A Merger‐Formation Bow Echo Caused by Low‐Level Mesovortex in South China

Abstract

AbstractThis work studied a high winds‐producing bow echo that occurred over South China in the pre‐rainy season using radar observations and high‐resolution analyses from the Variational Doppler Radar Analysis System. The bow echo developed from a quasi‐linear convective system (QLCS) and acquired a well‐defined bow shape after merging with a pre‐line convective cell (CC). Interestingly, the rear‐inflow jet (RIJ), which has been well recognized to play a key role in the formation of a bow echo, was not found in this merger‐formation bow echo (MFBE). The reason is that the QLCS developed in the monsoon environment of high humidity and weak vertical wind shear, leading to a weak cold pool and line‐end vortices. A new pathway of bow echo formation was proposed which highlighted the importance of the low‐level mesovortex (MV) on the leading edge of the QLCS. Vertical vorticity budget analyses revealed that the MV originated from a weak vertical vorticity band ahead of the QLCS and grew rapidly during the merger, because of the enhanced low‐level convergence in between which led to vortexgenesis via vertical stretching of vertical vorticity. The up‐scale growth of the MV produced a RIJ‐like flow wrapping cyclonically from north of the QLCS which forced the system to bulge out and finally evolve into a bow echo. This MV contributed foremost to the near‐surface gales as well. These findings shed light on the diverse formation mechanisms of bow echoes and associated high winds in the moist monsoon environment of South China.