An Innovative Tool for Measuring Sunlight Propagation Through Different Snowpacks

Sunlight penetration in the snowpack plays a fundamental role in many environmental processes, ranging from local energy balance to 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, light penetration through the snowpack is currently almost only modeled numerically, frequently using severe assumptions. The lack of experimental data and dedicated studies leave a remarkable scientific gap in snow research. In this paper, we propose a novel custom-made sensor to assess sunlight propagation through the snowpack in three different spectral bands with high spatial resolution (3 mm). The probe has been designed to be very compact and lightweight and therefore easily transportable. Measurements were performed across multiple sites with different altitudes and geographic exposure, illumination conditions and snowpack characteristics. We report how the snowpack physical properties have a significant impact on the characteristic sunlight penetration length, ranging from 37.4 $\pm$ 0.1 mm up to 75.2 $\pm$ 0.4 mm in the green spectral range (550 nm central wavelength, 90 nm FWHM) varying with environmental conditions. Data are compared to numerical simulations from the “Snow, Ice and Aerosol Radiative” (SNICAR) code. This approach provides important constraints to model the snowpack characteristics, deriving values for snowpack density and average snow grain size that are very consistent with field observations. In addition, this also allows us to extrapolate the radiative information to the UV range (300 nm). UV fluxes exhibit slopes that are systematically smaller than the green ones by about 25$\%$, consistently with the fact that UV radiation penetrates deeper than visible light. Nevertheless, in some cases the comparison between our measurements and model runs suggests complex light penetration dependent on the snowpack peculiar characteristics that SNICAR 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, as well as to a quantitative description of all those processes that occur in the uppermost layers of the snowpack.