Fine Observation Characteristics and Causes of "9·7" Extreme Heavy Rainstorm over Pearl River Delta,China

On 7-8 September 2023,the Pearl River Delta experiences an extremely heavy rainstorm,known as "9·7" extreme rainstorm.Multi-source data are comprehensively utilized,including high-density automatic weather station data,sounding data,wind profiler data,Doppler radar data,high-resolution measurements from FY-4B satellite,and the fifth-generation European Centre for Medium-Range Weather Forecasts（ECMWF） atmospheric reanalysis（ERA5）,to analyze the fine precipitation characteristics and causes of this case.Results indicate that the extremely heavy rainstorm is characterized by area of coverage,wide coverage area,long duration,and substantial rainfall.The extremely heavy rainstorm is caused by the combined interaction of 200 hPa upper-level divergence,the middle-level weak guiding flow,the lower-level southwest monsoon,and the residual vortex of Typhoon Haikui（2311）.It is generated by the long-term horizontal scale of about 100 km banded mesoscale convective complex,with significant train effect and warm cloud precipitation characteristics.The centroid of intense echoes with an intensity greater than 45 dBZ is located below 4 km during the most intense precipitation stage,while intense echoes with an intensity greater than 30 dBZ can last for up to 21 hours in Shenzhen.In terms of raindrop distribution characteristics extreme rainfall is mainly caused by a high density of small and medium-sized raindrops.When the rainfall intensity exceeds 20 mm·h-1,the size of raindrop particles increases,but the numerical concentration significantly decreases.Results in an increase in raindrop size but a decrease in the number of concentrations.The duration,intensity,and area of extreme rainstorms have a strong correlation with the fluctuation of the low-level jet in the boundary layer and the location of the core area of the jet.Heavy rainfall occurs within 1-2 hours after a rapid strengthening of the low-level jet index.After the low-level jet index decreases,the intensity of heavy precipitation diminishes.Variations in the low-level jet and lowlevel jet index have significant implications for heavy rainfall.The prolonged presence of Typhoon Haikui residual vortex in the Pearl River Delta is the synoptic-scale cause of this extremely heavy rainstorm.The residence time of the lingering vortex exceeds 16 hours.During that time,the deep boundary layer lowlevel jet continuously transfers warm water vapor to the lingering vortex.Simultaneously,the water vapor from the western Pac