Simulations of the Optical Properties of Dirty Snow

The surface albedo is an important factor in the radiative balance of earth. Seasonally varying, large terrestrial areas are covered in snow, contributing a high albedo to the radiative balance. However, as snow layers age, their composition and physical properties evolve. The question of how dark particles contribute to variations in the albedo of snow has been subject to several experimental and theoretical studies in recent years [1-5]. The process is complicated as localized changes in the microstructure can change the depth below the surface of dirt particles. A good understanding of the radiative properties of snow is necessary to improve atmospheric models used in weather or climate simulation.

We present simulation results on backscattered, transmitted, and absorbed intensity of shortwave radiation by a dirty snow layer. This gives insight into the microphysical factors that influence this scattering behavior. A two-dimensional geometric ray-tracing algorithm was developed to study the scattering of light in a snow layer. The snow model is comprised of randomized cross-sections of plate and column ice crystals. Periodic boundary conditions allow for a large horizontal extension of the snow layer. Incident light is subject to reflection, refraction, and absorption in those particles. Each incident ray is traced until it either leaves one of the surfaces or has decreased in intensity below a threshold. We present our findings on (i) the role of reality-based geometrical shapes versus spheres, (ii) the influence of light-absorbing particles (LAPs) in various depths and densities on the reflection and transmission properties of the snow layer.

Cross sections of hexagonal prisms include triangles, squares, pentagons, and hexagons. When generating the snow layer, these shapes were randomly chosen, rotated, stretched, and placed as to emulate an experimentally observed particle density and packing of snow. The system contains then between 200 and 500 ice particles with periodic boundary conditions, corresponding to a depth between 20 and 50 monolayers of ice particles between top and bottom surface of the model. Three wavelength-dependent indices of light were considered allowing some degree of spectral resolution in the scattering processes. The backscattered, transmitted, and absorbed intensities were collected for an assessment of the radiative behavior of the snow layer. One of the questions of interest targeted the influence of using geometrically realistic shapes versus spherical particles in such a simulation. We present a comparison of the two types of snow systems. To track how LAP’s depth and density impact albedo, the snowpack was divided into ten layers with varying depth and number of LAPs. Our data collection is still ongoing at time of abstract submission. The results will be presented in our AMS poster.

Sources:

1. Dumont, M., et al., Contribution of light-absorbing impurities in snow to Greenland/'s darkening since 2009. Nature Geosci, 2014. 7(7): p. 509-512.
2. Wang, T., et al., Impacts of Satellite-Based Snow Albedo Assimilation on Offline and Coupled Land Surface Model Simulations. PLoS ONE, 2015. 10(9): p. 1-19.
3. Tuzet, F., et al., A multilayer physically based snowpack model simulating direct and indirect radiative impacts of light-absorbing impurities in snow. Cryosphere, 2017. 11(6): p. 2633-2653.
4. Qu, X. and A. Hall, On the persistent spread in snow-albedo feedback. Climate Dynamics, 2014. 42(1/2): p. 69-81.
5. Malik, M.J., et al., Semi-empirical approach for estimating broadband albedo of snow. Remote Sensing of Environment, 2011. 115(8): p. 2086-2095.