DISPERSION IN COMPLEX TERRAIN
A Report of a Workshop Held
at Keystone, Colorado
May 17-20, 1983
ATMOSPHERIC SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISPERSION IN COMPLEX TERRAIN
A Report of a Workshop Held
at Keystone, Colorado
May 17-20, 1983
by
Bruce A. Egan, Editor
Environmental Research & Technology, Inc.
for the
American Meteorological Society
45 Beacon Street
Boston, Massachusetts 02108
Cooperative Agreement 810297
Project Officer
Francis A. Schiermeier
Meteorology and Assessment Division
Atmospheric Sciences Research Laboratory
Research Triangle Park, North Carolina 27711
ATMOSPHERIC SCIENCES RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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NOTICE
The information in this document has been funded by the
United States Environmental Protection Agency under
Cooperative Agreement 810297 to American Meteorology
Society. It has been subject to the Agency's peer and
administrative review, and it has been approved for
publication as an EPA document.
Mention of trade names or commercial products does not
constitute endorsement or recommendation for use.
ii
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PREFACE
Ong of the most formidable challenges to the meteorological community is
to satisfy the need for a capability to reliably describe, explain, and
predict the behavior of the atmosphere in complex terrain. The infinite
variations in the size, shape, slope and orientation of ridges and
valleys, coupled with land and water interfaces, cause radical alterations
in the characteristics of the atmosphere's boundary layer over short
distances and time periods. Natural resources that are important to
economic development and to recreational needs are often concentrated in
such locales. Consequently, interests of those seeking to preserve
certain amenities of life for themselves and future generations are
often at variance with the interests of those seeking industrial growth.
Equitable resolution of disputes over the use of resources and the
degree of protection of the amenities frequently centers on the capability
to assess the circulation and dispersion characteristics of the near
surface portion of the atmosphere. The magnitude of the impact on air
quality by human activity is a direct function of the atmosphere's
behavior in complex terrain.
This report contains the thoughts and judgments of 32 atmospheric
scientists who gathered to exchange recently acquired technical information
and research results on atmospheric processes in complex terrain and to
comment on matters relating to adjustments in current air quality
modeling practices.
Since 1979, the American Meteorological Society (AMS) has collaborated
with the Environmental Protection Agency (EPA) through a cooperative
agreement to improve the scientific basis of air quality modeling. The
organisation of this Workshop on Dispersion in Complex Terrain has been
carried out under this cooperative program.
ill
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I am indebted to the following people who participated in this workshop:
S.P.S. Arya, N'orth Carolina State University; S. Barr, Los Alamos Scientific
Laboratory; W. Blumen, University of Colorado; N. Bowne, TRC Environmental
Consultants, Inc.; L. Crow, Denver, CO; R. Fisher, EPA, Denver; D. Fox,
ğ
USDA Forest Service; S. Hanna, Environmental Research $ Technology,
Inc.; D. Henderson, U.S. Park Service, Denver; T. Lavery, Environmental
Research § Technology, Inc.; R. McNider, Department of Environmental
Management, State of Alabama; R. Meroney, Colorado State University;
W. Ohmstede, U.S. Army Atmospheric Sciences Laboratory; R. Petersen, NHC
Wind Engineering; R. Pielke, Colorado State University; D. Randerson,
NOAA Weather Service Nuclear Support Office; K. Rao, NOAA Atmospheric
Turbulence & Diffusion Lab.; R. Rowe, University of Calgary; F. Schiermeier,
EPA, Research Triangle Park; H. Slater, University of North Carolina;
M. Smith, Meteorological Evaluation Services, Inc.; R. Smith, Yale
University; N. Snyder, EPA, Research Triangle Park; G. Start, NOAA Air
Resources Laboratories; R. Sykes, ARAP, Inc.; J. Tikvart, EPA, Durham;
J. Weil, Martin Marietta Laboratories; F. White, National Research
Council; D. Whiteman, Battelle Pacific Northwest Laboratories; G. Wooldridge,
Utah State University; and J. Wyngaard, National Center for Atmospheric
Research.
Further, I would like to thank Lindalu Miller of the USDA Forest Service
and Margit Kohl of Environmental Research $ Technology, Inc. for their
most capable assistance in the production of this report.
Bruce A. Egan
Workshop Chairman
IV
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ABSTRACT
Since 1979, the American Meteorological Society has collaborated with the
Environmental Protection Agency through a cooperative agreement to improve
the scientific basis of air quality modeling. Under this continuing agree-
ment, the American Meteorological Society conducted a workshop on dispersion
in complex terrain in Keystone, CO, during May 17-20, 1983. The purpose of
the workshop was to encourage atmospheric scientists working in the area of
complex terrain dispersion modeling to exchange recently acquired informa-
tion on atmospheric processes in mountainous terrain and to make recommenda-
tions regarding both the present application of air quality models to
complex terrain settings and the research necessary to meet future needs.
This report contains the thoughts and judgments of 32 atmospheric scientists
who gathered to exchange such technical information and research results on
atmospheric processes in complex terrain and to comment on matters relating
to adjustments in current air quality modeling practices.
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TABLE OF CONTENTS
Preface iii
Abstract v
Executive Summary 1
1. Introduction 6
2. State of the Science Review 9
A Phenomena of Importance 9
(1) Interaction of Elevated Plumes with Windward Facing 9
Terrain Features
(2) Lee Side Effects 19
(5) Valley Situations 22
B Physical Modeling Capabilities 26
(1) Comparisons of Field Data with Laboratory Results 27
(2) Complex Terrain Applications of Physical Modeling 28
C Mathematical Modeling Capabilities 29
(1) Current EPA Recommended Models 30
(2) Windward Flow about Isolated Hills 31
(3) Mesoscale Flow Models 35
(4) Land and Sea Breeze Interaction with Complex Topography 36
(5) Modeling of Slope and Mountain/Valley Flows 37
3. Recommendations 39
A Use of the Science 39
[1) Comments on Current Research Efforts 39
(2) Need to Quantify Uncertainties in Model Predictions 40
(3) Meteorological Input Requirements for Complex Terrain Models 42
(4) Suggestions on Improving Present Screening Models 45
(5) Suggestions on the Use of Physical Modeling Techniques 47
B Future Research and Development Needs 48
References
Appendix (1) List of Workshop Attendees 64
" (2) Workshop Schedule 66
VII
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EXECUTIVE SUMMARY
The presence of mountainous terrain introduces significant complexities
into the atmospheric transport and diffusion processes affecting ambient
air quality concentrations in a given area. Terrain acts to distort
otherwise organized flow patterns resulting in the creation of regions
of converging and diverging flows, enhanced shear effects, and turbulent
eddies. These alterations affect both flow trajectories and ambient
turbulence levels to a large degree. Terrain also determines the development
of local circulations in mountain-valley settings. The net effect on
air pollution concentrations depends critically on the specific geometric
and topographic relationships and on the characteristics of the flow
fields. For similar source sizes and release heights, emissions from an
air pollution source located in complex terrain may result in concentrations
on nearby high terrain several times larger than the maximum concentration
expected in the absence of the high terrain.
Similarly, stagnation effects in confined valleys can result in the
build-up of concentrations much higher than those which would be observed
under similar large scale meteorological conditions over flat terrain.
For these reasons, the prediction of air quality concentrations in
regions of complex terrain has remained a key area of concern for regulatory
agencies and methods for quantifying atmospheric dispersion processes
have received considerable attention over the past several years.
This workshop was convened by the American Meteorological Society via a
cooperative agreement with the U.S. Environmental Protection Agency.
Its purpose was to encourage atmospheric scientists working in this area
to exchange recently acquired information on atmospheric processes in
complex terrain and to make recommendations regarding both the present
application of air quality models to complex terrain settings and the
research necessarv to meet future needs.
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The following summarises the major conclusions of the workshop about the
present state-of-understanding of dispersion in complex terrain.
(1) Interaction of Elevated Plume with Windward Facing Terrain Features
The largest ground-level concentrations associated with elevated
releases near terrain rising above plume height are often associated
with stable atmospheric conditions. The dynamics of the air flow
depends upon the Froude number of the upstream flow based on hill
height. A related parameter, the dividing or critical streamline
height appears to vertically separate a flow regime which can
transport a plume up and over a terrain object from a flow regime
which constrains plumes to stagnate or to pass around the sides of
a terrain feature. Verification of the above concepts has emerged
from physical modeling efforts for a variety of terrain shapes and
the concepts are supported by field measurement results. Mathematical
models are presently being refined for these situations. Further
verification is needed, however, to understand the applicability of
the concept to a broader variety and larger scales of terrain
geometries.
(2) Turbulent Dispersion Rates in Regions of Complex Terrain
Atmospheric turbulent dispersion rates in complex terrain can often
be expected to exceed those over flat terrain for otherwise similar
conditions. This is especially true under stably stratified conditions
and results from the presence of gravity-driven drainage flows,
gravity waves, the production of shear from flow, deformation, and
the creation of eddies from upwind terrain features. The effect on
ground-level concentrations depends upon the specific source-
receptor geometry.
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(5) Lee Side Effects
The flow on the lee side of terrain features can, under certain
conditions, also cause high ground-level concentrations. During
neutral conditions, flow "separation" can occur on leeward slopes,
causing poor ventilation of emissions from lee side sources within
the wake cavity region. Under stably stratified conditions,
streamlines passing over the crest may pass closest to the surface
on the leeward side. This could give rise to highest ground-level
concentrations on the lee side from plumes originating upwind of a
terrain feature. No widely accepted models exist for these situations.
(4) Valley Situations
A number of air pollution phenomena are associated with the constraints
on ventilation that valleys impose or with the gravity-driven local
flows created by valley side walls. The most severe effects are
the occurrence of multi-day air pollution episodes within deep
valleys during periods when high pressure systems stagnate over a
region. Mathematical models for air pollution applications in deep
valleys are largely in the development stage at the present time.
(5) Physical Modeling Capabilities
The use of physical modeling principles with wind tunnels and
towing tanks has increased markedly over the past several years.
These techniques allow systematic investigations of flow situations
under controlled conditions not practically achievable with field
studies. Properly interpreted, the results of these tests add
substantially to our understanding of phenomena. Limitations
remain for the study of two-dimensional terrain features under
stable conditions and in the spectral range of turbulence which can
be simulated.
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(6) Mathematical Modeling Capabilities
As new information is emerging from theoretical efforts, physical
modeling and field experiments, better and more refined mathematical
models are being developed. Models have been developed or are
under development to simulate many of the phenomena identified as
important. No model exists which can simulate all of the phenomena
for a given setting. The verification of models is difficult due
to the general lack of extensive data bases. The trend of lower
computer costs with time will encourage the development of more
advanced models capable of using more extensive input data.
