The genesis tendency for a sea of clouds to occur at night in the Japanese Alps region derived by surface observation and satellite data

The tendency to form a sea of clouds (SOC) with surface meteorological conditions was observed for 3-year warm periods at the foot of Mt. Yatsugatake by time-lapse camera and meteorological instruments at Fujimi Panorama ski resort, Nagano prefecture. In situ observation revealed large- and small-scale SOCs in the valley. Large-scale SOCs were commonly observed in the early morning, while small-scale SOCs in the eastern valley corresponded with low-level orographic clouds ascending over the slopes of Yatsugatake. An empirical algorithm was developed to detect the occurrence of nocturnal low-level clouds, corresponding to large-scale morning SOCs, using Hiamari-8 images on hourly basis with references to in situ camera observation. SOCs frequently occurred in the large-scale valley or basin in the inland areas in the Japanese Alps region, referred as 12 target areas, and they were infrequent in the coastal areas or high elevations over 2000 m. When we defined days of wide-ranging SOC occurrence, in which SOCs occurred in the half or more of target areas, 67% were associated with a subsidence inversion layer by a synoptic-scale high pressure system. The low-level cloud-top height determined by two camera images at different altitudes almost corresponded with the height of the inversion layer observed by radio-sounding data at Wajima station. We concluded that the synoptic-scale subsidence inversion layer plays an important role in forming large-scale SOCs in the Japanese Alps region in addition to nocturnal radiative cooling conditions.

(Publlished in Japanese on Tenki 68, 2021: https://www.metsoc.jp/tenki/year.php)