Abstract
Abstract. When contaminated by absorbing particles, such as refractory black carbon (rBC) and continental dust, snow's albedo decreases and thus its absorption of solar radiation increases, thereby hastening snowmelt. For this reason, an understanding of rBC's affect on snow albedo, melt processes, and radiation balance is critical for water management, especially in a changing climate. Measurements of rBC in a sequence of snow pits and surface snow samples in the eastern Sierra Nevada of California during the snow accumulation and ablation seasons of 2009 show that concentrations of rBC were enhanced sevenfold in surface snow (~25 ng g–1) compared to bulk values in the snowpack (~3 ng g–1). Unlike major ions, which were preferentially released during the initial melt, rBC and continental dust were retained in the snow, enhancing concentrations well into late spring, until a final flush occurred during the ablation period. We estimate a combined rBC and continental dust surface radiative forcing of 20 to 40 W m−2 during April and May, with dust likely contributing a greater share of the forcing.
Highlights
Most water resources in the western US originate as mountain snow in higher elevations and climate warming will likely lead to enhanced winter snowmelt and earlier springtime release (Bales et al, 2006)
We use measurements of refractory black carbon (rBC), continental dust, soluble ions, and some physical properties taken from a sequence of snow pits in the eastern Sierra Nevada in 2009, together with radiation modeling, to (1) characterize concentrations of rBC in Sierra Nevada snow; (2) investigate changes in concentration and movement of rBC during snow accumulation and ablation; and (3) simulate surface radiative forcing from measured rBC and continental dust in snow during winter and spring, using the Snow, Ice, and Aerosol Radiative (SNICAR) model (Flanner et al, 2007)
While atmospheric and other meteorological measurements are needed to resolve the relative roles of these processes, we find that the alternative explanations – ablation season precipitation enriched in rBC or dry deposition increasing during late spring – are unlikely to explain the observed 5 times increase in surface rBC during the ablation season (Table 1)
Summary
Most water resources in the western US originate as mountain snow in higher elevations and climate warming will likely lead to enhanced winter snowmelt and earlier springtime release (Bales et al, 2006). The graphitic portion and primary absorbing component of carbonaceous aerosols, rBC is the product of the incomplete combustion of fossil fuels and biomass (Goldberg, 1985; Guggenberger et al, 2008) These particles readily absorb solar radiation in the visible wavelengths, and when mixed with highly transparent snow crystals, even minute concentrations reduce snow albedo and enhance melt (Warren and Wiscombe, 1980). The vertical distribution of these impurities in the snowpack determines their influence on radiative forcing, both after initial deposition and during snow metamorphism and melt (Flanner and Zender, 2006) Such aerosols have atmospheric residence times ranging from days to weeks and can be transported across oceans (Hadley et al, 2007). We use measurements of rBC, continental dust, soluble ions, and some physical properties taken from a sequence of snow pits in the eastern Sierra Nevada in 2009, together with radiation modeling, to (1) characterize concentrations of rBC in Sierra Nevada snow; (2) investigate changes in concentration and movement of rBC during snow accumulation and ablation; and (3) simulate surface radiative forcing from measured rBC and continental dust in snow during winter and spring, using the Snow, Ice, and Aerosol Radiative (SNICAR) model (Flanner et al, 2007)
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