United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S3-87/013 June 1987
&EPA Project Summary
Rocky Mountain Acid
Deposition Model
Assessment: Review of
Existing Mesoscale Models for
Use in Complex Terrain
R. E. Morris and R. C. Kessler
Existing mesoscale meteorological
and acid deposition models are sur-
veyed, reviewed, and evaluated for po-
tential application to a complex terrain
region within the Rocky Mountain re-
gion. The purpose of the review it to
choose meteorological and acid deposi-
tion models that might be indued in a
mesoscale acid deposition model for
the Rocky Mountain region. This acid
deposition model would then be used
by the western regulatory agencies to
estimate the amounts of acidic deposi-
tion from proposed new sources at PSD
class 1 and acid-sensitive areas.
Typical application scenarios consist
of shale oil plants and gas treatment
plants that emit both sulfur and nitro-
gen oxides. Thus it will be important to
correctly define the source-receptor re-
lationship of both sulfur and nitrogen
deposition over mesoscale distances in
complex terrain. The project report in-
cludes a review of meteorological mod-
eling in complex terrain and acid depo-
sition processes, a survey of over 60
existing mesoscale meteorological and
acid deposition models, and a 'discus-
sion of the procedures used to select
candidate meteorological and acid de-
position models for final evaluation.
Among the candidate models, no one
meteorological or acid deposition
model was significantly superior to the
others; all the candidate models con-
tained different features that would be
desirable attributes in an acid deposi-
tion model for the Rocky Mountain re-
gion. The conceptual design of the
mesoscale acid deposition model
makes use of modules from the candi-
date meteorological and acid deposi-
tion models.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Acid deposition has recently become
an increasing concern in the western
United States. Although this problem
may not be as acute in the western
United States as it is in the eastern
United States, it is currently a concern
of the public and regulatory agencies
because of the high sensitivity of west-
ern lakes at high altitudes and the rapid
industrial growth expected to occur in
certain areas of the West. An example of
such an area is the region known as the
Overthrust Belt in southwestern Wyo-
ming. Several planned energy-related
projects including natural gas sweeten-
ing plants and coal-fired power plants
may considerably increase emissions of
acid precursors in northeastern Utah
and northwestern Colorado and signifi-
cantly affect ecosystems in the sensitive
Rocky Mountain areas.
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Under the 1977 Clean Air Act, the U.S.
Environmental Protection Agency
(EPA), along with other federal and state
agencies, is mandated to preserve and
protect air quality throughout the coun-
try. As part of the Prevention of Signifi-
cant Deterioration (PSD) permitting
processes, federal and state agencies
are required to evaluate potential im-
pacts of new emission sources. In par-
ticular, Section 165 of the Clean Air Act
stipulates that, except in specially regu-
lated instances, PSD increments shall
not be exceeded and air quality-related
values (AQRV's) shall not be adversely
affected. Air-quality-related concerns
range from near-source plume blight to
regional-scale acid deposition prob-
lems. By law, the Federal Land Manager
of Class I areas has a responsibility to
protect air-quality-related values within
those areas. New source permits cannot
be issued by the EPA or the states when
the Federal Manager concludes that ad-
verse impacts on air quality or air-
quality-related values will occur. EPA
Region VIM contains some 40 Class I
areas in the West, including two Indian
reservations. Several of the remaining
26 Indian reservations in the region are
considering similar designations. State
and federal agencies, industries, and
environmental groups in the West need
accurate data concerning western
source-receptor relationships.
To address this problem, EPA Region
VIII needs to design an air quality model
for application to mesoscale pollutant
transport and deposition over the com-
plex terrain of the Rocky Mountain re-
gion for transport distances ranging
from several km to several hundred km.
The EPA recognizes the uncertainties
and limitations of currently available air
quality models and the need for contin-
ued research and development of air
quality models applicable over regions
of complex terrain. Therefore, the ob-
jective of the project reported here is to
select and assemble the best air quality
models available for application to the
Rocky Mountain area.
Air quality modeling of the Rocky
Mountain region is especially difficult
due to the complex airflow patterns
over the Rocky Mountains and the diffi-
culty of predicting acid deposition lev-
els. Available data bases are inadequate
for thorough model evaluation studies.
The primary objective of this project
is to assemble an interim air quality
model based primarily on models or
modules currently available for use by
the federal and state agencies in the
Rocky Mountain region. The EPA has
formed an atmospheric processes sub-
group of the Western Atmospheric Dep-
osition Task Force, referred to as
WADTF/AP, to develop criteria for
model selection and subsequent model
evaluations. This group comprises rep-
resentatives from the National Park
Service, U.S. Forest Service, EPA, Re-
gion VIII, the National Oceanic and At-
mospheric Administration, and other
federal, state, and private organizations.
