Abstract
AbstractCurrent glacier ablation models have difficulty simulating the high‐melt transition zone between clean and debris‐covered ice. In this zone, thin debris cover is thought to increase ablation compared to clean ice, but often this cover is patchy rather than continuous. There is a need to understand ablation and debris dynamics in this transition zone to improve the accuracy of ablation models and the predictions of future debris cover extent. To quantify the ablation of partially debris‐covered ice (or ‘dirty ice’), a high‐resolution, spatially continuous ablation map was created from repeat unmanned aerial systems surveys, corrected for glacier flow in a novel way using on‐glacier ablation stakes. Surprisingly, ablation is similar (range ~5 mm w.e. per day) across a wide range of percentage debris covers (~30–80%) due to the opposing effects of a positive correlation between percentage debris cover and clast size, countered by a negative correlation with albedo. Once debris cover becomes continuous, ablation is significantly reduced (by 61.6% compared to a partial debris cover), and there is some evidence that the cleanest ice (<~15% debris cover) has a lower ablation than dirty ice (by 3.7%). High‐resolution feature tracking of clast movement revealed a strong modal clast velocity where debris was continuous, indicating that debris moves by creep down moraine slopes, in turn promoting debris cover growth at the slope toe. However, not all slope margins gain debris due to the removal of clasts by supraglacial streams. Clast velocities in the dirty ice area were twice as fast as clasts within the continuously debris‐covered area, as clasts moved by sliding off their boulder tables. These new quantitative insights into the interplay between debris cover characteristics and ablation can be used to improve the treatment of dirty ice in ablation models, in turn improving estimates of glacial meltwater production. © 2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd
Highlights
Ablation of partially debris-covered ice is thought to be higher than ablation of both clean and completely debris-covered ice (Reid and Brock, 2010; Evatt et al, 2015)
It was found that very high-resolution (0.04m ground sample distance (GSD)), accurate (RMSE 1.45mm w.e. per day) and spatially continuous maps of ablation can be produced by combining repeat unmanned aerial systems (UAS) imagery with field measurements of the horizontal and vertical components of glacial flow
The resulting ablation map was of a high enough quality to distinguish patterns of ablation attributed to variations in percentage debris cover and the influence of sub-debris streams
Summary
Ablation of partially debris-covered ice (dirty ice) is thought to be higher than ablation of both clean and completely debris-covered ice (Reid and Brock, 2010; Evatt et al, 2015). When the cover of debris is thin (defined by being less than the ‘critical thickness’), ablation is higher than the ablation rate for clean ice, whereas debris thicker than the critical thickness reduces ablation compared to clean ice (see figure 2 in Kirkbride and Dugmore, 2003). This critical thickness is dependent on weather conditions and the bare ice albedo (Hagg et al, 2008; Lejeune et al, 2013).
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