Linking snow optical properties to snow microstructure

Snow plays a crucial role in the climate system, as its high albedo is unique among the Earth’s surface materials. Several microstructural properties such as the snow specific surface area strongly modulate the optical properties of snow. Light penetration and scattering by ice particles are also impacted by other microstructural parameters, such as the grain shape. Nevertheless, most radiative transfer models still treat snow as a medium composed of idealized and simplified geometries, which limits the understanding of how the snow microstructure impacts the snow optical properties. Assuming geometric optics and weak ice absorption, only two parameters are needed to describe the snow grain shape in the diffusion regime. These are the absorption enhancement parameter B and the geometric asymmetry factor g^G. Here we aim to understand the relationship between the snow microstructure properties and the shape parameters, B and g^G.

 

To do so, we combine ray-tracing Monte Carlo methods with 3D images of the actual microstructure of snow, obtained with X-ray imaging and computed microtomography (µCT). The existing Rough Surface Ray-Tracer (RSRT) model, originally designed to simulate snow albedo over rough surfaces, has been adapted to trace light propagation in microstructure 3D images. This approach allows getting rid of the simplified representation of snow in radiative transfer models, and benefits from the accurate ray-tracing calculations. We present here our initial findings and results, which compare well with the results of the advanced radiative transfer theories that relate snow optical properties to the chord length distribution in snow microstructure.