Ice cliff contribution to the tongue-wide ablation of Changri Nup Glacier, Nepal, central Himalaya

Abstract

Abstract. Ice cliff backwasting on debris-covered glaciers is recognized as an important mass-loss process that is potentially responsible for the “debris-cover anomaly”, i.e. the fact that debris-covered and debris-free glacier tongues appear to have similar thinning rates in the Himalaya. In this study, we quantify the total contribution of ice cliff backwasting to the net ablation of the tongue of Changri Nup Glacier, Nepal, between 2015 and 2017. Detailed backwasting and surface thinning rates were obtained from terrestrial photogrammetry collected in November 2015 and 2016, unmanned air vehicle (UAV) surveys conducted in November 2015, 2016 and 2017, and Pléiades tri-stereo imagery obtained in November 2015, 2016 and 2017. UAV- and Pléiades-derived ice cliff volume loss estimates were 3 % and 7 % less than the value calculated from the reference terrestrial photogrammetry. Ice cliffs cover between 7 % and 8 % of the total map view area of the Changri Nup tongue. Yet from November 2015 to November 2016 (November 2016 to November 2017), ice cliffs contributed to 23±5 % (24±5 %) of the total ablation observed on the tongue. Ice cliffs therefore have a net ablation rate 3.1±0.6 (3.0±0.6) times higher than the average glacier tongue surface. However, on Changri Nup Glacier, ice cliffs still cannot compensate for the reduction in ablation due to debris-cover. In addition to cliff enhancement, a combination of reduced ablation and lower emergence velocities could be responsible for the debris-cover anomaly on debris-covered tongues.

Field Study of Mass Balance, and Hydrology of the West Khangri Nup Glacier (Khumbu, Everest)

“…We exploited here a rather complete, mostly unexploited dataset, including weather, glaciological, hydrological, and isotopic data gathered during recent field campaigns (2010-2014) to thoroughly investigate the present state of the bare WKN glacier, which is a relatively little, and yet representative ice body nested in the Khumbu glacier of the Everest region [79]. We demonstrated that (i) significant snow accumulation is not retained recently, (ii) the last measured snowpack was recent (fall-winter 2013-2014), (iii) large, widespread ice losses occurred on the glacier recently, and (iv) most of the hydrological contribution from this area comes from ice melt, and glaciers depletion may imply a large reduction of glacier runoff in this basin (e.g., [6][7][8]).…”

“…The West Khangri Nup is a small, debris-free glacier that is likely not representative of the full spectrum of glaciers of the Himalayas. Recent evidences (see e.g., [79]) suggest that debris-covered glaciers in the Himalayas appear to have thinning rates similar to debris-free glaciers, in spite of the well-known insulating effect of debris cover on ice beneath [80], which is a phenomenon called "debris cover anomaly". Among others, Brun et al [79] used data from an unmanned aerial vehicle (UAV), and from satellites to quantify ice loss (i.e., using stereophotogrammetry) from the debris-covered Khangri Nup, north of the WKN here, during 2015-2017.…”

“…Recent evidences (see e.g., [79]) suggest that debris-covered glaciers in the Himalayas appear to have thinning rates similar to debris-free glaciers, in spite of the well-known insulating effect of debris cover on ice beneath [80], which is a phenomenon called "debris cover anomaly". Among others, Brun et al [79] used data from an unmanned aerial vehicle (UAV), and from satellites to quantify ice loss (i.e., using stereophotogrammetry) from the debris-covered Khangri Nup, north of the WKN here, during 2015-2017. Based on their results, they hypothesize that the debris-cover anomaly could be a result of (i) lower emergence velocities (upward ice flux), (ii) reduced ablation, and (iii) formation of ice cliffs and supraglacial ponds, which may contribute disproportionately to the average tongue ablation over debris-covered glaciers., and lead to thinning rates comparable to those observed on clean ice glaciers.…”

A high-resolution image time series of the Gorner Glacier – Swiss Alps – derived from repeated unmanned aerial vehicle surveys

“…Glacier surface features evolve continuously, and these changes provide insights into the structure, internal dynamics, and mass balance of the glacier. Important efforts have been made to monitor glacier surfaces, from early stakes measurements at the end of the 19th century (Chen and Funk, 1990) to present-day in situ topographic surveys (Ramirez et al, 2001;Aizen et al, 2006;Dunse et al, 2012;Benoit et al, 2015) and remotely sensed data acquired from diverse platforms: ground-based devices (Gabbud et al, 2015;Piermattei at al., 2015), aircraft (Baltsavias et al, 2001;Mertes et al, 2017), or satellites (Herman et al, 2011;Kääb et al, 2012;Dehecq et al, 2015;Berthier et al, 2014). Recently, the development of unmanned aerial vehicles (UAVs) has enabled glaciologists to carry out their own aerial surveys autonomously, rapidly, and at reasonable costs (Whitehead et al, 2013;Immerzeel et al, 2014;Bhardwaj et al, 2016;Jouvet et al, 2017;Rossini et al, 2018).…”

“…Alternatively, the complete time series of orthomosaics and DEMs can be used for detection and quantification of changes at the surface of the glacier (Barrand et al, 2009;Berthier et al, 2016;Fugazza et al, 2018). Finally, the time lapse coupled with the MMs is an interesting tool to monitor ice surface velocity and deformation (Ryan et al, 2015;Kraaijenbrink et al, 2016), and in turn ice flow dynamics at the glacier surface (Brun et al, 2018). The MMs provide a quantification of the ice velocity at every location on the surface of the glacier, which can be used to calibrate or to validate ice flow models, especially for the Gorner Glacier, which was extensively used as a modeling benchmark (see for instance Riesen et al, 2010;Sugiyama et al, 2010;Werder et al, 2013).…”