Abstract
AbstractAn overview of the Community Earth System Model Version 2 (CESM2) is provided, including a discussion of the challenges encountered during its development and how they were addressed. In addition, an evaluation of a pair of CESM2 long preindustrial control and historical ensemble simulations is presented. These simulations were performed using the nominal 1° horizontal resolution configuration of the coupled model with both the “low‐top” (40 km, with limited chemistry) and “high‐top” (130 km, with comprehensive chemistry) versions of the atmospheric component. CESM2 contains many substantial science and infrastructure improvements and new capabilities since its previous major release, CESM1, resulting in improved historical simulations in comparison to CESM1 and available observations. These include major reductions in low‐latitude precipitation and shortwave cloud forcing biases; better representation of the Madden‐Julian Oscillation; better El Niño‐Southern Oscillation‐related teleconnections; and a global land carbon accumulation trend that agrees well with observationally based estimates. Most tropospheric and surface features of the low‐ and high‐top simulations are very similar to each other, so these improvements are present in both configurations. CESM2 has an equilibrium climate sensitivity of 5.1–5.3 °C, larger than in CESM1, primarily due to a combination of relatively small changes to cloud microphysics and boundary layer parameters. In contrast, CESM2's transient climate response of 1.9–2.0 °C is comparable to that of CESM1. The model outputs from these and many other simulations are available to the research community, and they represent CESM2's contributions to the Coupled Model Intercomparison Project Phase 6.
Highlights
The Community Earth System Model Version 2 (CESM2) is the latest generation of the coupled climate/Earth system models developed as a collaborative effort between scientists, software engineers, and students from the National Center for Atmospheric Research (NCAR), universities, and other research institutions
The first coupled climate model to achieve a stable present‐day control simulation without any flux corrections after long multicentury integrations was the Climate System Model (CSM; Boville & Gent, 1998). This latter effort was followed by the Community Climate System Model Version 2 (CCSM2; Kiehl & Gent, 2004), the CCSM3 (Collins et al, 2006), and the CCSM4 (Gent et al, 2011)
The Whole Atmosphere Community Climate Model Version 6 (WACCM6) in CESM2 is configured identically to Community Atmosphere Model Version 6 (CAM6), except that it uses 70 vertical levels and its model top is at 4.5 × 10‐6 hPa
Summary
The Community Earth System Model Version 2 (CESM2) is the latest generation of the coupled climate/Earth system models developed as a collaborative effort between scientists, software engineers, and students from the National Center for Atmospheric Research (NCAR), universities, and other research institutions. This latter effort was followed by the Community Climate System Model Version 2 (CCSM2; Kiehl & Gent, 2004), the CCSM3 (Collins et al, 2006), and the CCSM4 (Gent et al, 2011) Additional capabilities such as interactive carbon‐nitrogen cycling, global dynamic vegetation and land use change due to anthropogenic activities, a marine biogeochemistry module, a dynamic ice sheet model, and new chemical and physical processes for direct and indirect aerosol effects were added to develop CCSM into an Earth system model. Output from numerous simulations using CCSM and CESM has been routinely used in studies to better understand the processes and mechanisms responsible for climate variability and change Most of these studies make use of CCSM's and CESM's contributions to the various phases of the Coupled Model Intercomparison Project (CMIP). Solutions from the PI control and historical simulations as well as many other CESM2 DECK and MIP simulations are analyzed and documented in more detail in the manuscripts collected as part of the AGU CESM2 Virtual Special Issue
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