On the basis of our review of the model-
ing needs identified by the WADTF/AP,
the specific requirements of the model
proposed in this project are as follows:
The anticipated use of the model is
to analyze permit applications by cal-
culating acid deposition impacts at
sensitive receptors from specified
sources. Thus the primary need is to
estimate long-term averages of wet
and dry nitrogen and sulfur deposi-
tion amounts. However, there is also
a need to estimate short-term (3-hour,
24-hour) S02 and TSP impacts for
PSD increment consumption. Thus
the model should be primarily con-
cerned with a mesoscale region
within the Rocky Mountain region.
The modeling system will include a
mesoscale meteorological model,
which creates wind fields in complex
terrain, as a driver for an acid deposi-
tion model. Since the primary interest
is in longer-term averages, this mete-
orological model will be required to
generate these wind fields in a cost
effective manner.
The acid deposition model will be
primarily concerned with estimating
incremental acid deposition and am-
bient concentration impacts from the
specified sources only.
A mathematical modeling system for
describing the various physical and
chemical processes associated with
acid deposition must consist of several
components or modules. These mod-
ules describe processes such as wind
transport, chemical reactions, plume
rise, and wet/dry deposition. Although
the modeling system must be an inte-
grated, internally consistent package, it
can be conveniently divided into two
distinct parts:
Simulation of meteorological proc-
esses
Simulation of pollutant dispersion,
chemical reactions, and deposition.
Procedure
After an extensive literature review,
65 mesoscale meteorological and 75
acid deposition/air quality models were
identified as possible candidates for in-
corporation into an acid deposition
model for the Rocky Mountain region.
These models were classified according
to their model formulation, and
parameterizations of the major proc-
esses that describe airflows or acid dep-
osition over complex terrain. Those me-
teorological models that did not treat
complex terrain were eliminated
from further consideration. The remain-
ing models were then subjected to a
technical merit analysis in which objec-
tive scores are assigned to the modeling
approaches to the processes that lead to
airflows over complex terrain and acid
deposition. A model ranking was ob-
tained by summing the scores the
model receives for the treatment of the
major processes.
The needs and desires of the potential
users were considered in the selection
process. The most technically advanced
model may not be the most appropriate
choice if it does not meet the needs of
the users. Based on such nontechnical
criteria, the final candidate meteorologi-
cal and acid deposition models were
chosen for further analysis.
The mesoscale meteorological mod-
els were subjected to a comparative
evaluation and then exercised using £
hypothetical terrain obstacle at a scale
typically found in the Rocky Mountair
region. The acid deposition model;
were evaluated by a detailed analysis o
the methods used by the models to trea
transport, diffusion, chemical transfer
mation, and dry and wet deposition.
Results and Discussion
Review of Mesoscale Meteoro-
logical Models
The alteration of large-scale wim
flows by terrain features can be roughl
divided into kinematic, dynamic, am
thermal-radiational effects. The kine
matic effects are a result of the deflec
tion of the synoptic wind due by the tei
rain. Kinematic effects include blockinc
channeling, and orographic lifting. D^
namic effects are caused by the interac
tion of the mountain topography an
the atmosphere. Under stable cond
tions, air forced vertically over a ridg
may oscillate in an internal gravity wav
called lee waves. Another dynamic e
feet involves the degree of coupling b<
tween synopytic winds, and the wine
within valleys and canyons. Thermal e
fects are caused by the heating or coc
ing of the ground surface, which caus<
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air parcels near the ground to rise or
fall. Examples of thermal effects include
katabatic, or drainage winds, that are
driven by the gravity force as the higher
terrain becomes an elevated heat sink
due to long-wave radiation, the colder
air moves downslope. Similarly, up-
slope or anabatic winds during the day
are driven by buoyancy forces as the
higher terrain becomes an elevated
heat source in response to solar heat-
ing. In reality, airflows in complex ter-
rain are a result of a combination of the
synoptic wind and the kinematic, dy-
namic, and thermal effects. The degree
to which a mesoscale meteorological
model can simulate these phenoma is
an important factor in the selection of
the model.
We identified 65 models that have the
potential of describing meteorological
processes in the atmosphere. These
meteorological models consisted of 50
prognostic models based on the primi-
tive equations or the vorticity formula-
tion, 12 diagnostic models based pri-
marily on the conservation of mass
equation, and three objective analysis
interpolation schemes. The prognostic
models are technically more rigorous
because they explicitly solve the cou-
pled differential equations of conserva-
tion of mass, heat, water, and momen-
tum. Although less technically sound
than the prognostic models, the diag-
nostics models have been used more
extensively in the past for air pollution
studies because they are able to pro-
duce mass-consistent wind fields that
match observations in a cost-effective
manner. The objective analysis interpo-
lation procedure has also been used fre-
quently in air pollution models in the
past; however, the procedure requires
extensive measurements to properly
describe airflows, and may not be ap-
propriate for applications to complex
terrain where observations are limited.