Recommendations emerging from the workshop were in several areas.
(1) Recommendations on the Use of the Science
The workshop participants supported the approach taken by
current major research efforts in gathering field and laboratory
experimental data for ultimate use in developing better mathematical
models.
A need to quantify uncertainty in model predictions was identified
together with the recognition that this involved obtaining
more information about flow conditions than is generally
needed to estimate mean values from deterministic models.
A viewpoint was taken that the dispersion of pollutants in
complex topography involved interactions of air flows and
terrain structure leading to certain natural fluid dynamic
time and space scales which would not necessarily be important
for simpler (e.g., flat) terrain problems. If such time
scales are larger than the averaging time of application
interest, estimates of concentrations by deterministic models
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uould contain larger uncertainties. This resulted in the
suggestion that the stochastic nature of the phenomena be
recognized and accepted by those developing or applying models
for these circumstances.
The meteorological input data needs for complex terrain models
is greater than those required for level terrain models.
Specific requirements for vertical profiles of temperature and
velocity emerge from the need to estimate Froude numbers and
dividing streamline height. On-site turbulence measures were
also identified as being especially appropriate for complex
terrain modeling efforts.
(2) Recommendations on Research and Development Needs
The workshop participants identified a number of specific technical
areas needing further research to further advance our ability to
predict ground level concentrations of pollutants in complex topography.
Table 1 lists the topics in summary fashion and identifies the
status of available observational data, physical conceptualizations,
and modeling efforts. The order of topics in this list does not
signify the order of priorities.
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Summation of the Keystone Workshop
May, 1983
1. Introduction
The need to estimate reliably the impact of emission sources in regions
of complex terrain for regulatory decision-making purposes remains as a
key challenge to the meteorological community. It has been identified
as an area for further study arid as a controversial area at all of the
public hearings on the EPA Air Quality Modeling Guidelines, and at
numerous professional society meetings and conferences. The report of
the AMS/EPA Cooperative Agreement "Air Quality Modeling and the Clean
Air Act" (AMS, 1981) identified dispersion in regions of mountainous
terrain as one of the most important areas for further research and
development activities.
The subject has in fact received considerable professional attention
over the past several years as is evident from the large number of
papers presented at AMS and APCA meetings as well as other specialty
meetings and workshops. In July of 1979, the EPA sponsored a "Workshop
on Atmospheric Dispersion Models in Complex Terrain" (Hovind et al.,
1979) for purposes of developing specific recommendations to EPA with
respect to the design of a multi-year program to address complex terrain
modeling problems. On the basis of this, the EPA subsequently began a
multi-phased program of field studies, physical modeling experiments and
mathematical model development. Independently, the Electric Power
Research Institute (EPRI), as part of their Plume Model Validation and
Development (PMV§D) study (Bowne et al., 1983), has undertaken a field
program which will include two experiments in complex terrain. The
Department of Energy's Atmospheric Studies in Complex Terrain [ASCOT)
program (Dickerson et al., 1980, 1983) has been investigating the problem
from the perspective of energy development needs including geothermal
power and oil shale. Although none of the above large research efforts
is completed, they are at the stage where considerable field and laboratory
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experience has been gained. Also, a number of mathematical model development
activities have been pursued and findings from these experiences are
rapidly becoming available. For these reasons, the Steering Committee
of the AMS/EPA Cooperative Agreement decided that it would be timely to
review the progress made in this field and to encourage a discussion
about what has been learned, what the implications are to present practice
in dispersion modeling, and what these experiences might suggest with
respect to possible augmentations to current research efforts.
To achieve these objectives, this Workshop on Dispersion in Complex
Terrain was organized and conducted. The agenda for the workshop and a
listing of the attendees are included in the Appendix. The specific
purpose was to provide a forum for the exchange of technical information
on atmospheric dispersion processes in mountainous terrain and the
relationship of this information to the modeling of air quality concentrations
in terrain settings. A specific focus for the workshop was to provide
scientific information for use in the activities of EPA. The workshop's
agenda emphasized measurements and observational data at the beginning.
This focused attention on the new information obtained from field studies
over the past few years and on the importance of understanding meteorological
phenomena for model development in regions of complex terrain.
Following presentations on observed phenomena, the workshop proceeded to
foster discussions focusing on modeling techniques - mathematical and
physical. These topics were divided into three sections:
(1) Flow Field Modeling. Presentation and discussion on theory and
practice of simulating the wind and temperature fields in complex
terrain settings.
(2) Physical Modeling. Presentation and discussion on the use of and
results from physical modeling experiments in wind tunnels and
towing tanks.
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(5) "Regulatory" Modeling. Presentation and discussion focusing on
experiences and practice in predicting ambient concentrations for
use in regulatory decision-making.
The final sessions of the workshop involved dividing the participants
into two groups to address: (1) how recent findings should affect current
practice in the application of models; and (2) future information and
research needs. These included the development of recommendations or
identification of consensus on modeling approaches, identification of
areas needing further information, and recommendations to the technical
and regulatory community at large regarding how to achieve future needs.
This document presents the conclusions made regarding the state-of-the-
science of dispersion modeling in complex terrain, recommendations made
on the use of the science for the application, and recommendations on
future research needs.
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I. State of the Science Review
A Phenomena of I_mpo_rtan_ce
In this chapter, phenomena which are of special importance to the
problem of estimating atmospheric dispersion processes in or near
complex terrain are described. Where the phenomena are easily
described by mathematical algorithms, these are presented.
(1) Interaction of Elevat_ed Plumes with Windward Facing Terrain Features
Flow Parameters Affecting Flow Trajectories. If a source of
emissions is located near a hill that is taller than its stack
or release height, the possibility exists that the highest
concentrations to be expected in the area will occur on the
facing hillside when the airflow is from the stack toward the
hill. These high concentrations would be expected either by
direct plume impaction during strongly stable conditions, or
by near misses as the streamlines pass close to the hill
during less stable, neutral or unstable conditions. This
section of the report addresses our knowledge of flow conditions
on windward facing terrain features.
The presence of mountainous terrain has several effects on the
flow upwind and above the terrain. It acts to distort the
flow field causing accelerations/decelerations and associated
turning, contractions and expansions of air parcels as they
pass by terrain features. It also acts to alter the structure
of turbulence within the region of flow near the surface. The
dynamics of the ambient flow field upstream of a hill depend
critically on the ambient density (or temperature) stratification.
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In very stable conditions, vertical motion is opposed by
restoring buoyancy forces. The stratification effect can be
characterized by a Froude number Fr given by
Fr=U/Nh (1)
where U is a characteristic wind speed for the upstream flow;
N is the Brunt-Vaisala frequency given by
- ' ^ ff'1/2 m
where g is the gravitational acceleration, P is the air density,
and h is the hill height. The Froude number can be interpreted
as a ratio of inertia! to buoyancy forces in a fluid. Moderate
to neutral stability [dominated by inertial effects) includes
the range 1
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This formula is consistent with a simple energy balance argument
for an air parcel as first put forth by Sheppard (1956) .
Sheppard postulated that for a given environmental lapse rate,
one could calculate the value of horizontal velocity far
upwind which would enable air to just surmount a hill by
equating the kinetic energy of a fluid parcel upwind to the
potential energy change associated with lifting the parcel to
the hillcrest. Snyder et al. (1985) have extended this
argument to arbitrary velocity and density profiles and to a
wide range of hill shapes with the result
1/2 oU (H ) ğ I pN (h-Z)dz
A* * f* .Ğ U*
where H must in general be determined iteratively.
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For two-dimensional ridges or very long finite ridges, the
fluid can be blocked (i.e., effectively become stagnant) ahead
of the obstacle. For these geometries, a well accepted formula
for the dividing streamline height does not yet exist. In
addition, there is ambiguity on the upstream extent of the
blocked-fluid region and the variation in depth of the region
with upstream distance. Thus, while solid evidence exists for
flow field and H behavior for three-dimensional hills of not
c
too large an aspect ratio, further experimental laboratory and
field work is required to extend the dividing streamline
concept to more general topography.
In addition to more experimental work, a more complete model
is required for the dividing-streamline height. The simple
energy argument of Sheppard assumes that the air parcel has a
zero horizontal velocity at hilltop. However, this assumption
is inconsistent with observed flow fields which show that
fluid speeds up at the top of the hill. A more complete model
is required to adequately explain the above speedup phenomena
and to ensure reliable extension of the Hc concept to more
complex geometries. For example, in less strongly stratified
flows (Froude number greater than, but near unity), the hydrostatic
solution of Smith (1930) is probably more applicable than
potential flow. Whereas potential flow suggests symmetrical
streamline patterns upwind and downwind of a symmetrical
terrain feature, the hydrostatic solution dees not.
In Smith's model, about halfway between h and H , stronger
lateral streamline deflections will begin. . These deflections
are very sensitive to initial upstream elevation, so that a
plume will spread into a dome shape covering the top of the
mountain.
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Near Hc the flow will be brought to rest and the flow will
split. The plume will probably go to one side or the other
and may oscillate back and forth, being very sensitive to
upstream flow direction. Large concentrations are expected
near this level as well as large apparent lateral diffusivity.
Dispersion in Strongly Stratified Flow. Strongly stratified
flow below Hc has insufficient kinetic energy to pass over the
hillcrest and, neglecting wind shear effects, such flows can
be considered essentially horizontal as they pass around a
hill. When the plume is flowing along the stagnation streamline,
the plume will "impinge" on the hill resulting in surface
concentrations nearly as large as those in the elevated plume's
center (Snyder and Hunt, 1984) .
Along the stagnation streamline, the flow diverges as it
approaches the hill. Hunt et al., (1979) show that the large
increase in the crosswind dispersion coefficient, a~y, caused
by diverging streamlines is almost compensated for by the
decrease in wind speed, U, as the stagnation point is approached,
and that at the stagnation point, o~T,U is approximately the
same as it would have been in the absence of the hill. These
arguments suggest that the concentration at the stagnation
point is approximately equal to that which would occur in the
atmosphere without the hill. Full doubling of the concentration
values due to surface "reflection" effects does not occur
under these conditions.
