Abstract
At clear, calm nights, the strongly stable layer called cold air lake is formed in the bottom of the basin and at the lower part of the slope. In order to make clear the processes of the formation of cold air lake and the relation between cold air lake and local circulations including cold air drainage, observations were carried out in the Sugadaira basin and on the slope of Mt. Ohmatsu in Nagano Prefecture, Japan, from May 6 to 10, 1982. During the observation period, the self-recording bimetal thermometers were set at 8 points in the study area. Wind directions and velocities were recorded at 2 points in the same area. Simultaneously, the vertical wind and air temperature distributions were observed by means of the tethered balloons up to 200m height above the ground (Fig. 1).
The results of the observations are summarized as follows:
1. At night, three sub-layers were distinguished above the bottom of the basin. The lowest sub-layer is thermally stable, in which wind direction is along short axis of the basin and its velocity is less than 0.0-0.5m/s. The middle sub-layer is also stable and the winds formed on the surrounding slopes blow into it, when the daily minimum temperature occurs in the basin. The third sub-layer is almost neutral and the general wind prevails. In this paper, the lower and middle sub-layers are defined as cold air lake (Fig. 5).
2. After sunset, wind becomes weaker than in the daytime and its direction becomes along the short axis of the basin in the air layer near the ground in the bottom of the basin and at the lower part of the slope. The air drainagee occurs at the upper part of the slope. Under these conditions, the cold air lake is formed in the basin within two hours after sunset (Figs. 2, 3, 4, 5 and 6).
3. The cold air lake has its own wind systems and circulations. The general wind has an effect on the height of the cold air lake but little effect on the inside of it. So the time variation of the air temperature (potential temperature) in the upper sub-layer is almost constant, but in the inside of the cold air lake, it decreases by 7_??_10°C (Fig. 5) . 4. In the period when the general, synoptic-scale wind is weak in the upper sub-layer, the thickness of the cold air lake reaches up to 80_??_90m. This height is about 1/2 of the ridges surrounding the basin (Fig. 5).
5. When the daily minimum temperature occurs, the difference of the air temperature (potential temperature) between the top of the cold air lake and the bottom of the basin becomes 7_??_12°C (Fig. 5).
6. When the general, synoptic-scale wind becomes strong occasionally, the thickness of the cold air lake becomes low down to 20_??_30 m and air temperature rises on the: slope. This regime is named “break” in most cases, which continues 2_??_3 hours and commonly observed at clear, calm nights. During the break, air temperature continues to decrease in the cold air lake (Fig. 5).
7. When cold air drainage flows down on the slope, air temperature decreases only by 3_??_4°C. So the difference of air temperature between the slope and the bottom of the basin reaches almost 5_??_7°C (Figs. 4 and 6).
8. When cold air drainage flows down on the slope, the fluctuations of wind velocity appear as surges with periods ranging 40_??_50 minutes and about 20 minutes, and air temperature 50_??_60 minutes, 30_??_40 minutes and about 20 minutes (Fig. 7).
