Radiative Transfer through the Black Carbon-Snow System: Fundamentals and Applications

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Black carbon (BC) has been identified as an important warming agent in the atmosphere. However, its radiative forcing through reducing the snow/ice albedo is currently associated with large uncertainty. The study seeks to address this uncertainty by improving the understanding of the BC-snow-radiation interactions and developing and implementing a parameterization for the spectral optical properties of BC-snow system accounting for the non-spherical snow grain shapes, and close-packed snow grains with BC-snow stochastic mixing. The parameterization will be implemented in a land surface model to improve our understanding of BC-snow-radiation and feedbacks on the surface temperature, hydrological cycle, and surface water budget. This study will conduct fundamental research on radiative transfer through atmospheric black carbon (BC) and snow systems to understand snow albedo reduction and its potential application to land surface models. This study will investigate the impact of snow-grain packing on the conventional independent scattering approximation, commonly employed for radiative transfer in planetary atmospheres, critical to snow albedo reduction. The study will also develop a physically-based parameterization for the spectral (0.2 - 5 micron) optical properties involving the coupling of atmospheric BC particles, with sizes ranging from 0.01 - 0.1 micron and core-shell structures, and snow grains, with sizes spanning from 50 - 1000 micron and shapes from snowflakes, through plates/columns, to spheroids. Geometric-optics and surface-wave (GOS) approach will be used, verified with laboratory measurements and other numerical methods, to derive necessary and sufficient databases to organize suitable lookup tables and polynomial-exponential functions readily for incorporation into a land surface model. The mixing state of the BC-snow system will also be investigated following the approach developed under a previous EAGER award for external (dry deposition) and internal (wet deposition) multiple mixtures of BC particles in snow grains using single and core-shell spheres serving as building blocks. A fourth objective includes the use of solar radiative transfer through the BC atmosphere and penetrating snow layers, using the exact adding/doubling method, but with the 2-stream approximation for consistence with the methodology included in land surface models. Noah multi-parameterizations (Noah-MP) will be used within the context of the Weather Research and Forecasting (WRF) model.