Convective triggering and organization through colliding outflow boundaries and vortex merger in the lee of a mesoscale mountain ridge

This study investigates the triggering and organization of a mesoscale event in the lee of a mesocale mountain ridge over Eastern China during the field campaign of the Observation, Prediction and Analysis of severe Convection of China (OPACC). Both the radar and surface observations are assimilated into the Variational Doppler Radar Analysis System (VDRAS), developed by NCAR, to obtain the high spatiotemporal kinematic and thermodynamic structure of the convective event which evolves from a squall line to a mesoscale convective vortex. It is found that the triggering of the convection event may have resulted from colliding of three distinct outflow boundaries. The initial collision that leads to subsequent convection initiation occurs between two outflow boundaries originated respectively from an east-west-oriented Mei-front rainband and the leeside convection. After collision, the two cold pools merge into one that becomes larger and stronger. This enhanced cold pool boundary moves southeastward to produce the second collision with the ridge of the mountain at the south and the third collision with the easterly inflow at the east. As a result of both collisions, convection is triggered at both the east and south sides of the enhanced cold pool. A mesoscale vortex is also observed to develop below 3 km in the lee of the mesoscale mountain ridge which intensifies with the newly triggered and developing convection. This mesoscale vortex then merges with a pre-existing mesoscale convective vortex (MCV) embedded within the Meiyu rainband to form a stronger MCV. This enhanced MCV ultimately leads the development and organization of a south-north-oriented squall line. This study provides the unique observations on how the convection is triggered and organized by the interaction of Meiyu-front and the local terrains in East China. Ongoing research is further examining the convective triggering and organization with high-resolution convection-permitting simulations.