United States
Environmental Protection
Agency
Atmospheric Sciences
Research Laboratory
Research Triangle Park NC 27711
Research and Development
EPA/600/S9-85/031 Jan. 1986
Project Summary
Dispersion in Complex
Terrain: A Report of a
Workshop Held at Keystone,
Colorado, May 17-20, 1983
Bruce A. Egan
Since 1979, the American Meteoro-
logical Society has collaborated with
the Environmental Protection Agency
through a cooperative agreement to
improve the scientific basis of air qual-
ity modeling. Under this continuing
agreement, the American Meteorologi-
cal Society conducted a workshop on
dispersion in complex terrain held 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 re-
cently acquired information on atmo-
spheric processes in mountainous ter-
rain and to make recommendations
regarding the present application of air
quality models to complex terrain set-
tings and the research necessary to
meet future needs.
The workshop report contains the
thoughts and judgments of 32 atmo-
spheric scientists who gathered to ex-
change technical information and re-
search results on atmospheric
processes in complex terrain and to
comment on matters relating to adjust-
ments in current air quality modeling
practices.
This Project Summary was devel-
oped by EPA's Atmospheric Sciences
Research Laboratory, Research Triangle
Park, NC, to announce key findings of
the research project that is fuily docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
The presence of mountainous terrain
introduces significant complexities in
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 re-
gions of converging and diverging
flows, enhanced shear effects, and tur-
bulent eddies. These alterations affect,
to a large degree, flow trajectories and
ambient turbulence levels. Terrain also
determines the development of local
circulations in mountain-valley settings.
The net effect on air pollution concen-
trations depends critically on the
specific geometric and topographic re-
lationships 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 ground-level con-
centration expected in the absence of
the high terrain.
Similarly, stagnation effects in con-
fined valleys can result in the buildup 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 regula-
tory agencies, and methods for quan-
tifying atmospheric dispersion proc-
esses have received considerable
attention over the past several years.
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*%*
A workshop on dispersion in complex
terrain was convened by the American
Meteorological Society under a cooper-
ative agreement with te U.S. Environ-
mental Protection Agency. Its purpose
was to encourage atmospheric scien-
tists working in this area to exchange
recently acquired information on atmo-
spheric processes in complex terrain
and to make recommendations regard-
ing the present application of air quality
models to complex terrain settings and
the research necessary to meet future
needs.
Conclusions
This section summarizes the major
conclusions of the workshop partici-
pants concerning the present state-of-
understanding of dispersion in complex
terrain.
A. Interaction of Elevated Plume with
Windward-Facing Terrain Features
The largest ground-level concentra-
tions associated with elevated release
near terrain rising above plume height
are often associated with stable atmo-
spheric conditions. The dynamics of the
air flow depend on the Froude number
of the upstream flow based on hill
height. A related parameter, the divid-
ing or critical streamline height, ap-
pears to separate vertically a flow
regime that can transport a plume up
and over a terrain object from a flow
regime that constrains plumes to stag-
nate or to pass around the sides of a
terrain feature. Verification of these
above concepts has emerged from
physical modeling efforts for a variety
of terrain shapes and from field mea-
surement results. Mathematical models
are presently being refined to accom-
modate these situations. Further verifi-
cation is needed, however, for various
scales of terrain geometries.
B. Turbulent Dispersion Rates in Re-
gions of Complex Terrain
Atmospheric turbulent dispersion
rates in complex terrain can often be
expected to exceed those over flat ter-
rain with otherwise similar conditions.
This is especially true under stably strat-
ified conditions, resulting from pres-
ence of gravity-driven drainage flows,
gravity waves, production of shear from
flow deformation, and creation of ed-
dies from upwind terrain features. The
effect on ground-level concentrations
depends on the specific source-receptor
geometry.
C. Lee Side Effects
The flow on the lee side of terrain fea-
tures can, under certain conditions, also
cause high ground-level concentra-
tions. 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 pass-
ing 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 ex-
ist for these situations.
D. Valley Situations
A number of air pollutions phenom-
ena are associated with the constraints
on ventilation that valleys impose or
with the gravity-driven local flows cre-
ated by valley side walls. The most
severe effects are the occurrence of
multi-day air pollution episodes within
deep valleys during periods in which
high pressure systems stagnate over a
region. Mathematical models for air
pollution applications in deep valleys
are in the developmental stage at the
present time.
