The Formulation and Atmospheric Simulation of the Community Atmosphere Model Version 3 (CAM3)

TLDR

CAM3 is a new atmospheric general circulation model that integrates land, ocean, and sea‑ice components and has been released to the climate community. CAM3’s dynamics and physics have been substantially revised, offering Eulerian spectral, semi‑Lagrangian, and finite‑volume formulations, coupled simulation options, adjustable time‑step and cloud parameters, and major updates to moist, radiation, and aerosol parameterizations. The revisions improve tropical tropopause temperatures, boreal winter land surface temperatures, surface insolation, clear‑sky polar radiation, and ENSO‑related cloud radiative forcing, but systematic biases remain in tropical variability, oceanic surface fluxes, Southern Hemisphere heat transport, storm‑track surface stress, 500‑mb height, and the Aleutian low.

Abstract

Abstract A new version of the Community Atmosphere Model (CAM) has been developed and released to the climate community. CAM Version 3 (CAM3) is an atmospheric general circulation model that includes the Community Land Model (CLM3), an optional slab ocean model, and a thermodynamic sea ice model. The dynamics and physics in CAM3 have been changed substantially compared to implementations in previous versions. CAM3 includes options for Eulerian spectral, semi-Lagrangian, and finite-volume formulations of the dynamical equations. It supports coupled simulations using either finite-volume or Eulerian dynamics through an explicit set of adjustable parameters governing the model time step, cloud parameterizations, and condensation processes. The model includes major modifications to the parameterizations of moist processes, radiation processes, and aerosols. These changes have improved several aspects of the simulated climate, including more realistic tropical tropopause temperatures, boreal winter land surface temperatures, surface insolation, and clear-sky surface radiation in polar regions. The variation of cloud radiative forcing during ENSO events exhibits much better agreement with satellite observations. Despite these improvements, several systematic biases reduce the fidelity of the simulations. These biases include underestimation of tropical variability, errors in tropical oceanic surface fluxes, underestimation of implied ocean heat transport in the Southern Hemisphere, excessive surface stress in the storm tracks, and offsets in the 500-mb height field and the Aleutian low.