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Factors Affecting the Inland and Orographic Enhancement of Sea-Effect Snowfall in the Hokuriku Region of Japan


 The Hokuriku region along the west coast of the Japanese island of Honshu receives exceptionally heavy snowfall accumulations, exceeding 500 cm from December to February near sea level and 1300 cm at high elevation sites, much of which is produced by sea-effect systems. Though the climatological enhancement of snowfall is large, the lowland–upland snowfall distribution within individual storms is highly variable, presenting a challenge for weather forecasting and climate projections. Utilizing data from a C-band surveillance radar, the ERA5 reanalysis, and surface precipitation observations, we examine factors affecting the inland and orographic enhancement during sea-effect periods in the Hokuriku region during nine winters (December–February) from December 2007 to February 2016. The distribution and intensity of precipitation exhibits strong dependence on flow direction due to three-dimensional terrain effects. For a given flow direction, higher values of boundary layer wind speed and sea-induced CAPE favor higher precipitation rates, a maximum displaced farther inland and higher in elevation, and a larger ratio of upland to lowland precipitation. These characteristics are also well represented by the nondimensional mountain height H^, with H^<1 associated with a precipitation maximum over the high elevations and a larger ratio of upland to lowland precipitation, and H^>1 having the opposite effect. Nevertheless, even in high enhancement periods, precipitation rates decline as one moves inland from the first major mountain barrier, even over high terrain. These results highlight how the interplay between sea-effect and orographic processes modulates the distribution and intensity of precipitation in an area of complex and formidable topography.