effects of snow grain shape on climate simulations: sensitivity tests with the norwegian earth system model
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2017
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Abstract
Snow consists of non-spherical grains of various shapes and
sizes. Still, in radiative transfer calculations, snow grains are
often treated as spherical. This also applies to the computation of
snow albedo in the Snow, Ice, and Aerosol Radiation (SNICAR) model
and in the Los Alamos sea ice model, version 4 (CICE4), both of
which are employed in the Community Earth System Model and in the
Norwegian Earth System Model (NorESM). In this study, we evaluate
the effect of snow grain shape on climate simulated by NorESM in a
slab ocean configuration of the model. An experiment with spherical
snow grains (SPH) is compared with another (NONSPH) in which the
snow shortwave single-scattering properties are based on a
combination of three non-spherical snow grain shapes optimized
using measurements of angular scattering by blowing snow.
The key difference between these treatments is that the asymmetry parameter
is smaller in the non-spherical case (0.77–0.78 in the visible
region) than in the spherical case ( ≈ 0.89). Therefore, for
the same effective snow grain size (or equivalently, the same
specific projected area), the snow broadband albedo is higher when assuming
non-spherical rather than spherical snow grains, typically by 0.02–0.03.
Considering the spherical case as the baseline, this results in an
instantaneous negative change in net shortwave radiation with a
global-mean top-of-the-model value of ca. −0.22 W m−2.
Although this global-mean radiative effect is rather modest, the
impacts on the climate simulated by NorESM are substantial. The
global annual-mean 2 m air temperature in NONSPH is 1.17 K
lower than in SPH, with substantially larger differences at high
latitudes. The climatic response is amplified by strong snow and
sea ice feedbacks. It is further demonstrated that the effect of
snow grain shape could be largely offset by adjusting the snow grain
size. When assuming non-spherical snow grains with the
parameterized grain size increased by ca. 70 %, the climatic
differences to the SPH experiment become very small.
Finally, the impact of assumed snow grain shape on the radiative effects
of absorbing aerosols in snow is discussed.
| Reference Key |
risnen2017theeffects
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| Authors | ;P. Räisänen;R. Makkonen;A. Kirkevåg;J. B. Debernard |
| Journal | journal of applied polymer science |
| Year | 2017 |
| DOI |
10.5194/tc-11-2919-2017
|
| URL | |
| Keywords |
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