LAUSR

This study focused on a quasi‐stationary convergence zone over the Sea of Japan (Japan Sea polar airmass convergence zone, JPCZ) in early winter and examined how the Changbai Mountains and the Korean Peninsula play an influential role in the formation and maintenance of the JPCZ and associated precipitation amounts in the coastal regions of central and western Japan. We performed numerical simulations with and without modified topography of the Changbai Mountains and the Korean Peninsula using a regional atmospheric model. The results show that the presence of the mountains is largely responsible for the JPCZ formation and usually contributes to the reduction of precipitation in the coastal regions. If specific conditions are satisfied, however, it can trigger heavy precipitation. One condition is the rapid growth of mesoscale eddies off the Sea of Japan coast under strong low‐level wind convergence. The precipitation band involved in the enhanced eddies slowly migrates southward and reaches the coastal regions, causing heavy snowfall. Another condition is the change in background monsoonal flows associated with synoptic‐scale cyclone development due to the orographic effect. The resultant background flows may help sustain a significant amount of precipitation in the coastal regions. Under the prevailing JPCZ, the mountains play two contradictory roles in modulating precipitation amounts in the coastal regions.