Line-shaped Mesoscale Convective Systems in Northern Japan: Climatology and Predictive Diagnostics

Abstract This study presented a statistical analysis of line-shaped mesoscale convective systems (LS-MCSs, called “Senjo-Kousuitai”, Kato 2020) in northern Japan using a 64-year high-resolution reanalysis dataset. A significant increase in LS-MCSs frequency and rainfall intensity is identified, particularly along the Pacific side, suggesting a growing risk of extreme precipitation in northern Japan. The synoptic-scale circulation patterns exhibited a strong correspondence with the spatial orientation of LS-MCSs. Pacific-side events were frequently associated with tropical cyclones, whereas the Sea of Japan side events were linked to quasi-stationary frontal systems. An environmental diagnostic analysis, based on the recently-proposed criteria for LS-MCS occurrence (so-called 6-conditions) revealed that approximately 60% of the events satisfy all conditions at the time of initiation. Upper-level moistening and ascent meet thresholds before initiation, while low-level moisture transport and convective inhibition become increasingly satisfied their criteria toward the onset. The results also demonstrate sensitivities to threshold values. Composite analysis further indicates that moisture flux convergence and upstream disturbances, such as mesoscale low-pressure systems, contribute to LS-MCSs organization. Overall, this study provides new insights into the climatology and environmental mechanisms of LS-MCSs in northern Japan, contributing to improved understanding and prediction of extreme rainfall events.