Search

Snow microstructure is crucial for radia- tive transfer and mechanical interactions inside the snow cover and at the snow surface. The aim of this thesis is to determine which simplifications to general radiative trans- fer theory and to the true 3D snow microstructure can be assumed to still guarantee an adequate description of radiative transfer in snow. The analysis focuses on the optical properties reflectance and transmittance at near-infrared wavelengths and on the snow mi- crostructural parameters specific surface area (equivalent to snow grain size) and density. Three different approaches to radiative transfer are analyzed. They are characterized by (i) assuming an equivalent microstructure of a random collection of ice spheres with the same specific surface area and density as the true microstructure, (ii) ray tracing applied to the true microstructure or the equivalent microstructure and (iii) an asymptotic solu- tion for infinite snow blocks of the equivalent microstructure. Reflectance measurements can only be explained within the natural snow variability of 18% by a combination of models (i) and (ii) if the exact geometry of the compact measuring tool is implemented. Transmittance measurements of natural snow agree well with results for approach (i) and (ii). Only transmittance of highly anisotropic machine-made snow cannot be de- scribed by model (i). The results show that specific surface area and density are the two crucial properties that determine radiative transfer in snow. Finally, a combined high- resolution measurement method of these two microstructural parameters in the field is presented based on snow-profile photography.

[1] A multiscale methodology for the determination of the macroscopic optical properties of snow is presented. It consists of solving the coupled volume-averaged radiative transfer equations for two semi-transparent phases - ice and air - by Monte Carlo ray tracing in an infinite slab via direct pore-level simulations on the exact 3D microstructure obtained by computed tomography. The overall reflectance and transmittance are computed for slabs of five characteristic snow types subjected to collimated and diffuse incident radiative flux for wavelengths 0.3-3 μm. The effect of simplifying the snow microstructure and/or the radiative transfer model is elucidated by comparing our results to (i) a homogenized radiation model and considering a particulate medium made of optical equivalent grain size spheres (DISORT), or (ii) a multiphase radiation model considering a packed bed of identical overlapping semi-transparent spheres. The calculations are experimentally validated by transmittance measurements. Significant differences in the macroscopic optical properties are observed when simplifying the snow morphology and the heat transfer model (i.e., homogenized versus multiphase). The proposed approach allows - in addition to determine macroscopic optical properties based on the exact morphology and obtained by advanced heat transfer model - for detailed understanding of radiative heat transfer in snow layers at the pore-scale level. © 2012. American Geophysical Union. All Rights Reserved.