Synoptic-scale water vapor transport associated with regionally concentrated rainfall in Sanyo, western Japan

Abstract In the Sanyo region of western Japan, which is characterized climatologically by low precipitation owing to surrounding mountains, relatively intense rainfall occurs occasionally, leading to floods and landslides that can have considerable socioeconomic consequences. However, the synoptic-scale factors behind the regionally concentrated rainfall in this region have not yet been clarified. In this study, we focused on days when regionally stronger precipitation was observed within the Sanyo region, compared with other areas of western Japan, and investigated the characteristics of the synoptic-scale pressure patterns and moisture transport pathways that contributed to the occurrence of the observed heavy rainfall. By extracting typical pressure patterns using a self-organizing map technique, it was found that the pressure pattern on 50% of the days with localized heavy rainfall was classified as the North Pacific High (NPH) pattern, while that on 37% of the days was classified as the far low-pressure (FL) pattern. Composite analysis revealed that on NPH-pattern days, moisture was transported from the west in the lower-to-middle troposphere along the northwestern edge of the westward-extended North Pacific High. On FL-pattern days, moisture was transported from the south in the lower troposphere on the eastern side of the low-pressure system over the western North Pacific. These synoptic-scale atmospheric circulation patterns differ from the typical patterns that bring widespread heavy rainfall to western Japan and represent unique features that cause relatively intense rainfall in the Sanyo region.