This work documents version two of the Department of Energy's Energy Exascale Earth SystemModel (E3SM). E3SMv2 is a significant evolution from its predecessor E3SMv1, resulting in a model that is nearly twice as fast and with a simulated climate that is improved in many metrics. We describe the physical climate model in its lower horizontal resolution configuration consisting of 110 km atmosphere, 165 km land, 0.5° river routing model, and an ocean and sea ice with mesh spacing varying between 60 km in the mid-latitudes and 30 km at the equator and poles. The model performance is evaluated with Coupled Model Intercomparison Project Phase 6 Diagnosis, Evaluation, and Characterization of Klima simulations augmented with historical simulations as well as simulations to evaluate impacts of different forcing agents. The simulated climate has many realistic features of the climate system, with notable improvements in clouds and precipitation compared to E3SMv1. E3SMv1 suffered from an excessively high equilibrium climate sensitivity (ECS) of 5.3 K. In E3SMv2, ECS is reduced to 4.0 K which is now within the plausible range based on a recent World Climate Research Program assessment. However, a number of important biases remain including a weak Atlantic Meridional Overturning Circulation, deficiencies in the characteristics and spectral distribution of tropical atmospheric variability, and a significant underestimation of the observed warming in the second half of the historical period. An analysis of single-forcing simulations indicates that correcting the historical temperature bias would require a substantial reduction in the magnitude of the aerosol-related forcing.

Clouds play an essential role in global climate through interactions with radiation and hydrological cycle. The extensive coverage and strong radiative effects make clouds an important modulator of the energy budget at the surface and top of the atmosphere. Cloud radiative effects are controlled by cloud optical depth and other optical properties that are closely related to cloud microphysical properties such as amount, size, shape, and thermodynamic phase of cloud hydrometeors (Curry & Ebert, 1992;Curry et al., 1996;Shupe & Intrieri, 2004). Cloud albedo is more sensitive to variations in cloud liquid water than cloud ice water. The shortwave radiative cooling effect due to liquid water usually dominates the net cloud radiative effect in mixed-phase clouds, highlighting the importance of cloud thermodynamic phase on cloud radiative forcing (Sun & Shine, 1994). In addition, differences in microphysical properties between liquid and ice are critical for global precipitation. Satellite observations have demonstrated that most of the Earth's precipitation originates from the ice phase and mixed-phase cloud processes, while warm rain mechanisms are more critical for precipitation over tropical and subtropical oceans (Field & Heymsfield, 2015;Heymsfield et al., 2020;Mülmenstädt et al., 2015). The distinct roles of cloud liquid and cloud ice on precipitation formation make cloud phase one of the key factors influencing the hydrological cycle in the Earth system. Moreover, the cloud liquid and ice microphysical properties in the present-day climate can also have a significant impact on the Arctic amplification (Middlemas et al., 2020) and future global climate change (Bjordal et al., 2020;Lohmann & Neubauer, 2018;Tsushima et al., 2006). For example, it has been found that the predicted Arctic amplification strength is highly sensitive to the ice particle size and number concentration of ice nucleating particles in the present-day environment (Tan & Storelvmo, 2019;Tan et al., 2022). Across the globe, if clouds in the present-day climate have a lower ice water amount, the phase transition from ice to liquid would be less significant in a future warmer climate, which would result in a weaker

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Clouds cover 80%-90% of the SO region from an annual average perspective as indicated by satellite retrievals (

This study evaluates high-latitude stratiform mixed-phase clouds (SMPC) in the atmosphere model of the newly released Energy Exascale Earth System Model version 2 (EAMv2) by utilizing one-year-long ground-based remote sensing measurements from the U.S. Department of Energy Atmospheric Radiation and Measurement (ARM) Program. A nudging approach is applied to model simulations for a better comparison with the ARM observations. Observed and modeled SMPCs are collocated to evaluate their macro- and microphysical properties at the ARM North Slope of Alaska (NSA) site in the Arctic and the McMurdo (AWR) site in the Antarctic. We found that EAMv2 overestimates (underestimates) SMPC frequency of occurrence at the NSA (AWR) site nearly all year round. However, the model captures the observed larger cloud frequency of occurrence at the NSA site. For collocated SMPCs, the annual statistics of observed cloud macrophysics are generally reproduced at the NSA site, while at the AWR site, there are larger biases. Compared to the AWR site, the lower cloud boundaries and the warmer cloud top temperature observed at NSA are well simulated. On the other hand, simulated cloud phases are substantially biased at each location. The model largely overestimates liquid water path at NSA, whereas it is frequently underestimated at AWR. Meanwhile, the simulated ice water path is underestimated at NSA, but at AWR, it is comparable to observations. As a result, the observed hemispheric difference in cloud phase partitioning is misrepresented in EAMv2. This study implies that continuous improvement in cloud microphysics is needed for high-latitude mixed-phase clouds.