1. Introduction

The Community Atmosphere Model version 7.0 (CAM7.0) is released as the active atmosphere component of the Community Earth System Model version CESM-3.0. It is the latest in a series of global atmosphere models whose development is guided by the Atmosphere Model Working Group (AMWG) of the Community Earth System Model (CESM) project. CAM can be run in many configurations within the CESM; it is the atmosphere component in the B, E, F, Q, and P compsets. The term “standalone CAM” is often used to refer to a compset which does not include prognostic ocean and sea ice models. When one speaks of “doing CAM simulations” the implication is that it’s a standalone configuration that is being used. When CAM is coupled to prognostic land, ocean, and sea ice models then we refer to doing “fully coupled CESM simulations” which are implemented in the B compsets.

Scientific studies with CAM should always be done via a released CESM version. To get started running CAM refer to the CESM2 Quick Start Guide and the Building and Running CAM section of this User’s Guide. Running CAM using the CIME scripts provides a high level of support for doing production runs of predefined experiments on supported platforms.

CAM development takes place in the ESCOMP/CAM github repository. The development versions utilize standalone CAM configurations and are available from github, see Downloading CAM standalone.

CAM provides the basic atmospheric physics for several other models included in this release:

  • CAM-chem: Community Atmosphere Model with Chemistry

  • WACCM: Whole Atmosphere Community Climate Model

  • WACCM-X: Whole Atmosphere Community Climate Model with thermosphere and ionosphere extension

Throughout this document, we will use the name CAM in a generic sense and directions provided will be useful for CAM-chem, WACCM and WACCM-X also.

Note

To facilitate research on the effects of physical and chemical parameterizations on model simulations, it is possible to configure CAM7 to use the physics and chemistry packages used in the earlier CAM versions 4, 5, and 6. It is not, however, possible to reproduce the climates obtained from those earlier CAM versions as released in earlier CESM versions. This is due to many factors including code changes in CAM’s parameterizations and dynamical cores, as well as changes in the other CESM system components. If users wish to reproduce a climate from earlier CAM runs, they should use the CESM version corresponding to that CAM version release.

1.1. What’s new in the CAM7.0 release?

New dycore/physics features:

  • The Spectral-Element (SE) dynamical core replaces the Finite Volume (FV) dycore used in CAM4,5,6. This provides:

    • Better scalability (strong scaling) needed for variable and high resolution grids.

    • More accurate transport.

    • More advanced thermodynamics.

    • Horizontal molecular diffusion and thermal conductivity operators for WACCM-X.

  • The vertical resolution for the low-top (about 40km) version of CAM has increase to 58 levels (from 32).

  • The PUMAS microphysics code base replaces MG. This adds missing processes and hydrometeor types. PUMAS is GPU-enabled.

  • Updated L-scale CLUBB code with prognostic momentum transport.

  • The physics sequence was reordered moving the call to CLUBB to after returning from the coupler. This alleviates spurious surface wind oscillations.

  • The RTE+RRTMGP radiation code replaces RRTMG. The new code has improved algorithms and gas optics.

  • GPU support may be enabled via OpenACC directives in the PUMAS, CLUBB, and RTE-RRTMGP parameterizations.

  • New gravity wave drag parameterization adds missing sources to improve high-top stratospheric wind biases.

  • The topography software was updated to improve the ocean-atmosphere coupling. Ease of using the software has also improved.

  • Add a convective gustiness parameterization.

  • The ZM deep convection scheme has a modified convective parcel calculation to adapt to higher boundary layer resolution.

  • The Richardson number based free-atmosphere mixing from CAM4 is used to augment mixing in areas where CLUBB is not active. This improves model stability.

  • The physics parameterizations have been modified as necessary to support the height based vertical coordinate used by the MPAS dynamical core. This supports the SIMA/EarthWorks project.

  • A new simpler models configuration has been added: Moist Held-Suarez.

New chemistry features:

  • The default GHG chemistry now uses a simple prognostic scheme instead of being entirely prescribed. There is improved SOA and aerosol descriptions, and coupling with biogenic SOA precurors.

  • OASISS, DMS emissions based on Online Air-Sea Interface for Soluble Species.

  • Improved chemical mechanisms TS2,3, SLH chemistry. Different science questions require chemical mechanisms of different complexity. Air quality studies require more tropospheric chemistry, short-lived halogens are important for improved climate and ozone.

  • Option to use CARMA, a comprehesive sectional aerosol model as an alternative to the default modal aerosol model.

  • GEOS-Chem is an alternative chemistry module.

  • Alternative emissions from HEMCO which allows online regridding and incorporating regional inventories into global inventories.

  • The CAMchem MPAS configuration uses a non-hydrostatic dynamical core to allow simulations at 5km horizontal resolution.

  • TUV-x improves the representation of photochemical decomposition of reactive species generating different radicals (O1D, OH, Cl/Br/I atoms) as well as the penetration of downwelling radiation to the lower troposphere. Also improves treatment of aerosols.

New WACCM features:

  • Inline TUV-x to improve representation of photolysis and heating rates.

  • Use SE dynamical core for both WACCM and WACCM-X to maintain consistency with CAM and enable high resolution simulations.

1.2. Features removed from CAM7.0 code base:

  • The Eulerian spectral dycore.

  • The Super-parameterized CAM (SPCAM).