The regulatory default option is controlled from the MODELOPT keyword on the CO pathway.  As its name implies, this keyword controls the selection of modeling options.  It is a mandatory, non‑repeatable keyword, and it is an especially important keyword for understanding and controlling the operation of the ISC models.  As noted in Section 1, the regulatory default options, as specified in the Guideline on Air Quality Models, are truly the default options for the ISC models.  That is to say that, unless specified otherwise through the available keyword options, the ISC models implement the following regulatory options:

·    Use stack‑tip downwash (except for Schulman‑Scire downwash);

·    Use buoyancy‑induced dispersion (except for Schulman‑Scire downwash);

·    Do not use gradual plume rise (except for building downwash);

·    Use the calms processing routines;

·    Use upper‑bound concentration estimates for sources influenced by building downwash from super‑squat buildings;

·    Use default wind profile exponents; and

·    Use default vertical potential temperature gradients.

     Rather than specifying options with numeric switches, the parameters used for the MODELOPT keyword are character strings, called "secondary keywords," that are descriptive of the option being selected.  For example, to ensure that the regulatory default options be used for a particular run, the user would include the secondary keyword "DFAULT" on the MODELOPT input.  The presence of this secondary keyword tells the model to override any attempt to use a non‑regulatory default option.  The model will warn the user if a non‑regulatory option is selected along with the DFAULT option, but will not halt processing.  For regulatory modeling applications, it is strongly suggested that the DFAULT switch be set, even though the model defaults to the regulatory options without it.

     For any application in which a non‑regulatory option is to be selected, the DFAULT switch must not be set, since it would otherwise override the non‑regulatory option.  The non‑regulatory options are also specified by descriptive secondary keywords, such as "NOBID" to specify the option not to use buoyancy‑induced dispersion.  (A programmer note:  these modeling option keywords also correspond to the Fortran logical variable names used to control the options in the ISC computer code.  This is one reason why they are limited to six  characters, .e.g., DFAULT instead of DEFAULT, since the standard Fortran language (ANSI, 1978) only allows variable names up to six characters in length).

     The MODELOPT keyword, which is also used to specify the selection of rural or urban dispersion parameters, and concentration or deposition values, is described in more detail in the Section 3.2.2.



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