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| Titel |
Limitations of using a thermal imager for snow pit temperatures |
| VerfasserIn |
M. Schirmer, B. Jamieson |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1994-0416
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| Digitales Dokument |
URL |
| Erschienen |
In: The Cryosphere ; 8, no. 2 ; Nr. 8, no. 2 (2014-03-11), S.387-394 |
| Datensatznummer |
250116075
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| Publikation (Nr.) |
 copernicus.org/tc-8-387-2014.pdf |
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| Zusammenfassung |
| Driven by temperature gradients, kinetic snow metamorphism plays an import
role in avalanche formation. When gradients based on temperatures measured
10 cm apart appear to be insufficient for kinetic metamorphism, faceting
close to a crust can be observed. Recent studies that visualised small-scale
(< 10 cm) thermal structures in a profile of snow layers with an infrared
(IR) camera produced interesting results. The studies found melt-freeze
crusts to be warmer or cooler than the surrounding snow depending on the
large-scale gradient direction. However, an important assumption within these
studies was that a thermal photo of a freshly exposed snow pit was similar
enough to the internal temperature of the snow. In this study, we tested this
assumption by recording thermal videos during the exposure of the snow pit
wall. In the first minute, the results showed increasing gradients with time,
both at melt-freeze crusts and artificial surface structures such as shovel
scours. Cutting through a crust with a cutting blade or shovel produced small
concavities (holes) even when the objective was to cut a planar surface. Our
findings suggest there is a surface structure dependency of the thermal
image, which was only observed at times during a strong cooling/warming of
the exposed pit wall. We were able to reproduce the hot-crust/cold-crust
phenomenon and relate it entirely to surface structure in a
temperature-controlled cold laboratory. Concave areas cooled or warmed more
slowly compared with convex areas (bumps) when applying temperature
differences between snow and air. This can be explained by increased
radiative and/or turbulent energy transfer at convex areas. Thermal videos
suggest that such processes influence the snow temperature within seconds.
Our findings show the limitations of using a thermal camera for measuring
pit-wall temperatures, particularly during windy conditions, clear skies and
large temperature differences between air and snow. At crusts or other
heterogeneities, we were unable to create a sufficiently planar snow pit
surface and non-internal gradients appeared at the exposed surface. The
immediate adjustment of snow pit temperature as it reacts with the atmosphere
complicates the capture of the internal thermal structure of a snowpack with
thermal videos. Instead, the shown structural dependency of the IR signal may
be used to detect structural changes of snow caused by kinetic metamorphism.
The IR signal can also be used to measure near surface temperatures in a
homogenous new snow layer. |
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