Dynamics of two episodes of high winds produced by an unusually long-lived quasi-linear convective system in South China

Abstract

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 first 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 jet (RIJ). The RIJ was initially elevated, 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. Vertical momentum budget revealed that the downdrafts were initiated by the negative buoyancy of 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 finally merged with the QLCS. The merger greatly enhanced the low-level convergence, leading to 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 flow. The findings provide new insights into the generation of high winds by QLCS-MCS merger, highlighting the importance of low-level vortices in addition to the RIJ.