In the Nobi Plain, snowfall events were brought about by band-shape snow clouds flowing from Wakasa Bay during cold air outbreaks. Previous studies have shown that these snow clouds are enhanced by the mountainous between Wakasa Bay and the Nobi Plain and also develop over the Nobi Plain. However, the detailed mechanisms of this phenomenon remain unclear. The purpose of this study is to clarify the dynamical and microphysics mechanisms of snowfall enhancement over the Nobi Plain. This study analyzed the result of numerical simulations for three cases in which accumulated snowfall amount exceeded 10 cm at Nagoya City.

In the from December 17 to 18, 2014 and January 16, 2011, cases, lee waves generated by the Suzuka Mountains and Ryohaku Mountains clearly analyzed over the Nobi Plain. In the ascending regions of the lee waves, cloud water is formed, as a result, snow also increased by deposition and riming growth and advected by northwesterly winds with descending over the Nobi Plane, precipitation maxima are formed in the descending regions of the lee waves. The Scorer parameters over the Nobi Plain was analyzed, revealing that the atmospheric environment was favorable for the formation of lee waves up to an altitude of 4 km. To further investigate the effect of lee waves generated by the Suzuka Mountains, sensitivity experiments were conducted by removing the Suzuka Mountains. In these experiments, precipitation in Nagoya decreased, and the updrafts and downdrafts associated with the lee waves generated by the Suzuka Mountains were also weakened. These results indicate that the lee waves caused by the Suzuka Mountains enhanced precipitation in Nagoya by approximately 50%. Sensitivity experiments removing the Suzuka Mountains revealed that the lee waves caused by the Suzuka Mountains enhanced precipitation in Nagoya by approximately half. What was clarified for the first time in this study is the enhancement mechanisms of snowfall events contributed by lee waves in the Nobi Plain, which had not been pointed out in previous studies.

In the February 2, 2012, case, a band-shaped precipitation zone with low-level convergence extending from Wakasa Bay to the Nobi Plain (Wakasa-Nobi Convergence Zone; WNCZ) was analyzed in the simulation results. Relatively warm and moist west- northwesterly winds ascend relatively cold and dry northwesterly winds along the low-level convergence zone. Updrafts along the convergence grew condensates (cloud water, snow), as a result, snowfall enhances in the Nobi Plain. The cold pool in the lower layer on the northeastern side of the WNCZ contributes to the maintenance and intensification of the WNCZ. Sensitivity experiments without cloud microphysical processes and those removing the terrain of the Japanese Islands suggested that the formation of the WNCZ was closely related to the Japan Sea Polar airmass Convergence Zone (JPCZ).