RADIATIVE COOLING AND GROUND INVERSION ON THE SLOPE OF MT. OMATSU, SUGADAIRA, CENTRAL JAPAN

Abstract

Microclimatological observations were carried out in order to make clear the formation of the ground inversion on the mountain slope accompanied with rapid decrease of net radiation before and after sunset. The observations were made on the northeastern slope of Mt. Omatsu (1, 648.7 m) in the northern part of Nagano Prefecture, during the period from July 29 to August 1, 1978. Seven points for observation were selected on the slope between about 1, 250 m and 1, 450 m a. s. 1. The horizontal distance is about 1, 600 m, between the highest station No. 1 (1, 450 m) and the lowest No. 7 (1, 250 m) and the average inclination is approximately 7°. Net radiation measured by economical net radiometer or net radiometer, wind speed and direction by photo-electronic wind vane and anemometer at the height of 1.0 m above the ground, and air temperature and relative humidity by Assmann ventilated psychrometer at the height of 1.3 m and 0.3 m above the ground were observed every 2 minute during the nights from July 29 to August 1, 1978. The difference of air temperature between two levels is defined as “the degree of inversion ”. Furthermore, the difference of temperature falling between two levels is defined as “the vertical difference of cooling”.The results of the observation are summarized as follows: (1) “The degree of inversion” on the slope of Mt. Omatsu before and after sunset correlates negatively with the net radiation at stations No. 3 (1, 320 m) and No. 5 (1, 275 m). On the other hand, the correlation at station No. 7 has a positive slope. (2) Maximum. value of temperature falling at the height of 0.3m was observed at station No. 3 for two and half hours from 17 h 30 m to 20 h 00 m, and it was at stations No. 3 and No. 5 that the temperature falling was recorded greater at the height of 0.3 m than at the height of 1.3 m. (3) During the time from 17 h 30 m to 20 h 00 m, “ the vertical difference of cooling ” which was calculated every 30 minute varied slightly within a range from -0.2°C to 0.0°C at stations No. 1 and No. 2. Furthermore the value at station No. 7 fluctuated largely from positive to negative, and again to positive value. From these facts it is considered that cooling of the surface boundary layer is disturbed by the advective air from outside of the slope. On the other hand, it indicates the high positive value (max 0.5°C) on the slope from stations No. 3 to No. 5, where the surface boundary layer is considered to be cooled due to the infrared radiation from lower layer. These results nearly agreed with the observation result which was carried out on the same slope of Mt. Omatsu, August 20, 1976 (Nakamura, 1978).These considerations lead to the conclusion that in the area ranging from station No. 3 through No. 5, the lower part of the slope of Mt. Omatsu, the ground inversion developed strikingly before and after sunset.