MULTI-SCALE OBSERVATIONAL CHARACTERISTICS AND EVOLUTION OF THE 22 MAY 2020 EXTREME RAINFALL IN THE GUANGDONG-HONG KONG-MACAO GREATER BAY AREA

LI Zhuofan; WANG Donghai; ZENG Zhilin

doi:10.16032/j.issn.1004-4965.2023.078

Abstract：In the early morning of May 22,2020,an extremely heavy rainfall process occurred in the Guangdong-Hong Kong-Macao Greater Bay Area.The maximum hourly and three-hour rainfall broke the historical record during the"Dragon Boat Water"period in Guangdong province.This paper uses multi-source observation data and ERA5 reanalysis data to study the evolution process and mesoscale characteristics of the mesoscale convective system(MCS)causing the local extreme precipitation.The results are shown as follows.(1)The heavy precipitation process mainly concentrated in the early morning hours,with the characteristics of short duration,extreme local rainfall intensity,and large accumulated rainfall.Two precipitation peaks appeared in the three-hour precipitation process.(2)This process is a torrential rain process in the warm sector.A low-level southwest monsoon provides sufficient warm and humid water vapor,and a low vortex shear line provides good uplifting conditions.The process of meso-α-scale monsoon cloud clusters was quasi-stationary and grew by upscaling into MCC,triggering the local nighttime extreme precipitation in the Greater Bay Area.(3)Two stages of peak precipitation in the process is closely related to the propagation of meso-γ-scale convective cells in the life cycle and the change of the motion vector.Several meso-γ-scale convective cells form a multi-cell storm pattern,which is arranged in the northwest-southeast lateral direction.These cells are triggered to develop successively and have their own ground convergence lines.The first stage of peak precipitation is caused by the convective cell moving to the southeast under the guidance of the ambient wind.Subsequently,the convergence lines of different cells are connected,and the propagation of convection to the southwest is significantly accelerated,which changes the movement vector of the convective system,thus causing the peak precipitation in the second stage.(4)There are obvious differences in the structure of the meso-γ-scale convective cells that cause the two periods of peak precipitation.The strong echoes of the peak precipitation in the first stage have a higher extension height with supercell-like structural characteristics such as inclination,inflow gap,and echo overhang,and the echoes of the second stage of peak precipitation are characterized by convective,low mass cores,causing more extreme and localized short-duration heavy precipitation.