The program ICON prepares chemical initial conditions (ICs) for the CCTM from either ASCII vertical profiles or from an existing CCTM output concentration (CONC) file. ICON creates an output file with a single time step that represents the chemical conditions in each grid cell at the beginning of a CCTM simulation. The ICs can be either spatially uniform or variable across the model grid, depending on the source of the initial chemical concentration data. If deriving ICs from the ASCII vertical profiles, ICON can only create spatially uniform ICs within each model layer; it can create different initial conditions across model layers. ICON can extract spatially varying ICs from CONC files on either the same grid cell resolution, as a windowed modeling domain, or for a finer resolution model grid, as for a nested simulation.

Two distinct modes of operation exist for ICON, depending on the nature of the input data. When creating ICON executables, the user must specify whether the input data will be ASCII vertical profiles or a CONC file by selecting either “profile” or “m3conc”, respectively, for the setting of the ModInpt variable. This variable determines the input module to use when creating a ICON executable.