Abstract
The Nakayama wind-hole in Shimogo, Fukushima, is a famous natural monument where cold air vents from the foot of the tales along the mountain during summer. The wind-hole with various species of alpine plants is a wellknown tourist attraction, but has been decaying in recent years. The local government has replanted the surrounding forest to protect the natural monument. However, protection efforts might result in further destruction if we do not understand the mechanism of the wind-hole well. The first step in protecting the important natural monument is thus to understand the mechanism of the cold air vent at the Nakayama wind-hole.
In this study, we carried out extensive in situ observations of the wind-hole in both summer and winter in 2001. In summer, cold air blows from the cold wind-holes at the foot of the talus. It has been inferred that the air inhaled at the warm wind-hole up the slope cools in the tales and blows out from the cold wind-hole. To confirm this speculation, we conducted a tracer experiment using CO2, which is harmless to the environment. As a result, it was found that the air inhaled at the warm wind-hole is exhaled from the cold wind-hole 140 m downslope only after 90 minutes. Based on the resulting mass flux of 2.6cm/s and the known cooling rate of the air, we found that a total of 4×1012J of thermal energy is stored in the tales during summer.In winter, on the other hand, warm moist air blows from the warm wind-holes at the top of the tales. Using thermographic instruments, we found that such warm wind-holes are located ubiquitously at high altitudes, especially around Sekichu-no-oka. We measured the surface heat budget at the warm wind-hole and found that 150W/m2 of heat flux arises from the underground section of the talus. The heat flux is multiplied by the total area of the warm wind-holes, and a total of 5×1012J of thermal energy is released from the talus during winter. The result is consistent, at least in terms of the order, with the amount of heat accumulated during summer.
Hence, we conclude that the cold air gently blowing from the cold wind-hole during summer can be quantitatively explained by the cold accumulated during winter. Convective overturning under the unstable stratification in winter and stable stratification in summer behaves as an effective thermal filter which accumulates only cold in the tales. This mechanism of a natural thermal filter is essential to maintain the cold air vent from the wind-hole during summer. Based on this conclusion regarding the mechanism of the wind-hole, we suggest cutting trees and opening vents around the warm wind-holes after identifying them thermographically to protect the important natural monument of the Nakayama wind-hole.
