29 Sep 2023
 | 29 Sep 2023

Surface heat fluxes at coarse-blocky Murtèl rock glacier (Engadine, eastern Swiss Alps)

Dominik Amschwand, Martin Scherler, Martin Hoelzle, Bernhard Krummenacher, Anna Haberkorn, Christian Kienholz, and Hansueli Gubler

Abstract. 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 parametrisation of the turbulent heat fluxes. Seasonally contrasting atmospheric conditions occur in the Murtèl 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.

Dominik Amschwand et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2023-2109', Anonymous Referee #1, 29 Oct 2023
  • RC2: 'Comment on egusphere-2023-2109', Anonymous Referee #2, 31 Oct 2023

Dominik Amschwand et al.

Dominik Amschwand et al.


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Short summary
Rock glaciers are coarse-debris permafrost landforms that are comparatively climate-resilient. We estimate the surface energy balance of rock glacier Murtèl (Swiss Alps) based on a large surface and sub-surface sensor array. During the thaw seasons 2021 and 2022, 90 % of the net radiation was exported via turbulent heat fluxes and only 10 % was transmitted towards the ground ice table. However, early snow melt and droughts make these permafrost landforms more vulnerable to climate warming.