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.
Highlights
Arctic permafrost areas represent a vast region which is expected to be strongly impacted by global warming in the coming decades to centuries
While the infiltration dynamics is not reproduced, the employed model still allows a good approximation of the amount of water reaching the bottom of the snow pack, which is the relevant quantity for the thermal regime of the soil
The timescale of infiltration in wet snow is on the order of minutes to hours (e.g. Singh et al, 1997), while water can persist at the bottom of the snow pack for several weeks (Sect. 4.1), so that the infiltration dynamics is of minor relevance for the thermal regime of the permafrost
Summary
Arctic permafrost areas represent a vast region which is expected to be strongly impacted by global warming in the coming decades to centuries. Simulations of future climate using General Circulation Models (GCM’s) suggest a warming of the near-surface air temperature of up to 10 K in the Arctic in the coming century, which is significantly more than the global average (Solomon et al, 2007). Significant advective heat transfer has been documented in spring, when meltwater infiltrates and subsequently refreezes in the frozen ground, causing a rapid increase of soil temperatures This phenomenon only occurs during a very limited period of the year, so that it is generally not considered in permafrost models
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