Surface concentrations at an assumed point of impingement or
stagnation can be estimated during plume meandering conditions
by integration over the changes in wind direction. During any
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quasi-steady period, the concentration at the stagnation point is
given by C.(9, 9 ) where 6 is the wind direction and 9
is the stagnation wind direction. The hourly concentration is given
by:
Cmax - ' C.(6,ed)P(e)de (5)
where P(6) is the frequency distribution of wind directions
(Strimaitis et al., 1983). If the plume spread is due principally
to plume meander, then
C = CP(9.)a ,
max d s
where
C = Q/(/2iro Ua x),
z s
e = e.-Hs /2,
d s
and x is the distance from the release to the stagnation point. If
P(6) is Gaussian, then
z 8 2erQ
where a is the standard deviation of the wind direction about
Q
the mean wind direction 9 . P(9) can also be specified
explicitly by the observed distribution of winds during the hour.
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Experimental Evidence. Snyder et al. (1982, 1985) have shown
agreement of Eqn. 4 with laboratory simulations under stably
stratified conditions. These tests were made at the EPA Fluid
Modeling Facility and in the stratified wind tunnel at the
Japanese National Institute for Environmental Studies. The
concept of Hc was examined for a bell-shaped hill, a cone, a
hemisphere, triangular ridges, a sinusoidal ridge, vertical
fences and for a scale model of Cinder Cone Butte (CCB) near
Boise, Idaho. Snyder and his co-workers have concluded that
the integral equation for estimating Hc accurately predicted
the separation of the flow regimes for a wide range of hill
shapes and profiles.
The EPA-sponsored Small Hill Impaction Studies, conducted at
Cinder Cone Butte (CCB), Idaho, and Hogback Ridge (HBR), New
Mexico, (Lavery et al., 1983) have also shown that the integral
formula for HC discriminates between the flow regimes for both
an isolated axisymmetric hill (CCB) and a very long ridge
(HBR). Photos of oil-fog plumes and ground-level concentration
patterns of two tracers released clearly distinguishes between
the horizontal flow and the flow that goes over the hills.
The HC concept and its ability to predict whether plumes
impinge upon a hill and pass around it or travel up and over a
hill was also found by Ryan and Lamb (1984) and Ryan et al.
(1984) to be valid at Steptoe Butte, a large (335 m), isolated
hill in eastern Washington.
An analysis of the observed tracer gas concentrations and the
meteorological data at CCB showed that the_highest X/Q occurred
when the release height, H, was near or slightly higher than
H . During this situation the plume was transported directly
toward the hill and produced high ground-level concentrations.
Lower releases tended to be transported around the hill sides
and releases above Hc were transported up and over. The
highest normalized concentration occurred when H~H .
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The HBR experiment also showed that Hc discriminates between
the flow regimes, although the nature of the flow below Hc is
currently under investigation. A preliminary analysis of 34
tracer-hours showed that the highest VQ occurred when H < Hc.
Further field observations showing horizontal flow about
larger-scale terrain features at low Fr have been reported by
Williams and Cudney (1976). Field experiments by Rowe et al.,
1982 also tended to validate the dividing-streamline concept
for a ridge with a length about 50 times its height of SO m.
Thus, there appears to be consistent agreement between field
and laboratory observations on the flow structure over a
three-dimensional hill, in particular the horizontal nature of
the flow below Hc and the dependence of Hc on stratification.
This is true for axisymmetric hills and for hills with small
aspect ratios (with width/height ratios as large as 16) (Snyder
et al., 1985) .
Field experiment verification is still needed for the validity
and applicability of the dividing-streamline concept for large
terrain features (greater than several hundred meters).
Flow over Terrain during Neutral Conditions. It is generally
accepted that the first-order effects of terrain on the flow
on the windward face during neutral conditions can be estimated
using modifications to potential flow theory (Hunt and Mulhearn,
1973; Hunt et al., 1979). For regulatory applications, a
practical approach involves the superposition of a Gaussian
plume model (steady state or "puff" type) onto trajectories
determined by potential flow approximations (Isaacs et al.,
1979; Hunt et al., 1979). Egan (1975) demonstrated that the
"half-height" terrain correction factor followed from approximations
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of the effects of potential flow over a hemisphere but a
"terrain-following" plume path assumption provided first order
estimates for neutral flow over a two-dimensional (ridge-like)
shape. .Neutral conditions often are associated with high wind
speeds and synoptically persistent meteorological conditions.
Thus neutral conditions can be of importance to the maintenance
of 24-hour average ambient air standards or Prevention of
Significant Deterioration (PSD) increments, especially where
"channeling" effects of terrain features on the wind fields
are important.
Dispersion during Unstable Conditions. For most regulatory
applications, "worst case" conditions for sources close to
high terrain are expected to occur during stable or neutral
conditions. For this reason, phenomena during unstable conditions
have not been studied in depth. Because of the differential
heating of mountain slopes during daylight hours, convection
effects do result in sustained and significant updrafts and
downdrafts. Also "fumigation" of pollutant material onto
hilltops, in mid to late mornings has been observed to result
in short durations of high concentrations. Currently available
models generally use the flow trajectories for neutral conditions
also for unstable conditions.
Turbulence Levels in Regions of Complex Terrain. In general,
turbulence levels over complex terrain are expected to be
higher over complex terrain than over level terrain for the
same atmospheric stability classification. These increased
turbulence levels are most likely a result of the following
phenomena.
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(a) Nocturnal, radiational cooling which produces surface
inversions is often coupled with very low wind speeds in
level terrain and is likely to result in the generation
of gravity-driven drainage flows in complex terrain.
These flows result in the mechanical production of turbulence
and time-dependent nonstationary secondary motions which
periodically sweep the terrain.
(b) Topographic alteration of flow direction and speed will
result in the production of shear in all directions which
not only contributes to the production of turbulence but
also results in large flow meandering.
(c) In complex terrain the presence of flow stratification is
a key element in the production of regions of rapid flow
acceleration and deceleration, waves, rotors, and "hydraulic"
jumps which tend to produce shearing motions, and associated
turbulence.
The effects of the above relative to flat terrain seem to
be largest under stable conditions. During neutral and
unstable atmospheric conditions, the effects of terrain
on increasing turbulence levels generally appear to be
smaller (Start et al., 1975; Egan, 1975).
Lateral turbulence levels in complex terrain are generally enhanced to
a greater extent than vertical turbulence. Plume meandering and
uplifting as a result of interactions with upwind terrain are reasons
cited for an increase in horizontal dispersion rates. The extent to
which such rates are larger at higher elevations above terrain is not
well quantified. When the atmosphere is stably stratified, generalizations
are difficult. Air parcels downwind of a ridge may be rapidly dispersed
upward (or even upwind) by rotor zones or other eddy motions associated
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with lee side phenomena. Note, however, that terrain features,
especially ridge-shaped features, can contribute to large
scale stagnation of the air flow (blocking) at base locations
and can result in very low winds with little net transport of
pollutant material into or out of an area.
(2) Lee-Side Effects
Previous discussion has focused on the phenomena of importance
in determining ambient air quality concentrations on upwind-
facing slopes. Mathematical and physical modeling shows that
high concentration can also be expected, for certain meteorological
conditions, on the leeward slopes of mountains downwind of a
source. Field measurements are uncommon for these situations
as regulatory requirements have generally focused on obtaining
information on the upwind facing slopes nearest to a pollution
source.
This section provides a brief overview of the current understanding
of flow in the lee of hills.
Neutral Flow. Simple flow field models, e.g., potential flow
coupled with rapid distortion theory, work reasonably well for
predicting surface concentrations on the upwind faces of
hills, both two-dimensional and three-dimensional. However,
these simple models are inadequate for predicting wake effects,
even for hills of moderate slope (i.e., 15 degrees for two-
dimensional hills and 25 degrees for three-dimensional hills),
let alone for steep hills with separated wakes. Flows on the
lee sides of hills are among phenomena expected to cause high
ground-level concentrations in the vicinity of terrain and for
which no routine model simulation techniques are available.
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Snyder (1983) summarized a variety of idealized neutral-flow
wind tunnel studies of plumes from stacks located both upwind
and downwind of various terrain shapes. The results show that
ground level concentrations on the lee side of the obstacle
can be many times higher than would be expected if the terrain
were not present. This increased concentration over the "no-
obstacle" concentration when expressed in a ratio is termed
the "terrain amplification factor" (taf). It is specifically
defined as the ratio of the maximum concentration occurring in
the presence of the hill to the maximum concentration that
would occur from the same (elevated) source if it were located
in flat terrain.
Physical modeling experiments using simulated atmospheric
boundary layers show that plumes released downwind of variously
shaped two-dimensional hills all resulted in taf's of 10 to
15, whereas plumes released upwind or on top of the hills
produced taf's of 2-4. Two lee-side phenomena were observed
to produce high taf's. One was the reverse flow within a
cavity that recirculated plumes from stacks as high as the
hill. The other was flow that had not separated but due to
the low transport speeds and high turbulence levels, plumes
dispersed rapidly to the ground farther downwind. High taf's
were also observed from sources located on the lee-sides of
three-dimensional hills. The highest taf's occurred when the
source was placed approximately on the separation-reattachmert
streamline. In the 3-D cases, higher crosswind aspect ratios
(across wind length of hill divided by hill height) generally
produced higher taf's for downwind sources and smaller taf's
for upwind sources.
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Physical modeling results indicate that typical lengths of
reversed flow regions are of order 10 hill heights for two-
dimensional hills and 2 to 10 hill heights for three-dimensional
hills. In addition, strong trailing vortices downwind of
three-dimensional hills have been observed in laboratory
studies. The strong downwash caused by these vortices can
result in large surface concentrations.
While no specific mathematical models can be suggested at this
time, it is recommended that sources located 10 hill heights
or closer downwind of a terrain feature be investigated for
possible high impacts due to the obstacle. While physical
modeling provides a cost-effective means for examining such a
system systematically, field studies based upon detailed
tracer releases can serve to identify maximum ground level
concentrations. Snyder (1981) and references in Snyder
(1983) provide information on the proper use of physical
models to account for lee side influences.
Stratified Flow. Under strongly stratified flows, pollutants
released below the dividing-streamline height on the lee side
of hills have been observed to be recirculated to the hill
surface and to cover a narrow vertical band spread over roughly
a 180 degree sector of the hill surface. Whereas instantaneous
concentrations are observed to be considerably lower than
those associated with impaction (from upwind sources), long-
term-average concentrations may be larger than impaction
concentrations because the wind meander will significantly
reduce the time-averaged impaction concentrations, but not the
lee-side concentrations. Even simple models for predicting
lee-surface concentrations from downwind sources are not
available.
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(3) Valley Situations
Many air pollution sources such as cities, roads, industrial
operations and energy production facilities are located in
mountain valleys. It has been recognized for at least 40
years (Hewson and Gill, 1944) that air pollution problems can
arise from these sources due to the special meteorological
processes that occur in valleys.
The workshop attendees separated the discussion of the dynamics
of individual plumes interacting with high terrain from discussions
of the special flow conditions associated with valley settings.
