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| Titel |
Influence of grain shape on light penetration in snow |
| VerfasserIn |
Q. Libois, G. Picard, J. L. France, L. Arnaud, M. Dumont, C. M. Carmagnola, M. D. King |
| 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 ; 7, no. 6 ; Nr. 7, no. 6 (2013-11-26), S.1803-1818 |
| Datensatznummer |
250085184
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| Publikation (Nr.) |
 copernicus.org/tc-7-1803-2013.pdf |
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| Zusammenfassung |
| The energy budget and the photochemistry of a snowpack depend greatly on the
penetration of solar radiation in snow. Below the snow surface, spectral
irradiance decreases exponentially with depth with a decay constant called
the asymptotic flux extinction coefficient. As with the albedo of the snowpack,
the asymptotic flux extinction coefficient depends on snow grain shape. While
representing snow by a collection of spherical particles has been successful
in the numerical computation of albedo, such a description poorly explains the
decrease of irradiance in snow with depth. Here we explore the limits of the
spherical representation. Under the assumption of geometric optics and weak
absorption by snow, the grain shape can be simply described by two
parameters: the absorption enhancement parameter B and the geometric
asymmetry factor gG. Theoretical calculations show that the
albedo depends on the ratio B/(1-gG) and the asymptotic flux
extinction coefficient depends on the product B(1-gG). To
understand the influence of grain shape, the values of B and
gG are calculated for a variety of simple geometric shapes
using ray tracing simulations. The results show that B and
(1-gG) generally covary so that the asymptotic flux extinction
coefficient exhibits larger sensitivity to the grain shape than albedo. In
particular it is found that spherical grains propagate light deeper than any
other investigated shape. In a second step, we developed a method to estimate
B from optical measurements in snow. A multi-layer, two-stream, radiative
transfer model, with explicit grain shape dependence, is used to retrieve
values of the B parameter of snow by comparing the model to joint
measurements of reflectance and irradiance profiles. Such measurements were
performed in Antarctica and in the Alps yielding estimates of B between
0.8 and 2.0. In addition, values of B were estimated from various
measurements found in the literature, leading to a wider range of values
(1.0–9.9) which may be partially explained by the limited accuracy of the
data. This work highlights the large variety of snow microstructure and
experimentally demonstrates that spherical grains, with B = 1.25, are
inappropriate to model irradiance profiles in snow, an important result that
should be considered in further studies dedicated to subsurface absorption of
short-wave radiation and snow photochemistry. |
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