The Community Earth System Model version 2 (CESM2) is the latest generation of the coupled Earth system models developed as a collaborative effort between scientists, software engineers, and students from NCAR, universities, and other research institutions. It is an open-source, comprehensive model used in simulations of the Earth’s past, present, and future climates. Simulations with the model have been done as contributions to the Coupled Model Intercomparison Project phase 6 [CMIP6; Eyring et al. 2016] using the nominal 1° horizontal resolution configuration with both the low-top (40 km with limited chemistry) and high-top (140 km with comprehensive chemistry) versions of the atmospheric component. CMIP6-required core simulations are the Diagnostic, Evaluation, and Characterization of Klima (DECK) experiments that consist of a long pre-industrial control simulation; an abrupt quadrupling of CO2 concentration simulation; a 1% per year CO2 concentration increase simulation; and a simulation forced with prescribed observed sea surface temperatures and sea-ice concentrations. These are complemented by multiple simulations of the historical (1850-2014) period. In addition, CESM2 is participating in more than 20 Model Intercomparison Projects (MIPs) within CMIP6.

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. The improvements include major reductions in low latitude precipitation (see Figure) and short-wave cloud forcing biases; better representation of the Madden-Julian Oscillation; better El Niño – Southern Oscillation-related teleconnections; a global land carbon accumulation trend that agrees remarkably well with observations; a more realistic representation of Greenland's evolving ice sheet; and better simulations of how crops interact with the larger Earth system. Most features of the low- and high-top simulations are very similar to each other, so these improvements are present in both model configurations [Danabasoglu et al. 2019]. Solutions from the CESM2 CMIP6 simulations are analyzed and documented in the manuscripts collected as part of the AGU CESM2 Virtual Special Issue (also see http://www.cesm.ucar.edu/publications).

Figure: Shows the 1985-2014 average total precipitation (in mm day-1) from (top) the Global Precipitation Climatology Project [GPCP; Huffman et al. 2009] observations and (left) low-top CESM2(CAM6), high-top CESM2(WACCM6), and CESM1 Large Ensemble CESM1(LENS) simulations. The right panels show the respective model minus observations difference distributions. The global-means for each panel and the root-mean-square (rms) error for the difference distributions are also provided. Both sets of CESM2 simulations clearly depict a systematic reduction of most biases from CESM1(LENS) to CESM2. The improvements arise primarily from the reduction of the major tropical wet biases of the Indian Ocean, the South Pacific Convergence Zone, and the tropical Atlantic Inter-Tropical Convergence Zone. Over land, the dipole of Andean wet and Amazon dry biases is decreased, and excessive precipitation over Australia and Western US is reduced.

To expedite the use of CESM2 by the community primarily for CMIP6-related science and simulations, two incremental releases of CESM2 with the same base code were made available in December 2018 (CESM2.1.0) and in June 2019 (CESM2.1.1) from which many of these simulations can be run as out-of-the-box configurations. These model versions as well as the previous versions are freely available here. The CESM2 CMIP6 solutions are also freely available from the Earth System Grid Federation (ESGF) at https://esgf-node.llnl.gov/search/cmip6/ or from the NCAR Digital Asset Services Hub (DASH) at https://data.ucar.edu/

References

• Danabasoglu, G., and Coauthors, 2019: The Community Earth System Model version 2 (CESM2). J. Advances in Modeling Earth Systems (submitted).
• Eyring, V., S. Bony, G. A. Meehl, C. A. Senior, B. Stevens, R. J. Stouffer, and K. E. Taylor, 2016: Overview of the Coupled Model Intercomparison Project phase 6 (CMIP6) experimental design and organization. Geosci. Model Dev., 9, 1937-1958, doi: 10.5194/gmd-9-1937-2016.
• Huffman, G. J., R. F. Adler, D. T Bolvin, and G. Gu, 2009: Improving the global precipitation record: GPCP Version 2.1. Geophys. Res. Lett., 36, L17808, doi: 10.1029/2009GL040000.