The processes identified to be of special importance to the
valley situations include nocturnal drainage flows, fumigation,
flow channeling by valley sidewalls, and persistent low wind
speed stable flows. For purposes of presentation, it is
convenient to distinguish between relatively shallow valleys,
deep, draining valleys, and closed valleys.
Shallow Valleys. Shallow valleys were defined by comparison
of valley sidewall height to the effective height of a plume
from a source affecting air quality in the valley. A valley
is "shallow" if the plume is significantly higher than the
terrain features. Under these conditions, the plume is cut
off from the valley boundary layer during stable conditions
and reacts in a manner analogous to a plume over flat terrain.
Although the trajectory of the plume may be steered somewhat
by the valley orientation, the centerline of the plume is
higher than the valley sides or ridges forming the valley.
Preliminary results reported at the workshop and described by
Reynolds et al., [1984) from the EPRI Plume Model Validation
and Development project tracer studies at Bull Run, Tennessee
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(a shallow valley), indicated that terrain influences on
lifting of the plume paths were not observed as the plumes
from the facility usually followed trajectories that were
parallel to the valley axes. Other analyses (e.g., Turner and
Irwin, 1982) indicate that flat terrain models overestimate
the ground level concentrations on terrain if the plume centerline
is not deflected upward.
Deep, Draining Valleys
Scientific investigations of valley flows have, so far, focused
primarily on improving our understanding of the physics of
valley meteorology, and only a few important research studies
have focused directly on air pollution investigations (e.g.,
Start et al, 1975; Hewson and Gill, 1944). An improved understanding
of valley nocturnal drainage flows is now becoming available
from the U.S. Department of Energy's Atmospheric Studies in
Complex Terrain (ASCOT) program (Dickerson, 1980, 1983).
Other work has focused on the breakup of nocturnal valley
temperature inversions in deep valleys (Whiteman, 1982). This
work has led to a thermodynamic model of temperature inversion
breakup (Whiteman and McKee, 1982) and, more recently, to an
initial model of air pollution concentrations produced on the
valley floor and sidewalls due to post-sunrise fumigations of
elevated nocturnal plumes (Whiteman and Allwine, 1983) . In
these studies the effects of convective boundary layers which
grow over heated valley surfaces after sunrise, the effect of
upslope flows produced over the sidewalls, and the effect of
compensating subsiding motions over the valley floor have been
simulated but need field evaluations.
In actual valleys, topographic complications can be expected
to greatly influence the development of local circulations and
the dispersion of pollutants emitted within the valley. The
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diversity of valley shapes, orientations, presence of tributary
valleys, and presence of terrain constrictions along the
valley axes can be expected to influence the development of
the along-valley circulations, turbulence levels, and other
important aspects of valley meteorology.
Closed Valleys. Certain valleys with weak or obstructed
outflow have been characterized as trapping valleys in contrast
with draining valleys with vigorous outflow. The accumulation
of cool air draining from the sides of the trapping valleys
will build up a deep, stable layer during nighttime hours
that is capped by a stronger inversion at the interface with
the above-valley air near the top of the cold pool. Pollution
plumes emitted into this domain are likely to be confined
below the inversion and between the valley sidewalls.
In addition to diurnal trapping regimes, certain synoptic
conditions produce stagnation episodes. High pressure systems
characterized by low wind speeds, clear or foggy skies, subsidence
inversions and nocturnally-produced ground level inversions,
may exist for 4-5 day periods. During these episodes, additional
emissions are not compensated by flushing so that air quality
can continually degrade to the point of threatening human
health. The pollution conditions are not stationary, as there
may be sloshing of air masses within the valley. The DOE
ASCOT program characterized diurnal pooling and stagnation
within one California valley in 1979 and 1980. Murphy et al.,
(1984) studied the accumulation of smoke from space heating in
a trapping valley in Colorado and Marlatt et al. (1981)
studied the structures of a trapping valley inversion.
The shape of a closed valley creates unique flow regimes.
Limited field data show that nighttime radiational cooling of
the surrounding mountain slopes can create a downs lope drainage
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flow that appears to reach maximum strength just prior to
local sunrise. The drainage flow tends to move toward the
lowest point in the valley. Available data are inadequate to
determine if a gyre usually develops over the valley low point
prior to sunrise. Some observations from the Nevada test site
(Randerson, 1984) indicate that material released at ground
level within a closed valley at night can be transported out
of the valley. The implications of this situation for air
quality modeling are clear. An effluent released into a
stratified environment can be transported out of the valley,
and this is difficult to explain physically. Mo data are
available to permit a quantitative description of the behavior
of effluents released into a thermally-stratified valley.
Dispersion over Complex Terrain with Superimposed Convective Circulation.
In flat terrain, convective flows frequently lead to the
fumigation of pollutants trapped aloft or the early downwash
of a looping plume resulting in high ground level concentrations.
Convective flows developing over hills, ridges, or more complex
terrain may significantly alter streamline patterns, separation
and stagnation locations, and hill wake turbulence. It is
possible that non-homogeneous radiative heating caused by
slope orientation could result in different convective scales
than commonly associated with horizontal terrain. These
perturbed spatial or time scales could result in worst-case
ground-level concentrations from lee impaction, sudden subsidence
or downdrafts.
Orgill (1981) cites the pioneering studies by Hewson and Gill
(1944) and describes two types of fumigation phenomena within
valleys. Type I, a simple diurnal fumigation, occurs during
the early morning hours when surface heating develops convection
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up to an elevated plume. Pollution reaches the valley bottom
nearly simultaneously throughout the valley in this case.
Type II, a dynamical fumigation (Tyson, 1968), may also be
related to the morning transition when an up-valley wind
undercuts the well developed, down valley, mountain wind. A
marked discontinuity in the wind and turbulence fields is
produced, leading to severe dynamic fumigations. In this case
the fumigation event propagates up-valley with time. Whiteman
and McKee (1979) have identified wind and temperature structures
associated with the conditions for dynamic fumigation.
Associated with the 1982 ASCOT field program, Whiteman et al.,
(1984) performed tracer experiments to further document the
time and space character of the fumigation.
One of the major aims of the DOE ASCOT program is to contribute
to a better understanding of the relationship of the details
of these fumigation phenomena to the structure of the winds,
temperature, turbulence and valley topography.
Physical Modeling Capabilities
The report of the 1979 Workshop on Complex Terrain (Hovind et
al., 1979) has a fairly detailed discussion on the background
and similarity criteria for physical modeling in mountainous
terrain. In addition, the workshop document discusses the
relative merits of the wind tunnel and towing tank. Since the
workshop, the EPA has published a document (Snyder, 1981) that
describes in detail the fundamental principles, practical
applications and the hardware associated with physically
modeling atmospheric dispersion. EPA also has published a
guideline document (Huber, 1981) that specifies the necessary
elements of a physical modeling study for assessing the stack
height required so that a plume is not adversely affected by
wakes and eddies of nearby terrain (or buildings).
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(1) Comparisons of Field Data with Laboratory Results
Since the 1979 workshop, several comparisons of physical model
simulations with actual field observations have been made.
Weil et al., (1981) described field and wind tunnel experiments
conducted for the Westvaco Pulp Mill situated in the hilly-
terrain of Western Maryland. The wind profiles measured at
similar locations under unstable stratification in the wind
tunnel and in the field showed good agreement as did the
ground-level concentrations on the windward side of a nearby
hill. The observed concentrations in the wind tunnel were
consistently lower than those in the field on the leeward side
of the hill but only by a factor of about 1.6.
Snyder and Lawson (1981) reported on a series of experiments
attempting to simulate, in a towing tank, field results from
the Cinder Cone Butte, Idaho experiment. In particular, one
hour from the field experiment that represented very stable
atmospheric conditions was simulated. The results of the
tests showed that the crosswind concentration distributions in
the towing tank were exceedingly narrow and that maximum
concentrations were 5 to 10 times larger than those observed
in the field. This was attributed to the fact that low frequency
fluctuations in wind speed and wind direction are not present
in the towing tank. An ad hoc attempt was .made to simulate
the low frequency wind fluctuations by superimposing concentration
patterns from tows done at a series of discrete wind speeds
and wind directions. This attempt was moderately successful
in that 80% of the model concentrations were within a factor
of 2.5 of the field concentrations. Snyder and Lawson felt
agreement would have been better had wind data at plume altitude
been available.
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Other field/laboratory comparisons for complex terrain settings
include studies by Meroney (1980) and Meroney et al. (1980),
which simulated wind speed and direction measurements at sites
in New Zealand and Hawaii. The measurements in the laboratory
correlated well with those in the field.
(2) Complex Terrain Applications of Physical Modeling
Since the promulgation of the stack height regulation in 1979,
numerous physical modeling studies have been conducted to
evaluate the effect of terrain wakes and eddies on dispersion
(Greenway et al., 1981; Petersen and Cermak, 1979; Petersen,
1981; to list a few). These assessments have demonstrated
that physical modeling can be used in a regulatory environment
to define good-engineerir.g-practice stack heights.
Mountain-valley and drainage wind simulations were conducted
(Petersen et al., 1980; Cermak and Petersen, 1981) to provide
qualitative information about flow in deep valleys and quantitative
information for use in refining and calibrating a numerical
model. The physical simulation was also used to assist in the
design of a field experiment.
Petersen and Twombly (1982) conducted a wind tunnel experiment
to evaluate maximum concentrations on elevated terrain under
stable stratification. They compared the EPA Valley model and
a new model that included potential flow theory and the dividing
streamline concept with the wind tunnel simulation. The
average ratio of Valley model prediction to wind tunnel observation
was 2.6 whereas the ratio for the new model was 1.2.
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Snyder (1983) conducted neutral flow wind tunnel experiments
with a stack situated upwind, on-top and downwind of three
different shaped hills. Ground-level concentrations were then
measured with and without the hill present. An amplification
factor was defined to be the ratio of maximum concentration
with and without the hill. Snyder summarized these amplification
factors as a function of hill shape, stack location and stack
height.
These studies by no means represent all the applications of
physical modeling since 1979. It is apparent, however, that
physical modeling has been used to assess concentrations in
complex terrain for the following situations: 1) plumes
impacting elevated terrain under neutral, unstable and stable
stratification, 2) plumes affected by the wakes and eddies of
upwind terrain, 3) plumes released in deep valleys under
stable nighttime conditions.
Mathematical Modeling Capabilities
One of the more important effects of complex terrain on the
plume transport and diffusion problem is the modification of
the ambient flow field. Flow field assumptions in existing
diffusion models for regulatory applications are quite crude.
