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

Abstract

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-the-atmosphere (TOA) energy balance in the modified model can be attained without retuning the model. 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 slabocean 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 20.003 W m 22 K 21, assuming flat ice emissivity as sea ice emissivity, and 0.002 W m 22 K 21 , assuming coarse snow emissivity as sea ice emissivity, which are two orders of magnitude smaller than the surface albedo feedback.

Local Radiative Feedbacks Over the Arctic Based on Observed Short‐Term Climate Variations

“…Several feedbacks such as albedo feedback (Manabe & Wetherald, ), temperature feedback (Pithan & Mauritsen, ), and cloud feedback (Vavrus, ) have been reported to contribute to AA. The surface longwave emissivity and associated feedbacks are also found to have discernable impact on the outgoing longwave radiation and surface temperature in the Arctic (Feldman et al, ; Huang et al, ). There are, however, uncertainties and disagreement about the role of different feedbacks.…”

“…Several feedbacks such as albedo feedback (Manabe & Wetherald, 1975), temperature feedback (Pithan & Mauritsen, 2014), and cloud feedback (Vavrus, 2004) have been reported to contribute to AA. The surface longwave emissivity and associated feedbacks are also found to have discernable impact on the outgoing longwave radiation and surface temperature in the Arctic (Feldman et al, 2014;Huang et al, 2018) has often been considered as the main contributor (e.g., Screen & Simmonds, 2010;Taylor et al, 2013). However, Winton (2006) reported that it is not a dominating factor in the simulated AA, and AA still exists in a coupled climate model with locked albedo (Graversen & Wang, 2009).…”

Intercomparison and improvement of two-stream shortwave radiative transfer schemes in Earth system models for a unified treatment of cryospheric surfaces

“…8). Improved treatment of surface cryospheric radiative properties in the thermal infrared has recently been shown to remediate significant climate simulation biases in polar regions (Huang et al, 2018). It is hoped that adoption of improved and consistent treatments of solar radiative properties for snow-covered surfaces as described in this study will further remediate simulation biases in snow-covered regions.…”

Using AIRS and ARM SGP Clear‐Sky Observations to Evaluate Meteorological Reanalyses: A Hyperspectral Radiance Closure Approach

“…Given that the sounding profiles extend only to ~20 km and no ozone profiles were observed, the temperature and humidity profiles above 20 km and ozone profiles from MERRA‐2 are used in conjunction with the ARM SGP data for the forward radiative transfer calculation in this study. In addition, to best represent the surface radiative properties, upward and downward longwave (LW) fluxes at the surface measured by a pyrgeometer at a tower 10 m above the surface (Rutan & Charlock, ) are used to derive surface skin temperatures ( T s ) by numerically solving the following equation (Huang et al, ), where and are LW upward and downward fluxes at the surface; B v ( T s ) is the Planck's function at T s ; ε v and are surface spectral emissivity and the surface LW broadband emissivity, respectively. is an average of ε v weighted by B v ( T s ).…”