Dynamics of Two Episodes of High Winds Produced by an Unusually Long-Lived Quasi-Linear Convective System in South China

Using radar observation and convection-permitting simulation, this work studies the storm-scale dynamics governing the generation of two episodes of high winds by an unusually long-lived quasi-linear convective system (QLCS) in South China on 21 April 2017. The fi rst episode of high winds occurred at the apex of a bowing segment in the southern QLCS due to the downward transport of high momentum by a descending rear-inflow fl ow jet (RIJ). The RIJ was initially elevated which was generated as low-frequency gravity wave response to the thermal forcing in the QLCS leading convective line. It descended to the surface owing to the enhancement of low-level diabatic cooling which strengthened the downdrafts at the RIJ leading edge. The vertical momentum budget revealed that the downdrafts were initiated by the negative buoyancy of the cold pool and strengthened by the weakened buoyancy-induced upward pressure gradient force in the boundary layer and enhanced hydrometeor loading above. The second episode of high winds occurred in the decaying stage of the QLCS which, however, redeveloped as its northern part interacted with an intensifying large-scale shear line to the east. A zonal convective line developed along the shear line and fi nally merged with the QLCS. The merger greatly enhanced the low-level convergence, leading to the downward development of the line-end vortex via vertical stretching of vertical vorticity. The area of high winds was notably increased by the superposition of the ambient translational wind with the vortex rotational fl ow. The f indings provide new insights into the generation of high winds by the QLCS-MCS merger, highlighting the importance of low-level vortices in addition to the RIJ. SIGNIFICANCE STATEMENT: Quasi-linear convective systems (QLCSs) are prolific fi c producers of high winds at the surface. While conceptual models have been established for high winds produced by single QLCSs, this is not the case for the high winds generated by QLCSs merging with other convective systems. This work revealed different dynamics for two episodes of high winds produced by a long-lived QLCS merging with an MCS in South China. The fi rst episode resulted from the descending rear-inflow fl ow jet (RIJ) as in the case of single QLCSs. The second and stronger one was due to the superposition of ambient fl ow with a line-end vortex that developed downward given the enhanced lowlevel convergence by the merger. This fi nding sheds light on the importance of mergers in high-wind generation which has even greater damaging potential than the RIJ.