Abstract
ABSTRACTMass losses originating from supraglacial ice cliffs at the lower tongues of debris-covered glaciers are a potentially large component of the mass balance, but have rarely been quantified. In this study, we develop a method to estimate ice cliff volume losses based on high-resolution topographic data derived from terrestrial and aerial photogrammetry. We apply our method to six cliffs monitored in May and October 2013 and 2014 using four different topographic datasets collected over the debris-covered Lirung Glacier of the Nepalese Himalayas. During the monsoon, the cliff mean backwasting rate was relatively consistent in 2013 (3.8 ± 0.3 cm w.e. d−1) and more heterogeneous among cliffs in 2014 (3.1 ± 0.7 cm w.e. d−1), and the geometric variations between cliffs are larger. Their mean backwasting rate is significantly lower in winter (October 2013–May 2014), at 1.0 ± 0.3 cm w.e. d−1. These results are consistent with estimates of cliff ablation from an energy-balance model developed in a previous study. The ice cliffs lose mass at rates six times higher than estimates of glacier-wide melt under debris, which seems to confirm that ice cliffs provide a large contribution to total glacier melt.
Highlights
Debris-covered glaciers are notable features in the PamirKarakoram-Himalaya (PKH) region (e.g. Scherler and others, 2011; Bolch and others, 2012), covering ∼10% of the glacierized area (Bolch and others, 2012), and are of importance for melt and mass balance because they normally lay at low elevations
Given the limitation of tools like M3C2, we developed a specific method to estimate volumetric change associated with ice cliff backwasting, which we applied to selected cliffs on the debris-covered tongue of Lirung Glacier in the Nepalese Himalaya
We calculate a normalized volume loss, called hereafter average cliff backwasting rate, as the volume loss between t1 and t2 divided by the average of the cliff areas at t1 and t2, converted to cm w.e. assuming an ice density of 900 km m−3
Summary
Debris-covered glaciers are notable features in the PamirKarakoram-Himalaya (PKH) region (e.g. Scherler and others, 2011; Bolch and others, 2012), covering ∼10% of the glacierized area (Bolch and others, 2012), and are of importance for melt and mass balance because they normally lay at low elevations. A number of pioneering (e.g. Sakai and others, 2000, 2002) and more recent studies (Steiner and others, 2015; Buri and others, 2016; Miles and others, 2016) have suggested that supraglacial lakes and cliffs are responsible for larger than expected volume losses These features have low albedo and are exposed directly to the atmosphere, enhancing the radiative transfer and turbulent energy fluxes at the glacier surface (Buri and others, 2016; Miles and others, 2016). Most numerical models (based on calculation of the energy balance at the cliff surface) have been developed at the point scale (Sakai and others, 1998, 2000; Han and others, 2010; Reid and Brock, 2014; Steiner and others, 2015) and some have quantified the total contribution of cliffs to melt by extrapolation of point estimates of backwasting (Sakai and others, 1998, 2000; Han and others, 2010; Reid and Brock, 2014). A recent attempt has been made to develop a fully distributed model of cliff backwasting that could provide estimates of total cliff
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