The CAR modeling system currently incorporates 7 major complete cloud-aerosol-radiation packages from the latest global weather forecast and climate prediction models used in the key operational centers and research institutions worldwide. These include NCAR, GFDL, NCEP, NASA, ECMWF, CCCMA, CAWCR, and Fu-Liou-Gu (popular for ARM). It also contains other relatively simple packages often used in regional mesoscale modeling like WRF. As such, CAR integrates, and thus facilitates intercomparison of, the numerical representations of cloud-aerosol-radiation interactions commonly available in the modeling community.
More importantly, the CAR separates all packages from internally hard-wired codes into 7 distinct modules to build the true plug-and-play capability among individual parameterizations. It incorporates three main drivers (cloud, aerosol, radiation) to provide the hubs for mastering all alternative parameterizations for cloud properties (cover, water, radius, geometry), aerosol properties (type, profile), and radiation transfers (solar, infrared); three couplers (cld_2_rad for cloud optics, aer_2_rad for aerosol optics, aer_2_cld for aerosol impacts on cloud droplet nucleation) to interface their interactions (cloud radiative forcings, aerosol direct and indirect effects) across all spectral bands; and one external (rad_ext) to manage all external forcings, such as solar insolation, earth orbit variations, radiative gas concentrations, aerosol loadings, surface albedo, emissivity and topographic impacts, all of which can be predicted via coupling with other modules or specified by data inputs.
Given the modular design, the CAR enables all built-in cloud and aerosol parameterizations selectable and fully exchangeable to operate effectively with all radiation transfer schemes. It also allows us to have easily implemented numerous new parameterizations available in the literature (not in the listed packages) and to do so as advances continue. Thus, CAR depicts the quite comprehensive numerical representation for cloud and aerosol radiative forcings, and when corporated into climate models, it will be an useful system for better understanding the cloud-aerosol-radiation interactions. It includes free choices of alternative parameterization schemes for cloud cover (420), water (4), geometry (15), and effective particle radius (80); cloud optics for solar (264) and infrared (180); aerosol type (13), profile (8); aerosol optics for solar (24) and infrared (16); aerosol indirect effects (15); radiation transfer for solar (10) and infrared (9). It can be used to determine (via intercomparison across all schemes) or reduce (via the optimized ensemble integration) the range of the uncertainties caused by the likely cloud-aerosol-radiation interactions. That range accounts for the wide spread in climate sensitivities among GCMs and consequently their climate change projections (IPCC 2007).