Time and Spatial Variations of Heavy Snowfalls in the Japan Sea Coastal Region

[摘要] In Part I of the present study, spatial distributions of daily snowfall (precipitation) over Niigata Prefecture (the Japan Sea coastal side of Japan) are classified into three types (Mountain-, Normal- and Plain-type) in terms of EOF. In Part 2, we examine the large-scale situations in the heavy snowfall period (three days) of these three types, basing on composite analysis.One of the important features found commonly for heavy snowfalls of the three types is that the air in the upper trough is much colder than in moderate/light snowfall cases. The intrusion of very cold upper trough to south over the warm Japan Sea results in formation of thick layer of transformed airmass (mixed layer).M-type heavy snowfalls (heavy snowfalls concentrated in high mountain [-1, 000m] area) occur when a wedge-shaped deep upper trough propagates fast over the Japan Islands, accompanied with a rapidly developing surface depression. For this type of snowfalls, the formation of the thick mixed layer and its orographic lifting (low-level wind is perpendicular to the mountain range) will be essential.P-type snowfalls occur when a cold vortex slowly propagates to east after protruding southward over the Japan Sea. In this case the surface depressiondoes not rapidly develop. The mixed layer is very deep and active cumulus convection develop there. The orographic lifting is not primarily important because the wind is not perpendicular to the mountain ranges.

[学科分类] 大气科学