An innovative tool for measuring Sunlight propagation through different snowpacks

Abstract. Sunlight penetration in the snowpack plays a fundamental role in many environmental processes, including the local radiative energy balance, snow hydrology and snow microbiology and can potentially contribute to climate change. In addition, many photochemical reactions typically occur in the snowpack driven by solar radiation. Although a few measurements have been attempted in the past decades with several approaches, light penetration through the snowpack is currently almost only modelled numerically, frequently using severe assumptions and several parameters not always easy to be fixed. The lack of experimental data and dedicated studies leave a remarkable scientific gap in the snow research. In this paper, we propose a novel sensor, specifically designed and custom-made, to assess sunlight propagation through the snowpack in three different spectral bands. The probe has been designed to be very compact and lightweight and therefore easily transportable. We measure at different depths in the snowpack the scattered light propagating horizontally with respect to the surface with high spatial resolution (3 mm). Measurements were performed over the past two years across multiple sites with different altitudes and geographic exposure, different illumination conditions and snowpack characteristics. Data are compared to numerical simulations from the “Snow, Ice and Aerosol Radiative” (SNICAR) code, exploited here to extract the information about the light propagation at different depths. This approach provides important constraints to properly model the snowpack characteristics, allowing us to extrapolate this information to the UV radiation range. Nevertheless, in some cases the comparison between our measurement and model run suggest a more complex light penetration depending on the snowpack peculiar characteristics that SNICAR numerical simulations cannot capture. We believe that our tight experimental approach will strongly contribute to a better understanding of the radiative transfer process inside the snow layers, as well as to a quantitative description of all those chemical, physical and biological processes that occur in the uppermost layers of the snowpack.