A Long-distance Rainstorm Study Of A Typhoon In The Yangtze River Delta Region

An extraordinary heavy rain event caused by Typhoon Mangkhut occurred in the Yangtze River Delta region on 16 September 2018,with the maximum of 24-h accumulated rainfall at a single station reaching 297 mm.However,numerical models and official forecast failed to predict this typhoon remote rainstorm(TRR)accurately.Using a comprehensive data collection including the U.S.National Centers for Environmental Prediction(NCEP)FNL Operational Global Analysis data,the observation data from surface automatic weather stations and sounding stations in China,the precipitation data merged with the observation data in China and the Climate Prediction Center Morphing technique(CMORPH)data,and the Hybrid Single-Particle Lagrangian Integrated Trajectory(HYSPLIT)model,a diagnostic analysis was conducted on the causes of the TRR event.The reasons for the failed prediction of the TRR event by two numerical weather prediction(NWP)models were examined using the European Centre for Medium-Range Weather Forecasts(ECMWF)and the NCEP Global Forecast System(GFS)model outputs.In addition,a mesoscale analysis was performed for factors that were poorly predicted by the NWP models.The results showed that:(1)A large amount of unstable energy accumulated in East China due to muggy environmental conditions before the rainstorm.The coupling of upper-level westerly jet and low-level southerly jet was beneficial to the development of strong convection.There was a positive vorticity center in the rainstorm area connected to the main body of the typhoon,indicating that the typhoon provided dynamic conditions for the rainstorm.The cooling and humidifying effect of dry-cold air saturated the formerly unsaturated wet air,leading to precipitation increasing.Besides,there was a lowertropospheric moisture transport path connecting the typhoon and the rainstorm area,providing abundant moisture for the development of the rainstorm.The typhoon,subtropical high,upperand low-level jets,low-level convergence lines and the intrusion of dry and cold air were the key factors leading to the TRR event.The backward trajectory simulation showed that the moisture mainly originated from the lower troposphere over the Philippine Sea,the southern South China Sea,and the sea south of the Philippines.(2)The NCEP GFS model had forecast errors in the large-scale atmospheric circulation fields,and the forecasted moisture conditions and dynamic conditions were weak.However,the GFS model made better forecasting shills in cold air and local rainstorm than the ECMWF model,the magnitude of the rainstorm is smaller than the observation.The ECMWF model forecasted the large-scale atmospheric circulation situation closer to the observations,and the predicted moisture transport conditions were even more favorable for generating rainstorm than the observations;however,it still failed to predict the rainstorm.Therefore,it was suggested that weak cold air was a main reason for the failed prediction of the TRR event by two NWP models.(3)Low pressure and cold air infiltration near surface at meso-? scale were directly responsible for the generation of local rainstorms.