Model Change Bulletin

                                    MCB#9                       02/04/2002

                             ISCST3 (dated 02035)

    This Model Change Bulletin documents changes made to the Industrial
Source Complex Short Term (ISCST3) Dispersion Model (see Code Changes below).

[NOTE:  New ISC3 users are urged to read ISC3 MCB#1, Amendment #1 for
important information.  In addition, this change to the ISCST3 model does
not address the anomalous behavior of the dry deposition algorithm described
in MCB#2 and MCB#5.]

CODE CHANGES

    Changes from ISCST3 (dated 00101) include the following:

1.  Correction of a problem with allocating data storage for the
    SHRDOW emission factor option when all SHRDOW input records are in
    INCLUDED files.

2.  Correction to a potential problem with concatenated meteorological
    data files when the header record has been removed between the
    individual years.  The problem occurred when the surface station ID
    matched the year of the concatenated data file.

3.  A modification to the area source optimizations under the
    TOXICS option to address potential problems with the distance
    at which the algorithm switches to a point source approximation
    for very elongated area sources.

4.  Addition of a non-default option (HE>ZI) to address a potential
    problem that may occur for cases when the receptor elevation is
    below the stack base elevation.  In these cases the mixing height
    (ZI), which is terrain-following, may drop below the plume centerline
    height (HE), which is horizontal, resulting in anomalously large
    concentrations due to the actual plume approaching the centerline
    of one of the image plumes in the Vertical Term.  When the HE>ZI option
    is specified on the CO MODELOPT card, the model limits the plume
    centerline height (HE) to be less than or equal to the mixing height
    (ZI) when calculating the Vertical Term.  The model also generates
    informational messages that identify when this adjustment has been made.
    Since HE>ZI is a non-default option, it will be overridden if the
    DFAULT option is specified.



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                           *   IMPORTANT   *
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    This program will henceforth be identified by its name and a Julian date,
i.e.: ISCST3 (dated 02035).  Future changes to this model will be identified
by an updated Julian date found at the beginning of the model source code
and followed by a reference to a SCRAM Model Change Bulletin (MCB#n)
which will explain the nature and content of the changes.  All Model Change
Bulletins for this program will always be available for back reference on
this website.  In the future, notification of changes (MCBs) to this program
will be made in the SCRAM ALERTS section of this website.