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

This 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.