Sensitivity of Modeled Far-IR Radiation Budgets in Polar Continents to Treatments of Snow Surface and Ice Cloud Radiative Properties

Thursday, 18 December 2014
Xiuhong Chen, Xianglei Huang and Mark Flanner, University of Michigan, Ann Arbor, MI, United States
While most general circulation models assume spectrally independent surface emissivity and non-scattering clouds in their longwave radiation treatment, spectral variation of the index of refraction of ice indicates that, in the far-IR, snow surface emissivity can vary considerably and ice clouds can cause non-negligible scattering. These effects are more important for high-elevation polar continents where the dry and cold atmosphere is not opaque in the far-IR. We carry out two sensitivity studies to show that in a winter month over the Antarctic Plateau, including snow surface spectral emissivity and ice cloud scattering in radiative transfer calculation reduce net upward far-IR flux at both top of atmosphere and surface. The magnitudes of such reduction in monthly-mean all-sky far-IR flux range from 0.72 to 1.47 Wm-2, with comparable contributions from the cloud scattering and the surface spectral emissivity. The reduction is also sensitive to sizes of both snow grains and cloud particles. We hope that this study can raise enough attention in both modeling and observational communities to further look into these issues and improve the representations of these far-IR radiative properties and processes in GCMs.