An investigation of the thermo-mechanical features of Laohugou Glacier No. 12 in Mt. Qilian Shan, western China, using a two-dimensional first-order flow-band ice flow model

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.