Qiqing Liu

A heavy rainfall (HR) event caused by a bow echo struck South China on 11 April 2019. Two extremely HR periods were identified within this event, and the second rainfall period led to severe flooding in Shenzhen city, resulting in 11 fatalities. The first rainfall period was dominated by warm‐rain processes, while the development of the second period was closely related to the intensification of ice‐phase processes. The contribution of raindrops from the melting process played a crucial role in the formation of extreme rainfall, which achieved a high rain rate (RR) exceeding 120 mm hr⁻¹. The enhancement of the ice‐phase processes during the second rainfall period was found to be closely associated with the development of a low‐level mesoscale vortex (MV). Due to the complementary non‐linear dynamical accelerations induced by the MV, the vertical velocity within the convective system rapidly intensified, leading to a more upright and deeper convective organization. As a result, more water vapor and supercooled water were lifted above the freezing level, which increased the presence of ice‐phase particles with the potential to melt, subsequently contributing to the extreme high RR. This study investigates the microphysical characteristics of two periods of HR that occurred during and after the development of a MV within a bow echo event, and examines the key microphysical processes affected by the MV, which partially contributed to the second HR period.

Classical bow echoes (CBEs) normally develop due to rear‐inflow jets, while merger‐formation bow echoes (MFBEs) evolve from the merger between linear systems and preline convection. MFBEs are rarely documented before, and the key processes remain unclear because of the small‐scale and fast‐evolving merging processes. Using radar observations during 2015–2019, this study examines the characteristics of these two types of bow echoes over South China. Eighteen MFBEs and 11 CBEs are identified, accounting for 62% and 38% of the total 29 bow echoes, respectively. MFBEs commonly occur over coastal regions in the afternoon, where southerly sea breezes provide favorable thermodynamic conditions. About half CBEs develop over mountainous areas in the morning, and northerly winds enhance the local baroclinity and kinematic convergence. A new Merger‐Classical index is proposed to distinguish the environments for the development of two types of bow echoes, disclosing the more favorable near‐surface thermal conditions for MFBEs. In addition, more severe weather occurs in metropolitan area near the Pearl River Delta region during MFBEs, in which the accumulated precipitation is about 2.5 times higher than that during CBEs. This study reveals the discrepancies between MFBEs and CBEs over South China, emphasizing the MFBEs account for more than half of the total bow echoes and cause more high‐impact weather in the metropolitan area. This study proposes a new perspective on the environment in which bow echo forms by merger process over South China, urging for more research to explore the underlying processes.

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.