Deriving the Østrem curve to quantify supraglacial debris-related melt-altering effects on the Djankuat Glacier, Caucasus, Russian Federation

We have derived the glacier-specific Østrem curve to quantify the influence of a supraglacial debris cover on the mass and surface energy balance of the Djankuat Glacier, a northwest-facing and partly debris-covered temperate valley glacier in the Caucasus region (Russian Federation), which has been selected as a ‘reference glacier’ by the WGMS. A 2D energy balance model, in combination with meteorological data from automatic weather stations and ERA5-Land reanalysis data, is used to assess the melt-altering effect of supraglacial debris on the overall glacier runoff during 1 complete balance year. The main results show that both the surface energy balance and mass balance fluxes are modified significantly due to the presence of debris on the glacier surface, as the surface characteristics (albedo, emissivity, and roughness) and near-surface temperature, moisture and wind regimes are greatly altered when compare to bare ice surfaces.  As such, for very thin debris (< 3 cm), a slight relative melt-enhancement occurs due to a decreased surface albedo and/or the patchiness of the debris. If debris, however, further thickens (> 9 cm), the insulating effect becomes dominant and reduces the melt of the underlying ice significantly. Sensitivity experiments show that especially within-debris properties, such as the thermal conductivity and the vertical porosity gradient within the debris pack, highly impact the magnitude of the sub-debris melt rates. Moreover, the relative melt suppression of the debris cover is modelled to increase in a warming climate, regardless of debris thickness changes. The above-mentioned effects are found to be increasingly pronounced with an increasing thickness of the superimposed supraglacial debris cover and can be of great importance with respect to future glacio-hydrologic regimes and glacio-geomorphological processes. Quantifying such melt-modification effects is therefore also important to more accurately understand and assess the behavior of (partly) debris-covered glaciers under a future warming climate.