For example, diffusion models for computing concentrations on
the windward slope of isolated hills assume that the same
horizontal velocity exists everywhere, upstream as well as
over the crest of the hill, in contrast to the streamline
deformation and flow speedup which actually occurs. Some of
these models attempt to account for the closer passage of
streamlines to elevated terrain and deformation of plumes by
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using a "terrain correction" factor as discussed earlier, but
as noted, even these models ignore many details of the corresponding
velocity changes and deformation effects on plume spread.
In this section of the report a brief description of models
currently recommended by EPA as screening techniques is
followed by a discussion of mathematical modeling techniques
for simulating various flows in more detail.
1. Current EPA Recommended Models
EPA has not yet established a guideline model for use in
complex terrain settings where plume heights are expected to
be below the height of nearby terrain. EPA does recommend the
Valley, COMPLEX I and sometimes COMPLEX II models for use as
conservative screening techniques for these situations.
Research on the development of a refined model is currently
underway (Lavery et al., 1983). The following provides a
brief description of present screening models.
The Valley model (Burl:, 1977) uses a Gaussian plume dispersion
equation modified to include 22.5 "sector averaging" of the
crosswind distribution. Vertical dispersion is calculated
using Pasquill-Gifford dispersion curves. The model can be
run to calculate 24-hour average concentrations or annual
averages. Under stable conditions, the model assumes that a
plume travels toward terrain with no vertical deflection until
the plume centerline approaches to within 10 m of a terrain
surface. Thereafter a minimum stand-off distance of 10 m is
assumed. Full doubling of concentrations due to reflection
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occurs. At surface elevations above the plume height, concentrations
are assumed to decrease linearly with height, reaching a value
of zero at 400 m above the initial plume height. Under
neutral or unstable conditions, the plume trajectory assumed
is "terrain-following" - producing concentration values equivalent
to those which would be obtained in the absence of terrain.
For screening analyses, the Valley model is commonly used with
assumed worst-case meteorological inputs of a stack-top wind
speed of 2.5 m/sec. and Pasquill-Gifford stability class F
vertical dispersion coefficients. A 24-hour average concentration
is obtained by assuming that these conditions could occur for
6 hours within a 24-hour period.
The COMPLEX I (EPA, 1981a) and COMPLEX II (EPA, 1981b) models
are multiple point source models capable of using sequential
meteorological input data. Under stable conditions, the
algorithms for plume impaction within COMPLEX I are the same
as those within Valley. COMPLEX II differs from COMPLEX I by
using crosswind dispersion coefficients instead of 22.5
sector averaging. For neutral and unstable conditions the
models use a 0.5 terrain correction factor to lift the plume
centerline height above terrain height.
2. Windward Flow about Isolated Hills
The flow models discussed in this section, while highly simplified,
address some of the major effects of hills .on perturbing an
airstream, i.e., streamline convergence and divergence and the
associated fluid acceleration and deceleration. The models
are considered for two stratification regimes, neutral/unstable
and stable; all are based on inviscid flow.
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For neutral/unstable conditions, potential flow can be applied
following the work of Hunt and Mulhearn (1973), Jackson and
Hunt (1975), and Britter et al., (1981). These authors showed
that for two-dimensional hills and outside a thin "inner
region," the flow behaves like an inviscid shear flow. Furthermore,
they find that potential flow is a useful concept for flow
speedup, as confirmed with field and wind tunnel measurements.
The Jackson and Hunt theory has been extended to three-dimensional
hills by Mason and Sykes (1979a, b) and found to be in good
agreement with wind observations about a small hill.
Potential flow has been applied in a number of nonregulatory
diffusion models for a variety of terrain shapes: three-
dimensional hills (Hunt et al., 1979), simple combinations of
hemispherical and cylindrical shapes (Isaacs et al., 1979),
and arbitrarily-shaped three-dimensional hills (Weil et al.,
1981). The last application employs a potential flow code
developed by the McDonnell-Douglas Company (Hess and Smith,
1962) and is based on the surface source method. Further work
is required to adapt these methods to actual terrain sites for
use in regulatory applications. Of particular concern here is
how much terrain detail must be included to get an adequate
prediction of the flow field and the associated concentration
distribution.
For stably stratified flow about three-dimensional hills, we
consider two stratification regimes: moderately to weakly
stable (Fr > 1) and strongly stable (Fr
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mountain for both round hills and also near the end of a. long
finite ridge. For general finite amplitude hills (not necessarily
low) and Fr -^ 1, the momentum equations can also be linearized
in density stratifications, i.e., by considering a small
perturbation in stratification about the neutral, potential
state. In contrast to the more commonly used small-height
linearization, it has the great advantage of being applicable
to any terrain slope or height. The most general outline of
this approach is given in Drazin (1961). However, its only
application so far has been to two-dimensional hills (Baines
and Grimshaw, 1979). A central question with regard to the
latter approach, i.e., stratification perturbation to potential
flow, is to how small a Froude number it can be applied. This
question, as well as how well the above approach predicts flow
patterns about terrain obstacles, must be assessed experimentally.
Physical modeling would be of value in this assessment.
In strongly stable stratification (Fr< 1), the flow about
large three-dimensional hills is confined to horizontal planes
below a certain height H , which depends upon the stratification.
Such horizontal layering has been predicted theoretically by
Drazin (1961) and demonstrated in stratified towing tank
experiments by Riley et al., (1976), Hunt et al., (1978), and
Weil et al., (1981) for various terrain geometries. It has
also been observed in the atmosphere (Williams and Cudney,
1976; Rowe et al., 1982; and Wooldridge and Furman, 1984).
Drazin suggested that the two-dimensional potential flow could
serve as a useful ambient wind model for the horizontally
layered regime. It should be emphasized that potential flow
is applicable only on the windward side of the hill; it cannot
describe flow within the separated wake in the lee of the
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hill. A key problem in applying the model to real terrain
sites is the choice of appropriate lateral boundary conditions,
a problem also discussed below in the context of mesoscale
modeling.
Above the horizontally layered regime, fluid passes mostly
over the hill, as predicted by Drazin and as demonstrated in
the aforementioned laboratory experiments. Hunt et al.
(1984) suggest a useful ambient flow model for the fluid
passing over the hill. It is the linear stratified flow
theory (for Fr>>!) applied to a "cut-off" hill, i.e., that
portion of the hill above the horizontally layered regime
defined by H . Thus, one could use Smith's (1980) theory (for
the limit Frğ 1) and "push" it to FrĞ*l. We note that in
stable stratification, theory predicts that fluid flowing over
the hill does so asymmetrically, i.e., streamlines pass closer
to the surface on the leeward than on the windward side of the
hill. Thus, in some instances, for a plume passing over a
hill we should expect higher surface concentrations on the
leeward side as observed in the physical modeling experiments
(Snyder and Hunt, 1984).
Models for stratified conditions, when implemented for regulatory
applications, will require more detailed meteorological data
inputs than are generally available. Specifically, knowledge
of the temperature and velocity structure as a function of
height from the surface to heights above the terrain features
will be required.
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3. Mesoscale Flow Models
This discussion of mesoscale models considers flow models
which may be applied in complex terrain for distance scales of
the order of 50 kilometers. Major breakthroughs have been
achieved in recent years in the development of flow models
because of improved high speed computer systems. Concurrently,
there has been development of improved flow models, thus
making it feasible to use these models for applications in a
variety of complex situations.
Mesoscale flow models may be classified into three major
categories: 1) fluid dynamic models, 2) diagnostic models,
and 3) objective analysis models. There are many existing
models in each of these three categories.
Improved computer systems, and more efficient numerical schemes
which solve the complex equations, have made practical applications
of the mesoscale models more feasible. For example, applications
can be simplified by allowing topographic data, models and
graphical output to be integrated with "user friendly," menu-
driven codes (Fox et al., 1983). These mesoscale flow models
may be used in various ways to assist with air quality analyses.
However, there is a need for additional model evaluation
studies to compare the flow fields generated by the models
with wind flow observations. Flow models may be used to
calculate wind fields in complex terrain situations where
there are few meteorological data measurements (Pielke et al.,
1983). The models can be coupled with various advection/diffusion
schemes to simulate dispersion of pollutants. Insight into
air stagnation in basins and valleys with resultant air pollution
buildup, and flushing of pollutants from basins and valleys
may be analyzed. The characteristics of mountain and valley
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flows can be better understood by simulating cases where local
terrain drives the drainage or the upslope flows. Diagnostic
or mass consistent models (e.g., Sherman, 1978; Davis, et al.,
1984; King and Bunker, 1984) have been applied to produce flow
fields for three-dimensional complex terrain settings. These
models to not, however, include thermal effects directly. The
diagnostic models can be used to obtain a better understanding
of the controlling influences of strong synoptic flow patterns
on the local flow fields.
4. Land and Sea Breeze Interaction with Complex Topography.
There has been considerable investigation of sea- and land
breezes over flat terrain; however, the interaction of these
circulations with complex terrain along and inland from the
coast has been investigated relatively less. Recent studies
have documented the movement of pollution offshore in the land
breeze, its fumigation down to the surface layers, and its
subsequent movement back onshore (McRae et al., 1982). This
recirculation of pollution, which can occur for several days
or more at a time in the Los Angeles basin, can result in aged
pollutant plumes with high concentrations of secondary pollutants,
In other investigations, numerical studies have demonstrated
that sea and land breezes in complex terrain are not simply a
superposition of sea and land breezes over flat terrain, with
mountain-valley circulations in complex terrain. One reason
for this is that mountain-valley circulations near a coastline
can result in subsidence over the shoreline which will, in
turn, diminish the interaction of the sea breeze. Thus, such
raesoscale systems must be treated in their entirety.
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5. Modeling of Slope and Mountain/Valley Flows
a. Existing Models. Defant's (1951] descriptive model of
slope and mountain/valley flows, while rudimentary, provides
practical guidance for understanding thermally driven circulations
in well defined valleys. According to Defant, at night shallow-
downs lope flows on the valley sidewalls (directly driven by
temperature deficits near the surface) produce a deep pooling
of cool air in the valley. If there are variations in the
valley depth or width, an along-the-valley axis flow will
develop. The along-the-valley axis flow can be quite deep
depending on the depth of the pool of cool air. Defant's
descriptive model, while addressing the three-dimensionality
of the problems, cannot of course provide details of the flow,
handle interaction with synoptic winds, or address turbulent
structure.
Because the full problem is so complex, recent models have
separated the problem into components. Rao and Snodgrass
(1981), and Smith and Garrett (1985), and others have concentrated
on the slope drainage flows. These slope flow models appear
to reproduce quite well the observed temperature and velocity
structure.
