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| Titel |
Kilimanjaro ice cliff recession patterns derived from terrestrial photogrammetry |
| VerfasserIn |
Michael Winkler, W. Tad Pfeffer, Klaus Hanke, Nicolas J. Cullen, Georg Kaser  |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043686
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| Zusammenfassung |
| Ice cliffs are intriguing features of glaciers around the world, but little is known about
mechanisms of their formation and maintenance. Ice cliffs also characterize the plateau
glaciers on Kilimanjaro, Tanzania (3Ë S, 37Ë E). Their heights range from 3 to
more than 40 meters and they have at least persisted since the late 19th century
when early explorers documented them. Snow accumulation is only possible on the
flat parts of the plateau glaciers but not on the vertical or near-vertical cliffs. Dry
calving due to ice dynamics is unlikely and ice ablation through sublimation and
melt by far out weighs the negligible mass gain by deposition of water vapour.
Consequently, as soon as the cliffs are established, they are forced to retreat and thus,
the areal shrinkage of the glaciers on Africa’s highest peak is closely linked to
the existence of these cliffs. In order to extract climate change details from the
glaciers on Kilimanjaro, the sensitivity of the ice cliffs to climate fluctuations must be
understood.
Strikingly, the cliffs are mainly either north- or south-facing which entails at
this near-equatorial site that direct sunlight either always hits the cliff faces from
dawn till dusk, or not at all. The examination of the annual insolation patterns at a
25m high, south-facing sample cliff shows that it is not hit by direct shortwave
radiation from March to October. During this time not enough energy is available for
melting and only sublimation occurs. From November to February the cliff is sunlit
12 hours a day and its surface temperature can reach 0Ë C. Melting sometimes
occurs during some hours, although air temperature is almost always below freezing.
Point measurements at the sample cliff reveal a 20-30 times faster retreat during the
sunlit period because melting is a much more energy-efficient ablation process than
sublimation.
Repeat terrestrial photogrammetric surveys have been carried out at the transition dates
from the shaded to the sunlit period and vice versa. Using a calibrated DSLR camera
and photogrammetric software for automatic stereo matching it was possible to
derive digital surface models (DSMs) of the sample cliff with an accuracy of a
few cm. The comparison of the DSMs provides a laminar picture of the changes
at the cliff between the two retreat phases. Small scale variations in aspect and
slope as well as alterations in the surface roughness can be quantified. Additionally,
recession rates can be studied as a function of aspect, slope, height above ground, and
insolation phase. These findings can be used to validate future process-based mass
balance models for the ice cliff and will help to quantify the role of deformation. |
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