台地と谷すじの夜間冷却について

(2) 地形による冷却過程の差異

抄録

The nocturnal cooling process and the extent of cooling were observed in midwinter at Tomakomai Experiment Forest of Hokkaido University, located on the gently sloped plateau from the Shikotsu Caldera to the Pacific Ocean.
Comparing the nocturnal cooling process between on the valley and on the flat plateau, the extent of cooling was almost the same and the process was exponential as far as wind was extremely calm.
However, when the weak wind as much as 2-3m/s generated on the flat plateau, the air temperature near the snow surface of the flat plateau suddenly rose or ceased to cool. This behavior of variation of air temperature on the flat plateau resembled to that of the upper part of the valley.
The amount of compensating heat current due to the turbulent diffusion might frequently increase from above and the maximum amount of cooling at the snow surface on the flat plateau might become smaller than that on the bottom of the valley.
Changing in the scale of topographic relief of valleys, the narrower it was, the larger the amount of cooling at the snow surface of the bottom of the valley became, because the scale of air turbulence in the narrower valley might be smaller than in the wider valley.
As noted above the extent of cooling was the same both on the valley and on the flat plateau on the calm condition. And on the flat plateau there will be no inflow of colder and heavier air from the circumference for the sake of flatness. Besides, the amount of cooling due to net long wave radiation is almost the same both in the valley and on the flat plateau.
Whereby, it was concluded that the extreme cooling on the snow surface of the bottom of the valley in a calm and cloudless night was caused not by the inflow of colder air from above, but by local cooling due to only net long wave radiation.