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ALMA Memo No . 480 Thermal Properties of subsurface layer at Pampa La Bola

We monitored subsurface temperature for more than 2 years at Pampa La Bola every 20 cm down to 100 cm, and evaluated hourly temperature change, apparent thermal diffusivity, freezing points, and maximum depth to frost line. Hourly temperature changes at 0–40 cm is dominated by diurnal cycle, whereas longer timescale variation takes over at deeper layer. Annual temperature range remains 9◦C peak-to-peak even at the depth of 80 cm. The hourly temperature change at 80 cm and deeper remains at a level of ∼ 0.005◦Chr−1, and is mostly attributed to the annual cycle and practically cannot be overcome simply by digging deeper trenches. The apparent thermal diffusivity estimated from amplitude suppression of diurnal temperature change between 0 and 60 cm was smaller than 5.3× 10−7 m s−1 for more than 75% of time, with the median value of 4.6×10−7 m s−1. A significant seasonal variation was found, with higher values (up to ∼ 10 × 10−7 m s−1) during rainy summer, and with very low values (∼ 2× 10−7 m s−1) in autumn and spring when freezing-thawing transition layer appears near the surface. The freezing point deduced from statistical plots was −0.2◦C, and can be explained with 0.3% salinity. Annual mean subsurface temperature was higher than freezing point with slight increasing trend as a function of depth, and no permafrost will exist in the plateau. The maximum depth to the frost line is approximately 1m at Pampa La Bola, and is estimated to be 2m or more at Llano de Chajnantor because of lower mean temperature.