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X-ray microtomography reveals the optical shape of snow

X-ray microtomography and micrometre-scale simulations of the trajectory of sunlight as it reaches a snowpack shows what snow looks like from the photon s perspective, providing a more universal representation of snow in optical models. These can be used to deliver more accurate predictions in climate modelling.

Fig. 99: 3D microstructure of natural snow sample as revealed

by X-ray tomography at beamline ID19.

Once deposited on the ground, snow is a material composed of air and ice crystals, whose shape and arrangement vary greatly at the micrometre scale. This is known as the snow microstructure. This skeleton of ice and air controls the propagation of light within the snowpack through optical phenomena such as refraction and internal reflections in the ice phase. However, despite its extreme complexity and irregularity, natural snow is still represented in a simplistic manner in almost all optical models, including those implemented in climate models. These models typically depict snow as a collection of ice particles with perfect geometric shapes, mainly spheres. Among the many implications for the energy balance of snow, this simplification leads to significant uncertainties in climate modelling, with potential impacts of up to 1.2°C on global air temperature [1].

In this new study, using a Monte Carlo ray-tracing model, the propagation of light was accurately simulated in a collection of 3D images of snow microstructures obtained by X-ray microtomography. Very different snow types were investigated, from fresh snow (also called precipitation particles, PP) to refrozen melt-freeze forms (MF). Some images were obtained at the 3SR-Lab, whereas several snow microstructures required higher resolution and were acquired with X-ray microtomography at beamline ID19, using specifically dedicated cold cells. Figure 99 shows one of the images acquired at ID19, in this case the microstructure of fresh snow.