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The actual influence of the snow microstructure on the snow optical properties, namely the optical shape of snow, can be quantified through the estimation of two parameters: the absorption enhancement parameter B quantifies the lengthening of the photon s path resulting from refraction and internal reflections in the ice phase, and the asymmetry parameter gG quantifies the snow s tendency to scatter light forward versus backward [2].
The results of this study show that for sunlight, snow is not equivalent to spheres or other simple shapes, contrary to what is currently the case in many snow optical models (Figure 100). For the first time, accurate values of the
PRINCIPAL PUBLICATION AND AUTHORS
Unraveling the optical shape of snow. A. Robledano (a,b), G. Picard (a), M. Dumont (b), F. Flin (b), L. Arnaud (a), Q. Libois (c), Nat. Commun. 14, 3955 (2023); https:/doi.org/10.1038/s41467-023-39671-3 (a) Univ. Grenoble Alpes, CNRS, INRAE, IRD, Grenoble INP, IGE, Grenoble (France) (b) Univ. Grenoble Alpes, Université de Toulouse, Météo-France, CNRS, CNRM, Centre d Etudes de la Neige, Grenoble (France) (c) CNRM, Université de Toulouse, Météo-France, CNRS, Toulouse (France)
REFERENCES
[1] P. Räisänen et al., The Cryosphere 11, 2919-2942 (2017). [2] Q. Libois et al., The Cryosphere 7, 1803-1818 (2013).
optical shape of snow have been deduced, values that can be directly used in climate models instead of the sphericity assumption. By using this refined knowledge of the optical shape of snow, the uncertainties related to the shape in these models would be divided by three. At the same time, these results show that, despite the very different microstructures of snow, the distance travelled by sunlight in ice is, on average, always the same. In other words, snow is fundamentally ergodic. This work represents a paradigm shift in the way snow is represented in optical models. Beyond climate simulations, these results can be beneficial wherever snow optics is important, from snow photochemistry to remote sensing algorithms.
Fig. 100: Absorption enhancement parameter B and geometric asymmetry parameter gG (and combinations) of snow at 900 nm, retrieved with two different methods: the macroscopic method (a,b), and the geometric method (c,d). Note that albedo and light penetration depend on other factors than shape, in particular on grain size, so the representation in (b,d) must be interpreted at equal snow grain size. Dark symbols correspond to geometric shapes reported in the literature and to the two-phase random medium, labelled in (a) as Malinka (2014). Coloured symbols correspond to the natural snow
samples, depending on the snow type: precipitation particles (PP), decomposing and fragmented precipitation particles (DF), faceted crystals (FC), depth hoar (DH), rounded grains (RG) and melt forms (MF).
