Motivations: Longwave (LW) radiative transfer between the surface and atmosphere directly affects the surface energy budget and atmospheric radiative cooling. However, these radiative couplings have been modeled in an inconsistent and physically inaccurate manner in E3SM (and virtually all other ESMs as well). We have shown that such inconsistency is particularly important for the simulated high-latitude climate. Three structural biases in such LW radiative couplings are (1) Spectral consistency and spectral emissivity of the surface: the atmosphere model assumes all surfaces LW blackbodies but the surface models (including sea-ice) assume graybody surfaces, i.e., emissivity less than one and spectrally uniform. In reality, surface emissivity can vary strongly across different surfaces and wavelengths. (2) LW scattering media in the atmosphere: Clouds are assumed to be non-scattering in the LW in the E3SM while, in reality, their scattering effects sometimes can be substantial. Non-blackbody surfaces and cloud scattering allow multiple reflections between the surface and clouds, a process not represented in the E3SM at all. (3) 3D radiative effect of clouds: all models still employ the plane-parallel assumption for cloud and ignore horizontal radiative flux from the cloud while in reality, horizontal radiative flux from cloudy to clear regions can be non-negligible. All of these issues affect the fidelity of E3SM simulations.

Project Objectives and Technical Approaches: We propose the following tasks to address aforementioned structural biases: (1) Implement key improvements developed from the current ESM-SciDAC project into the E3SM, namely the treatments of surface spectral emissivity, the updated ice cloud optical properties, and the modified RRTMG_LW for LW scattering. (2) Update the sea-ice radiation code to use the same spectral bands as RRTMG_LW and to modify the coupler accordingly. (3) Conduct fully-coupled simulations with (1) and (2) to assess the impact on the simulated climate. Compare the new simulations with a suite of observations as well as default E3SM runs to understand to what extent the modifications improve the simulation and address known biases in the model. (4) Assess and test ECRAD, a radiation scheme developed by the ECMWF to account for 3-D radiative transfer. We plan to update the ice cloud optical properties in ECRAD for consistency with those used in (1) and to test its performance with respect to benchmark radiative transfer codes for a variety of cloud fields over the globe. (5) Implement ECRAD from (4) into the E3SM and test it in both single column (SCM) model and full model modes, making it as an optional radiative transfer package in E3SM. For SCM implementation, we will carry out simulations over several ARM IOP periods and assess its impact on simulated cloud fields and the radiation budget. If positive results are obtained from such SCM evaluations, we will further assess the impact on the E3SM fully coupled runs.

Targeted current Biases in the E3SM: In general, reducing structural biases in the LW radiation can help expose other factors that contribute to model biases. Specifically, our current ESM-SciDAC result shows that the inclusion of cloud LW scattering and realistic surface emission reduces all-sky OLR in the deep tropics. This can help reduce the bias of weak LW cloud radiative effect (CRE) in the E3SM. Taking cloud 3-D radiative effecting into account can affect the all-sky OLR and LW CRE in a similar way. Improved surface-atmosphere LW coupling can affect the simulated polar surface climate and help address identified biases of unrealistic freezing of the Labrador Sea in the E3SM, which in turn affects deep water formation.

Potential impacts and outcomes: The proposed work will improve the fidelity and consistency of simulated LW radiative coupling between surface and atmosphere and the fidelity of cloud LW radiative transfer in the E3SM. It will also provide an alternative LW radiation scheme for the E3SM. All these directly contribute to enabling the E3SM to simulate real-world conditions with improved fidelity. Knowledge gained from this project will be generally applicable to other earth system models as well. The proposal team has complementing expertise in radiative transfer, E3SM model development, and data-model comparisons. The synergy among team members makes this a uniquely strong team for improving radiation schemes in the E3SM.