Atmospheric Rivers landfalling in Japan: Climatology and physical characteristics causing heavy rainfall

Abstract. This study investigates the climatology and physical factors governing the precipitation efficiency of Atmospheric Rivers (ARs) landfalling in Japan. Using an ERA5 reanalysis-based AR database (1940–2023), we identified typical synoptic patterns of landfalling ARs via Self-Organizing Maps, which effectively categorize the regional moisture transport pathways. Climatological analysis revealed a significant increasing trend in AR frequency specifically in northern Japan. We further examined the relationship between AR characteristics and rainfall using nationwide high-resolution observations. While Integrated Water Vapor Transport (IVT) explains a substantial portion of the overall relationship (R = 0.71), considerable variability in precipitation amounts remains for similar IVT levels. Our analysis demonstrates that rainfall intensity is primarily modulated by a combination of strong moisture convergence (MVIMC), low convective inhibition (CIN), and orographic enhancement over high elevations. Furthermore, precipitable water (PW) emerged as the critical differentiator for the formation of quasi-stationary linear rainbands (QSLRBs), which consistently develop in close proximity to the AR axis. These findings suggest that the synoptic-scale AR provides the necessary environmental conditions for the organization of mesoscale extremes. Enhancing the predictive accuracy of AR landfall location and internal structure is thus a crucial prerequisite for improving the predictability of catastrophic localized rainfall in East Asia.