Recently, full three-dimensional primitive equation models
have been developed and applied to slope an.d valley flows,
e.g., Egger (1981), McNider (1981), and Yamada (1981).
Despite continuing initialization and boundary problems, these
models offer the promise of including the slope-valley flow
dependence, and of dynamically simulating the evolving thermal
and turbulent structure.
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b. Applications. While full treatment of the flow with
primitive equation models is possible, actual application of
the models has been limited primarily to idealized terrain,
neglecting synoptic interactions. Egger (1981), McNider and
Pielke (1981) and Bacler and McKee (1983) addressed a simple
three-dimensional valley opening onto a plain. The results
from these idealized applications appear to agree qualitatively
with recent observations, (e.g., Whiteman (1982)), and the
descriptive ideas of Defant.
c. Input Data. For mass-consistent models the data requirements
can be quite voluminous, especially if the flows are primarily
thermally driven, since thermal effects can only be included
through wind observations. For dynamic primitive equation
models the input data are fairly simple if the synoptic flow
is assumed spatially uniform and steady. The basic input
information required is the topography, initial temperature
distribution, and gradient wind speed and direction.
As mentioned, definite problems for dynamic models still exist
in the areas of flow initialization and boundary conditions.
Computation speed is also a major drawback, with computer
memory and speed requirements outside the reach of most applications,
d. Treatment of Dispersion Processes. Because of spatial
variations in both wind direction and turbulent structure, it
is unlikely that simple Gaussian-dispersion models can be
utilized with such numerical flow models. .Small-grid, finite-
difference, advection-diffusion calculations might be applicable.
Lagrangian particle models for point source transport and
dispersion are also promising.
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5. Recommendations
A number of recommendations evolved from the workshop. These
included statements regarding current research efforts and the
use of current technology, suggestions of specific research
needs focused on near-term needs (and which presumably could
be accomplished in the near-term) and suggestions regarding
the longer term research needs. As will be seen, some of
these latter suggestions may equally well relate to other
applications of geophysical fluid dynamics. The recommendations
are organized into two categories relating to recommendations
on the Use of the Science and recommendations on Future Research
Needs.
A Use of the Science
(1) Comments on Current Research Efforts. First of all, it should
be noted that there seemed to be a consensus among the workshop
participants that the approach taken over the past several
years with a focus on gathering more field data on transport
and diffusion processes in complex terrain settings was appropriate,
The need for large efforts to gather field data was strongly
recommended at previous workshops on this topic (e.g., EPA
Workshop on Atmospheric Dispersion Models in Complex Terrain
(Hovind et al., 1979), DOE Workshop on Research Needs for
Atmospheric Transport and Diffusion in Complex Terrain (Barr
et al., 1977)), and the research programs of EPA, DOE and EPRI
have been very responsive to these needs. The increased use
of wind tunnels and water channels in applying physical modeling
techniques to study flow phenomena was also supported by the
participants. The notion of performing scale model tests in
parallel with full scale field experiments performed in complex
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terrain was viewed as providing an excellent methodology to
understand the effects of physical scale on the phenomena of
concern and to investigate systematically the effects of
changes of dispersion patterns with flow conditions. Such
systematic investigations are difficult and expensive to
accomplish in field experiments. Emerging mathematical modeling
techniques for predicting concentration values in complex
terrain account for the physical phenomena of importance
better than earlier models. A number of these developments
have been made hand-in-hand with the increased data bases
emerging from the field and physical modeling programs.
(2) Need to Quantify Uncertainties in Model Predictions. One of
the recurring issues surfacing at this workshop was the need
for modelers to incorporate uncertainty of predictions into
the process of using models. The group recognized the regulatory
application needs of having decisions made on the basis of
certainty and on the basis of computations made for both short
and long averaging times and for infrequent events. An admonition
to include uncertainty into the modeling evolves from the fact
that the flow over complex terrain, like any other atmospheric
flow, is turbulent in most regions at most times. This
turbulence thus has both temporal and spatial scales. These
scales are not determined simply by the universal flow physics;
they are also strongly influenced by the structure of the
topography, by the interactions of this topography with the
ambient, larger-scale atmospheric circulation, and, in dispersion
applications, by the nature and geometry of the pollution
sources.
The stochastic nature of the atmosphere coupled with its
nonstationarity and spatial complexity leads to significant
uncertainty in any prediction of its behavior. It is virtually
-40-
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impossible to measure initial conditions in a manner which
represents this complex flow. The measurement problem is
magnified in complex topography where small scale boundary
forcing is related to small scale variability of the surface.
In idealized conditions (flat terrain) a recent AMS/EPA workshop
(Fox, 1984) recognized that inherent uncertainty would remain
even if the measurement problem could be removed due simply to
the difference between individual realizations of seemingly
identical flows. In practice, physical models offer an opportunity
to investigate components of this source of variance. It is
possible that the physical constraints of topography might
tend to reduce this variance because of the more frequent
occurrence of locally organized secondary flows (slope flows,
etc.). To the extent this may occur, such inherent variability
may be less over complex terrain. This notion, however, is
hypothetical at this time - it has not been demonstrated.
The recommendation to include uncertainty into the modeling
process has several implications. Firstly, it will not be a
simple task. Deterministic models have evolved from development
techniques attempting to achieve "best fits" of predictions
with observations without (until rather recently) attempting
to match uncertainties of predictions with uncertainties in
the observations. Further, the amount of information required
to quantify uncertainty is greater than that needed to estimate
"mean" values since uncertainty of all the various input
values and parameters needs to be assessed. For example,
theoretical and observational work on uncertainty indicates a
strong dependence of concentration fluctuations on the source
exit conditions even at large downwind distances (Sawford,
-41-
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1983; Fackrell and Robins, 1982). Secondly, there follows a
suggestion that the effects of uncertainty in input data and
parameters should be studied with physical models where a
degree of control of variables with time can be maintained.
Thirdly, the recommendation is supportive of pursuits of
higher-order closure modeling which includes consideration of
higher statistical moments of distributions and of large-eddy
simulation modeling wherein uncertainty can be quantified by
interpretation of output results of multiple simulations.
Finally, the recommendation is consistent with the charge to
rely on field data (with appropriate consideration of stochastic
variability) to evaluate the statistical performance of models.
(3) Meteorological Input Requirements for Complex-Terrain Models.
An underlying concept in the field of dispersion modeling is
that one cannot expect a dispersion model to perform well
without appropriate input data. Generally, "appropriate" is
interpreted as being of good quality, being representative of
the flow field of interest and providing reliable information
on the flow parameters of importance. Some specific recommendations
appropriate for dispersion models as applied to regulatory
applications are provided below.
The set of meteorological data collected must be representative
for the site of the source and the probable paths of pollution
diffusion, including those paths which are highly repeatable
and those that are expected to produce the highest levels of
ground level impact for the time periods that have regulated
pollution standards. The data sets must be adequate to be
used for the modeling of the annual, 24-hour, and 3-hour
averaging times of pollution impacts.
-------
All sites in complex terrain will have unique characteristics
and will deserve careful examination of the surrounding topography
The initial plan for collection of meteorological data should
be discussed with the meteorological staffs of applicable
regulatory bodies before data collection begins, so that any
special modeling input needs can be met.
For an elevated source, an adequate data set would probably
include, but would not be limited to, the following:
At least one instrumented tower with the data feeding
to a data logger and strip chart backup for wind and
temperature. The instruments should include wind
direction and speed at two or.more levels, turbulence
measurements at the top of tower (as near to plume
height as practical), temperature measurements and
delta T measurements.
Solar radiation measurements near the base of tower.
Collection of additional wind data on a continuous
basis at other locations at or near the suspected
points of high impact on air quality.
Radar or theodolite-tracked tetroons carrying minisondes
They will describe three-dimensional trajectories at
specific sites over ranges of a few to some tens of
kilometers. The temperature sensor will provide
information on the energetics of flow along slopes
and in mountain valleys.
-43-
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To apply the integral form of calculating H ,
vertical profiles of wind U(Z) and temperature T(Z)
measured upwind of the terrain to heights at least
as high as the terrain height are desired. It is
impractical to construct towers to heights of 200 or
300 m or higher, so remote sensing devices are
needed. Doppler acoustic sounders can provide
information on wind profiles. Alternatively, tethersondes
or minisondes could be used with a shorter tower to
measure temperature profiles.
Possibilities for special studies to augment the basic data
described above might include tracer studies over limited time
periods during the season(s) of primary concern. Such a study
would involve release of a tracer gas from the effluent point
of interest along with samplings at sites in the region of
anticipated impact. By identifying "worst-case" atmospheric
conditions, tracer releases would be necessary on only a
limited number of days. Wind tunnel studies allow systematic
investigation of cause and effect and provide a means of
extrapolation from field information (see Section 2B).
While the above suggestions relate rather specifically to a
single category of sources, there appeared to be a consensus
among workshop participants that detailed meteorological data
including direct measures of turbulence intensity should be
required for use in complex terrain dispersion models. The
need for such data is greater for mountainous settings than
for flat terrain settings because of the importance of site
specific phenomena on turbulent dispersion processes.
-44-
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.Another recommendation relating to the use of meteorological
input data in models evolved from the recognition that more
advanced models will probably require more detailed inputs.
Current regulatory practice requires that one or more years of
hour-by-hour values of wind speed and direction, stability
class, and mixing depth be input sequentially, so that all
sets of three-hour and daily averages are computed. The
purpose of this procedure is to allow evaluation of the highest
and "highest, second-highest" concentration averages. Concern
is expressed that the associated volume of computations may be
excessive unless simplifications are made in the simulation
algorithms to reduce total computation costs. An alternative
approach which should be considered would be to be selective
in identifying worst-case meteorological scenarios to be used
and thereby encourage the use of more computation time-intensive
algorithms if they were deemed to be superior.
(4) Suggestions on Improving Present Screening Models
(a) For impingement on high terrain, the group recognized the
importance of screening models, as applied by regulatory
agencies, to eliminate the need for detailed assessments for
facilities having minor ambient air impacts. A screening
model was defined in this context as a conservative model
which can be used without any meteorological data, e.g.,
worst-case meteorological assumptions are made. Worst-case
Valley is the present EPA screening model for terrain higher
than a source height and assumes a specific wind speed, a
specific duration of wind direction, specified temperature
profiles and dispersion rates, and arbitrary "miss" distances
for plume paths. The group felt that because of concerns over
the arbitrariness and elementary nature of the assumptions
-45-
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already in Valley, that it would not provide a good basis for
improvement. Adding more complexity would not necessarily
make this model any more reliable, because further arbitrary
meteorological assumptions would be required. Use of the
critical streamline height concept, which was raised specifically
in discussion (Rowe, 1982), requires assumptions about the
vertical variations of temperature and velocity. Members of
the group thought it unwise to add complexity requiring more
detailed meteorological data inputs to a model which would be
used in the absence of such data.
