Abstract. By combining in situ measurements and a twodimensional thermomechanically coupled ice flow model, we investigate the thermomechanical features of the largest valley glacier (Laohugou Glacier No. 12; LHG12) on Qilian Shan located in the arid region of western China. Our model results suggest that LHG12, previously considered as fully cold, is probably polythermal, with a lower temperate ice layer overlain by an upper layer of cold ice over a large region of the ablation area. Modelled ice surface velocities match well with the in situ observations in the east branch (main branch) but clearly underestimate those near the glacier terminus, possibly because the convergent flow is ignored and the basal sliding beneath the confluence area is underestimated. The modelled ice temperatures are in very good agreement with the in situ measurements from a deep borehole (110 m deep) in the upper ablation area. The model results are sensitive to surface thermal boundary conditions, for example surface air temperature and near-surface ice temperature. In this study, we use a Dirichlet surface thermal condition constrained by 20 m borehole temperatures and annual surface air temperatures. Like many other alpine glaciers, strain heating is important in controlling the englacial thermal structure of LHG12. Our transient simulations indicate that the accumulation zone becomes colder during the last two decades as a response to the elevated equilibrium line altitude and the rising summer air temperatures. We suggest that the extent of accumulation basin (the amount of refreezing latent heat from meltwater) of LHG12 has a considerable impact on the englacial thermal status.

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The A’nyêmaqên Mountains have the largest concentration of glaciers in the Yellow River basin, which play a crucial role in regulating the runoff regime of the Yellow River. Thus, the quantification of glacier mass balance and its effects on river runoff is greatly required. However, current studies mainly focus on mass changes since 2000. Here, we report for the first time region-wide glacier elevation and mass changes, which were derived from digital elevation models (DEMs) produced from historical topographic maps (TOPO), SRTM retrievals, and ASTER L1A stereo imagery spanning the past 50 years. The results indicated a negative mass balance (−0.24 ± 0.05 m w.e. a−1) of all glaciers for the 1966–2018 timespan. The mass loss rapidly accelerated from −0.16 ± 0.09 m w.e. a−1 in 1966–2000 to −0.36 ± 0.06 m w.e. a−1 during the period from 2000–2018. The rise in mass loss rate from 2000 onwards was mainly associated with the rapidly increased summer warming.

Abstract. En-glacial thermal conditions are very important for controlling ice rheology. By combining in situ measurements and a two-dimensional thermo-mechanically coupled ice flow model, we investigate the present thermal status of the largest valley glacier (Laohugou No. 12; LHG12) in Mt. Qilian Shan in the arid region of western China. Our model results suggest that LHG12, previously considered as fully cold, is probably polythermal, with a lower temperate ice layer (approximately 5.4 km long) overlain by an upper layer of cold ice over a large region of the ablation area. Generally, modelled ice surface velocities match in situ observations in the east branch (mainstream) well but clearly underestimate the ice surface velocities near the glacier terminus because the convergent flow of the west branch is ignored. The modelled ice temperatures agree closely with the in situ measurements (with biases less than 0.5 K) from a deep borehole (110 m) in the upper ablation area. The model results were highly sensitive to surface thermal boundary conditions, for example, surface air temperature and near-surface ice temperature. In this study, we suggest using a combination of surface air temperatures and near-surface ice temperatures (following the work of Wohlleben et al., 2009) as Dirichlet surface thermal conditions to include the contributions of the latent heat released during refreezing of surface melt-water in the accumulation zone. Like many other alpine glaciers, strain heating is the most important parameter controlling the en-glacial thermal structure in LHG12.