Linking snow optical properties to snow microstructure
Authors: Robledano Alvaro , Picard Ghislain , Dumont Marie , Arnaud Laurent , Flin Frédéric 
Published: March 2022

&lt;p&gt;&lt;span&gt;Snow plays a crucial role in the climate system, as its &lt;/span&gt;&lt;span&gt;high albedo is&lt;/span&gt;&lt;span&gt; unique among the Earth&amp;#8217;s surface materials. &lt;/span&gt;&lt;span&gt;S&lt;/span&gt;&lt;span&gt;everal microstructural properties such as &lt;/span&gt;&lt;span&gt;the snow specific surface area &lt;/span&gt;&lt;span&gt;strongly modulate the optical properties &lt;/span&gt;&lt;span&gt;of snow&lt;/span&gt;&lt;span&gt;. &lt;/span&gt;&lt;span&gt;Light penetration and scattering by ice particles are also impacted by other microstructural parameters, such as the grain shape. &lt;/span&gt;&lt;span&gt;Nevertheless, most radiative transfer models still &lt;/span&gt;&lt;span&gt;treat&lt;/span&gt;&lt;span&gt; snow as &lt;/span&gt;&lt;span&gt;a medi&lt;/span&gt;&lt;span&gt;um&lt;/span&gt;&lt;span&gt; composed of &lt;/span&gt;&lt;span&gt;idealized and simplified geometries, which limits the understanding of how the snow microstructure impacts the snow optical properties. A&lt;/span&gt;&lt;span&gt;ssuming geometric optics and &lt;/span&gt;&lt;span&gt;weak ice&lt;/span&gt;&lt;span&gt; absorption, &lt;/span&gt;&lt;span&gt;only two parameters are needed to describe the snow grain shape &lt;/span&gt;&lt;span&gt;in the diffusion regime.&lt;/span&gt; &lt;span&gt;T&lt;/span&gt;&lt;span&gt;hese are the absorption enhancement parameter B and the geometric asymmetry factor g&lt;sup&gt;G&lt;/sup&gt;.&lt;/span&gt; &lt;span&gt;Here we &lt;/span&gt;&lt;span&gt;aim to understand the &lt;/span&gt;&lt;span&gt;relationship between &lt;/span&gt;&lt;span&gt;the snow microstructure &lt;/span&gt;&lt;span&gt;properties and&lt;/span&gt;&lt;span&gt; the&lt;/span&gt; &lt;span&gt;shape parameters, B and g&lt;sup&gt;G&lt;/sup&gt;.&lt;/span&gt;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;&lt;span&gt;To do &lt;/span&gt;&lt;span&gt;so&lt;/span&gt;&lt;span&gt;, we&lt;/span&gt;&lt;span&gt; combine ray-tracing Monte Carlo methods with 3D images of the actual microstructure of snow, obtained wit&lt;/span&gt;&lt;span&gt;h &lt;/span&gt;&lt;span&gt;X-ray imaging and computed microtomography (&amp;#181;CT)&lt;/span&gt;&lt;span&gt;. The existing &lt;/span&gt;&lt;span&gt;Rough Surface Ray-Tracer&lt;/span&gt; &lt;span&gt;(&lt;/span&gt;&lt;span&gt;RSRT&lt;/span&gt;&lt;span&gt;)&lt;/span&gt;&lt;span&gt; model, &lt;/span&gt;&lt;span&gt;originally &lt;/span&gt;&lt;span&gt;designed&lt;/span&gt;&lt;span&gt; to simulate snow albedo over rough surfaces,&lt;/span&gt;&lt;span&gt; has been adapted &lt;/span&gt;&lt;span&gt;to &lt;/span&gt;&lt;span&gt;trace light propagation in&lt;/span&gt;&lt;span&gt; microstructure 3D images. &lt;/span&gt;&lt;span&gt;This approach allows getting rid of the simplified representation of snow in radiative transfer models, and benefits from the accurate ray-tracing calculations. &lt;/span&gt;&lt;span&gt;We present &lt;/span&gt;&lt;span&gt;here our&lt;/span&gt;&lt;span&gt; initial &lt;/span&gt;&lt;span&gt;findings&lt;/span&gt; &lt;span&gt;and results, &lt;/span&gt;&lt;span&gt;which&lt;/span&gt;&lt;span&gt; compare well with the results of the advanced radiative transfer theories that relate snow optical properties to the chord length distribution in snow microstructure. &lt;/span&gt;&lt;/p&gt;