Toward Understanding Direct Absorption and Grain Size Feedbacks by Dust Radiative Forcing in Snow With Coupled Snow Physical and Radiative Transfer Modeling

The darkening of the snow surface by light-absorbing particles impacts snow albedo directly by increasing absorption of shortwave radiation in the visible wavelengths. This indirectly enhances the rate of snow grain coarsening, which determines absorption in the near-infrared wavelengths. In combination, these processes reduce snow albedo over the full range of snow reflectance, accelerating melt, and impacting regional climate and hydrology. Accurate parameterizations of snow albedo should represent both the direct and indirect radiative impacts. Here dust-influenced snow cover evolution was simulated at Senator Beck Basin Study Area, San Juan Mountains, CO with a multilayer physically based snow process model. The model was modified to track dust stratigraphy, and coupled to a snow/aerosol radiative transfer model to inform reflected shortwave radiation based on snow properties, dust concentrations, and region-specific dust optical properties. This varies from previous efforts to constrain the magnitude of accelerated melt due to dust by directly and physically representing the processes that determine the radiative impacts. Model outputs, including effective grain size, dust stratigraphy, timing of dust emergence, and albedo, were validated with a near daily snow and light-absorbing particle physical and optical property data set, and were well simulated. Daily mean radiative forcing ranged from 2 to 109 W/m(2) and was 30 W/m(2) on average over the full simulation, advancing snowmelt by 30 days. A partitioning of direct and indirect radiative impacts shows that direct absorption by dust contributes similar to 80% of total radiative forcing, with grain coarsening accounting for similar to 20%.