Abstract
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.
Highlights
Model studies indicate that black carbon (BC) deposited on snow and sea ice produces climatically significant radiative forcing at both global and regional scales by reducing surface albedo (“BC albedo forcing”) (e.g., Warren and Wiscombe, 1980; Hansen and Nazarenko, 2004; Jacobson et al, 2004; Flanner et al, 2007)
Coupling these model-derived BC mass deposition rates with observed precipitation rates can produce unrealistic values of MRBC both (1) where there are systematic biases in the prognostic model’s snowfall and (2) where the interannual variability in the model is decoupled from the observed snowfall rates used in the prescribedaerosol run or offline calculation
We argue that prescribing temporally and geographically smoothed surface BC deposition fluxes in a model where snowfall varies on typical meteorological timescales will produce high biases in timeaveraged surface snow BC mixing ratios
Summary
Model studies indicate that black carbon (BC) deposited on snow and sea ice produces climatically significant radiative forcing at both global and regional scales by reducing surface albedo (“BC albedo forcing”) (e.g., Warren and Wiscombe, 1980; Hansen and Nazarenko, 2004; Jacobson et al, 2004; Flanner et al, 2007). The mixing ratio of BC in the surface snow layer (MRBC) at each time step n is determined by the addition of BC through dry deposition (BCdepdry) and wet deposition (BCdepwet) and by the addition of new snowfall to www.atmos-chem-phys.net/14/11697/2014/. The CRU/NCEP data set specifies precipitation rates but not whether it is rain or snow, so we made the simple assumption that when the reported surface air temperature was 0 ◦C or lower the precipitation was snowfall In both cases, snow cover – the snow-water equivalent in the surface snow layer for each day and grid box – is the average across the 10 model years of the year 2000 CESM1-CAM4 run. Those grid boxes containing snow on land are included in the statistics presented below; snowfall on sea ice and BC in snow on sea ice are not considered here
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