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Title
Assessing the improved treatments of surface-atmosphere longwave coupling in the E3SM with M-PACE and AWARE observations
Authors
Xianwen Jing, Yi-Hsuan Chen, Xianglei Huang (University of Michigan), Ping Yang (Texas A&M), Wuyin Lin (BNL)
Abstract
Previous studies have identified surface spectral emissivity and cloud longwave scattering as key missing processes in the surface-atmosphere longwave coupling in the polar climate simulation. We have implemented a new ice-cloud radiation scheme into the E3SM and modified the RRTMG_LW to take longwave scattering into account. As a parallel effort to our assessment of the new schemes for coupled simulations, here we study the impact of the new schemes on the single-column model (SCM) simulations for two sub-polar campaigns by the ARM project, i.e., the M-PACE in October 2004 and AWARE in January 2016. Large-scale forcing is prescribed using the ARM observations for the M-PACE period and ERA-interim reanalysis tendency fields for the AWARE period, at a frequency of every three and one hours, respectively. The evolution of atmospheric temperature and water vapor are subject to physical tendencies in addition to the prescribed forcing, while the surface quantities (e.g., temperature, latent heat, and sensible heat) are entirely determined by the forcing. The SCMs from standard E3SM v1 and modified E3SM are both used in the simulation. The results are compared with each other as well as against the observations. Other sub-grid atmospheric physical processes are unchanged from the standard E3SM. Consistent with our findings from the fully coupled simulations, the new schemes that we implemented affect the downward longwave flux the most and have little impact on cloud and precipitation fields. The simulated changes of total water vapors and surface longwave fluxes due to the inclusion of both mechanisms are comparable to the counterparts from AMIP-type simulations. These findings suggest that the inclusion of cloud longwave scattering can improve the fidelity of the simulated energy budget but does not deteriorate other aspects of simulated climate in the high latitudes. Such an impact on the surface energy budget will be manifested through the atmosphere-surface longwave coupling, a mechanism not included in the SCM simulations.