Black carbon-induced snow albedo reduction over the Tibetan Plateau: uncertainties from snow grain shape and aerosol–snow mixing state based on an updated SNICAR model
<p>We implement a set of new parameterizations into the widely used Snow, Ice, and Aerosol Radiative (SNICAR) model to account for effects of snow grain shape (spherical vs. nonspherical) and black carbon (BC)–snow mixing state (external vs. internal). We find that nonspherical snow grains...
| Main Authors: | , , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-08-01
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| Series: | Atmospheric Chemistry and Physics |
| Online Access: | https://www.atmos-chem-phys.net/18/11507/2018/acp-18-11507-2018.pdf |
| Summary: | <p>We implement a set of new parameterizations into the widely used Snow, Ice,
and Aerosol Radiative (SNICAR) model to account for effects of snow grain
shape (spherical vs. nonspherical) and black carbon (BC)–snow mixing state
(external vs. internal). We find that nonspherical snow grains lead to
higher pure albedo but weaker BC-induced albedo reductions relative to
spherical snow grains, while BC–snow internal mixing significantly enhances
albedo reductions relative to external mixing. The combination of snow
nonsphericity and internal mixing suggests an important interactive effect on
BC-induced albedo reduction. Comparisons with observations of clean and
BC-contaminated snow albedo show that model simulations accounting for both
snow nonsphericity and BC–snow internal mixing perform better than those
using the common assumption of spherical snow grains and external mixing. We
further apply the updated SNICAR model with comprehensive in situ
measurements of BC concentrations in the Tibetan Plateau snowpack to
quantify the present-day (2000–2015) BC-induced snow albedo effects from a
regional and seasonal perspective. The BC concentrations show distinct and
substantial sub-regional and seasonal variations, with higher values in the
non-monsoon season and low altitudes. As a result, the BC-induced regional
mean snow albedo reductions and surface radiative effects vary by up to an
order of magnitude across different sub-regions and seasons, with values of
0.7–30.7 and 1.4–58.4 W m<sup>−2</sup> for BC externally mixed with fresh and aged
snow spheres, respectively. The BC radiative effects are further complicated by uncertainty
in snow grain shape and BC–snow mixing state. BC–snow internal mixing
enhances the mean albedo effects over the plateau by 30–60 % relative to
external mixing, while nonspherical snow grains decrease the mean albedo
effects by up to 31 % relative to spherical grains. Based on this study,
extensive measurements and improved model characterization of snow grain
shape and aerosol–snow mixing state are urgently needed in order to precisely
evaluate BC–snow albedo effects.</p> |
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| ISSN: | 1680-7316 1680-7324 |