Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model
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, ve...
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ftcopernicus:oai:publications.copernicus.org:tc59923 2023-05-15T18:18:18+02:00 Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model Räisänen, Petri Makkonen, Risto Kirkevåg, Alf Debernard, Jens B. 2018-09-27 info:eu-repo/semantics/application/pdf https://doi.org/10.5194/tc-11-2919-2017 https://tc.copernicus.org/articles/11/2919/2017/ eng eng info:eu-repo/grantAgreement/EC/FP7/265863 info:eu-repo/grantAgreement/EC/FP7/265148 doi:10.5194/tc-11-2919-2017 https://tc.copernicus.org/articles/11/2919/2017/ info:eu-repo/semantics/openAccess eISSN: 1994-0424 info:eu-repo/semantics/Text 2018 ftcopernicus https://doi.org/10.5194/tc-11-2919-2017 2020-07-20T16:23:30Z 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. Other/Unknown Material Sea ice Copernicus Publications: E-Journals The Cryosphere 11 6 2919 2942 |
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Open Polar |
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Copernicus Publications: E-Journals |
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ftcopernicus |
language |
English |
description |
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. |
format |
Other/Unknown Material |
author |
Räisänen, Petri Makkonen, Risto Kirkevåg, Alf Debernard, Jens B. |
spellingShingle |
Räisänen, Petri Makkonen, Risto Kirkevåg, Alf Debernard, Jens B. Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model |
author_facet |
Räisänen, Petri Makkonen, Risto Kirkevåg, Alf Debernard, Jens B. |
author_sort |
Räisänen, Petri |
title |
Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model |
title_short |
Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model |
title_full |
Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model |
title_fullStr |
Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model |
title_full_unstemmed |
Effects of snow grain shape on climate simulations: sensitivity tests with the Norwegian Earth System Model |
title_sort |
effects of snow grain shape on climate simulations: sensitivity tests with the norwegian earth system model |
publishDate |
2018 |
url |
https://doi.org/10.5194/tc-11-2919-2017 https://tc.copernicus.org/articles/11/2919/2017/ |
genre |
Sea ice |
genre_facet |
Sea ice |
op_source |
eISSN: 1994-0424 |
op_relation |
info:eu-repo/grantAgreement/EC/FP7/265863 info:eu-repo/grantAgreement/EC/FP7/265148 doi:10.5194/tc-11-2919-2017 https://tc.copernicus.org/articles/11/2919/2017/ |
op_rights |
info:eu-repo/semantics/openAccess |
op_doi |
https://doi.org/10.5194/tc-11-2919-2017 |
container_title |
The Cryosphere |
container_volume |
11 |
container_issue |
6 |
container_start_page |
2919 |
op_container_end_page |
2942 |
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1766194830705164288 |