CAM5 (Community Atmosphere Model version 5)

NCAR Community Atmosphere Model version 5



Initial contribute: 2019-10-13


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CAM has been modified substantially with a range of enhancements and improvements in the representation of physical processes since version 4 (CAM4). In particular, the combination of physical parameterization enhancements makes it possible to simulate full aerosol cloud interactions including cloud droplet activation by aerosols, precipitation processes due to particle size dependant behavior and explicit radiative interaction of cloud particles. As such the CAM5.0 represents the first version of CAM that is able to simulate the cloud-aerosol indirect radiative effects. More generally CAM5.0 forms the main atmopshere component of the COmmunity Earth System Model, version 1 (CESM1). The entensive list of physical parameterization improvements are described below:

A new moist turbulence scheme (Section pbl_uw) is included that explicitly simulates stratus-radiation-turbulence interactions, making it possible to simulate full aerosol indirect effects within stratus. It is based on a diagnostic Turbulent Kinetic Energy (TKE) forumlation and uses a 1^{st} order K-diffusion scheme with entrainment [BP09] originally developed at the University of Washington.. The scheme operates in any layer of the atmopshere when the moist Ri ( Richardson number ) is larger than its critical value.

A new shallow convection scheme (Section sec-shallow-convection) uses a realistic plume dilution equation and closure that accurately simulates the spatial distribution of shallow convective activity (Park and Bretherton 2009). A steady state convective updraft plume and small fractional area are assumed. An explicit computation of the convective updraft vertcial velocity and updraft fraction is performed using an updraft vertical momentum equation, and thus provides a representation of convective momentum transports. The scheme is specifically designed to interact with the new moist turbulence scheme in order to prevent double counting seen in previous CAM parameterizations. The deep convection parameterization is retained from CAM4.0 (Section Deep Convection).

Stratiform microphysical processes (Section sec-microphysics) are represented by a prognostic, two-moment formulation for cloud droplet and cloud ice with mass and number concentrations following the original parameterization of [MG08]. The implimentation in cam [GMG08] determines liquid and ice particle sizes from gamma functions and their evolution in time is subject to grid-scale advection, convective detrainment, turbulent diffusion and several microphysical processes. Activation of cloud droplets occurs on an aerosol size distribution based on aerosol chemistry, temperature and vertical velocity. A sub-grid scale vertical velocity is provided through a turbulent kinetic energy approximation. A number of mechanisms are calcuated for ice crystal nucleation [LPGW07] and combined with modifications to allow ice supersaturation [G+10].

The revised cloud macrophysics scheme (Section sec-macrophysics,:cite:PBR10) provides a more transparent treatment of cloud processes and imposes full consistency between cloud fraction and cloud condensate. Separate calculations are performed for liquid and ice stratiform cloud fractions which are assumed to be maximally overlapped. Liquid cloud fraction is based on an assumed triangular distribution of total relative humidity. Ice cloud fraction is based on [Get10] and allows supersaturation via a modified relative humidity over ice and the inclusion of ice condensate amount.

A new 3-mode modal aerosol scheme (MAM3, Section aerosols, [LG10]) provides internally mixed representations of number concentrations and mass for Aitkin, accumulation and course aerosol modes which are merged characterizations of the more complex 7-mode version of the scheme. Anthropogenic emissions, defined as originating from industrial, domestic and agriculture activity sectors, are provided from the [lamarque10] IPCC AR5 emission data set. Emissions of black carbon and organic carbon represent an update of [bond07] and [junker08]. Emissions of sulfur dioxide are an update of [SPW01], [smith04]. Injection heights, and size distribution of emissions data are not provided with the raw datasets so the protocols of [dentener06] are followed for CAM5.0. AEROCOM emission datastes are used for natural aeroso0l sources. All emission datasets required to run MAM for pre-industrial or 20th century scenarios are available for download. A full inventory of observationally based aerosol emission mass and size is provided in standard available datasets. The 7-mode version of the scheme is also available.

Calculations and specifications for the condensed phase optics (aerosols, liquid cloud droplets, hydrometeors and ice crystals) are taken from the microphysics and aerosol parmeterization quantities and provided as input to the radiation scheme (Section ref{sec:condensed_optics}). The radiation scheme (Section ref{sec:radiation}) has been updated to the Rapid Radiative Transfer Method for GCMs (RRTMG, [IDM+08][ mlawer]). It employs an efficient and accurate modified correlated-k method for calculating radiative fluxes and heating rates in the clear sky and for the condensed phase species. For each short-wave band calculation extinction optical depth, single scattering albedo and asymmetry properties are specified. For each long-wave band mass-specific absorption is specified. The aerosol optical properties are defined for each mode of the MAM as described by [GZ07]. Hygroscopicity characteristics are specified for soluable species. For volcanic aerosols a geometric mean radius is used. Optical properties of aerosols are combined prior to the radiative calculation. Liquid-cloud optics are calculated following [Wis96] and ice-cloud optics are calculated following [Mit02]. Ice-cloud size optics are extended to allow for radiatively active falling snow. Optical properties of clouds (including separate fractions and in-cloud water contents) are combined prior to the radiative calculation. RRTM separates the

short-wave spectrum into 14 bands extending from 0.2 \mum to 12.2 \mum, and models sources of extinction for H_2O, O_3, CO_2, O_2, CH_4, N_2 and Rayleigh scattering. Solar irradiance is now specified for the short-wave bands from the Lean dataset [WLS05]. The long-wave spectrum is separated into 16 bands extending from 3.1 \mum to 1000 \mum with molecular sources of absorption for the same species, in addition to CFC-11 (containing multiple CFC species) and CFC-12. RRTMG has extensive modifications from the original RRTM in order to provide significant speed-up for long climate integrations. Chief amongt these is the Monte-Carlo Independent Column Approximation [PM03]) that represnts sub-grid scale cloud variability. With these modifications RRTMG still retains superior offline agreement with line-by-line calculations when compared to the previous CAM radiation package (CAM-RT).



NCAR The Atmosphere Model Working Group (AMWG) (2019). CAM5 (Community Atmosphere Model version 5), Model Item, OpenGMS,


Initial contribute : 2019-10-13



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