Orographic Influences on the Formation and Maintenance of a Japan Sea Polar Airmass Convergence Zone (JPCZ)

The Sea of Japan (SOJ) coast and adjoining orography of central Honshu, Japan receive substantial snowfall each winter. A frequent contributor during cold-air outbreaks (CAOs) is the Japan Sea Polar-Airmass Convergence Zone (JPCZ), which forms downstream of the Korean Highlands, extends southeastward to Honshu, and generates a mesoscale band of precipitation. Mesoscale polar vortices (MPVs) ranging in horizontal scale from tens of kilometers (i.e., meso-β-scale cyclones) to several hundred kilometers (i.e., “polar lows”) are also common during CAOs and often interact with the JPCZ. Here we use satellite data and numerical Weather Research and Forecast (WRF) simulations to examine the formation and evolution of an MPV and subsequent JPCZ during a 2–7 February 2018 CAO. WRF diagnostics reveal how conditions in the locally warm, convergent, and convective environment in the lee of the Korean Highlands are conducive to MPV formation and that the developing JPCZ is continuous with a thermal gradient in the MPV’s wake. Air parcel trajectories depict how airstreams channeling through low passes and valleys, combined with terrain-induced convergence in the lee of the Korean Highlands, form the JPCZ and that the Korean Highlands’ influence is less apparent as the JPCZ approaches the Japanese SOJ coast. Secondary circulations across the JPCZ are similar to secondary circulations found in long-lake-axis parallel snowbands over elongated lakes and are more symmetric (asymmetric) when a cross-JPCZ thermal gradient does not (does) exist. These results provide new insights into how thermal boundaries, the Korean Highlands, and other geographic features influence the development and maintenance of the JPCZ.