Abstract
The distribution of nocturnal cooling above snow was observed over a wide area on the plateaus with shallow valleys gently sloping from the Shikotsu Caldera to the Ishikari-Tomakomai Lowland in Hokkaido Island (Fig. 1), and the mechanisms of radiative cooling were induced in a shallow valley and on a plateau by comparing the temperature distribution with the cooling process at three stations.
The same extent and simultaneous cooling occurred everywhere in the valley bottom B as shown in Fig. 2. This fact proved that so-called cold air inflow played little role in nocturnal radiative cooling, though weak down valley winds were observed. The same conclusion (Tanaka et al., 1983) was induced because the cooling process went on simultaneously in a valley and on a flat plateau on clear and calm night.
Along the gently sloped plateau A on clear and calm night, the air temperature increased by 1°C with increasing altitude by 20m as shown in Fig. 3.
The potential temperatures at the lower part of the plateau A were close upon those at the bottom of the valley, which were estimated at those of extreme cooling in this area on clear and calm night.
The potential temperature at the higher points on the plateau A was close upon that at the top of inversion layer on the lowland as shown in Fig. 5. These temperature profiles suggest that because the depth of inversion layer decreased with increasing altitude of the plateau, the cooling rate of the surface layer decelerated.
