Abstract
Abstract. Few surface energy balance models for debris-covered glaciers account for the presence of moisture in the debris, which invariably affects the debris layer's thermal properties and, in turn, the surface energy balance and sub-debris melt of a debris-covered glacier. We adapted the interactions between soil, biosphere, and atmosphere (ISBA) land surface model within the SURFace EXternalisée (SURFEX) platform to represent glacier debris rather than soil (referred to hereafter as ISBA-DEB). The new ISBA-DEB model includes the varying content, transport, and state of moisture in debris with depth and through time. It robustly simulates not only the thermal evolution of the glacier–debris–snow column but also moisture transport and phase changes within the debris – and how these, in turn, affect conductive and latent heat fluxes. We discuss the key developments in the adapted ISBA-DEB and demonstrate the capabilities of the model, including how the time- and depth-varying thermal conductivity and specific heat capacity depend on evolving temperature and moisture. Sensitivity tests emphasize the importance of accurately constraining the roughness lengths and surface slope. Emissivity, in comparison to other tested parameters, has less of an effect on melt. ISBA-DEB builds on existing work to represent the energy balance of a supraglacial debris layer through time in its novel application of a land surface model to debris-covered glaciers. Comparison of measured and simulated debris temperatures suggests that ISBA-DEB includes some – but not all – processes relevant to melt under highly permeable debris. Future work, informed by further observations, should explore the importance of advection and vapor transfer in the energy balance.
Highlights
Enhancing the melt of underlying ice when thin and inhibiting it when thick (Östrem, 1959), supraglacial debris is known to affect the surface energy balance and retreat patterns of mountain glaciers
Supraglacial debris covers 11 % of glacier area in high-mountain Asia (HMA) (Kraaijenbrink et al, 2017), a region that contains the highest volume of ice on Earth outside the polar regions and where glacier melt flows into rivers that deliver water to 800 million people (Pritchard, 2019)
Is a physically based scheme that solves both time- and depth-dependent heat and moisture diffusion numerically through mass- and heat-conserving implicit time schemes. It provides a convenient basis for simulating the surface energy balance of a supraglacial debris layer, after making modifications to account for the differences between soil and debris
Summary
Enhancing the melt of underlying ice when thin and inhibiting it when thick (Östrem, 1959), supraglacial debris is known to affect the surface energy balance and retreat patterns of mountain glaciers. Moisture has been largely unaddressed in glacier models, despite the fact that water and ice affect the thermal properties of a debris layer. Including the thickness-dependent wind dynamics in their energy balance model contributed to their reproduction of the thickness–ablation curve of Östrem (1959) Their model does not account for moisture beyond that which is evaporated; like the other models, it assumes that melt runs off and does not affect the system’s energy (except in the case of evaporation). We highlight the important physical processes that need to be accounted for in any debris-covered glacier melt model, such as conduction and phase change of water and ice in the debris. We discuss the limitations of our model and propose some further considerations for making improvements
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