Abstract
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.
Highlights
Snow albedo, defined as the fraction of incoming solar energy reflected upwards by the snow surface, plays a fundamental role in the surface energy budget of snow-covered regions
We evaluate the effect of snow grain shape on climate simulated by Norwegian Earth System Model (NorESM) in a slab ocean configuration of the model
– Snow albedo is higher when non-spherical rather than spherical snow grains are assumed, typically by 0.02– 0.03, which results from a lower asymmetry parameter in the non-spherical case
Summary
Snow albedo, defined as the fraction of incoming solar energy reflected upwards by the snow surface, plays a fundamental role in the surface energy budget of snow-covered regions. The high albedo of snow contributes to the cold climate of high latitudes. Decreasing snow cover on land and sea ice acts to reduce the surface albedo, thereby increasing the solar radiation absorbed by the underlying surface and accelerating the warming, both on the seasonal timescale (i.e. the snowmelt in spring) and in the context of climate change The treatment of snow albedo varies considerably among climate models Many of the albedo schemes are semi-empirical rather than based on radiative transfer modelling. A simple scheme that diagnoses snow broadband albedo as a function of temperature was used in the ECHAM5 model (Roeckner et al, 2003). In ECHAM6 (Giorgetta et al, 2013), it has been replaced with a more comprehensive parameterization originally adapted from the Biosphere-Atmosphere Transfer
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