Frost damage for young trees occur frequently in shallow valleys of the Tomakomai Experiment Forest of Hokkaido University, located on the gently sloped plateau from the Shikotsu Caldera to the Pacific Ocean.
To know the frost danger zone in and around the Experiment Forest distribution of nocturnal cooling was observed and compared with the meteorological data measured continuously at Forest Observation Tower of a height of 31m.
It turned out that nocturnal cooling occurred on a wide area of the gently sloped plateau on clear day through all seasons when the wind speed at a height of 33m from the bottom of a valley was lower 3.5m/s. The routine observation at key stations showed that differences of minimum temperature between a hill of a relative height 20m and a valley were 8°C in winter and 3°C in summer, on the average.
According to the observation of distribution of the minimum temperatures in winter, it was confirmed that the strong radiative cooling occurred in a shallow valley and on a flat plateau rather than on a hill, a hill side and on a gently sloped plateau.
Comparing the temperature profile observed at Observation Tower located in the bottom of a valley with that on the adjacent small side slope of 15m in a relative height, it was recognized that the temperature profile in the valley was almost same as that on the slope.
It turned out that on the side slope the temperature at 10cm in height from the surface of the ground sometimes was 0.5°C higher than that at 150cm in height.
On the same side slope and in the valley, periodic oscillations of nocturnal temperatures were observed through the night. This observation suggests that weak periodic downslope winds occur every about 10 minutes. Comparing the periodic oscillations of wind speeds with those of temperatures, the weak gusts of wind (<1m/s) were found to be accompanied by the decrease of the air temperature near the ground surface.

抄録全体を表示

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.

抄録全体を表示

霜害に及ぼす朝日の影響を調べるたあに, 5年生トドマッを絹いて実験室で融解速度と被害の開孫を,現在では霜日に朝日があたった場合の融解速度を調べた。
凍害に及ぼす融解速度の有害な作用は芽の開舒が進むにっれて著しくなる。芽が開野する直前のトドマッでは, -4°Cで1～2時聞凍結後, 15°C以下の塞中でゆっくりとかす時(融解速度0.8°C/分以下)は害が縁とんど認められない溺,約20°Cの外気温の時,直射日光にさらす時(融解速度約2.O°C/分),または5°Cの水中に浸す時(融解速度約4°C/分)には,トドマッの芽は著しい書を受ける。しかし, -4°Cで凍結したトドマツを0°Cに約50分おいて芽の温度が0°C近くになってから直射日光,また捻5°Cの水中に入れても害嬢まとんど認められない。さらに,-4°Cで凍結後-3°, -2°, -1°および0°Cまで温度をあげて1時闇各温度においてから5°Cの水中に入れる時は, -1°C以下,ことに-2°C以下の凍結状態から5°Cの水中に入れ愚時に著しい害があらわれる。
凍結に愛ぱす融解速度の影響は,芽だけでなく,皮層部,材部にも認められる。現在で霜日に凍っている芽に朝日があたってとける遮さは約0.5°C以下である。融解速慶が1°C/分をこえない時には,開鉦時期のトドマツは有害な影響をほとんど受けないという実験事実から考えて,霜日に朝日が有害な修用を及ぼすことは少ないものと考えられる。

抄録全体を表示