Graziano Coppa , Francesca Sanna, Luca Paro, Chiara Musacchio, Andrea Merlone

Permafrost degradation is a growing direct impact of climate change. Detecting permafrost reductions, in terms of its extension, deepening of the active layer and rise of the base is fundamental to capture the magnitude of trends and address actions and warnings. Temperature profiles in permafrost allow direct understanding of the status of the frozen ground layer and its evolution in time. The Sommeiller Pass permafrost monitoring station, at about 3000 m of elevation, is the key site of the regional network installed in 2009 during the European Project “PermaNET” in the Piedmont Alps (NW Italy). The station consists of three vertical boreholes 5, 10 and 100 m deep with different characteristics, equipped with thermometric chains for a total of 36 temperature sensors (thermistors type). The raw collected data shows an active layer 8-9 m thick, and a degradation of the base at approximately 60 m of depth since 2014, corresponding to about 0.03 °C/yr. In order to verify and better quantify this potential degradation, three on-site sensor calibrations campaigns were carried out aimed to understand the reliability of the measurements in progress. By repeated calibrations, two key results have been achieved: the profiles have been corrected for biases and the re-calibration allowed to distinguish the effective change of permafrost temperatures along the years, from possible drifts of the sensors, which can be of the same order of magnitude of the investigated thermal change. The reduction of permafrost starting from 2012 at a depth of ~60 m has been confirmed, with a rate of 0.042 ± 0.005 °C/yr. This paper reports the implementation and installation of the on-site metrology laboratory, the dedicated calibration procedure adopted, the calibration results and the resulting adjusted data, profiles and their evolution in time. It is intended as a further contribution to the ongoing studies and definition of best practices, to improve data traceability and comparability, as prescribed by the World Meteorological Organization Global Cryosphere Watch programme.