Model Change Bulletin

                               MCB#5                       8/19/93

                       CTDMPLUS (dated 93228)


     This Model Change Bulletin is to notify users of changes
made to correct problems in CTDMPLUS (dated 93076).  The specific
changes, the problems addressed, and the consequences of the
changes are summarized below.

     These revisions constitute CTDMPLUS (dated 93228) and
completely replace CTDMPLUS (dated 93076).  With this bulletin,
the source codes and executables for CTDMPLUS (dated 93228) are
being made available for downloading from the Air Quality Models
area of the File Transfer Section of SCRAM.  CTDMPLUS (dated
93076) is obsolete and, with this bulletin, will no longer be
available on SCRAM.  All copies of such should be discarded.

1.  Non-convergence in WPDF

PROBLEM:

      The WPDF subroutine determines the vertical velocity that connects
the source to the receptor through the terrain-disturbed flow.  Thirty
iterations are allowed to converge on a value for the vertical velocity.  The
resulting vertical velocity (wprime) is used in the probability density
function calculation and the y-coordinate of the position of the streamline is
used in the calculation of sigma-y.  In CTDMPLUS (dated 93076), when
convergence was not obtained, the last value of wprime was retained
correctly.  However, the position was incorrectly reset to the receptor
location.

SOLUTION:

      In CTDMPLUS (dated 93228), the position that is returned is the
one corresponding to the last guess for wprime.  For most cases, this coding
change will have no effect on predicted concentrations.  For cases in which
the non-convergence occurred, we can not predict the magnitude or the
direction of changes that could be expected.

SUBROUTINES MODIFIED:

      WPDF.FOR


2.  Wind speed used in WPDF subroutine

PROBLEM:

      In CTDMPLUS (dated 93076), the wind speed used in determining the
vertical velocity needed to connect the source to the receptor through the
terrain-disturbed flow was set to the wind speed at stack height.  This is
inconsistent since the wind speed at half the plume height is used in the
concentration equation.

SOLUTION:

      In CTDMPLUS (dated 93228), the wind speed at half the plume height is
used for all calculations.  The effect of this change will depend on the wind
speed profile between stack top and plume height.  This coding change will
have a greater effect with increased wind shear.

SUBROUTINES MODIFIED:
      WPDF.FOR, FLOWSP.FOR, DAYCALC.FOR


3.  Problem with circular ellipses

PROBLEM:

      In CTDMPLUS (dated 93076), the code for handling the special case of a
circular ellipse did not account for a value of the variable ARSQ less than
1.0.

SOLUTION:

      A check for a value of ARSQ greater than 1.0 has been added to
CTDMPLUS (dated 93228).  For most cases, this change will have no effect.  A
runtime error would have been reported if this problem had been encountered in
a particular run.

SUBROUTINES MODIFIED:
      SEQMOD.FOR


4.  New value for IUNSTA switch

        A new value for the IUNSTA switch has been added that allows the
user to model only unstable hours.  The possible values for this switch are
now:
         0 = model only stable hours
         1 = model both stable and unstable hours
         2 = model only unstable hours


5.  Changes made to the menu driver

      a.  DRIVEIT now deletes any existing output files (e.g. CONC, CTDM.OUT)
          before a new model run begins.

      b.  The error handling associated with running CTDMPLUS has been
          improved.  In addition to the detection of runtime errors that
          was a part of the last version, DRIVEIT now indicates when
          non-runtime errors (e.g.  lowest contour above base elevation) occur
          and refers you to the CTDM.OUT file for more information.

6.  Changes to RECGEN

      In some cases (especially multiple contours), RECGEN did not correctly
repaint the screen when the F2 key was pressed.  This has been corrected.


7.   Contacts for Questions (NO CHANGE)

     Questions related to the use of CTDMPLUS in specific
regulatory applications should be directed to the appropriate
regulatory agency or Regional Modeling Contact (see State/Local
Modeling Contacts in the Agency Communications Section of SCRAM).

     Generic questions related to the use and regulatory
applicability of the model should be directed to Desmond Bailey
via SCRAM EMAIL or phone (919) 541-5248.

     Questions or problems related to program execution should be
reported to Donna Schwede via SCRAM EMAIL or phone (919) 541-3255
or to Steve Perry phone (919) 541-1341.

     Users who would like to share comments, suggestions, or
helpful hints are encouraged to use the Model Conferencing area
on the SCRAM Bulletin Board (see Model Conferencing area in the
Public Communications Section of SCRAM).


     Technical correspondence should be directed to:

     Ms. Donna Schwede or
     Dr. Steven Perry
     Applied Modeling Research Branch
     ACMD/AREAL/USEPA
     MD-80
     Research Triangle Park, NC  27711

     Fax: (919) 541-1379