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The Cryosphere An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/tc-2017-118
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Research article
07 Aug 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal The Cryosphere (TC).
Effects of snow grain shape on climate simulations: Sensitivity tests with the Norwegian Earth System Model
Petri Räisänen1, Risto Makkonen2, Alf Kirkevåg3, and Jens Boldingh Debernard3 1Finnish Meteorological Institute, P.O. Box 503, 00101 Helsinki, Finland
2Dept. of Physics, University of Helsinki, P.O. Box 64, 00014 University of Helsinki, Finland
3Norwegian Meteorological Institute, P.O. Box 43, Blindern, 0313 Oslo, Norway
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 (ca. 0.89). Therefore, for a given snow grain size, the use of non-spherical snow grains leads to a higher snow broadband albedo, 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 Wm−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.

Citation: Räisänen, P., Makkonen, R., Kirkevåg, A., and Boldingh Debernard, J.: Effects of snow grain shape on climate simulations: Sensitivity tests with the Norwegian Earth System Model, The Cryosphere Discuss., https://doi.org/10.5194/tc-2017-118, in review, 2017.
Petri Räisänen et al.
Petri Räisänen et al.
Petri Räisänen et al.

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Short summary
While snow grains are non-spherical, spheres are often assumed in radiation calculations. Here, we replace spherical snow grains with non-spherical snow grains in a climate model. This leads to a somewhat higher snow albedo (by 0.02–0.03), increased snow and sea ice cover, and a distinctly colder climate (by over 1 K in the global mean). It also impacts the radiative effects of aerosols in snow. Overall, this work highlights the important role of snow albedo parameterization for climate models.
While snow grains are non-spherical, spheres are often assumed in radiation calculations. Here,...
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