Sub-surface processes and heat fluxes at coarse blocky Murtèl rock glacier (Engadine, eastern Swiss Alps): seasonal ice and convective cooling render rock glaciers climate-robust

Abstract

We measured sub-surface heat fluxes and calculated the energy budget of the coarse blocky active layer (AL) of Murtèl rock glacier, a seasonally snow-covered permafrost landform located in the eastern Swiss Alps. In the highly permeable AL, conductive/diffusive heat transfer, including thermal radiation, non-conductive heat transfer by air circulation (convection), and heat storage changes from seasonal build-up and melting of ground ice, shapes the ground thermal regime. Individual heat fluxes are quantified based on a novel in situ sensor array in the AL (operational from 2020-2023) and direct observations of the ground ice melt (in thaw seasons 2022-2024). The AL energy budget yields the first field-data-based quantitative estimate of the climate sensitivity of rock glaciers. The total Murtèl AL heat uptake during the thaw season has been increasing by 4-10 MJ m-2 per decade (4 %-11 % of the 2022 heat uptake of 94MJm-2), driven by earlier snow melt-out in June and increasingly hot and dry July-September periods. Two thaw-season processes render Murtèl rock glacier comparatively climate-robust. First, the AL intercepts ∼70 % (55-85 MJ m-2) of the thaw-season ground heat flux by melting ground ice that runs off as meltwater, ∼20 % (10-20 MJ m-2) is spent on heating the blocks, and only ∼10 % (7-13 MJ m-2) is transferred into the permafrost body beneath and causes slow permafrost degradation. Second, the effective thermal conductivity in the ventilated AL increases from 1.2Wm-1K-1 under strongly stable temperature gradients (weak warming) to episodically over 10Wm-1K-1 under unstable temperature gradients (strong cooling), favouring convective cooling by buoyancy-driven Rayleigh ventilation (thermal semiconductor effect). In winter, radiatively cooled air infiltrating through a discontinuous, semi-closed snow cover leads to strong AL cooling. The two characteristic parameters (effective thermal conductivity and intrinsic permeability) are sensitive to debris texture; hence the two undercooling processes are specific to highly permeable coarse blocky material.