Development of a Polar Mesocyclone and Associated Environmental Characteristics During the Heavy Snowfall Event in Sapporo, Japan, in Early February 2022

摘要

In early February 2022, the Sapporo region, north Japan, experienced record-breaking snowfall. During this period, a mesoscale cyclonic rotation corresponding to a polar mesocyclone (PMC) was observed over the Sea of Japan by a geostationary satellite. This study investigates important processes involved in this heavy snowfall event with attention to the role of the PMC using a 100-member regional ensemble forecast. The initial conditions for the ensemble forecast are obtained by a regional-scale ensemble data assimilation system that assimilates conventional observations only. Several ensemble members successfully predict heavy snowfall in the Sapporo region with an 18 hr lead time. Composite analyses reveal that these better ensemble members predict that a southward-moving PMC with a precipitation system develops from small vortices over the ocean and approaches to the Sapporo region, whereas worse members instead predict precipitation systems shifted to the north of the Sapporo region. Further composite analyses demonstrate that the better ensemble members have environmental characteristics such as a deeper and colder upper-level trough and large low-level baroclinicity, which are favorable for the development of PMCs. In particular, the latter is associated with reverse shear in which PMCs tend to move southward. Therefore, these environmental factors are key to predicting the heavy snowfall event in Sapporo associated with the PMC.