United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-87/058 Mar. 1988
vxEPA Project Summary
User's Guide to the Complex
Terrain Dispersion Model
Robert J. Paine, David G. Strimaitis, Michael G. Dennis,
Robert J. Yamartino, Michael T. Mills, and Elizabeth M. Insley
The Complex Terrain Dispersion
Model (CTDM) is a refined air quality
model for use in stable and neutral
conditions in complex terrain applica-
tions. Its use of meteorological input
data and terrain information is different
than current EPA models; considerable
detail for both types of input data are
required and are supplied by preproces-
sors specifically designed for CTDM.
CTDM requires the parameterization of
individual hill shapes using the terrain
preprocessor and the association of
each model receptor with a particular
hill (except for receptors in flat terrain,
which CTDM can also handle).
A central feature of CTDM is its use
of a critical dividing-streamline height
(Hc) to separate the flow in the vicinity
of a hill into two separate layers. Flow
in the upper layer has sufficient kinetic
energy to pass over the top of the hill
while streamlines in the lower layer are
constrained to flow in a horizontal
plane around the hill. Two separate
components of CTDM compute
ground-level concentrations resulting
from plume material in each of these
flows: LIFT handles the flow above Hc,
and WRAP handles the flow below Hc.
Hourly profiles of wind and temper-
ature measurements are used by CTDM
to compute plume rise, the value of Hc,
and the Froude number above Hc. The
profiles of turbulence data (ffeor crv and
<7W values) are used to compute plume
ay and crz values at plume height.
The model will calculate on an hourly
basis how the plume trajectory and
shape are deformed by each hill. The
computed concentration at each recep-
tor is then derived from the receptor
position on the hill and the resultant
plume position and shape.
The CTDM user guide is divided into
two volumes: Volume 1 describes the
model and how to use it, while Volume
2 contains code listings. Two auxiliary
user manuals describe the CTDM
terrain and meteorological pre-
processors.
This Project Summary was devel-
oped by EPA's Air and Energy Engi-
neering Research Laboratory. Research
Triangle Park, NC. to announce key
findings of the research project that is
fully documented in two separate
volumes of the same title (see Project
Report ordering information at back).
Introduction
The Complex Terrain Model Develop-
ment (CTDM) project was initiated by the
U.S. Environmental Protection Agency
(EPA) to develop a practical refined plume
model for elevated point sources near
complex terrain. The result of this effort
is the CTDM, which this user manual
describes.
Complex terrain models presently
recommended for regulatory use by EPA
have been designated as screening
models, as they tend to overpredict
ambient concentrations and can there-
fore screen out situations which may not
require further detailed analyses. These
screening models (VALLEY, COMPLEX I,
SHORTZ, and RTDM are deficient in
several areas:
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Deflection and diffusion of plumes in
the vertical is crudely treated by
means of a plume height factor.
Only the third-level screening model,
RTDM, incorporates the concept of a
dividing-streamline height. x
The screening models can accept only
one level of input meteorology. Wind
speed profiles are parameterized as a
function of stability class. Wind
direction is not allowed to vary with
height.
Explicit calculations of cry and az from
turbulence intensities are not done in
the screening models.
Plumes cannot be deflected horizon-
tally around obstacles; a straight-line
trajectory is assumed in the screening
models.
Screening models for complex terrain
applications generally employ sector
averaging in the horizontal for con-
centration calculations because of the
inability of the model to depict the
actual plume trajectory.
Plumes embedded in a stable layer
above a shallow unstable surface
layer are ignored.
The screening models make limited
use of knowledge of the terrain shape.
CTDM addresses these deficiencies:
The structure of the two-layer flow
(above/below the dividing-streamline
height) is explicit in the formulation,
and plume material that straddles the
interface remains in the respective
layers (the plume is not treated as if
it were all in one layer or the other).
Above HCl the material is deflected and
distored, and the rate of dispersion is
altered. Below Hc, the stagnation
streamline divides the flow, and only
material that diffuses onto the stag-
nation streamline is able to reach the
surface of the hill. The stagnation
streamline and the concentration
pattern wraps around the terrain.
Plumes that lie to one side of stag-
nation streamline pass around the
terrain.
The rate of plume growth depends on
the turbulence and, in the case of az,
it also depends on the degree of
stratification. Sector averaging in the
lateral direction is not used.
The focus of the model development
effort to date has been on the stable
plume impingement problem. As a result,
CTDM contains algorithms that are
suitable for neutral and stably-stratified
flows (i.e., Pasquill-Gifford stability
classes, D, E, and F). However, the
sequential hourly datasets required for
regulatory applications contain hours
characterized by classes A, B, and C as
well. Rather than include an untested
algorithm for these classes, or explicity
include an algorithm from an existing
screening model, CTDM writes out "null"
concentrations (-999) for these hours.
Model Applicability and
Technical Limitations
The CTDM is a point-source Gaussian
plume dispersion model designed to
estimate hourly-averaged concentra-
tions of plume material at receptors near
an isolated hill or near a well-defined
segment of an array of hills. Primary
emphasis is given to simulating situa-
tions in which the flow toward the hill
is stably stratified, and in which the
plume has not encountered significant
terrain upwind. Receptors on terrain
downwind of a point source will generally
be associated with the greatest esti mates
of ground-level concentrations in stable
conditions.
