Biases in modeled surface snow BC mixing ratios in prescribed-aerosol climate model runs

Abstract. Black carbon (BC) in snow lowers its albedo, increasing the absorption of sunlight, leading to positive radiative forcing, climate warming and earlier snowmelt. A series of recent studies have used prescribed-aerosol deposition flux fields in climate model runs to assess the forcing by black carbon in snow. In these studies, the prescribed mass deposition flux of BC to surface snow is decoupled from the mass deposition flux of snow water to the surface. Here we compare prognostic- and prescribed-aerosol runs and use a series of offline calculations to show that the prescribed-aerosol approach results, on average, in a factor of about 1.5–2.5 high bias in annual-mean surface snow BC mixing ratios in three key regions for snow albedo forcing by BC: Greenland, Eurasia and North America. These biases will propagate directly to positive biases in snow and surface albedo reduction by BC. The bias is shown be due to coupling snowfall that varies on meteorological timescales (daily or shorter) with prescribed BC mass deposition fluxes that are more temporally and spatially smooth. The result is physically non-realistic mixing ratios of BC in surface snow. We suggest that an alternative approach would be to prescribe BC mass mixing ratios in snowfall, rather than BC mass fluxes, and we show that this produces more physically realistic BC mixing ratios in snowfall and in the surface snow layer.