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 94 MJ m−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.