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The Cryosphere An interactive open-access journal of the European Geosciences Union
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research article 20 Feb 2019

Research article | 20 Feb 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal The Cryosphere (TC).

Inter-comparison and improvement of 2-stream shortwave radiative transfer models for unified treatment of cryospheric surfaces in ESMs

Cheng Dang1, Charles S. Zender1, and Mark G. Flanner2 Cheng Dang et al.
  • 1Department of Earth System Science, University of California, Irvine, CA, USA
  • 2Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI, USA

Abstract. Snow is an important climate regulator because it greatly increases the surface albedo of large parts of the Earth. Earth System Models (ESMs) often adopt 2-stream approximations with different radiative transfer techniques, the same snow therefore has different solar radiative properties depending whether it is on land or on sea ice. Here we inter-compare three 2-stream algorithms widely used in snow models, improve their predictions at large zenith angles, and introduce a hybrid model suitable for all cryospheric surfaces in ESMs. The algorithms are those employed by the SNow ICe and Aerosol Radiative (SNICAR) module used in land models, and by Icepack, the column physics used in the Los Alamos sea ice model CICE and MPAS-seaice, and a 2-stream discrete ordinate (2SD) model. Compared with a 16-stream benchmark model, the errors in snow visible albedo for a direct-incident beam from all three 2-stream models are small (<±0.005) and increase as snow shallows, especially for aged snow. The errors in direct near-infrared (near-IR) albedo are small (<±0.005) for solar zenith angles θ < 75°, and increase as θ increases. For diffuse incidence under cloudy skies, Icepack produces the most accurate snow albedo for both visible and near-IR (<±0.0002) with the lowest underestimate (−0.01) for melting thin snow. SNICAR performs similarly to Icepack for visible albedos, with a slightly larger underestimate (−0.02), while it overestimates the near-IR albedo by an order of magnitude more (up to 0.04). 2SD overestimates both visible and near-IR albedo by up to 0.03. We develop a new parameterization that adjusts the underestimated direct near-IR albedo and overestimated direct near-IR heating persistent across all 2-stream models for solar zenith angles > 75°. These results are incorporated in a hybrid model SNICAR-AD, which can now serve as a unified solar radiative transfer model for snow in ESM land, land ice, and sea-ice components.

Cheng Dang et al.
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Cheng Dang et al.
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