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| Titel |
Investigating ice cliff evolution and contribution to glacier mass-balance
using a physically-based dynamic model |
| VerfasserIn |
Pascal Buri, Evan Miles, Silvan Ragettli, Fanny Brun, Jakob Steiner, Francesca Pellicciotti |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250121739
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| Publikation (Nr.) |
 EGU/EGU2016-575.pdf |
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| Zusammenfassung |
| Supraglacial cliffs are a surface feature typical of debris-covered glaciers, affecting surface
evolution, glacier downwasting and mass balance by providing a direct ice-atmosphere
interface. As a result, melt rates can be very high and ice cliffs may account for a significant
portion of the total glacier mass loss. However, their contribution to glacier mass balance has
rarely been quantified through physically-based models. Most cliff energy balance models
are point scale models which calculate energy fluxes at individual cliff locations.
Results from the only grid based model to date accurately reflect energy fluxes and
cliff melt, but modelled backwasting patterns are in some cases unrealistic, as the
distribution of melt rates would lead to progressive shallowing and disappearance of
cliffs.
Based on a unique multitemporal dataset of cliff topography and backwasting
obtained from high-resolution terrestrial and aerial Structure-from-Motion analysis
on Lirung Glacier in Nepal, it is apparent that cliffs exhibit a range of behaviours
but most do not rapidly disappear. The patterns of evolution cannot be explained
satisfactorily by atmospheric melt alone, and are moderated by the presence of
supraglacial ponds at the base of cliffs and by cliff reburial with debris. Here, we
document the distinct patterns of evolution including disappearance, growth and
stability.
We then use these observations to improve the grid-based energy balance model,
implementing periodic updates of the cliff geometry resulting from modelled melt
perpendicular to the ice surface. Based on a slope threshold, pixels can be reburied by debris
or become debris-free. The effect of ponds are taken into account through enhanced melt
rates in horizontal direction on pixels selected based on an algorithm considering distance to
the water surface, slope and lake level.
We use the dynamic model to first study the evolution of selected cliffs for which
accurate, high resolution DEMs are available, and then apply the model to the entirety of
Lirung and Langtang glaciers to quantify the total contributions of cliffs to glacier mass
balance.
Observations and model results suggest a strong dependency of the cliffs’ life cycle on
supraglacial ponds, as the water body keeps the cliff geometry constant through a
combination of backwasting and calving at the bottom and maintenance of steep slopes in the
lowest sections. The absence of ponds causes the progressive flattening of the cliff, which
finally leads to complete disappearance. Modelled volume losses from May to October
2013 range from 2650 to 9415 m3 w.e., in agreement with the estimates with the
SfM-approach. Mean error of modelled elevation within the cliff outline ranges from -1.3 to
0.6m.
This work sheds light on mechanisms of cliffs’ changes by quantifying them for the first
time with a physically-based, dynamic model, and presents the first complete estimate of the
relevance of supraglacial ice-cliffs to total glacier mass-balance for two distinct glaciers. |
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