Short Description of CAM
Author: Phil Rasch
The Community Atmosphere Model (CAM3) represents the sixth generation of Atmospheric General Circulation Models (AGCMs) developed by the climate community in collaboration with the National Center for Atmospheric Research (NCAR).Like its predecessors, CAM is designed to be a modular and versatile model suitable for climate studies by the general scientific community. CAM3 can be run either as a stand-alone AGCM (coupled to the CLM land model) or as a component of the Community Climate System Model (CCSM). When coupled to the CCSM it interacts with fully prognostic land, sea-ice, and ocean models. It is also being employed in biogeochemical and physical-chemistry studies in experimental configurations.
CAM3 includes prognostic equations for three water substances: vapor, small cloud water drops, and cloud ice particles, and diagnoses the production of rain and snow mixing ratios (and their fluxes) by assuming the sources terms balance the sinks. The CAM3 employs an internally consistent formulation for the fractional condensation rate and a self-consistent treatment of the evolution of water vapor, heat, loud fraction, and both cloud condensate quantities. Condensed water detrained from shallow and frontal convection can either form precipitation or additional stratiform cloud water.
Advection and sedimentation of cloud droplets and ice particles are included in the equations governing cloud condensate.The ice effective radius used in the microphysics and radiation is a function of temperature.The effective radius and the prescribed number density of liquid droplets transition from polluted values over land surface to pristine values over ocean. This transition affects the radiative properties and microphysical evolution of these droplets.
The radiative parameterizations have been updated to include new treatments of the interactions of shortwave and longwave radiation with cloud geometry and with water vapor.The new, generalized formulation for clouds can calculate the radiative fluxes and heating rates for any arbitrary combination of maximum and random overlap. The type of overlap assumptions are independent of the the radiative parameterizations, and vary from one grid cell or time step to the next. The parameterizations are mathematically equivalent to the independent column approximation.
The absorption and emission of longwave radiation by water vapor have been updated using modern spectral line data bases and empirical approximations for the water-vapor continuum. The absorption of near-infrared radiation by water vapor has been updated using the same modern line data and approximation for the continuum.
In its default configuration, CAM3 includes the radiative effects of an aerosol climatology in its calculation of shortwave fluxes and heating rates. The CAM3 also includes an optional prognostic formulation for the evolution of natural and anthropogenic sulfate aerosol, and its interactions with clouds and radiation. CAM3 can employ any of three different formulations for the numerical representation of the atmospheric equations of motion, and dynamical transport. The standard scheme uses a traditional spectral/finite difference formulation for the model dynamics and a semi-Lagrangian formulation for the tracer transport. A complete semi-Lagrangian, and a flux form semi-lagrangian formulation are available as options. Tuned configurations of each of these formulations are available at a variety of spatial resolutions.