It was felt that better screening models could be developed by
a two-fold effort:
1. Examine existing data available from ambient air quality
or tracer measurements for a variety of sources in terrain
settings to see if uniformly-conservative screening
algorithms could be developed on the basis of comparing
peak observed and predicted concentrations.
2. Develop screening techniques which were consistently
conservative. That is, develop methods which would not
result in biases in the decision-making depending upon
differences of source-topographic situations (i.e.,
biased against sources located close to terrain vs.
further from terrain, etc.). The degree of overprediction
expected from such models should be quantified.
(b) At present, no screening model is available or can be recommended
for predicting maximum air pollution concentrations due to
diurnal fumigations or light wind high-stability dispersion
-46-
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conditions in deep valleys. Further research and*evaluation
of actual concentration data are necessary before such models
can be developed and used with confidence.
(5) Suggestions on the Use of Physical Modeling Techniques
The advances made in developing laboratory techniques and in
the understanding of the limits of applicability of physical
modeling techniques suggest that increased reliance be placed
on their use. Application of these techniques should be
considered for those situations where they are appropriate.
(a) Consideration for Screening Models. Physical modeling can
provide a source of useful information regarding the expected
maximum concentrations at a proposed site. In particular,
fluid modeling could be used to estimate the maximum short
term average (e.g., 1-hour or less) concentrations under plume
impaction conditions as a function of hill Froude number,
stack height, and stack location. The optimum stack location
and height could then be selected so that concentrations are
mimimized. In addition, fluid modeling could be used to
estimate the short term concentrations due to terrain wake or
cavity effects. For screening purposes, it would have to be
assumed that the conditions modeled would actually occur. If
on-site or representative off-site data are available, more
realistic estimates can be made. For mountain-valley or
drainage-type flows, the wind tunnel may not be able to simulate
the worst-case condition which appears to occur during transition.
Another technique would have to be employed for these situations.
-47-
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(b) Detailed Evaluations. If the screening evaluation suggests
that plume impact or terrain wake or cavity effects will cause
standards to be exceeded, a more refined modeling analysis
could be undertaken.
Physical modeling could be used during this evaluation to help
refine and validate a site-specific model (if the length
scales are not too large). The basic components of the model,
such as plume trajectory and dispersion parameters, could be
evaluated as a function of stability, wind direction and wind
speed. The model's ground-level concentration estimates could
then also be compared with the wind tunnel results. Once the
validated model (for hourly concentration estimates) is fully
tested, it can be applied using on-site meteorological data to
estimate concentrations for the prescribed averaging times (3-
hour, 24-hour, annual). Post-concentration monitoring is
suggested at the location predicted to give the highest concen-
trations. This information could be used to assess the performance
of the technique. Physical models can also be used to assist
in the design of field programs for complex terrain settings.
Future Research and Development Meeds
The workshop attendees elected to identify research needs, and
establish some priority associated with them, for the problems
of predicting ground-level concentration of air pollutants in
complex topography. These problems are focused on concerns
with the highest concentration values which can result, and of
course with understanding the physics that'contribute to these
high values.
-48-
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After considerable deliberations a set of problems were established,
which in the opinion of participants, bracketed the physical
situations which can lead to maximal ground-level concentrations
in complex terrain. Of the seven problems finally selected,
four involved stable stratification, two involved the unstable
planetary boundary layer, and three involved detailed flow
field considerations. The problems were further recognized to
be composed of subcomponents of a more fundamental nature.
They were qualified by, in addition to their subcomponents:
(a) whether some observational data had been collected on the
problems; (b) whether a physical conceptualization of the
problem had been developed; (c) whether physical or numerical
modeling efforts had been called for or already conducted; and
(d) whether a further major field effort was deemed necessary.
Table 1 lists the topics and summarizes these considerations.
The ordering of tasks does not necessarily imply priority. A
brief synopsis of the issues for each of the tasks is provided
below.
(1) Stable Plume Impaction. Physical modeling and field studies
have established that in stable flows a critical elevation
exists which differentiates the flow which goes around a hill
from the flow which goes up and over a hill. In combination
with information on plume height and dimensions, better estimates
of the location and magnitude of ground-level concentrations
can be made. Field studies should now be directed toward
establishing the validity of this concept in larger scale and
more complex (less idealized) terrain situations. It is
likely that mathematical modeling of this phenomenon would
yield additional insights not otherwise obtainable.
-49-
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(2) Lee-Side Effects Laboratory modeling of both neutral and
stable flows over 2-D and 3-D topography has shown that elevated
concentrations may be associated with a source located downwind
of the terrain. A sounder, physically based understanding of
these phenomena is needed. A combination of theoretical and
laboratory modeling is called for in an attempt to provide
such an understanding. Some field research will be important,
especially as the theoretical understanding develops. Numerical
models capable of predicting separation and reattachment
points for neutral conditions and two-dimensional hills are
presently feasible arid should be extended and evaluated with
data.
(3) Fumigation. The upset of a stable layer in a valley has been
identified for many years as contributing to high ground level
concentrations. Although the physical concepts of this process
are reasonably well understood, it was suggested that laboratory
modeling might help to refine the understanding. Equally
important is the conduct of a field experiment focused on
these events. Numerical models and fluid modeling (e.g., Bell
and Thompson, 1980) can be used to understand the diffusion
processes near the valley top where strong wind shears occur.
More generally, mathematical models need to be developed to
describe dispersion processes in the transition region just
above and below hill crests.
(4) Trapping Valley. There exists a class of valleys which because
of their orientation cause drainage winds to pool at their
base forming a thick stable air mass. When circumstances are
such that diurnal heating and momentum transport from above
are insufficient to break this inversion, very high concentrations
-50-
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may occur. A combination of laboratory and field experiments
was recommended to quantify the existing understanding of this
situation. Studies of how to develop adequate meteorological
inputs for these conditions are also needed. Further guidance
on the development of parameterizations of physical processes
could be provided by numerical simulation models. The
interactions between the evolving circulations which develop
on different length scales are thought to be one of the key
scientific problems in dealing with valley meteorology.
Another important need is data to document the depth of the
stable boundary layer over the valley. The relationship
between the depth of this stratification and the strength and
persistence of drainage winds should be determined.
Future studies should address both diurnal and synoptic stagnation
conditions in trapping valleys -- especially those with populations
and pollutant sources. Field and modeling studies should
address the critical topographic and meteorological conditions
for occurrence, as well as the resulting air quality degradation.
(5) Convective Boundary Layer over Complex Terrain. As a result of
intensive theoretical, numerical and physical modeling studies,
as well as definitive field experiments, we have achieved a
fairly complete understanding of the evolution of the convective
planetary boundary layer (PEL) over homogeneous terrain.
Similar systematic studies of the convective PBL over complex
topography are generally lacking. There are also no studies
of the interaction between the topographically induced motions
(such as in the separated cavity and wake regions) and the
buoyancy-driven convective motions and their combined effects
on the dispersion of pollutants in complex terrain.
-51-
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In order to understand the above problem, the following types
of studies should be conducted in a sequential manner:
a. Construct simple slab models for the convective PBL over
a uniformly but. significantly sloping terrain and over
2-D hills or ridges.
b. Conduct physical modeling studies of boundary layers over
isolated 2-D and 3-D hills of different slopes and shapes
in both neutral and unstable conditions.
c. Conduct numerical modeling (large eddy simulation) studies
of the convective PBL over 2-D and 3-D isolated hills.
The studies proposed here should be very useful in improving
the understanding of the spatially inhomogeneous convective
PBL in complex terrain and its parameterization in mesoscale
numerical models of flow and dispersion. These will also
indicate to what extent and under what types of topographical
situations our current knowledge of the homogeneous flat-
terrain convective PBL can be transferred or applied for
modeling purposes.
(6) Mesoscale and Longer Range Transport and Diffusion. Although
not contributing to the highest ground-level concentrations,
the transport of pollutants over scales upward of 15 km to as
far as 200 km is important because of Class I PSD increments.
The research necessary here is twofold. First, a more fundamental
understanding of the physics is probably necessary before much
significant progress can be made. Second, although a great
many models exist, they are all unverified. It is recognized
that chemical transformation and removal processes become
-52-
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important aspects of models applied to longer distance transport
issues. The problem of longer range transport and diffusion
in complex terrain requires the combination of a wind field
model with an appropriate diffusion scheme. Evaluation of
such a combined approach is not easily accomplished. Both the
wind field and the diffusion components require attention. As
was discussed earlier, various procedures are available to
simulate winds, all of which involve some compromise. The
best that can be expected is that ranges of appropriate performance
can be defined for each class of model. These can be developed
by combinations of theoretical and field studies which are
recommended. Evaluation of the diffusion component can rely
on work done near the source but will ultimately require a
tracer study conducted over complex terrain. The CAPTEX
experiment (CAPTEX, 1984) may provide this requirement for
eastern rolling terrain; however, there remains a need for
western complex terrain tracer studies.
(7) Sea/Complex Terrain Interface. Analysis of dispersion in
coastal terrain involves further meteorological complications
associated with the coupling of land-sea breezes with mountain-
valley flows, discontinuities of surface temperature, etc. It
was felt that although it is common for pollution sources to
be located in such regions, relatively little research has
been done in this area.
A number of more general concerns were expressed by the group during
discussions of the above research needs. There was concern that we
ought to better recognize that dispersion of pollutants in complex
topography is a special subset of the geophysical fluid dynamics of
stratified flows. As such, under stable stratification, influences of
topography can be felt well above, up and downwind of the topography.
-------
As a result a caution was raised regarding the extrapolation of conceptu-
alizations which are valid and useful for simple (flat) terrain problems
to problems in complex topography. Interactions between the physics of
stratified flow and the topographic structure lead to certain natural
time and space scales in the flow. In cases where such time scales are
larger than the averaging time of application interest, estimates of
concentration by deterministic models will contain larger uncertainties.
There were repeated references to the difficulty of developing any
deterministic and generalized solution. Hence it was suggested that the
basic stochastic nature of the phenomena should be accepted, and that
statistical approaches be used to characterize the uncertainty.
Consideration of the more fundamental aspects suggested that the seven
problems in Table 1 depended upon an understanding of the following,
more basic items:
a. The space and time scales of the flow as a function of
topography and other types of forcing or boundary conditions
b. The depth of the planetary boundary layer in complex
terrain.
c. The interaction of the planetary boundary layer with the
free air above it.
d. The space and time variation of turbulence as a function
of stability and topography.
e. The generalization of the dividing-streamline concept to
arbitrary shapes and scales of hills.
f. The effects of radiation on the flow field in complex
terrain.
