结果表明:(1)MCS-A发生在累积雨量大、持续时间长、降水对流性强且变化平稳、雨带偏北且稳定少动的暖区降水阶段(15日00-12时),而MCS-B则发生在累积雨量小、持续时间短、对流性较弱但变化剧烈、雨带中心偏南且东移迅速的锋面降水阶段(14-20时)。(2)MCS-A的云顶结构呈孤立发展起来的近MCC结构的准椭圆形云团,TBB≤-70℃的冷云中心偏于云体后部,生命史长、移动缓慢。其中低层对流结构显示为准静止后向建立形态,通过列车效应实现对流组织化;MCS-B的云顶结构则是由多个β中尺度MCS合并而来的带状?中尺度持续拉伸型对流系统,冷云中心偏于云体前部,生命史短、移动迅速,其中低层对流结构呈尾随层状形态,前向建立。(3)MCS-A由地面冷池和环境风之间形成的“八”字型辐合线触发后,通过冷池出流与低层垂直风切变相平衡组织化对流、低空急流风速脉动补充水汽和动力机制、对流风暴中平流和传播作用相互抵消等机制而呈准静止状态长时间维持;而MCS-B则由冷锋、地面辐合线、700hPa切变线等多尺度天气系统触发后,通过中低层较强的水汽输送和锋生作用、中高层的冷空气侵入而维持。由于低层垂直风切变较小、对流风暴中平流和传播的方向一致等因素,生命史短、移动迅速。

     

    Abstract: On July 15, 2022, two Meso-Scale Convective(MCS) systems affected the eastern part of Gansu province, causing regional extremely heavy rainfall which breaking the historical extreme value in the northern region and local heavy rainfall in the southeast region of Qingyang City. In this studying, based on multi-source data and ERA5 reanalysis data, the differences in environmental field characteristics , structural morphology , tigger and maintenance mechanism between the two MCSs were named MCS-A and MCS-B according to their chronological order was analyzed. The results show that: (1)MCS-A occured in a warm-zone precipitation phase (00:00-12:00 on the 15th)that was characterized by high cumulative rainfall, long duration, highly convective and smoothly variable precipitation, and a northerly rain band that is stable and less mobile,while MCS-B occured in the frontal precipitation phase (14:00-20:00 ) when the accumulated rainfall was small, short-lasting, weakly convective but drastically varying, and the center of the rain band is southward and rapidly shifting eastward.(2)The cloud top structure of MCS-A was a quasi-elliptical cloud cluster near MCC developed in isolation,and the cold cloud center of TBB≤-70℃ was biased to the rear of the cloud body, with a long life history and slow movement. The convection structure in the lower layer of it was Back-building type and organizes convection through trains effect.The cloud top structure of MCS-B was a persistent elongated convective system derived from the development of multiple MCSs,the cold center tended to the front of the cloud, with a short life history and rapidly movement.The convection structure in the lower layer of it was Trailing Stratiform type .(3)MCS-A was triggered by the formation of a “eight” convergence line between the ground-based cold pool and the ambient winds , which was maintained quasi-stationary for a long period of time through the mechanisms of organizing convection by balancing the outflow from the cold pool with the low-level vertical wind shear, the supplemental water vapor and dynamical mechanisms of the wind speed pulsation in the low-altitude rapids, and the offsetting of advection and propagation effects in the convective storms.MCS-B, on the other hand, was triggered by multi-scale weather systems such as cold fronts, surface convergence lines, and 700hPa shear lines to trigger convective priming, and then sustained by stronger moisture transport and frontal generation in the lower and middle levels, and by the intrusion of cold air in the middle and upper levels. The life history is short and fast-moving due to factors such as small vertical wind shear in the lower levels and consistent direction of advection and propagation in convective storms.

     

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