The candidate mesoscale meteoro-
logical models are first classified ac-
cording to their mathematical formula-
tion and the parameterizations of the
major processes. Since the primary pur-
pose of the meteorological models will
be to generate wind fields over complex
terrain to drive an acid deposition
model, we identified three minimum re-
quirements for the meteorological
model for this project: (1) the model
must be able to predict a three-
dimensional wind field, (2) it must be
able to accommodate complex terrain,
and (3) it must be currently operational.
Of the 65 mesoscale meteorological
models surveyed, 17 do not predict a
three-dimensional wind field. These
models are either one-dimensional,
slab-symmetric (x,z), single-layer mod-
els (x,y), or models with a Lagrangian
formulation. Of the remaining 48 mod-
els, 11 do not explicitly account for com-
plex terrain. These models include
urban meteorological models, and
models used for hurricanes or tropical
storms in which terrain can be ade-
quately treated through increased fric-
tional effects. Of the remaining 37 mod-
els, 15 were classified as being currently
operational. A model was classified as
being non-operational if it was devel-
oped outside of the United States and
the model code or simulation results
were deemed too difficult to obtain, or if
it was reported as a research-grade
model in the literature.
The remaining 15 models consisted
of six prognostic models based on the
primitive equations, one diagnostic
model based on the primitive equa-
tions, and eight diagnostic models
based on mass continuity. Candidate
mesoscale meteorological models were
selected on the technical merits of the
models and the needs of the potential
users. The need for generating wind
fields on a long-term temporal scale
precludes the use of the prognostic
models, and the diagnostic model
based on the primitive equations. The
choice of the final candidate meteoro-
logical models was based on a technical
merit analysis in which scores were as-
signed to the models' mathematical for-
mulation, data reliance, and treatment
of the phenomena of blocking, kine-
matic effects, mountain waves, and up-
slope/downslope effects.
The evaluation of the four final candi-
date mesoscale meteorological models
consisted of a comparative description
of the models' treatment of the initial-
ization procedure and adjustment to
mass conservation, and by simulations
using a hypothetical terrain obstacle at
a scale found in the Rocky Mountains.
The results of the evaluation of the can-
didate meteorological models indicated
that no one model was superior to the
others. If there is a total lack of observa-
tional data, the SAI/CTWM alone among
the candidate models is able to gener-
ate wind fields. However, the SAI/
CTWM is also the only model formu-
lated in a Cartesian coordinate system,
an undesirable attribute, and the ability
of the model to use more than one wind
observation is questionable. For the
treatment of blocking and deflection of
air flows, both the PNL/MELSAR-MET
and the SAI/CTWM contain Froude
Froude number adjustments. If reason-
able vertical velocities are desired, then
the LANL/ATMOS1, which attempts ad-
justment of the vertical velocity based
on stability criteria, may be a better
choice. If input data is plentiful and rep-
resentative, the flexibility of the CIT/
WINDMOD interpolation scheme may
be of value.
The design of the meteorological
model as a driver for an acid deposition
model for the Rocky Mountain region
requires the generation of other meteo-
rological variables, in addition to wind
fields. Of the final candidate meteoro-
logical models, only the PNL/MELSAR-
MET also generates gridded fields of
boundary layer heights, temperatures,
and other meteorological variables.
Thus the conceptual design makes use
of the PNL/MELSAR-MET code as a
basis for the meteorological driver.
However, the wind field module within
the PNL/MELSAR-MET may be replaced
by another candidate model formulated
in a terrain-following coordinate sys-
tem. Whichever model is chosen, some
of the unique upslope/downslope
parameterizations within the SAI/
CTWM will be added.
Review of Existing Acid
Deposition/Air Quality Models
Acid deposition/air quality models
can be divided into long-term and
episodic models. Long-term models
generally are either deterministic mod-
els, which calculate the average of indi-
vidual simulations of all episodes for
the time period of interest, or statistical
models, which calculate a long-term av-
erage based on the mean of individual
trajectory calculations or long-term av-
erage (climatological) data. Episodic
models can be classified according to
the fixed-frame Eulerian models, the
moving-framework Lagrangian models,
or hybrid models that merge aspects of
both Eulerian and Lagrangian models.
