Radiative transfer (RT) plays a critical role across multiple components in Earth system models (ESMs) such as the Energy Exascale Earth System Model (E3SM). Small column water vapor abundance in high latitudes implies a close coupling of longwave radiation between the surface and the atmosphere, especially in the far-IR. This trait, absent from the tropics and mid-latitudes, demands careful consideration of surface spectral emissivity and cloud longwave scattering for a faithful simulation of high-latitude climates. 3-D cloud radiative effects become more significant in high-resolution simulation. The treatment of biogeochemical processes and their interactions with the physical climate system, such as snow algae growth, requires the radiative impacts of biological components to be incorporated into the ESMs as well. Funded by the previous round of the funding opportunities focusing on E3SM development, this multi-institutional team has incorporated the surface spectral emissivity and longwave ice cloud scattering treatments into E3SM v1 and v2 alpha versions and demonstrated its early success. Building on this, the team aims to refine representations of the E3SM surface-atmosphere radiative coupling—with a special focus on the high latitudes—as follows: the team will 1) adapt their implementation to RRTMGP, a radiation code optimized for GPUs and undergoing consideration for E3SM; 2) use diagnose surface spectral emissivity at each time step from modeled land surface type compositions using the predefined surface spectral emissivity that they have developed, enabling fully capture of the radiative impacts of land surface change; 3) modify the coupler to find an efficient way to pass the spectral fluxes through different E3SM components to ensure spectrally consistent treatments of the radiative fluxes across all the E3SM components; 4) continue testing ECRAD, a radiation scheme to account for 3-D radiative transfer in the high-resolution climate modeling and 5) incorporate snow algae radiative properties into the SNICAR-AD radiation scheme that calculates snow and ice spectral albedo in E3SM, allowing the cryospheric components of E3SM to interface with the treatment of ice algae-biogeochemistry schemes.

The proposed research activities will improve the fidelity and consistency of simulated radiative coupling between the surface and the atmosphere. Such improvements in radiative couplings have great potential in alleviating biases in the E3SM polar surface climate. This work will also provide a 3-D radiation scheme for the high-resolution E3SM model and include snow algae radiative properties into the cryosphere radiation scheme. All these efforts contribute to enabling the E3SM to simulate real-world conditions with improved fidelity. By working with the RRTMGP and the coupler in the E3SM, the team will adopt the software practices and workflows most suitable for future exascale computing implementation of E3SM. Knowledge gained from this project will be generally applicable to other ESMs as well. The team has complementary expertise in radiative transfer, E3SM model development, and data-model comparisons and is uniquely qualified for the proposed tasks in improving radiation schemes in multiple components of the E3SM.