Microphysical Evolution of Heavy Rainfall During a Bow Echo Event in South China: Characteristics and the Mesovortex-Related Impacts

Abstract

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.