21 Oct 2021
21 Oct 2021
Status: this preprint is currently under review for the journal TC.

A generalized photon-tracking approach to simulate spectral snow albedo and transmissivity using X-ray microtomography and geometric optics

Theodore Letcher, Julie Parno, Zoe Courville, Lauren Farnsworth, and Jason Olivier Theodore Letcher et al.
  • US Army Corps of Engineers Cold Regions Research and Engineering Laboratory,Hanover, NH

Abstract. A majority of snow radiative transfer models (RTM) treat snow as a collection of idealized grains rather than a semi-organized ice-air matrix. Here we present a generalized multi-layer photon-tracking RTM that simulates light transmissivity and reflectivity through snow based on x-ray microtomography, treating snow as a coherent structure rather than a collection of grains. Notably, the model uses a blended approach to expand ray-tracing techniques applied to sub-1 cm3 snow samples to snowpacks of arbitrary depths. While this framework has many potential applications, this study's effort is focused on simulating light transmissivity through thin snowpacks as this is relevant for surface energy balance applications and sub-nivean hazard detection. We demonstrate that this framework capably reproduces many known optical properties of a snow surface, including the dependence of spectral reflectance on snow grain size and incident zenith angle and the surface bidirectional reflectance distribution function (BRDF). To evaluate how the model simulates transmissivity, we compare it against spectroradiometer measurements collected at a field site in east-central Vermont. In this experiment, painted panels were inserted at various depths beneath the snow to emulate thin snow. The model compares remarkably well against the spectroradiometer measurements. Sensitivity simulations using this model indicate that snow transmissivity is greatest in the visible wavelengths and is limited to the top 5 cm of the snowpack for fine-grained snow, but can penetrate as deep as 8 cm for coarser grain snow. An evaluation of snow optical properties generated from a variety of snow samples suggests that coarse grained low density snow is most transmissive.

Theodore Letcher et al.

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on tc-2021-310', Quentin Libois, 19 Nov 2021
    • AC1: 'Reply on RC1', Theodore W. Letcher, 18 Jan 2022
  • RC2: 'Comment on tc-2021-310', Anonymous Referee #2, 09 Dec 2021
    • AC2: 'Reply on RC2', Theodore W. Letcher, 18 Jan 2022

Theodore Letcher et al.

Theodore Letcher et al.


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Short summary
We present a radiative transfer model that uses ray-tracing to determine optical properties from computer generated 3D renderings of snow resolved at the microscale and to simulate snow spectral reflection and transmission for visible and near-infrared light. We are able to expand ray-tracing techniques applied to sub-1 cm3 snow samples to model an entire snowpack column. The model is able to reproduce known snow surface optical properties and simulations compare well against field observations.