The model cycles thru netCDF data files containing all the data the coupler expects from an atmosphere model. Each file contains one month of data consisting of one month's worth of daily mean fields. This input data must have the same domain (resolution and coordinate arrays) as the datm model. The model can cycle thru a multi-year sequence of data. Generally the input data files are created from history files that are output from the CCSM's active atmosphere component model (CAM or CCM). These files can be replaced by other data files that are in the same format. Because netCDF files are self describing, one can query the file itself for specifics about the file format.
On startup, the model reads in domain data from a netCDF file. Data exchanged with the coupler will be on the model domain. This file contains x & y coordinate arrays. The model uses a rectilinear grid with 2d coordinate arrays x(i,j) & y(i,j). Because the input atmosphere data (above) is on the same domain as the datm model, and because the input atmosphere data contains domain information, any input atmosphere data file can also serve as a domain data file. In fact, the first file in the input data file sequence is used as the domain data file
On startup, the model reads an input namelist parameter file from stdin. The model has very few input namelist parameters. Typically the only input parameters that are required are those that specify how often the model will communicate with the coupler, and those that specify the sequence of input atmosphere data files. See the section on input namelist variables for a complete list and description of namelist parameters.