the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
Cast shadows reveal changes in glacier thickness
Monika Pfau
Georg Veh
Wolfgang Schwanghart
Abstract. Increased glacier melt rates call for accurate local estimates of glacier elevation change to predict future changes in glacier runoff and their contribution to sea level rise. Glacier elevation change is typically derived from digital elevation models (DEMs) tied to surface change analysis from satellite imagery. Yet, the rugged topography in mountain regions can cast shadows onto glacier surfaces, making it difficult to detect local glacier elevation changes in remote areas. However, most optical satellite images offer precise time-stamped meta data of the solar position and angle during the acquisition. These data are useful to simulate shadows from a given DEM. Accordingly, any differences in shadow length between simulated and mapped shadows in satellite images could indicate a change in glacier elevation relative to the acquisition date of the DEM. We tested this hypothesis at five selected glaciers with long-term monitoring programs. For each glacier, we projected cast shadows on the glacier surface from freely available DEMs and compared simulated shadows to cast shadows mapped in ~40 years of Landsat images. We validated the relative differences with in situ geodetic measurements of glacier elevation change. We find that shadow-derived glacier elevation changes are consistent with independent photogrammetric and geodetic surveys in shadowed areas. Our method shows that Baltoro Glacier (Karakoram, Pakistan) gained slightly in elevation between 1987 and 2020, while Great Aletsch Glacier (Switzerland) recorded the most negative melt rates of about 1 m per year. While hydrological mass balances are averaged over a given glacier, we provide local glacier thickness changes, a vital information to quantify variances in melt rates in the accumulation or ablation zone. Our appraisal hinges on the precision of the DEM as the geometry of ridges and peaks constrain the shadow that we cast on the glacier surface. Future generations of DEMs with higher resolution and accuracy will improve our method, enriching the toolbox for estimating glacier mass balance especially in remote glacier areas with difficult field access.
Monika Pfau et al.
Status: final response (author comments only)
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RC1: 'Comment on tc-2022-194', Anonymous Referee #1, 06 Dec 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-194/tc-2022-194-RC1-supplement.pdf
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AC1: 'Reply on RC1', Monika Pfau, 14 Feb 2023
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-194/tc-2022-194-AC1-supplement.pdf
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AC1: 'Reply on RC1', Monika Pfau, 14 Feb 2023
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RC2: 'Comment on tc-2022-194', Anonymous Referee #2, 16 Dec 2022
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-194/tc-2022-194-RC2-supplement.pdf
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AC2: 'Reply on RC2', Monika Pfau, 14 Feb 2023
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-194/tc-2022-194-AC2-supplement.pdf
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AC2: 'Reply on RC2', Monika Pfau, 14 Feb 2023
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CC1: 'Comment on tc-2022-194', Niccolò Dematteis, 23 Dec 2022
Dear authors, I read with interest your work, as I am involved in similar research. I would like to inform you about a very recent publication of ours (https://ieeexplore.ieee.org/document/9997554). Our work focuses on the same topic that is treated in your manuscript, starting from the same concept but solving the issue differently. As evident, in the very last years, different research teams worked on this topic independently, thus proving the interest in this theme among the scientific glaciological community.
I have a curiosity about your work: are the Landsat images orthorectified on the same DEM? If not, can a certain bias verify if the case of the adopted DEMs are acquired at large temporal distances (since the glacier elevation where the satellite images are projected has likely varied)?
Citation: https://doi.org/10.5194/tc-2022-194-CC1 -
AC3: 'Reply on CC1', Monika Pfau, 14 Feb 2023
The comment was uploaded in the form of a supplement: https://tc.copernicus.org/preprints/tc-2022-194/tc-2022-194-AC3-supplement.pdf
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AC3: 'Reply on CC1', Monika Pfau, 14 Feb 2023
Monika Pfau et al.
Monika Pfau et al.
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