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We estimate the surface energy balance (SEB) of the Murtèl rock glacier, a seasonally snow-covered permafrost landform with a ventilated coarse blocky active layer (AL) located in the eastern Swiss Alps. We focus on the parameterisation of the turbulent heat fluxes. Seasonally contrasting atmospheric conditions occur in the Murtèl cirque, with downslope katabatic jets in winter and a strongly unstable atmosphere over the heated blocky surface in summer. We use a novel comprehensive sensor array both above the ground surface and in the coarse blocky AL to track the rapid coupling by convective heat and moisture fluxes between the atmosphere, the snow cover, and the AL for the time period September 2020–September 2022. The in situ sensor array includes a sonic anemometer for eddy-covariance flux above-ground and sub-surface long-wave radiation measurements in a natural cavity between the AL blocks. During the thaw seasons, the measurements suggest an efficient (∼ 90 %) export of the available net radiation by sensible and latent turbulent fluxes, thereby strongly limiting the heat available for melting ground ice. Turbulent export of heat and moisture drawn from the permeable AL contributes to the well-known insulating effect of the coarse blocky AL and partly explains the climate resiliency of rock glaciers. This self-cooling capacity is counteracted by an early snow melt-out date, exposing the low-albedo blocky surface to the intense June–July insolation and causing reduced evaporative cooling due to exacerbated moisture scarcity in the near-surface AL during dry spells. With climate change, earlier snowmelt and increased frequency, duration, and intensity of heat waves and droughts are projected. Regarding the parameterisation of the turbulent fluxes, we estimated the year-round turbulent fluxes using a modified scheme. The monthly SEB is closed within 20 Wm-2 except during the snowmelt months and under katabatic drainage winds in winter. Detected sensible turbulent fluxes from nocturnal ventilation processes, although a potentially important ground cooling mechanism, are within our 20 Wm-2 uncertainty because nighttime wind speeds are low. Wintertime katabatic wind speeds needed to be scaled to close the SEB, which hints at the limits of parameterisations based on the Monin–Obukhov similarity theory in complex mountain terrain and katabatic drainage winds. The present work contributes to the process understanding of the SEB and climate sensitivity of coarse blocky landforms. [ABSTRACT FROM AUTHOR]

We estimate the sub-surface energy budget and heat fluxes in the coarse-blocky active layer (AL) of the Murtεave;l 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 accretion and melting of ground ice shape the ground thermal regime. We quantify individual heat fluxes based on a novel in-situ sensor array in the AL and direct observations of the ground ice melt in the years 2020–2022. Two thaw-season mechanisms render Murtεave;l rock glacier comparatively climate-resilient. 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 1.2 W m−1 K−1 under strongly stable temperature gradients to episodically over 10 W m−1 K−1 under unstable temperature gradients, favouring convective cooling by buoyancy-driven Rayleigh ventilation (thermal semiconductor effect). In winter, radiatively cooled air infiltrating through a discontinuous, semi-closed snowcover leads to strong AL cooling. The two characteristic parameters (effective thermal conductivity and intrinsic permeability) are sensitive to debris texture, hence these convective undercooling processes are specific to highly permeable coarse-blocky material. [ABSTRACT FROM AUTHOR]

We estimate the surface energy balance (SEB) of the Murtεave;l rock glacier, a seasonally snow-covered permafrost landform with a ventilated coarse-blocky active layer (AL) located in the eastern Swiss Alps. We focus on the parametrisation of the turbulent heat fluxes. Seasonally contrasting atmospheric conditions occur in the Murtεave;l cirque, with down-slope katabatic jets in winter and a strongly unstable atmosphere over the heated blocky surface in summer. We use a novel comprehensive sensor array both above ground surface and in the coarse-blocky AL to track the rapid coupling by convective heat and moisture fluxes between the atmosphere, the snow cover and the AL for the time period September 2020–September 2022. The in situ sensor array includes a sonic anemometer for eddy-covariance flux above ground and sub-surface long-wave radiation measurements in a natural cavity between the AL blocks. During the thaw seasons, the measurements suggest an efficient (ca. 90 %) export of the available net radiation by sensible and latent turbulent fluxes, thereby strongly limiting the heat available for melting ground ice. Turbulent export of heat and moisture drawn from the porous/permeable AL contributes to the well-known insulating effect of the coarse-blocky AL and partly explains the climate resiliency of rock glaciers. This self-cooling capacity is counteracted by an early snow melt-out date, exposing the low-albedo blocky surface to the intense June–July insolation, and reduced evaporative cooling due to exacerbated moisture scarcity in the near-surface AL during dry spells. With climate change, earlier snow melting and increased frequency, duration and intensity of heat waves and droughts are projected. Regarding the parametrisation of the turbulent fluxes, we successfully estimated the year-round turbulent fluxes using a modified Louis 1979 scheme despite seasonally contrasting atmospheric conditions and closed the monthly SEB within 20 W m−2, except during the snow melt-out months. Detected sensible turbulent fluxes from nocturnal ventilation processes, although a potentially important ground cooling mechanism, are within our 20 W m−2 uncertainty, because nighttime wind speeds are low. Wintertime katabatic wind speeds had to be scaled to close the SEB, which hints at the limits of parametrisations based on the Monin–Obukhov theory in complex mountain terrain and katabatic drainage flows. The present work contributes to the process understanding of the SEB and climate sensitivity of coarse-blocky landforms. [ABSTRACT FROM AUTHOR]