# Cold Air Outbreak

Cold air outbreaks over East Asia are now sorted by which side of Mt. Baitoushan the cold airmass flux passes, not by intensity alone. Using JRA-3Q reanalysis for winters from 1979/80 to 2023/24, researchers computed the flux of cold airmass and applied self-organizing maps to group 2,052 outbreak cases into Western, Northern, and Intermediate groups, then defined a pathway index as the log ratio of flux west and east of the mountain. The index correlates negatively with 925 hPa wind convergence over the Sea of Japan, so the western route favors the Japan-Sea Polar airmass Convergence Zone and heavier coastal snowfall.

Cold air outbreaks over East Asia are now sorted by which side of Mt. Baitoushan the cold airmass flux passes, not by intensity alone. Using JRA-3Q reanalysis for winters from 1979/80 to 2023/24, researchers computed the flux of cold airmass and applied self-organizing maps to group 2,052 outbreak cases into Western, Northern, and Intermediate groups, then defined a pathway index as the log ratio of flux west and east of the mountain. The index correlates negatively with 925 hPa wind convergence over the Sea of Japan, so the western route favors the Japan-Sea Polar airmass Convergence Zone and heavier coastal snowfall.