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Influence of grain shape on light penetration in snow
- Source :
- The Cryosphere, The Cryosphere, 2013, 7 (6), pp.1803-1818. ⟨10.5194/tc-7-1803-2013⟩, The Cryosphere, Copernicus 2013, 7 (6), pp.1803-1818. ⟨10.5194/tc-7-1803-2013⟩, The Cryosphere, Vol 7, Iss 6, Pp 1803-1818 (2013)
- Publication Year :
- 2013
- Publisher :
- HAL CCSD, 2013.
-
Abstract
- 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.
- Subjects :
- Materials science
010504 meteorology & atmospheric sciences
Irradiance
Geometric shape
01 natural sciences
010309 optics
Atmospheric radiative transfer codes
Optics
0103 physical sciences
[SDU.STU.GL]Sciences of the Universe [physics]/Earth Sciences/Glaciology
Exponential decay
lcsh:Environmental sciences
0105 earth and related environmental sciences
Earth-Surface Processes
Water Science and Technology
lcsh:GE1-350
[PHYS.PHYS.PHYS-AO-PH]Physics [physics]/Physics [physics]/Atmospheric and Oceanic Physics [physics.ao-ph]
Geometrical optics
business.industry
lcsh:QE1-996.5
Snowpack
Molar absorptivity
Snow
Computational physics
lcsh:Geology
13. Climate action
Astrophysics::Earth and Planetary Astrophysics
business
Subjects
Details
- Language :
- English
- ISSN :
- 19940424 and 19940416
- Database :
- OpenAIRE
- Journal :
- The Cryosphere, The Cryosphere, 2013, 7 (6), pp.1803-1818. ⟨10.5194/tc-7-1803-2013⟩, The Cryosphere, Copernicus 2013, 7 (6), pp.1803-1818. ⟨10.5194/tc-7-1803-2013⟩, The Cryosphere, Vol 7, Iss 6, Pp 1803-1818 (2013)
- Accession number :
- edsair.doi.dedup.....ce10a837525b65ac1d7953b95a89d0b9