Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model

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.

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

“…This is because the four subregions (NETP, SETP, CTP, and HIMA) are dominated by the strong BC emissions in the non-monsoon season (particularly winter and spring) over South and East Asia (Lu et al, 2012;Zhang et al, 2015;Yang et al, 2018) and the efficient wet removal of BC in Asia in the monsoon season (Xu et al, 2009;He et al, 2014a). In contrast, the high concentrations during the monsoon period over NWTP and NOTP are primarily caused by the enrichment of BC via sublimation and/or melting of snow (Ming et al, 2009;Yang et al, 2015) and emissions from Central Asia and Middle East (Kopacz et al, 2011;Schmale et al, 2017).…”

“…To estimate BC-induced snow albedo effects over the TP, previous studies often used observed BC concentrations in snow/ice as inputs to snow albedo models by assuming spherical snow grains and BC-snow external mixing (e.g., Ming et al, 2013;Jacobi et al, 2015;Schmale et al, 2017;Zhang et al, 2018). This simplified treatment of BC-snow interactions has been widely used in snow albedo modeling over various snow-covered regions (e.g., Warren and Wiscombe, 1980;Aoki et al, 2011).…”

The regional climate model REMO (v2015) coupled with the 1-D freshwater lake model FLake (v1): Fenno-Scandinavian climate and lakes

“…Snow albedo is an important part of the energy cycle and at least in global climate models, the simulated temperature can be quite sensitive to relatively small changes in snow albedo (e.g., Räisänen et al, 2017). Lakes in the Nordic countries are usually always covered with snow during winter.…”

Resolving Size Distribution of Black Carbon Internally Mixed With Snow: Impact on Snow Optical Properties and Albedo

“…Black carbon (BC), the most important light‐absorbing aerosol (Bond et al, ), has been observed as a strong driver to accelerated glacier retreat and snowmelt over high mountains (Di Mauro et al, ; Painter et al, ), midlatitude seasonal snowpack (Sterle et al, ; Wang et al, ), and polar regions (McConnell et al, ; Pedersen et al, ). The impacts of BC on snow albedo can be affected by a number of factors, including BC content in snow, BC and snow particle properties, and environmental conditions (e.g., He et al, ; He, Takano, & Liou, ; Kokhanovsky, ; Räisänen et al, ; Warren & Wiscombe, ). Thus, accurate predictions of BC effects on snow albedo and regional climate require comprehensive understanding and quantification that integrates these key factors.…”