Modeling the impact of wintertime rain events on the thermal regime of permafrost

Abstract

Abstract. In this study, we present field measurements and numerical process modeling from western Svalbard showing that the ground surface temperature below the snow is impacted by strong wintertime rain events. During such events, rain water percolates to the bottom of the snow pack, where it freezes and releases latent heat. In the winter season 2005/2006, on the order of 20 to 50 % of the wintertime precipitation fell as rain, thus confining the surface temperature to close to 0 • C for several weeks. The measured average ground surface temperature during the snow-covered period is −0.6 • C, despite of a snow surface temperature of on average −8.5 • C. For the considered period, the temperature threshold below which permafrost is sustainable on long timescales is exceeded. We present a simplified model of rain water infiltration in the snow coupled to a transient permafrost model. While small amounts of rain have only minor impact on the ground surface temperature, strong rain events have a long-lasting impact. We show that consecutively applying the conditions encountered in the winter season 2005/2006 results in the formation of an unfrozen zone in the soil after three to five years, depending on the prescribed soil properties. If water infiltration in the snow is disabled in the model, more time is required for the permafrost to reach a similar state of degradation.

Ensemble-based assimilation of fractional snow-covered area satellite retrievals to estimate the snow distribution at Arctic sites

“…We use a linear transition to delineate between snowfall and rainfall (You et al, 2014), with thresholds given in Table 2. We only consider rainfall as a positive contribution to the mass balance during non-melting conditions when the rainwater generally refreezes in the snowpack (Westermann et al, 2011a). For melting conditions (where D m > 0), we assume that rainfall directly becomes runoff.…”

“…A randomized array of sample points was employed for Bayelva in most years, except for the first 2 years where transects were used. Basal ice layers resulting from rain-on-snow events (Kohler and Aanes, 2004;Westermann et al, 2011a) occur in the area and can constitute a major source of uncertainty for SWE measurements. In 2016, the depth of basal ice layers was measured using ice screws, and their contribution to the SWE was accounted for.…”

“…This area has been the subject of extensive studies spanning permafrost Boike et al, 2008;Westermann et al, 2011a), the surface energy balance (Boike et al, 2003;Westermann et al, 2009), CO 2 exchange (Lüers et al, 2014;Cannone et al, 2016), ecology (Kohler and Aanes, 2004), snow (Bruland et al, 2001;Gisnås et al, 2014;López-Moreno et al, 2016), hydrology (Nowak and Hodson, 2013) and satellite retrieval validation (Westermann et al, 2011b. The surface cover at Bayelva and Kvadehuksletta alternates between bare soil, rocks and sparse low vegetation (Westermann et al, 2009), while the more elevated Steinflåen plateau is predominantly covered by loose rocks.…”

Transient modeling of the ground thermal conditions using satellite data in the Lena River delta, Siberia

“…The model physics of CryoGrid 2 does not account for a range of processes that may influence the ground thermal regime in permafrost areas, such as infiltration of water in the snow pack and soil (Weismüller et al, 2011;Westermann et al, 2011a;Endrizzi et al, 2014), or thermokarst and ground subsidence due to excess ground ice melt. The latter can strongly modify the ground thermal regime, as demonstrated by Westermann et al (2016), which makes a comparison of model results to in situ measurements at thermokarstaffected sites (Kurungnakh, Sardakh, Sect.…”

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

“…It influences the physical properties of the subsurface (e.g., ice content, thermal conductivity, heat capacity, hydraulic conductivity, electrical conductivity, and permittivity), the energy and water exchange processes with the atmosphere (e.g., changing albedo, evaporation, infiltration rates, refreezing rates, latent heat release, ground heat flux, runoff, Hinkel et al, 2001;Boike et al, 2003Boike et al, , 2008Westermann et al, 2009Westermann et al, , 2011Scherler et al, 2010) and the characteristics of different permafrost landforms (e.g., differences of the above for soil, bedrock, rock glacier with coarse-or finegrained blocky material, talus slopes, steep, or flat terrain, Rist and Phillips, 2005).…”