E. Physical Modeling Capabilities
The use of physical modeling princi-
ples with wind tunnels and towing tanks
has increased markedly over the past
several years. These techniques allow
systematic investigations of flow situa-
tions under controlled conditions not
practically achievable with field studies.
Properly interpreted, the results of
these tests add substantially to the
understanding of phenomena. Limita-
tions remain for the study of two-
dimensional terrain feaures under sta-
ble conditions and in the spectral range
of turbulence, which can be simulated.
F. Mathematical Modeling Capabilities
As new information emerges from
theoretical efforts, physical modeling,
and field experiments, better and more
refined mathematical models are being
developed that simulate many of the
phenomena, although no model as yet
can simulate all of the phenomena for a
given setting. The verification of models
is difficult due to the general lack of ex-
tensive data bases. The trend of lower
computer costs will encourage the de-
velopment of more advanced models
capable of using more extensive input
data.
Recommendations
The following section summarizes the
major recommendations of the work-
shop participants regarding use of the
science and needs for further research «
and development.
A. The workshop participants sup-
ported the approach by current
major research efforts in gathering
field and laboratory experimental
data for ultimate use in developing
better mathematical models.
B. A need to quantify uncertainty in
model predictions was identified,
with the recognition that more infor-
mation about flow conditions is
needed to estimate mean values
from deterministic models.
C. Participants believed that the disper-
sion of pollutants in complex topog-
raphy involves interactions of air
flows and terrain structure leading
to certain natural fluid dynamic time
and space scales that are not neces-
sarily as important in simpler (e.g.,
flat) terrain problems. If such time
scales are larger than the averaging
time of application interest, esti-
mates of concentrations by deter-
ministic models would contain
larger uncertainties. Thus, it was
suggested that the stochastic nature
of the phenomena be recognized
and accepted by those developing or
applying models for these circum-
stances.
D. The meteorological input data needs
for complex terrain models are
greater than those required for level
terrain models. Specific require-
ments for vertical profiles of temper-
ature and velocity emerge from the
need to estimate Froude numbers
and dividing streamline height. On-
site turbulence measures were also
identified as being especially appro-
priate for complex terrain modeling
efforts.
E. The workshop participants identified
a number of specific technical areas
needing further research to advance
our ability to predict ground-level
concentrations of pollutants in com-
plex topography. Table 1 lists the
topics in summary fashion and iden-
tifies the status of available observa-
tional data, physical conceptualiza-
tions, and modeling efforts. The
order of topics in this list does not
signify the order of the priorities.
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Table 1. Recommended Future Research Topics
Topic
Stable Plume
Impact/on
Effects on Lee Side
Sources
Fumigation in
Complex Terrain
Hypotheses or
Subcomponents to
be Tested
Dividing Streamline
Height; Scales of
Applicability
Downwash; Dividing
Streamline Height
Boundary Layer Depth;
Turbulence
Characteristics
Data
Gathering
Status/Needs
Data Available or
Scheduled to Be
Gathered
Field Data
Needed
More Field Data
Needed
Physical Model
Status/Needs
Partially Accomplished
Some Accomplished
Modeling Needed
Mathematical Modeling
Some Accomplished
Development Needed
Some Accomplished
Stagnant Air Masses
within Valleys
Convective Flows in
Complex Terrain
Interaction of Boundary
Layer with Free
Atmosphere; Sloshing
and Net Ventilation Rates
Interaction of Boundary
Layer with Slopes
Field Data Not Yet
Needed
Field Data Not Yet
Needed
More Modeling
Needed
Exploratory Modeling
Needed
More Development
Needed
Some Accomplished
Mesoscale
Transport
Coastal/Complex
Terrain Interface
Flow Field Models;
Boundary Layer Depths
Boundary Layer Depths;
Data Representativeness
More Data
Needed
Field Data Not Yet
Needed
Not Applicable
Modeling Needed
Some Accomplished
Modeling Needed
Bruce A. Egan is with Environmental Research & Technology, Inc., Concord, MA
01742.
Francis A. Schiermeier is the EPA Project Officer (see below).
The complete report, entitled "Dispersion in Complex Terrain: A Report of a
Workshop Held at Keystone, Colorado, May 17-20, 1983," (Order No. PB 86-
122 694/AS; Cost: $11.95, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Atmospheric Sciences Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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