Mesoscale convective systems (MCSs) occasionally develop over the East China Sea (ECS) in the Baiu frontal zone under both the atmospheric and oceanic influence. The factors that determine their predictability have not been fully understood yet. This study investigates the uncertainties affecting two MCSs observed by research vessels on 19 June 2022 using regional ensemble simulations. These MCSs have contrasting features: the first was triggered by an atmospheric mesoscale disturbance, while the second was induced by the boundary layer destabilization over the warm Kuroshio current.

The first MCS shows high variability in the synoptic-scale uncertainties detected by the breeding ensemble. The best-performing member successfully represents the strong meso-β-scale cyclone and the frontal structure with deep moist layers. The ensemble simulations are less skillful for the second MCS than the first. The enhanced surface turbulent heat flux in the sea surface temperature (SST) frontal zone is found to be significantly correlated to the precipitation due to the second MCS despite the cold bias of SST that is commonly imposed on all members. The dense upper-air information from the vessels significantly improves the representation of the sharp frontal structure associated with the first MCS, but has little impact on the second MCS probably due to the underestimation of the boundary layer moistening. This case study indicates that the predictability of MCSs over the ECS depends on their development mechanisms, and that the incorporation of uncertainties in both the atmosphere and ocean are important for the ensemble forecasting of these MCSs.