Abstract
Abstract. The goal of this research is to constrain the influence of ice wedge polygon microtopography on near-surface ground temperatures. Ice wedge polygon microtopography is prone to rapid deformation in a changing climate, and cracking in the ice wedge depends on thermal conditions at the top of the permafrost; therefore, feedbacks between microtopography and ground temperature can shed light on the potential for future ice wedge cracking in the Arctic. We first report on a year of sub-daily ground temperature observations at 5 depths and 9 locations throughout a cluster of low-centered polygons near Prudhoe Bay, Alaska, and demonstrate that the rims become the coldest zone of the polygon during winter, due to thinner snowpack. We then calibrate a polygon-scale numerical model of coupled thermal and hydrologic processes against this dataset, achieving an RMSE of less than 1.1 ∘C between observed and simulated ground temperature. Finally, we conduct a sensitivity analysis of the model by systematically manipulating the height of the rims and the depth of the troughs and tracking the effects on ice wedge temperature. The results indicate that winter temperatures in the ice wedge are sensitive to both rim height and trough depth, but more sensitive to rim height. Rims act as preferential outlets of subsurface heat; increasing rim size decreases winter temperatures in the ice wedge. Deeper troughs lead to increased snow entrapment, promoting insulation of the ice wedge. The potential for ice wedge cracking is therefore reduced if rims are destroyed or if troughs subside, due to warmer conditions in the ice wedge. These findings can help explain the origins of secondary ice wedges in modern and ancient polygons. The findings also imply that the potential for re-establishing rims in modern thermokarst-affected terrain will be limited by reduced cracking activity in the ice wedges, even if regional air temperatures stabilize.
Highlights
It has long been understood that the formation of ice wedge polygons is intimately linked with thermal contraction ground stresses (Leffingwell, 1915; Lachenbruch, 1962; Mackay, 2000)
In contrast to research on ice wedge cracking, relatively few investigations have explored systematic variation in ground temperatures associated with polygon microtopography
The results of the rank sum tests (Table 4) confirm that the difference between minimum winter temperature in the rims and in the centers is significant at all depths (p < 0.025, indicating a low probability that variations could be explained by random processes), with median differences varying from 3.2 ◦C at 10 cm to 2.3 ◦C at 50 cm depth
Summary
It has long been understood that the formation of ice wedge polygons is intimately linked with thermal contraction ground stresses (Leffingwell, 1915; Lachenbruch, 1962; Mackay, 2000). One consistently observed trend is that the rims of a low-centered polygon tend to become several degrees colder in winter than the center or troughs (Mackay, 1993; Christiansen, 2005; Morse and Burn, 2014; Atchley et al, 2015) This effect is attributed to the thinner snowpack on top of the rims, as wind-driven redistribution of snow enhances accumulation in microtopographic lows. Consistent with these observations, previous conceptual models of the thermal regime of the active layer in ice wedge polygons have incorporated the idea that cooling is enhanced in raised zones, such as rims (Christiansen, 2005; Morse and Burn, 2014) and impeded in low ones (Gamon et al, 2012). It has been suggested that development of relief in the rims drives precisely the opposite effect, by increasing snow entrapment in the troughs, thereby enhancing insulation of the ice wedges (Lachenbruch, 1966)
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