The Energy Exascale Earth System Model Version 3: 1. Overview of the Atmospheric Component

Abstract This paper describes the atmospheric component of the US Department of Energy's Energy Exascale Earth System Model (E3SM) version 3. Significant updates have been made to the atmospheric physics compared to earlier versions. Specifically, interactive gas chemistry has been implemented, along with improved representations of aerosols and dust emissions. A new stratiform cloud microphysics scheme more physically treats ice processes and aerosol‐cloud interactions. The deep convection parameterization has been largely improved with sophisticated microphysics for convective clouds, making model convection sensitive to large‐scale dynamics, and incorporating the dynamical and physical effects of organized mesoscale convection. Improvements in aerosol wet removal processes and parameter re‐tuning of key aerosol and cloud processes have improved model aerosol radiative forcing. The model's vertical resolution has increased from 72 to 80 layers with the extra eight layers added in the lower stratosphere to better simulate the Quasi‐Biennial Oscillation. These improvements have enhanced E3SM's capability to couple aerosol, chemistry, and biogeochemistry and reduced some long‐standing biases in simulating tropical variability. Compared to its predecessors, the model shows a much stronger signal for the Madden‐Julian Oscillation, Kelvin waves, mixed Rossby‐gravity waves, and eastward inertia‐gravity waves. Aerosol radiative forcing has been considerably reduced and is now better aligned with community best estimates, leading to significantly improved skill in simulating historical temperature records. Its simulated mean‐state climate is largely comparable to E3SMv2, but with some notable degradation in shortwave cloud radiative effect, precipitable water, and surface wind stress, which will be addressed in future updates.Plain Language Summary This study is part of a series describing the newly released version 3 of the US Department of Energy's Energy Exascale Earth System Model (E3SMv3), focusing on updates to its atmospheric component model (EAMv3). Substantial improvements have been made in representing atmospheric chemistry, aerosols, clouds, convective processes, and their interactions in the model. The model's vertical resolution in the lower stratosphere has increased to better simulate the Quasi‐Biennial Oscillation. These updates strengthen E3SM's ability to model aerosol, chemistry, and biogeochemistry, and reduce biases in tropical variability. The model now shows stronger signals for phenomena like the Madden‐Julian Oscillation and Kelvin waves. Aerosol radiative forcing is better aligned with community estimates, improving the model's skill in simulating historical temperatures. The model's simulated mean‐state climate is largely comparable to its predecessor model EAMv2.Key Points Significant updates were made to Earth System Model version 3 atmospheric physics, including gas phase chemistry, aerosols, clouds, and convection Improved cloud, convection, and vertical resolution largely improved tropical variability simulation in troposphere and stratosphere Improved aerosol representation and aerosol‐cloud interactions have led to a much‐reduced and realistic aerosol radiative forcing