United States	Atmospheric Sciences
Environmental Protection	Research Laboratory
Agency	Research Triangle Park NC 27711
Research and Development	EPA'600/S8-88/003 Feb. 1988
wEPA Project Summary
The Complex Terrain Dispersion
Model (CTDM) Preprocessor
System: User Guide and
Program Description
Michael T. Mills, Elizabeth M. Insley, Robert J. Paine, and Bruce A. Egan
This report describes the theory
and operation of a terrain
preprocessor computer program
which approximates actual terrain
features with mathematical functions.
The best-fit parameters for these
functions are used by the Complex
Terrain Dispersion Model (CTDM) in
the calculation of lateral and vertical
streamline displacement, an
important step in the calculation of
concentrations at hill receptor
locations.
This Project Summary was
developed by EPA's Atmospheric
Sciences Research Laboratory,
Research Triangle Park, NC, to
announce key findings of the
research project that is fully
documented in a separate report of
the same title (see Project Report
ordering information at back).
Introduction
The Complex Terrain Dispersion
Model (CTDM) is a model designed to
estimate ground-level concentrations on
elevated terrain during periods in which
the atmosphere is stably stratified. The
model provides concentration estimates
for receptors on several isolated hills for
multiple averaging periods. The model
can accept multiple terrain features;
however, the flow is only influenced by
one hill at a time.
The central feature of CTDM is its use
of a critical dividing-streamline height
(Hc) to separate the flow into two discrete
layers. This basic concept was
suggested by theoretical arguments and
was demonstrated through laboratory
experiments. The flow below Hc is
restricted to lie in a nearly horizontal
plane, allowing little motion in the vertical
Consequently, plume material below Hc
travels along and around the terrain.
rather than up and over the terrain. The
flow above Hc is allowed to rise up and
over the terrain. Two separate
components of CTDM compute ground-
level concentrations resulting from
material in each of these flows
An important step in the calculation of
concentrations at receptors above Hc is
the determination of lateral and vertical
streamline displacement. The calculation
of these displacements for a hill of
arbitrary shape would require the use of
an elaborate numerical model and
significant computing resources, neither
of which can be-justified on the basis of
increased accuracy of the concentration
predictions. The current version of the
model is designed to run on a
microcomputer.
If one assumes that the portion of the
hill above Hc can be fit to a simple
mathematical surface, then the lateral
and vertical streamline displacements
can be estimated from analytical
expressions which can be rapidly
evaluated For CTDM to make use of this
idealized terrain, the model must have

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access to hill-fit parameters for a range
of Hc values.
Requirements for a CTDM
Terrain Preprocessor
CTDM requires much more
information about hills than other
Gaussian models. CTDM needs a
three-dimensional representation of
each hill. Therefore, the Terrain
Preprocessor produces an analytical
description of the hill shape. Although
CTDM will accept several distinct hills,
the Terrain Preprocessor will processs
only one hill at a time. Hence, the Terrain
Preprocessor must be run for each hill
and the resulting files may be appended
to one large rain file for input to the
CTDM. One constraint of CTDM is that in
the calculations, only one isolated hill is
considered at a time. A discussion whose
purpose is to aid the user in selecting
distinct terrain features is included in an
appendix to the report.
Summary of Preprocessor
Operation
Two programs must be run to
generate terrain input parameters to
CTDM for a given hill. A third program
allows the user to display the contours
for the actual and fitted hills. The first
program, FITCON, asks the user to
define in terms of its name, identification
number, maximum elevation and x,y
coordinates of the hill center. The user
then specifies the name of a master file
of digitized contour data and a file to be
used for diagnostic output during the
fitting process. In the master file, the
following data are provided for each
contour:
•	Contour identification number
•	Contour elevation
•	Number of digitized plots
•	A code indicating whether a contour is
input as complete or incomplete
•	x, y coordinates of the digitized
contour points.
The user chooses one of the following
three methods for selection of contours
from the master file: (1) all contours
selected, (2) contours selected based
upon a range of user-specified contour
identification numbers, or (3) the
specification of a file containing the
contour identification numbers for the hill
in question. Before a contour is accepted
for processing, it must pass a number of
tests Incomplete contours are closed by
a reflection of points through the hill
center or contour centroid. The program
provides special processing for those
contours which are to be a series of
multiple contours at the same elevation
After qualification and editing, the area
and centroid coordinates of each contour
are determined by numerical integration.
Each contour is then fit to an ellipse by
first finding the slope of the line through
the centroid in the plane of the contour,
which gives the largest second moment
for the area within the contour. In the
determination of this maximum second
mament for a contour to 10° resolution,
eighteen lines having equal angular
spacing are used. The line associated
with the maximum second moment is
assumed to define the orientation of the
minor axis of the ellipse representing the
contour. The lengths of the semi-major
and semi-minor axes for this ellipse are
calculated from the analytical
expressions for the area and second
moment of an ellipse.
These fitted ellipse parameters for
each contour are input to the second
preprocessor program, HCRIT, which
determines, for the portion of the hill
above a given critical elevation, the
best-fit inverse polynomial profiles
along the hill major and minor axes. The
center coordinates of the fitted hill are
calculated as the mean of the ellipse
center coordinates for those contours
above a given critical elevation. The
orientation of the fitted hill is calculated
as a vector average of the ellipse
orientations, weighted by the ellipse
eccentricity. The user can specify the
critical elevations to be used by HCRIT
in two ways. The first option is to have
each contour elevation, with the
exception of the uppermost, serve as a
critical elevation. Alternatively, the user
can specify a number of equally-spaced
critical elevations between the lowest and
uppermost contour. In the inverse
polynomial fit to the hill profile, a critical
elevation is treated as an effective hill
base. HCRIT provides an input file for
CTDM which contains the following
information for each critical elevation:
•	Ellipse parameters corresponding to
the contour at the critical elevation
(these parameters are interpolated in
the case where a critical elevation
does not correspond to a contour
elevation)
•	Coordinates of the center of the fitted
hill
•	Orientation of the major axis of the
fitted hill with respect to north
•	The length scales and exponents for
the inverse polynomial fits along the
hill's major and minor axes
The third preprocessor program,
PLOTCON, uses plot files from FITCON
and HCRIT to generate the following
screen displays which air in
evaluation of the hill-fitting process:
•	Map of digitized contours eithe
they were input or after they h*
been qualified and edited
•	Map of the digitized contours and their
associated fitted ellipses
•	For each critical cutoff elevation, a
map showing the digitized contours
and the contours of the fitted hill at
elevations corresponding to the
elevations of those digitized contours
above the critical elevation

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Michael T. Mills, Elizabeth M. Insley, Robert J. Paine, and Bruce A. Egan are with
ERT, inc., Concord, MA 01742.
Peter Finkelsteiri is the EPA Project Officer (see below).
The complete report, entitled "The Complex Terrain Dispersion Model (CTDM)
Preprocessor System: User Guide and Program Description," (Order No. PB 88-
162 094iAS: Cost: $19.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
	Research Triangle Park, NC 27711
U.S. OFFICIAL MAIL
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
; 0 .2 2 --

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Official Business
Penalty for Private Use $300
EPA 600 S8-88 003

•U.S. GPO 548-013*

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