Atmospheric and oceanic factors on short-term extreme heavy snowfall in Tokachi Plain, Hokkaido, Japan: Case study（北海道十勝平野における顕著な短時間大雪における大気と海洋の要因：事例解析）

抄録

This study investigates atmospheric, cloud, and precipitation processes that intensified snowfall during a short-term heavy snow event in the Tokachi Plain on 3–4 February 2025, focusing on the influence of a marine heatwave (MHW). A case study was conducted using snow profile observations, other observational and analysis data, and numerical simulations. Results showed that the event was caused by three systems: stratiform precipitation from the southeast from the night of 3 to early morning on 4, convective precipitation associated with a north-south oriented linear system in the early morning on 4, and stratiform precipitation from the linear system toward the morning on 4. A cold-air layer formed by nocturnal radiative cooling was maintained by cloud cover, snowfall diabatic cooling, and cold-air advection. Along a coastal front formed south of this layer, supercooled water clouds developed and enhanced snowfall through the seeder–feeder mechanism. This mechanism was particularly effective during the convective phase with intense snowfall, which developed along a convergence line south of Hokkaido under enhanced low-level water vapor inflow. These temporal variations were consistent with the snow grain characteristics observed in the snow profile. Sensitivity experiments indicate that the effects of topography explain about 30% of the precipitation enhancement, and the MHW explains approximately 17%. The MHW strengthened the convergence line and promoted seeder cloud development, while maintaining the cold-air layer and warming and moistening the lower atmosphere favored feeder cloud development. These effects likely intensified seeder–feeder enhancement and caused short-term heavy snowfall in the Tokachi Plain.