Seasonal changes in the cold-air pool formation in a subalpine hollow, central Japan

An observational study has been conducted into the seasonal characteristics of the cold-air pool formation in a hollow in the subalpine area of the Yatsugatake Range during the warm seasons from 1995 through to 1997. The uniqueness of the present study is a detection of seasonal differences in the formation processes based on a 3-year observation. The maximum inversion (maximum difference of nighttime air temperature between the bottom and summit of the hollow) is used to determine the cold-air pool formation, which frequently occurred on more than half of the observational days, and to examine its seasonal variation and intensity. The classification of the type of pressure pattern under which the cold-air pool is formed reveals that the North Pacific anticyclone (Type 5) and migratory anticyclone (Type 3) are closely associated with remarkable cold-air pool formation. The cold-air pool exhibits a seasonal variability and can be categorized into two types: the summer and the autumn types. As for the summer type, cold-air pools are formed gradually, from sunset to midnight, and the intensity of the inversion is relatively weak. This weak inversion may be due to increased downward long-wave radiation, which is inferred from the increased cloud amounts and water vapour content. The high content of water vapour, associated with cloud formation, is advected by the thermally induced winds. On the other hand, the autumn type is characterized by conditions favourable for cold-air pool formation, with fine weather and dry air conditions throughout the day. In brief, cold-air pool formation is governed by both the effects of local humidity and synoptic weather conditions in central Japan.