Evaluation of WRF-based Convection-Permitting Ensemble Forecasts for an Extreme Rainfall Event in East China during the Meiyu Season

This study focuses on an extreme rainfall event in East China during the Meiyu season, in which the capital city (Nanjing) of Jiangsu Province experienced a maximum 14-hour rainfall accumulation of 209.6 mm and a peak hourly rainfall of 118.8 mm. The performance of two sets of convection-permitting forecast systems (CEFSs), each with 30 members and a 3-km horizontal grid spacing, is evaluated. The CEFS_ICBCs, using multiple initial and boundary conditions (ICs and BCs), and the CEFS_ICBCsPhys, which incorporates both multi-physics schemes and ICs/BCs, are compared to the CMA-REPS with a coarser 10-km grid spacing. The two CEFSs demonstrate more uniform rank histograms and lower Brier scores (with higher resolution), improving precipitation intensity predictions and providing more reliable probability forecasts, although they overestimate precipitation over Mt. Dabie. It is challenging for the CEFSs to capture the evolution of mesoscale rainstorms, which is related to the errors in predicting the southwesterly low-level winds. Sensitivity experiments reveal that the microphysics and radiation schemes introduce large uncertainty in predicting the intensity and location of heavy rainfall in and near Nanjing and Mt. Dabie, while the ACM2 planetary boundary layer scheme combined with the Pleim-Xiu surface layer scheme tends to produce a biased northeastward-extending boundary-layer jet, contributing to the northeastward bias of heavy precipitation around Nanjing in the CEFS_ICBCs.