Topography such as mountains plays an important role in snowfall phenomena and is a key factor responsible for heavy snowfall along mountainous regions on the Sea of Japan side of Japan during the winter monsoon. Numerous studies have investigated the relationship between topography and snowfall; however, the effects of topography on airflow are complex depending on factors such as mountain height and width, wind strength, and atmospheric stability. Consequently, the cloud microphysical processes of snow clouds associated with terrain-induced airflow are still not fully understood.

A recent study conducted hydrometeor observations using a precipitation particle imaging sonde over the Uonuma Hills in southern Niigata Prefecture. In that study, aggregates and graupel were observed above the hills, suggesting in situ particle formation over the hilly terrain. However, the airflow around the hills and the spatial distribution of hydrometeors have not been clarified, and thus the influence of the hills on snowfall formation remains insufficiently understood.

The present study aims to analyze the airflow around the Uonuma Hills and to investigate its effects on the formation and distribution of snow and graupel by conducting numerical simulations using the Cloud Resolving Storm Simulator (CReSS) for a snowfall event that occurred from 15 to 16 January 2025. The control simulation successfully reproduced the event and revealed the presence of updrafts along the windward slope of the hills, from the Shiozawa area to the slopes of the Echigo Mountains. The airflow structures exhibited different characteristics and spatial scales depending on altitude. In the lower layer, persistent updrafts were formed in association with convergence zones along the slope, whereas in the upper layer, wave-like airflow structures were likely excited by large-scale topography such as the Northern Alps and the Echigo Mountains. Analysis of condensate distributions indicated that supercooled cloud droplets were generated by orographic updrafts along the windward slope of the hills. These droplets contributed to the formation of snow and graupel near the hilltops through the Bergeron process and riming. Furthermore, differences in horizontal wind speed led to changes in the distribution of snow and graupel, with stronger winds shifting both formation and fall regions farther downstream. In addition, due to differences in fall velocity between snow and graupel, graupel was suggested to fall relatively farther upwind compared to snow.