June 8, 1989






MEMORANDUM

SUBJECT:Policy Interpretation - Modeling for Intermediate Terrain

FROM:   Joseph A. Tikvart, Chief
        Source Receptor Analysis Branch, TSD (MD-14)

TO:     Alan J. Cimorelli, Lead Meteorologist
        Region III (3AM12)

    In response to your request, the Model Clearinghouse has
reviewed your position regarding modeling procedures that should
be used for "in-between" terrain, which we choose to call
"intermediate" terrain.  Our understanding of your position is as
follows.  When on-site meteorological data are available,
receptors that are located in intermediate terrain, i.e., between
stack height and plume height, should be modeled with both a
simple terrain model (with terrain "cut-off" at stack height) and
a complex terrain model and the highest of the two estimates
chosen on an hour-by-hour basis.  Estimates for averaging times
longer than 1 hour would be determined in a standard fashion and
may contain a mixture of simple terrain and complex terrain model
estimates.  This procedure would be used for both single and
multiple stack situations.

    When on-site meteorological data are not available and only
the Valley screen is available for the complex terrain estimates,
your recommendation is to allow for a case-by-case analysis where
judgments can be made on whether the controlling (design)
concentration would be associated with the simple terrain model
estimates or the Valley model estimates.  In those cases where
judgmental considerations do not lead to a probable conclusion in
that regard, it may be necessary to require the source to collect
1 year of on-site meteorological data so that the procedure in
the previous paragraph can be used.

    If the above restatement of your position is correct, then
we agree that it is appropriate.  Initially it was believed that
the language in the "Guideline on Air Quality Models," could be
satisfied by processing 1 year of data with both a simple terrain
model and a complex terrain model; the higher of the two design
concentrations (assuming that the design concentration indeed
occurs in intermediate terrain) was to be used in setting the
emission limits.

    During FY-88 a number of situations arose, involving
multiple stacks of varying heights, where it became clear that
the above procedure would not logically satisfy the guidance.  As
you point out, in a multiple source situation for a given hour a
specific receptor may be an intermediate terrain receptor for one
source while for a second source it may be either a complex
terrain or simple terrain receptor.  If one applies the above
procedure to this situation, the second source which should be
modeled using, say, a complex terrain model for the simple
terrain portion of the analysis will, for the hour in question,
be modeled in conflict with our guidance.  Because of these
difficulties we determined that the only logical way to satisfy
the guidance was to conduct the comparison on an hour-by-hour
basis when multiple stacks are involved.  A summary of that
position is contained in the FY-88 Model Clearinghouse Report. 
Finally, in your recent memorandum you point out that modeling
multiple source situations differently from single sources is not
equitable and that the hour-by-hour modeling should be required
for single stack situations as well.  As indicated above, we
agree with that position.

    Of course, the eventual availability of CTDMPLUS will
ameliorate this problem.  CTDMPLUS should be applicable to all
receptors above stack height; it will do away with the need for
using two different models and comparing the estimates.  While
there will be some ambiguity in multi-stack situations, we are
working jointly to develop straightforward guidance for such
situations.  However, for the present, we agree that your
position is the only logical approach available. 

    A related concern is the resources required to perform
modeling in complicated situations with more than one model. 
Possible approaches to deal with this problem might be:

    1.  acquire data and do analyses to substantiate/refute the
need for estimates from both models,

    2.  on a case-by-case basis make a proposal to apply the
original version of CTDM to all receptors above stack height, and

    3.  develop a general hybrid model or post-processing
software to make the analysis less resource consumptive.

Alternatives 1 and 3, while perhaps desirable, require resources
to complete; such resources are not currently identified.  For
Alternative 2, it is doubtful that very many sources will want
to, or have the data bases to, apply CTDM at the present time. 
Thus, for the foreseeable future we will need to implement the
guidance as we have in the past, using existing simple terrain
models and complex terrain models in the fashion described in
your memorandum.  Modelers will need to develop software to
process the data on a case-by-case basis.

    If you have any questions, please contact me.

cc: D. Grano, AQMD (MD-15)
    S. Reinders, TSD (MD-14)
    D. Wilson, TSD (MD-14)