Effects of large-scale atmospheric conditions on summer nocturnal cooling in high mountains in central Japan

Abstract

The present study investigated atmospheric conditions producing summer-time nocturnal cooling in high mountain areas of central Japan. The focus was on the cold-air pool observed in a small hollow in the Yatsugatake range during 1997. Differences in cooling mechanisms between the high mountains and lower plains/basins were examined in terms of atmospheric conditions and land-surface environments. The cold-air pool, formed in the hollow immediately after the Baiu rainy season (late July), was as strong as that during autumn. The enhanced cold-air pool during late July was observed when an anticyclone with extremely dry air and weak winds covered central Japan for one week. The dry air mass led to a large deficit in the longwave radiation balance, i. e., an intense radiative cooling. Although other dry air conditions occurred during late August due to migratory anticyclones, they did not produce intense cooling because of strong ambient winds. In low-elevation plains and basins, nocturnal cooling was stronger during late August than during the post-rainy season. This may be due in part to surface moisture conditions: wet soil conditions during the post-rainy season versus drier soils in late summer. In brief, the nocturnal cooling in the high mountains is primarily affected by atmospheric conditions, such as air dryness, cloudiness, and ambient winds. This is because the smaller scale high-elevation valley is exposed to the free atmosphere. In contrast, the land-surface wetness appears to play a major role in cooling in low-elevation plains and basins.