Quoted from: https://www.cgd.ucar.edu/cms/ccm3/history.shtml 

Over the last decade, the NCAR Climate and Global Dynamics (CGD) Division has provided a comprehensive, three-dimensional global atmospheric model to university and NCAR scientists for use in the analysis and understanding of global climate. Because of its widespread use, the model was designated a community tool and given the name Community Climate Model (CCM). The original versions of the NCAR Community Climate Model, CCM0A (Washington, 1982) and CCM0B (Williamson et al., 1983), were based on the Australian spectral model (Bourke et al., 1977; McAvaney et al., 1978) and an adiabatic, inviscid version of the ECMWF spectral model (Baede 1979). The CCM0B implementation was constructed so that its simulated climate would match the earlier CCM0A model to within natural variability (e.g., incorporated the same set of physical parameterizations and numerical approximations), but also provided a more flexible infrastructure for conducting medium-- and long--range global forecast studies. The major strength of this latter effort was that all aspects of the model were described in a series of technical notes, which included a Users' Guide (Sato et al., 1983), a subroutine guide which provided a detailed description of the code (Williamson et al., 1983) a detailed description of the algorithms (Williamson, 1983), and a compilation of the simulated circulation statistics (Williamson and Williamson, 1984). This development activity firmly established NCAR's commitment to provide a versatile, modular, and well--documented atmospheric general circulation model that would be suitable for climate and forecast studies by NCAR and university scientists. A more detailed discussion of the early history and philosophy of the Community Climate Model can be found in Anthes (1986).

The second generation community model, CCM1, was introduced in July of 1987, and included a number of significant changes to the model formulation which were manifested in changes to the simulated climate. Principal changes to the model included major modifications to the parameterization of radiation, a revised vertical finite-differencing technique for the dynamical core, modifications to vertical and horizontal diffusion processes, and modifications to the formulation of surface energy exchange. A number of new modeling capabilities were also introduced, including a seasonal mode in which the specified surface conditions vary with time, and an optional interactive surface hydrology which followed the formulation presented by Manabe (1969). A detailed series of technical documentation was also made available for this version (Williamson 1987; Bath 1987a; Bath Williamson and Williamson, 1987; Hack 1989) and more completely describe this version of the CCM.

The most ambitious set of model improvements occurred with the introduction of the third generation of the Community Climate Model, CCM2, which was released in October of 1992. This version was the product of a major effort to improve the physical representation of a wide range of key climate processes, including clouds and radiation, moist convection, the planetary boundary layer, and transport. The introduction of this model also marked a new philosophy with respect to implementation. The CCM2 code was entirely restructured so as to satisfy three major objectives: much greater ease of use, which included portability across a wide range of computational platforms; conformance to a plug-compatible physics interface standard; and the incorporation of single-job multitasking capabilities.

The standard CCM2 model configuration was significantly different from its predecessor in almost every way, starting with resolution where the CCM2 employed a horizontal T42 spectral resolution (approximately 2.8 x 2.8 degree transform grid), with 18 vertical levels and a rigid lid at 2.917 mb. Principal algorithmic approaches shared with CCM1 were the use of a semi-implicit, leap frog time integration scheme; the use of the spectral transform method for treating the dry dynamics; and the use of a bi-harmonic horizontal diffusion operator. Major changes to the dynamical formalism included the use of a terrain-following hybrid vertical coordinate, and the incorporation of a shape-preserving semi-Lagrangian transport scheme (Williamson and Rasch, 1993) for advecting water vapor, as well as an arbitrary number of other scalar fields (e.g., cloud water variables, chemical constituents, etc.). Principal changes to the physics included the use of a delta-Eddington approximation to calculate solar absorption (Briegleb, 1992); the use of a Voigt line shape to more accurately treat infrared radiative cooling in the stratosphere; the inclusion of a diurnal cycle to properly account for the interactions between the radiative effects of the diurnal cycle and the surface fluxes of sensible and latent heat; the incorporation of a finite heat capacity soil/sea ice model; a more sophisticated cloud fraction parameterization and treatment of cloud optical properties (Kiehl et al., 1994); the incorporation of a sophisticated non-local treatment of boundary-layer processes (Holtslag and Boville, 1992); the use of a simple mass flux representation of moist convection (Hack, 1994), and the optional incorporation of the Biosphere-Atmosphere Transfer Scheme (BATS) of Dickinson et al. (1986). As with previous versions of the model, a User's Guide (Bath et al., 1992) and model description (Hack 1993) and the CCM1 Datasets and Circulation Statistics (Williamson et al., 1992) were provided to completely document the model formalism and implementation.