Lagrangian models can be further di-
vided into forward trajectory (source
oriented), or backward trajectory (re-
ceptor oriented) models. For the pur-
pose of simulating source-receptor rela-
tionships over mesoscale distances in
complex terrain, the use of statistical
models may not be appropriate. Since
nonlinear chemical transformation can-
not be simulated with the Lagrangian
backward trajectory approach, the most
appropriate modeling methodology ap-
pears to be the deterministic approach
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within either an Eulerian or source-
oriented Lagrangian modeling frame-
work.
The physical and chemical processes
that determine the fafe of natural and
anthropogenic acid precursors are nu-
merous, complex, and intertwined. Suc-
cessful modeling of pollutant deposi-
tion requires simulating the most
important of the processes and interac-
tions. For the purpose of constructing
an acid deposition model for the Rocky
Mountain region, the most important
processes can be divided into transport,
dispersion, chemical transformation,
and dry and wet deposition.
Over 75 acid deposition/air quality
models were surveyed and reviewed as
to their input/output data requirements,
transport and dispersion algorithms,
formulation of chemical transformation,
and treatment of wet and dry deposi-
tion. In order to rank the models, a tech-
nical merit analysis assigned scores to
the methods used to treat the major
processes that lead to acid deposition.
Based on technical merit alone, the 10
highest-ranking acid deposition/air
quality models were retained for further
consideration.
The requirements of the potential
users are that only source-specific im-
pacts be calculated and that these esti-
mates must be made in a cost-effective
manner. Although Eulerian models may
be technically superior in simulating cu-
mulative acid deposition, they are not
as cost-effective as Lagrangian models
for calculating source-specific impacts.
Thus, based on the needs of the poten-
tial users, the candidate acid deposition
models for the Rocky Mountain region
are the four highest ranking Lagrangian
models: ERT/MESOPUFF-II, PNL/
MELSAR-POLUT, SAI/CCADM, and SAI/
RIVAD.
The four candidate models were eval-
uated for their treatment of transport,
dispersion, chemical transformation,
and dry and wet deposition. None of the
models contained a superior treatment
in all of these categories. Thus the con-
ceptual design of the acid deposition
model for the Rocky Mountin region will
make use of components from each of
these models. The conceptual design
makes use of the Gaussian puff formu-
lation as used in the POLUT and
MESOPUFF-II. Transport of the puff
would be defined by the meteorological
model. The dispersion coefficients
within the POLUT model will be used for
diffusion since they contain the best
representation of dispersion over com-
plex terrain. Chemical transformation
will be obtained from a parameterized
version of the chemistry module in
CCADM. This model contains one of the
most up to date gas-phase and
aqueous-phase chemical kinetic mecha-
nisms. Dry deposition would be based
on modules in the MESOPUFF-II or the
CCADM. The dry deposition algorithms
in these models use the preferred resis-
tance approach. The selection of the fi-
nal module will depend on tests using
data characteristics from the Rocky
Mountain region. Wet deposition will be
based on the scavenging coefficient
concept. This approach is more consis-
tent with the Lagrangian model formu-
lation and the varying effects of scav-
enging due to rain, snow, and storm
type can be easily incorporated.
Conclusions and
Recommendations
The acid deposition model for the
Rocky Mountain region developed in
this project will have the following at-
tributes: (1) the model is designed for
use by Western regulatory agencies to
estimate source-specific acid deposition
impacts at acid-sensitive receptors;
(2) the model is based on existing mod-
els and modules; (3) the model uses a
Lagrangian model formulation; and
(4) the model is designed specifically for
complex terrain.
The model is intended to be used as
an aid in deciding PSD permits. The
model should undergo testing and eval-
uation in order to establish confidence
in the results. The meteorological
model should be tested against field
data and the performance of other mod-
els using a wide variety of meteorologi-
cal conditions. The acid deposition
should be evaluated against field tracei
data in flat and complex terrain, both
with and without the meteorologica
driver. During simplified conditions (flai
terrain, inert tracer) the model shoulc
produce results that are similar to those
generated by existing regulatory mod
els. Finally, the model should be sub
jected to an extensive sensitivity analy
sis in order to understand the relative
importance of the definition of the inpu
parameters. The authors recognize th<
limitations of these types of models.
/?. £". Morris and R. C. Kessler are with Systems Applications, Inc.. San Rafael,
CA 94903.
Alan H. Huber is the EPA Project Officer (see below).
The complete report entitled "Rocky Mountain Acid Deposition Model
Assessment: Review of Existing Mesoscale Models for Use in Complex
Terrain," (Order No. PB 87-180 584/AS; Cost: $36.95, 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
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