APPEARANCE AND DRAINAGE AREAS OF THE COLD AIR STREAM ON THE SLOPE OF Mt. OMATSU, SUGADAIRA, CENTRAL JAPAN

Abstract

In order to make clear the appearance and drainage areas of cold air stream on the mountain slope before and after sunset, the microclimatological observations were carried out on the northeastern slope of Mt. Omatsu (1, 648.7m) in the north of Nagano Prefecture, during the period of Aug. 18-20, 1976. They were taken at seven points stationed between about 1, 250m and 1, 450m M. S. L. along the slope (Figs. 1, 2). The horizontal distance between the highest Station No. 1 (1, 450m) and the lowest No. 7 (1, 250m) is about 1, 600 m, and the average inclination is about 7°. The observations of the air temperature at the heights of 1.3m and 0.3m (by Assmann ventilated psychrometer), and the wind velocity and direction at the height of 1.0m (by photo-electronic wind vane and anemo-meter) were carried out every minute during the periods of 18h00m-20h00m Aug. 18 and 17h30m-20h00m Aug. 19 and 20, 1976 respectively. Air temperature difference between these two heights (1.3m and 0.3m above the ground) is defined as “the degrees of inversion”. The time transition of distribution of “the degrees of inversion” was analyzed to clarify the appearance and drainage areas of the cold air stream on the slope. The results of the observations are summarized as follows: (1) Grass temperature on the slope of Mt. Omatsu before and after sunset falls with altitude when the wind velocity is about 2_??_3m/s. These tendencies, however, are not found in case of light wind or calm. (2) Vertical distribution of grass temperature on the slope of Mt. Omatsu before and after sunset is estimated as Fig. 6. (3) “The degrees of inversion” is always positive at the Stations No. 3(1, 320m) and No. 5(1, 275m) respectively when the general wind velocities are less than about 3m/s. The weaker the general wind blows, the more “the degrees of inversion” increases. So far as the observations during this period is concerned, the extreme value of “the degrees of inversion” runs into+1.5°C. (4) The lowest grass temperature is found around the Station No. 1 (1, 450m). It is, however, pointed out that “the degrees of inversion” is negative at that station when the general wind is strong, on the contrary, is positive when it is weak. (5) The appearance and drainage areas of the cold air stream are defined as follows: Appearance areas: the area where (1) “the degrees of inversion” reaches greater than +0.5°C, (2) air temperatures at the height of 0.3m correlate positively with the wind velocities at the height of 1.0m, (3) “the degrees of inversion”, however, correlates negatively with the wind velocities at the height of 1.0m, and moreover, (4) there is a drainage area found downwards of the slope. Drainage area: the area where (1) “the degrees of inversion” is greater than+0.5°C, (2) air temperatures at the height of 0.3m have a negative correlation with the wind velocities at the height of 1.0m, (3) the wind velocities at the height of 1.0 m are less than 3.0m/s and (4) a fall-wind is observed.
These considerations lead to the conclusion that in the evening (17h30m-20h00m JMT) of Aug. 20, 1976 a cold air stream appeared around Station No. 3 (1, 320m) during the period of 19h00m-1 9h20m and spread downward to Station No. 5 (1, 275m) during 19h14m to 19h27m.