1. Introduction

The Community Atmosphere Model version 6.3 (CAM6.3) is released as the active atmosphere component of the Community Earth System Model version CESM-2.2. 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 it as a “fully coupled CESM simulation” which are implemented in the B compsets.

To get started running CAM refer to the CESM2 Quick Start Guide and the Building and Running CAM within CESM section of this User’s Guide. Running CAM using the CESM scripts provides a high level of support for doing production runs of predefined experiments on supported platforms. This is the place to start for most users.

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.

It should be noted that in CAM6, we are unable to support reproducibility of CAM4 and CAM5 numerical results with what a user would get running those configurations in CESM1.2 or prior. This is due to many factors including code changes and namelist settings. While a user is still able to set the “-phys” namelist setting to either cam4 or cam5, the results will differ with what a user would get using those settings in CESM1.2. Due to these changes, a number of compsets specific to CAM4 or CAM5 have been removed. We recommend that if a user wants a pure CAM4 or CAM5 run, that they use CESM1.2 for those runs. WACCM-X which utilizes CAM4 does not have this issue and it should be run using the CESM2 release of CAM.

1.1. What’s new in the CAM6.3 release?

The CAM6.3 is a developmental release mostly centered around new dynamical core configurations and capabilities. New configurations/capabilities include:

  • The Spectral-Element (SE) dynamical core has been further developed since the CESM2.0 release. Algorithmic improvements include:
    • New Exner function pressure gradient formulation and new reference profiles for damping to improve flow over orography.
    • New vertical remapping algorithms.
  • Extensive new capabilities for variable resolution SE applications which include three new resolutions/configurations (CONUS, ARCTIC and ARCTICGRIS) and tools for making new grids are.
  • Developmental setups for coupled simulations using CAM-SE and CAM-SE-CSLAM (Conservative Semi-Lagrangian multi-tracer transport scheme).
  • NOAA’s FV3 dynamical core used operationally by EMC (Environmental Modeling Center) has been coupled with CAM6 physics.
  • A new simpler models configuration has been added: Moist Held-Suarez.

These new configurations are not fully scientifically supported in the sense of extensive tuning, testing and vetting. Hence they are referred to as developmental configurations.