Verification and error analysis of regional ensemble forecasts for the torrential rain on 20 July 2021 in Henan province
In July 2021,Henan province experienced widespread,intense,and concentrated extremely heavy rainfall,resulting in significant damage.This paper evaluates the forecast performance of the China Meteorological Administration(CMA)Regional Ensemble Forecast System(CMA-REPS)during the heaviest precipitation period from 1400 to 2000 China Standard Time on 20 July 2021,using the CMA Multisource Precipitation Analysis System(CMPAS)-V2.1 product and the fifth generation European Centre for Medium-Range Weather Forecasts(ECMWF)reanalysis(ERA5).The study findings reveal that shorter lead times yield improved precipitation forecasts in terms of ensemble mean and probabilistic forecasts.However,notable forecast biases persist in precipitation intensity and spatial coverage of heavy rainfall.The best and worst ensemble members were identified by combining several precipitation verification scores.An analysis of atmospheric circulation and water vapor conditions was conducted to explore potential causes of precipitation forecast errors.The study identified that a good ensemble member successfully predicted convective precipitation,accounting for 30％of total precipitation in the Zhengzhou area,whereas a bad member failed to forecast convective precipitation.The overall precipitation distribution in both members aligned with the performance of non-convective precipitation.The precipitation area of the good member shifted northeastward,which is associated with the eastward deviation of the forecasted subtropical high position,the northward track deviation of typhoon"Cempaka",and the stronger south wind.In contrast,the precipitation area of the bad member shifted westward,consistent with the deviation of the maximum relative humidity area,possibly due to a stronger low-level east wind transported by the forecasted typhoon"Infa".At the 925 hPa level,the good member accurately predicted a strong convergence zone on the windward slope of the western mountains near Zhengzhou,causing intense rainfall exceeding 25 mm·(6 h)-1,extending from the foothills to elevations above 800 m.Conversely,the bad member predicted a small and weak convergence zone,leading to heavy rainfall confined to the foothill areas below 600 m.Overall,the forecast errors of CMA-REPS for this heavy rain event are primarily attributed to the simulated deviation of atmospheric circulation and the complex orographic effect.
ensemble forecastthe torrential rain in July 2021 in Henan provinceprobabilistic forecastforecast verification
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