Impact of Synoptic‐Scale Factors on Rainfall Forecast in Different Stages of a Persistent Heavy Rainfall Event in South China

Abstract

Abstract This study investigates the stage‐dependent rainfall forecast skills and the associated synoptic‐scale features in a persistent heavy rainfall event in south China, Guangdong Province, during 29–31 March 2014, using operational global ensemble forecasts from the European Centre for Medium‐Range Weather Forecasts. This persistent rainfall was divided into two stages with a better precipitation forecast skill in Stage 2 (S2) than Stage 1 (S1) although S2 had a longer lead time. Using ensemble‐based sensitivity analysis, key synoptic‐scale factors that affected the rainfall were diagnosed by correlating the accumulated precipitation of each stage to atmospheric state variables in the middle of respective stage. The precipitation in both stages was found to be significantly correlated with midlevel trough, low‐level vortex, and particularly the low‐level jet on the southeast flank of the vortex and its associated moisture transport. The rainfall forecast skill was mainly determined by the forecast accuracy in the location of the low‐level jet, which was possibly related to the different juxtapositions between the direction of the movement of the low‐level vortex and the orientation of the low‐level jet. The uncertainty in rainfall forecast in S1 was mainly from the location uncertainty of the low‐level jet, while the uncertainty in rainfall forecast in S2 was mainly from the width uncertainty of the low‐level jet with the relatively accurate location of the low‐level jet.