The diurnal variation in ground temperature is negligibly small at depths below 40 to 50 cm. The lapse rate of the ground temperature at 50-cm depth (GT50) during summer in the seasonal-frost zone is close to that of the air temperature, because the GT50 is principally controlled by ground heat flux for some days (Fig. 4). However, the GT50 lapse rate in the permafrost zone during summer is much larger than that of air temperature, because the GT50 is controlled by not only ground heat flux but cooling from permafrost. For example, Fujii and Higuchi (1972) and Fujii et al. (1999) estimated the permafrost lower limit based on the tendency for change in the GT50 lapse rate at Mt. Fuji, Japan, in August of 1973, 1998, and 1999 (Fig. 4).
Figure. 4 Ground temperatures at 50-cm depths during August 1998 on a south-facing slope of Mt. Fuji (modified Fujii et al., 1999).
Fujii and Higuchi (1976) also measured the GT50 at approximately 100 movement points on slopes of all aspects between 4500 and 5300 m in the Khumbu Himal in summer 1973. Because research period of GT50 measurements reached several weeks, time series variations in GT50 were observed during the research period. The time series variations in GT50 were revised using fixed-point GT50 measurements at Lhajung (4420 m) at 3-h intervals; revised GT50 (Tre) values were calculated in °C using the following equation:
Tre Tm Tha, where Tm is the GT50 at the movement points in °C and Tha represents the time series temperature variations for GT50 at Lhajung in °C.
Fukui et al. (2007a) took GT50 measurements at 20 movement points from 8 to 21 October 2004 on a south-facing slope in the Khumbu Himal between 4200 and 5600 m ASL
(Fig. 2) with a digital thermometer (testo110; testo AG, Lenzkirch, Germany), which has a resolution of 0.1°C and accuracy of 0.2°C. Shallow pits were dug and the thermometer probe was inserted in the pit at 50-cm depth just after excavation. GT50 was also monitored at Lhajung at 15-min intervals from 8 to 22 October using a miniature temperature data logger (TR-52; T and D Corp., Nagano, Japan) and revised the time series variations in the GT50.
To determine the frozen layer in the permafrost, three pits (0.7-1.4 m deep) were dug around Kala Pattar (Fig. 3b; 5554 m) on 20 and 21 October 2004 (Fukui et al., 2007a). The ground temperature profiles and stratigraphy were observed in the pits.
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