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.