Reflectance Modeling for Real Snow Structures Using a Beam Tracing Model

Dominik Bänninger,Margret Matzl,Claude Bourgeois,Martin Schneebeli

doi:10.3390/s8053482

Dominik Bänninger, Margret Matzl + Show 2 more

Open Access

https://doi.org/10.3390/s8053482

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Abstract

It is important to understand reflective properties of snow, for example for remote sensing applications and for modeling of energy balances in snow packs. We present a method with which we can compare reflectance measurements and calculations for the same snow sample structures. Therefore, we first tomograph snow samples to acquire snow structure images (6 × 2 mm). Second, we calculated the sample reflectance by modeling the radiative transfer, using a beam tracing model. This model calculates the biconical reflectance (BR) derived from an arbitrary number of incident beams. The incident beams represent a diffuse light source. We applied our method to four different snow samples: Fresh snow, metamorphosed snow, depth hoar, and wet snow. The results show that (i) the calculated and measured reflectances agree well and (ii) the model produces different biconical reflectances for different snow types. The ratio of the structure to the wavelength is large. We estimated that the size parameter is larger than 50 in all cases we analyzed. Specific surface area of the snow samples explains most of the difference in radiance, but not the different biconical reflectance distributions. The presented method overcomes the limitations of common radiative transfer models which use idealized grain shapes such as spheres, plates, needles and hexagonal particles. With this method we could improve our understanding for changes in biconical reflectance distribution associated with changes in specific surface area.

Highlights

The radiative transfer properties of snow are highly relevant for estimating the energy balance [1, 2], for interpreting remote sensing data [3] and for biological applications [4]
The variation of the penetration hardness was about 15% for all samples. From this result we conclude that each snow sample is homogeneous so that we can compare the reflectance measurement taking at the top of the snow sample with the results calculated from the cylindrical tomographed sample
With this study we could demonstrate by the direct comparison of reflectance measurements and radiative transfer modeling for the same snow structure that the modeling approach is very promising

Summary

Introduction

The radiative transfer properties of snow are highly relevant for estimating the energy balance [1, 2], for interpreting remote sensing data [3] and for biological applications [4]. Two opposite approaches are possible to calculate the radiative transfer properties of a snow sample, i.e. a medium with a complex geometry: (i) The snow structure is simplified to such a degree that the scattering of electromagnetic waves can be solved exactly by radiative transfer theory [6,9], (ii) the snow structure is described with a high accuracy at the expense of simplifying the physics of light scattering to be able to calculate the radiative transfer. It is the second approach we invest in this study. The motivation for this decision were the findings that the reflectance of snow in the near infrared (NIR) somehow depends on the specific surface area, a measure which is used to characterize snow structure [10, 11]

Results

Discussion

Conclusion