United States
Environmental Protection
Agency
Atmospheric Research and Exposure
Assessment Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S8-90/087 Feb. 1991
vyEPA Project Summary
User's Guide to CTDMPLUS:
Volume 2. The Screening Mode
(CTSCREEN)
Steven G. Perry, Donna J. Burns, and Alan J. Cimorelli
The EPA's Technology Transfer
Workgroup has developed CTSCREEN,
a screening version of the Complex
Terrain Dispersion Model, CTDMPLUS.
CTSCREEN uses an array of predeter-
mined meteorological conditions to
model the user-supplied source-terrain
configuration. CTSCREEN yields esti-
mates of maximum 1-h, 3-h, 24-h, and
annual impacts that are conservative
with respect to CTDMPLUS estimates
(with a full year of on-site data). In
comparison with other complex terrain
screening models, CTSCREEN provides
estimates that most consistently reflect
those of CTDMPLUS.
This Project Summary was developed
by EPA's Atmospheric Research and
Exposure Assessment Laboratory, Re-
search 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
CTDMPLUS is a point-source, steady-
state air quality model that is applicable to
pollutant sources located in or near com-
plex topography. The model accounts for
the three-dimensional nature of plume and
terrain interaction and therefore requires
detailed topographic and meteorological
descriptions of the modeling domain. Al-
though the terrain data may be obtained
from topographic maps and digitized for
use in CTDMPLUS, the required meteoro-
logical data may not be as readily avail-
able.
Since meteorological input requirements
can limit the application of CTDMPLUS, a
methodology was developed to use the
advanced techniques of CTDMPLUS in
situations where on-site meteorological
measurements are limited or unavailable.
This approach uses CTDMPLUS in a
"screening" mode (CTSCREEN) in which
actual source and terrain characteristics
are modeled with an extensive array of
predetermined meteorological conditions.
CTSCREEN obtains the necessary me-
teorological information from two matrices
of meteorological variables: one for stable/
neutral conditions and one for unstable/
convective conditions.
Model Description
The technical basis of the CTDMPLUS
and CTSCREEN models is the same and
is described in the User's Guide to
CTDMPLUS: Volume 1 (EPA/600/8-89/41).
They are applicable to situations with mul-
tiple sources and multiple hills. Both mod-
els yield identical 1-h estimates for the
same meteorological conditions. The user
supplies the terrain, source, and receptor
information identically to both. The mod-
els differ in the manner in which the me-
teorological inputs are obtained. For input
to CTDMPLUS, meteorological data are
collected on site. With CTSCREEN, me-
teorology is predetermined.
Wind direction is a critical variable for
estimating the maximum impacts from a
point source in complex terrain. The
source-hill geometry and plume height
greatly influence the concentration on the
terrain surface. Since this geometry
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changes with meteorology, CTSCREEN
automatically calculates the optimum wind
direction (that yields highest impacts) for
each combination of meteorological vari-
ables. Users are also given the option to
specify wind directions.
Since CTSCREEN uses different meth-
odologies for modeling stable/neutral and
unstable/convective atmospheric stabilities,
the combinations of meteorological vari-
ables required for each were developed
separately. Values for the meteorological
variables were determined from analyses
of model sensitivities, typical distributions
of meteorological conditions, and the
ranges of conditions associated with high
concentrations at actual field monitoring
sites. The stable/neutral algorithms of
CTSCREEN require the following meteo-
rological variables to compute concentra-
tions: wind speed at plume height, stan-
dard deviation of the lateral wind speed,
standard deviation of the vertical wind
speed, vertical potential temperature gra-
dient, and wind direction. Ambient tem-
perature at stack top is set to 293 K.
CTSCREEN requires the following me-
teorological variables to compute concen-
trations for unstable/convective conditions:
wind speed at half plume height, mixing
height, friction velocity, Monin-Obukhov
length, and potential temperature gradient
above the mixing height.
Although CTSCREEN calculates maxi-
mum 1-h impacts at all receptor locations,
it is also designed to provide conservative
estimates of worst-case 3-h and 24-h
highest-second-high (HSH) and annual
impacts. The results of a comparison study
between CTSCREEN and CTDMPLUS
were used to select appropriate factors
for conversion from 1-h to 3-h HSH, from
1-h to 24-h HSH, and from 1-h to annual
estimates of worst case impacts. The
comparisons included a wide variety of
source and terrain types and source-terrain
configurations. A conservative conversion
factor of 0.7 is used to convert CTSCREEN
1-h maxima to 3-h HSH estimates, a fac-
tor of 0.15 to convert 1-h maxima to 24-h
HSH estimates, and a factor of 0.03 to
convert 1-h maxima to annual estimates.
Comparison with Other
Screening Models
In order to evaluate the usefulness of
CTSCREEN as a screening tool, predicted
concentrations from CTSCREEN were
compared with those from CTDMPLUS
and from two established regulatory
screening models, COMPLEX-I and VAL-
LEY, for 22 different potential plume im-
pact ion scenarios. Predictions from CT-
SCREEN were always higher than those
of CTDMPLUS, and generally lower than
those of COMPLEX-I and VALLEY for all
averaging periods.
Summary of CTSCREEN
Operation
CTSCREEN is used in the same manner
as CTDMPLUS, with the exception of the
meteorological inputs. All necessary me-
teorological data are provided with
CTSCREEN; users do not need to create
any additional meteorological input files.
Files containing information on sources,
terrain, and receptors are identical to those
of CTDMPLUS.
As with CTDMPLUS, CTSCREEN is
designed for use on an IBM®-compatible
personal computer (PC) system, but with
a few simple modifications, it can be run
on other computer systems. The model is
written in ANSI standard FORTRAN. A
math coprocessor chip and a core space
of about 480K bytes is required to run the
model on a PC system.
.S. GOVERNMENT PRINTING OFFICE: 1991/548-028/20163
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The EPA author Steven G. Perry (also the E PA Project Officer, see below) is on
assignment to the Atmospheric Research and Exposure Assessment Laboratory,
Research Triangle Park, NC 27711, from the National Oceanic and Atmospheric
Administration; Donna J. Burns is with Computer Sciences Corporation, Research
Triangle Park, NC 27709; EPA author Alan J. Cimorelli is with Region III,
Philadelphia, PA 19107.
The complete report, entitled "User's Guide to CTDMPLUS: Volume 2. The Screening
Mode (CTSCREEN)," (Order No. PB91-136564/AS; Cost: $17.00, 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 Research and Exposure Assessment Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
United States
Environmental Protection
Agency
Center for Environmental
Research Information
Cincinnati, OH 45268
BULK RATE
POSTAGE & FEES PAID
EPA
PERMIT No. G-35
Official Business
Penalty for Private Use $300
EPA/600/S8-90/087
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