The following restrictions and assump-
tions about CTDM should be understood:
CTDM contains no wake algorithms
for simulating the mixing and recir-
culation found in cavity zones in the
lee of a hill.
CTDM contains no global flow calcu-
lation that accounts for the presence
of many hills. The path taken by a
plume through an array of hills cannot
be simulated by the model. It relies
on measurements of the flow obtained
in the neighborhood of the source to
define the incident flow field for each
of the terrain segments inde-
pendently.
All hills that are explicitly modeled are
done so in isolation; any changes to
the plume size caused by one hill are
not carried forward to subsequent
simulations downwind.
For situations when an individual hill
is difficult to isolate from a complex
terrain structure, caution must be
used in interpreting the CTDM results.
Real terrain features are approxil
mated by ideal shapes. CTDM consid-
ers Gaussian shaped hills.
CTDM does not simulate calm mete-
orological periods.
CTDM assumes that the meteorolog-
ical data are representative of the
entire averaging period, and apply to
the entire spatial domain. Spatial
variability resolved by an array of
meteorological towers cannot be used
directly in the model.
CTDM is designed for neutral to stable
flow conditions; therefore the model
does not predict concentrations dur-
ing unstable hours when any plume
is within the convective mixed layer.
Hill slopes are assumed not to exceed
15°, so that the linearized equations
of motion for Boussinesq flow are
applicable.
These restrictions are consistent with the
primary purpose of the model, namely,
that of estimating pollutant concentra-
tions on nearby terrain during stable
(generally nighttime) atmospheric
conditions.
Overview of CTDM
Components
The CTDM package consists of:
the Complex Terrain Dispersion Mode
a terrain preprocessor (Mills et al.
1987)
a meteorological preprocessor (Paine
1987)
a receptor coordinate generator
a graphical concentration displa
program
an interactive program which allow
the user to modify model inputs easil
and run CTDM in a case-study mode
All the programs, except the thre
graphics programs, PLOTCON, RECGEI
and CHIDIS, are written in FORTRAN 11
The graphics programs are written i
BASIC.
The CTDM terrain preprocessor i
made up of 3 programs which proces
digitized contour data to provide hi
shape parameters in a format suitabl
for direct input to CTDM. The fin
program, FITCON, asks the user to defir
a hill in terms of its maximum elevatic
and the (x,y) coordinates of the hill inpi
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from a master file. After evaluation and
editing, each contour is processed by
numerical integration to determine the
following parameters for an equivalent
ellipse: semi-major and semi-minor axis
lengths; contour centroid coordinates;
and the orientation of the ellipse. The
parameters are input to the second
terrain preprocessor program, HCRIT,
which determines, for the portion of the
hill above several given elevations, the
best-fit inverse polynominal profiles
along the hill major and minor axes.
CTDM uses the contour representations
to provide hill shape information above
the critical dividing streamline height for
each hour and each hill using interpo-
lation between two look-up table values.
The third program, PLOTCON, generates
screen displays to aid in the evaluation
of the hill fitting process.
The CTDM meteorological preproces-
sor, METPRO, uses routine measure-
ments to estimate the vertical structure
of wind, temperature and turbulence in
the lower atmosphere using surface
layer similarity theory. Estimates of the
friction velocity, uğ, the Monin-Obukhov
length, L, and the mixed layer height, h,
are provided by METPRO to CTDM. In the
absence of measurements of profiles of
wind, temperature, and turbulence,
these variables are used by CTDM to
compute values of wind, temperature
and turbulence at any height within the
nocturnal surface layer.
The receptor generator program, REC-
GEN, in an interactive program written
in BASIC which will compute receptor
coordinate information for receptors
along user selected hill contours. The
program will create a file containing the
receptor information which can be
directly input to CTDM. RECGEN will also
display the contour lines and receptor
locations on the terminal screen. The
user can easily add receptors in addition
to those produced by RECGEN using a
text editor.
The main CTDM program performs the
plume transport and dispersion calcula-
tions for the entire period of simulation.
It takes the files prepared by the mete-
orological and terrain preprocessors
together with the source and receptor
files, and executes under the control of
the options specified in an input file.
Modeled concentrations can be stored in
a binary or an ASCII output file, if desired.
The model lists all of the control data
describing the simulation, and pertinent
source, receptor, and terrain data to the
output list file. In case-study mode.
extensive tables of selected variables are
also listed for computations performed
for each source, hill, and receptor.
The concentration display postproces-
sor consists of two separate programs.
Both programs are interactive and will
ask the user for the necessary file names
and input data. These programs will
display on the screen the predicted
concentrations from CTDM on a map of
unedited hill contours. Concentrations
for a series of hours can be displayed
sequentially, but each hill must be done
individually.
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Robert J. Paine, Michael G. Dennis, and Michael T. Mills are with ERT, Inc.,
Concord MA 01742; David G. Strimaitis, Robert J. Yamartino, and Elizabeth
M. Insley are with Sigma Research Corp., Lexington, MA 02173.
Peter L. Finkelstein is the EPA Project Officer (see below).
The complete report consists of two volumes, entitled "User's Guide to the
Complex Terrain Dispersion Model:"
"Volume 1. Model Description and User Instructions," (Order No. PB 88-
162 128/AS; Cost: $19.95)
"Volume 2. Model Code Listings," (Order No. PB 88-162 136/AS; Cost:
$25.95)
The above reports will be available only from: (cost subject to change)
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
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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