Observation analysis of "5.22" extreme rainfall event in Guilin

Abstract

An extreme heavy rainfall event struck the northern part of the Guilin urban area in the early morning of May 22, 2023, breaking local historical records for both hourly and three-hourly precipitation, resulting in severe urban waterlogging. Analysis was conducted using data from routine observation, ground-based dense automatic stations, Doppler weather radar, and ERA5 reanalysis. The results are as follows. (1) This event occurred on the edge of the subtropical high-pressure system, with the continuous strengthening of the southwest jet stream providing abundant moisture and energy for the extreme rainfall under the influence of the low-level shear line and the southward movement of the surface cold front. (2) The echoes of heavy rainfall evolved into a linear convection, which changed the direction and shape when timely met with the surface cold air. Meanwhile, the continuously developing new individual cells on its western side joined to form a "train effect," leading to extreme rainfall. The echo exhibited characteristics of low core and high efficiency. (3) The timely intrusion of weak cold air intensified convective rainfall and enhanced cold pool outflow, triggering new convection in the warm and moist region ahead of it, which favored the development and persistence of heavy rainfall. (4) During the eastward movement of individual convective cells, they merged through cloud bridges and expanded through cloud development, forming new cloud clusters and rapidly intensifying precipitation. The interaction and merging of convective cells were the main mechanisms for the prolonged and intensified heavy rainfall. (5) Various numerical models underestimated the intensity of this event, with biases in forecasting the rainfall center mainly due to deviations in the timing of forecasting the influence of surface cold air.

Keywords

• extreme heavy rainfall
• cold air
• cell merging
• predictability