FORMATION OF COLD AIR LAKE IN SUGADAIRA BASIIN

Abstract

On a clear and calm night, a cold air lake is formed in such a basin as Sugadaira, mainly by radiative cooling. In order to identify the formation process of a cold air lake, we made observations from the evening of 8, May, 1981, at the Sugadaira basin in Nagano Prefecture. We used seventeen sets of the self recording bimetal thermometers to observe the distribution of temperature in the basin. Vertical temperature distributions were observed by utilizing the captive baloons with thermistor thermometers. These baloons were moved up to 100 meters height above the ground three times in this night at three points, two of these were located on the northeast slope of Mt. Omatsu and the other one was located at the bottom of the basin. Wind directions and velocities were also recorded on the same slope (Fig. 1). By these observations, we could see the horizontal and vertical temperature distributions of the cold air lake and their changes with time. The characteristic process of the cold air lake formation was as follows: After sunset, surface air temperature began to fall rapidly, especially at the bottom of the basin. But a zone of rising temperature appeared on the slope of Mt. Omatsu around 9 p. m. in the night (Fig. 2). And the change in temperature which were observed on the slope and the bottom of the basin showed very different patterns with each other. At the bottom of the basin, temperature has gradually fallen. On the other hand, temperature on the slope showed repeated rise and fall. Thus, the air on the slope was not so cooled as it is at the bottom of the basin (Fig. 3). In Fig. 4, it was shown that air temperature above the slope was rising during RUN 2 and RUN 3. It is suggested that the draining down of the cold surface layer air mass has occurred during these observations. At this time, the temperature in the cold air lake