Improved Representation of Surface Spectral Emissivity in a Global Climate Model and its Impact on Simulated Climate

TitleImproved Representation of Surface Spectral Emissivity in a Global Climate Model and its Impact on Simulated Climate
Publication TypeJournal Article
Year of Publication2018
JournalJournal of Climate
Date Published02/2018
Abstract / Summary

Surface longwave emissivity can be less than unity and vary significantly with frequency. However, most climate models still assume a blackbody surface in the longwave (LW) radiation scheme of their atmosphere models. This study incorporates realistic surface spectral emissivity into the atmospheric component of the Community Earth System Model (CESM) version 1.1.1 and evaluates its impact on simulated climate. By ensuring consistency of the broadband surface longwave flux across different components of the CESM, the top of atmosphere (TOA) energy balance in the modified model can be attained without retuning the model. The inclusion of surface spectral emissivity, however, leads to a decrease of net upward longwave flux at the surface and a comparable increase of latent heat flux. Global-mean surface temperature difference between the modified and standard CESM simulation is 0.20 K for the fully coupled run and 0.45 K for the slab-ocean run. Noticeable surface temperature differences between the modified and standard CESM simulations are seen over the Sahara desert and polar regions. Accordingly, the climatological mean sea ice fraction in the modified CESM simulation can be less than that in the standard CESM simulation by as much as 0.1 in some regions. When spectral emissivities of sea ice and open ocean surfaces are considered, the broadband LW sea-ice emissivity feedback is estimated to be -0.003 Wm-2 per K (assuming flat ice emissivity as sea ice emissivity) and 0.002 Wm-2 per K (assuming coarse snow emissivity as sea ice emissivity), two orders of magnitude smaller than the surface albedo feedback.

URLhttp://dx.doi.org/10.1175/jcli-d-17-0125.1
DOI10.1175/jcli-d-17-0125.1
Journal: Journal of Climate
Year of Publication: 2018
Date Published: 02/2018

Surface longwave emissivity can be less than unity and vary significantly with frequency. However, most climate models still assume a blackbody surface in the longwave (LW) radiation scheme of their atmosphere models. This study incorporates realistic surface spectral emissivity into the atmospheric component of the Community Earth System Model (CESM) version 1.1.1 and evaluates its impact on simulated climate. By ensuring consistency of the broadband surface longwave flux across different components of the CESM, the top of atmosphere (TOA) energy balance in the modified model can be attained without retuning the model. The inclusion of surface spectral emissivity, however, leads to a decrease of net upward longwave flux at the surface and a comparable increase of latent heat flux. Global-mean surface temperature difference between the modified and standard CESM simulation is 0.20 K for the fully coupled run and 0.45 K for the slab-ocean run. Noticeable surface temperature differences between the modified and standard CESM simulations are seen over the Sahara desert and polar regions. Accordingly, the climatological mean sea ice fraction in the modified CESM simulation can be less than that in the standard CESM simulation by as much as 0.1 in some regions. When spectral emissivities of sea ice and open ocean surfaces are considered, the broadband LW sea-ice emissivity feedback is estimated to be -0.003 Wm-2 per K (assuming flat ice emissivity as sea ice emissivity) and 0.002 Wm-2 per K (assuming coarse snow emissivity as sea ice emissivity), two orders of magnitude smaller than the surface albedo feedback.

DOI: 10.1175/jcli-d-17-0125.1
Citation:
Huang, X, X Chen, M Flanner, P Yang, D Feldman, and C Kuo.  2018.  "Improved Representation of Surface Spectral Emissivity in a Global Climate Model and its Impact on Simulated Climate."  Journal of Climate.  https://doi.org/10.1175/jcli-d-17-0125.1.