Semi-automated calibration method for modelling of mountain permafrost evolution in Switzerland

Abstract

Abstract. Permafrost is a widespread phenomenon in the European Alps. Many important topics such as the future evolution of permafrost related to climate change and the detection of permafrost related to potential natural hazards sites are of major concern to our society. Numerical permafrost models are the only tools which facilitate the projection of the future evolution of permafrost. Due to the complexity of the processes involved and the heterogeneity of Alpine terrain, models must be carefully calibrated and results should be compared with observations at the site (borehole) scale. However, a large number of local point data are necessary to obtain a broad overview of the thermal evolution of mountain permafrost over a larger area, such as the Swiss Alps, and the site-specific model calibration of each point would be time-consuming. To face this issue, this paper presents a semi-automated calibration method using the Generalized Likelihood Uncertainty Estimation (GLUE) as implemented in a 1-D soil model (CoupModel) and applies it to six permafrost sites in the Swiss Alps prior to long-term permafrost evolution simulations. We show that this automated calibration method is able to accurately reproduce the main thermal condition characteristics with some limitations at sites with unique conditions such as 3-D air or water circulation, which have to be calibrated manually. The calibration obtained was used for RCM-based long-term simulations under the A1B climate scenario specifically downscaled at each borehole site. The projection shows general permafrost degradation with thawing at 10 m, even partially reaching 20 m depths until the end of the century, but with different timing among the sites. The degradation is more rapid at bedrock sites whereas ice-rich sites with a blocky surface cover showed a reduced sensitivity to climate change. The snow cover duration is expected to be reduced drastically (between −20 to −37 %) impacting the ground thermal regime. However, the uncertainty range of permafrost projections is large, resulting mainly from the broad range of input climate data from the different GCM-RCM chains of the ENSEMBLES data set.

Soil Moisture Data for the Validation of Permafrost Models Using Direct and Indirect Measurement Approaches at Three Alpine Sites

“…For this study the semi-automated COUP model calibration procedure was completed by adjusting the porosity to match the best-guess porosity model used for the 4PM (see Table 3). The COUP model results were independently validated with the observed soil moisture values yielding a positive/negative bias of up to 30% (see Marmy et al, 2015). This bias was substantially reduced by manually adjusting the wilting point without worsening the thermal calibration .…”

“…The modeled processes of water infiltration into the frozen soil were implemented and described by Stähli et al (1996) and a complete description and test of water infiltration is given by Scherler et al (2010). The model has already been used in many studies focussing on various purposes, including soil moisture (e.g., Noroozvalashedi et al, 2012;Wu and Jansson, 2013) and permafrost (Engelhardt et al, 2010;Marmy et al, 2013Marmy et al, , 2015Scherler et al, 2013Scherler et al, , 2014Staub et al, 2015). Other studies specifically applied the COUP model to simulate the soil moisture in frozen grounds (Scherler et al, 2010;Xarpell et al, 2010;Python, 2015).…”

“…Here we use the COUP model as calibrated by Marmy et al (2015). In their study the COUP model was used at several high elevation study areas for the assessment of future long-term ground temperature evolution.…”

“…To analyze the water and ice content evolution in detail we used the heat and mass transfer model COUP (see Scherler et al, 2010Scherler et al, , 2013Marmy et al, 2013). Due to the complexity of the coupled hydrological and thermal processes in the ground, we used here the semi-automated calibration scheme proposed by Marmy et al (2015). The calibration only uses the borehole temperatures from SCH_5198 in order to have the measured soil moisture data as independent validation dataset.…”

“…In their study the COUP model was used at several high elevation study areas for the assessment of future long-term ground temperature evolution. Therefore, a semi-automated calibration procedure was used (see Marmy et al, 2015) and the COUP model was driven by on-site reconstructed meteorological data (air temperature, relative humidity, wind speed, global radiation, and precipitation) at daily resolution (see Rajczak et al, 2015) for the period 1999-2013 with a 18 year spin-up (1981-1999). The complete energy and mass balance are calculated along a soil column composed of 16 compartments of increasing thickness with depth, from which the water and ice content can be extracted.…”