Freezing of the Seasonally Thawed Layer in the Grøndalen, Svalbard

Abstract

The results of geocryological observations conducted during the winter of 2023/24 in the Grøndalen (Nordenskiöld Land, Spitsbergen, Svalbard), supplemented by model estimates of the freezing rates of thawed permafrost and the conditions for the onset of its degradation as a function of meteorological conditions and the thermo-physical properties of the ground are presented. Observations and calculations using a simple thermodynamic model showed that under the conditions of the winter of 2023/24, complete freezing of the ground in the valley occurs within approximately the first one and a half months of winter. Variations in air temperature from −5.7 to −9.7 °C and in permafrost temperature from −2.25 to −4.25 °C reduce the time required for complete ground freezing from about 75 to 30 days and from 55 to 40 days, respectively. In the absence of snow cover, an increase in the initial thickness of the active layer from 1 to 2 m leads to an increase in freezing time from 20 to 90 days. When snow depth and initial active-layer thickness exceed certain critical values (e.g., 1.5 m and 1.6 m, respectively), the formation of non-merging permafrost might occur. In snow-free conditions, the critical soil moisture content above which the freezing fronts do not merge during the cold period is close to 0.21. The dependence of freezing rate on soil porosity is relatively weak: increasing porosity from 0.24 to 0.44 reduces the freezing time from 62 to 32 days. Compared to loam, sandy loam and gravel-pebble soil are characterized by a shorter freezing time — approximately one and a half times shorter, from 45 to 32 days. Lowering the freezing onset temperature from 0 to −0.2 °C increases the time to merging of phase fronts from 46 to 50 days. Based on measurements and modeling results, the frozen ground in the study area is concluded to be thermally stable; with certain reservations, this conclusion may be extended to other snow-free polar desert regions of the archipelago.