-54-
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g. Effects of wakes in the flow over complex topography.
h. The identification and measurement of meteorological and
other data which are representative of the flows.
This list represents a very comprehensive set of basic issues stemming
from the fundamental complexity of stratified fluid dynamics, when
coupled with nongeneral specific topographic environments (real topography)
The associated uncertainty in the ability of simple models to simulate
these processes needs to be a key concept kept very much in the forefront
of any applications in complex topography.
-55-
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Clean Air Act. A Report Prepared under Cooperative Agreement with
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Bader, D.C., and T.B. McKee, 1983: Dynamical Model Simulation of the
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Bell, R.C., and Thompson, R.O.R.Y., 1980: Valley Ventilation by Cross Winds,
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Brighton, P.W.M., 1978: Strongly Stratified Flow Past Three-Dimensional
Obstacles. Quarterly Journal of the Royal Meteorological Society
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Burt, E.W., 1977: Valley Model User's Guide. EPA-450/2-77-018, U.S.
Environmental Protection Agency, Research Triangle Park, NC, 112 pp.
CAPTEX, 1984: Acid Rain: A Search for the Source. NOAA, winter '84, 9-11.
Cermak, J.E., and R.L. Petersen, 1981: Physical Modeling of Downslope
Mountain Wind and Atmospheric Dispersion, Proceedings of the Fourth
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Davis, C.G., S.S. Bunker, and J.P. Mutschlecner, 1984: Atmospheric Transport
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Petersen, R.L., 1981: Determination of GEP Stack Height for Bunker Hill
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Estimates for Stable Plume Impact on Elevated Terrain. Presented
at the 75th Annual Meeting of the Air Pollution Control Association,
June 20-25, 1982.
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APPENDIX 1
WORKSHOP ON DISPERSION IN COMPLEX TERRAIN
Keystone, Colorado
May 17-20, 1983
List of Attendees
Dr. S.P.S. Arya
Professor of Meteorology
Department of Marine, Earth $
Atmospheric Science
North Carolina State University
Raleigh, NC 27650
(919) 737-2210
Dr. Sumner Barr
Group H-8
Los Alamos Scientific Laboratory
Box 1663
Los Alamos, NM 87544
(505) 667-2636
Dr. William Blumen
Department of Astrogeophysics
Box 391
University of Colorado
Boulder, CO 80309
(305) 492-8770
Mr. Norm Bowne
TRC Environmental Consultants, Inc.
800 Connecticut Blvd.
East Hartford, CT 06108
(203) 289-8631
Mr. Loren W. Crow
Consulting Meteorologist
2422 South Downing Street
Denver, CO 80210
(305) 722-8665
Dr. Bruce A. Egan
Environmental Research
696 Virginia Road
Concord, MA 01742
(617) 369-8910
Mr. Richard W. Fisher
Region VIII, Air Programs
Environmental Protection Agency
1860 Lincoln Street
Denver, CO 80295
(303) 837-3763
Technology, Inc.
Dr. Douglas G. Fox
USDA Forest Service
Rocky Mountain Forest fğ Range
Experiment Station
240 West Prospect Street
Fort Collins, CO 80526
(303) 221-4390
Dr. Steven R. Hanna
Environmental Research
696 Virginia Road
Concord, MA 01742
(617) 369-8910
Mr. Don Henderson
U.S. Park Service -- Air
P.O. Box 25287
Denver, CO 80225
(303) 234-6620
Technology, Inc.
§ Technology, Inc.
Mr . Thomas F . La very
Environmental Research
696 Virginia Road
Concord, MA 01742
(617) 369-8910
Dr. Robert N. Meroney
Engineering Research Center
Colorado State University
Ft. Collins, CO 80523
(303) 491-8572
Dr. Richard T. McNider
Air Program/ Planning § Development Section
Department of Environmental Management
State Capitol
Montgomery, AL 36130
(205) 834-6-570
Mr. William Ohmstede
U.S. Army Atmospheric Laboratory
White Sands Missile Range, NM 88002
Dr. Ronald Peterson
NHC Wind Engineering
22477 72nd Ave. S
Kent, WA 98032
(206) 872-0218
64
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Dr. R. Pielke
Department of Atmospheric Science
Colorado State University
Fort Collins, CO 80523
(303) 491-8293
Dr. Darryl Randerson
NOAA Weather Service
Nuclear Support Office
P.O. Box 14985
Las Vegas, NV 89114
(702) 295-1260
Dr. K. Shankar Rao
NOAA Atmospheric Turbulence $ Diffusion
Laboratory
P.O. Box E
Oak Ridge, TN 37830
(615) 576-1238
Dr. Richard D. Rowe
Department of Mechanical Engineering
University of Calgary
2500 University Drive, NW
Calgary, Alberta, CANADA T2N 1N4
(403) 284-7185
Mr. Francis A. Schiermeier
Chief, Terrain Effects Branch
Meteorology & Assessment Division
Environmental Protection Agency
Research Triangle Park, NC 27711
(919) 541-4551
Mr. Herschel H. Slater
1310 Willow Drive
Chapel Hill, NC 27514
(919) 929-5889
Mr. Maynard E. Smith
Meteorological Evaluation Services, Inc.
134 Broadway
Amityville, NY 11701
(516) 691-3395
Dr. Ronald Smith
Department of Geology
Yale University
210 Whitney Avenue
New Haven, CT 06520
(203) 436-1077
Geophysics
Dr. William H. Snyder
Chief, Fluid Modeling Section
Meteorology § Assessment Division
Environmental Protection Agency
Research Triangle Park, NC 27711
(919) 541-2811
Mr. Gene Start
NOAA Environmental Research Laboratories
550 Second Street
Idaho Falls, ID 83401
(208) 526-2329
Dr. Ian Sykes
ARAP, Inc.
P.O. Box 2229
Princeton, NJ 08540
(609) 452-2950
Mr. Joseph A. Tikvart
Chief, Source Receptor Analysis Branch
(MD-14)
Monitoring § Data Analysis Division (OAQPS)
Environmental Protection Agency
Research Triangle Park, NC 27711
(919) 541-5561
Dr. Jeffrey C. Weil
flartin Marietta Laboratories
1450 South Rolling Road
Baltimore, MD 21227
(301) 247-0700
Dr. Fred D. White
3631 North Harrison Street
Arlington, VA 22207
(202) 334-3515
Dr. David Whiteman
Pacific Northwest Laboratories
Richland, WA 99352
(509) 375-3875
Dr. Gene Wooldridge
Department of Soil Science S,
Biometeorology
Utah State University (UMC 48)
Logan, UT 84321
(801) 750-2178, -2179
Dr. John Wyngaard
National Center for Atmospheric Research
P.O. Box 3000
Boulder, CO 80307
(303) 497-0632
65
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APPENDIX 2
WORKSHOP ON DISPERSION IN COMPLEX TERRAIN
Keystone, Colorado
May 17-20, 1983
Preliminary Schedule
Tuesday, May 17, 1985 "Review of Phenomena"
8:00 Introduction, Objectives of Workshop B. Egan
8:30-10:00 EPA-CTMD Study Experiences T. Lavery (1 hour Presentation,
h hour Discussion)
10:30-12:00 DOE-ASCOT Program Field S. Barr (1 hour Presentation,
Experience h hour Discussion)
1:30-3:00 EPRI Bull Run Study N. Bowne (1 hour Presentation,
Field Experience ^ hour Discussion)
3:15-5:30 Group Discussion on M.E. Smith, Moderator
Phenomena of Importance
Participants will be encouraged to present short Contributions
on this Topic to be followed by more general Discussion
Wednesday, May 18, 1983 "Review of Modeling" D. Randerson, Moderator
8:30-10:00 Flow Field Modeling R. Pielke (1 hour Presentation,
4 hour Discussion)
10:30-12:00 Physical Modeling W. Snyder (1 hour Presentation,
h hour Discussion)
1:30-3:00 Regulatory Modeling S.R. Hanna (1 hour Presentation,
% hour Discussion)
3:15-5:30 Group Discussion on Modeling D. Randerson,.Moderator
Approaches
Participants will be encouraged to present short Contributions
on this Topic to be followed by more general Discussion
5:30 Subgroup Assignments
66
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Thursday, May 1983 "Develop and Document Findings"
8:30-12:00 Meet as two working Subgroups to define Areas of Agreement
and Disagreement
Group 1: Suggestions for Model Applications, B. Egan, Moderator
Group 2: Suggestions for Future Information Needs,
D. Fox, Moderator
1:30-2:30 Reconvene entire Workshop to review Progress
3:00-5:30 Return to separate working Groups, Report Writing
Friday, May 20, 1983 "Wrap up Discussion"
8:30-10:00 Continue with Report Writing
10:30-12:00 Discussion of Workshop Findings in Draft Form
12:00 Departure
67
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
2.
3. RECIPIENT'S ACCESSION NO.
4. TITLE AND SUBTITLE
DISPERSION IN COMPLEX TERRAIN
A Report of a Workshop Held at
Keystone, Colorado, May 17-20, 1983
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Bruce Egan
8. PERFORMING ORGANIZATION REPORT NC.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
American Meteorological Society
45 Beacon Street
Boston, Massachusetts 02108
10. PROGRAM ELEMENT NO.
CDWA1A/02/0279 (FY-85)
11. CONTRACT/GRANT NO.
CR 810297
12. SPONSORING AGENCY NAME AND ADDRESS
Atmospheric Sciences Research Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
- RTF, NC
FY 83-85
14. SPONSORING AGENCY CODE
EPA/600/09
15. SUPPLEMENTARY NOTES
15. ABSTRAC1
A workshop was convened by the American Meteorological Society under a cooperative
agreement with the U.S. Environmental Protection Agency. The purpose of the workshop
was to encourage atmospheric scientists working in this area to exchange recently
acquired information on atmospheric processes in complex terrain and to make recom-
mendations regarding both the present application of air quality models to complex
terrain settings and the research necessary to meet future needs.
This report contains the thoughts and judgments of 32 atmospheric scientists who
gathered to exchange such technical information and research results on atmospheric
processes in complex terrain and to comment on matters relating to adjustments in
current air quality modeling practices.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COSATI Field/Group
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (Tins Report)
UNCLASSIFIED
21. NO. OF PAGES
20. SECURITY CLASS (This page)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (RĞv. 4-77) PREVIOUS EDITION is OBSOLETE
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