United States                     Atmospheric Sciences
                 Environmental Protection             Research Laboratory
                 Agency                         Research Triangle Park NC 27711

                 Research and Development            July 1987
V>EPA        PROJECT REPORT
                                                   OOfR87101
                 CONTRIBUTIONS  OF THE FLUID MODELING

                 FACILITY TO EPA's COMPLEX TERRAIN

                 MODEL  DEVELOPMENT PROGRAM

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     CONTRIBUTIONS OF THE FLUID MODELING FACILITY

TOEPA's COMPLEX TERRAIN MODEL DEVELOPMENT PROGRAM
                              by
                      WILLIAM H. SNYDER

               Meteorology and Assessment Division
             Atmospheric Sciences Research Laboratory
               U.S. Environmental Protection Agency
                Research Triangle Park, NC 27711
             Atmospheric Sciences Research Laboratory
                Office of Research and Development
                U.S. Environmental Protection Agency
                Research Triangle Park, NC 27711

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                                NOTICE
     This information in  this document  has been funded by the United States
Environmental Protection Agency.  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.
     The author, William  H.  Snyder, is a physical scientist in the Meteorology
and  Assessment  Division,  Atmospheric Sciences  Research  Laboratory,  U.S.
Environmental Protection Agency,  Research  Triangle Park,  NC.    He is on
assignment  from  the  National Oceanic  and  Atmospheric  Administration,  U.S.
Department of Commerce.

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                                FORWARD

     The Atmospheric Sciences Research Laboratory (ASRL) conducts intramural
and  extramural research programs  in  the  physical sciences to detect,  define,
and  quantify  air  pollution  and  its  effects  on  urban,  regional,  and  global
atmospheres and the subsequent impact on water quality and land use.  The
Laboratory  is  responsible for  planning, implementing, and  managing  research
and  development  programs  designed  to  quantify  the  relationships  between
emissions  of  pollutants  for   all   types   of  sources   with  air  quality  and
atmospheric effects,  and to uncover and  characterize  hitherto  unidentified air
pollution  problems.  Information from ASRL programs and from the programs of
other government  agencies, private  industry,  and the academic  community are
integrated by  the  Laboratory to develop the technical  basis  for  air pollution
control strategies for various pollutants.
     The Complex Terrain Model Development Program is designed  to develop
reliable  atmospheric  dispersion  models that are applicable to  large  pollutant
sources  located in complex terrain.  The  major field studies of  this six-year
program  were conducted during 1980 at Cinder Cone Butte near Boise,  Idaho,
during 1982 at Hogback Ridge near Farmington, New Mexico, and  during 1983-84
at the Tracy Power  Plant near  Reno,  Nevada.   Data from these field studies
along with  measurements of fluid  modeling simulations  performed  in  the  EPA
Fluid  Modeling  Facility  are  being  used  to quantify  the  effects  of  terrain
obstacles on  stable  plume dispersion.   A series of annual milestone reports
has been issued to describe the development of the Complex Terrain Dispersion
Model (CTDM) and to contrast the performance evaluation of the CTDM against
existing   complex  terrain  dispersion  models.     This   report  describes   the
contributions of the  Fluid Modeling Facility  to  the  Complex  Terrain  Model
Development Program.
                                    A.H. Ellison
                                    Director
                                    Atmospheric Sciences Research Laboratory
                                    in

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                                ABSTRACT

     The contributions of the EPA Fluid Modeling Facility (FMF) to the Complex
Terrain Model Development Program (CTMDP) are described.  These contributions
included a wide range of  laboratory  studies and a  limited amount of numerical
modeling  of  flow  and  diffusion  in  neutral  and  stably stratified conditions in
complex terrain. The  goal  of the  CTMDP  is the development of a  dispersion
model valid in  complex terrain, with emphasis on  plume impaction on  nearby
hills  during  nighttime  stable  conditions.   Work  at the  FMF  prior to  the
inception  of the program  provided  the basic framework  for  the  model  - the
dividing-streamline  concept  -  and  the focal point  around  which  the field
program was designed.  Throughout the course of the CTMDP, the FMF interacted
vigorously  with the model developers  by  providing  support  in  various  ways.
Early work  provided  direct  support as an aid  to  planning  the  details  and
strategies of the  field  experiments  and  testing the  limits  of applicability of
the  dividing-streamline  concept.   Later work  included  exercises  of  "filling in
the  gaps" in  the  field data,  furthering  the  understanding  of  the physical
mechanisms  important  to  plume  impaction in complex terrain  and  in  stably
stratified  flows in  general,  testing  various modeling  assumptions,  providing
data  for "calibration"  of various  modeling   parameters, and  testing  the  ability
of  the  laboratory  models  to simulate  full-scale  conditions.    Simultaneously,
the FMF responded to the needs of  the regulatory arm of EPA, the Office of Air
Quality  Planning  and  Standards  (OAQPS),  by providing  guidance  concerning
expected  terrain  effects and by  conducting  demonstration  studies.   Finally,
several supplemental studies were conducted, broadening and expanding upon the
specific requests of the model developers and the OAQPS.
                                     IV

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                                CONTENTS


Forward   	    ///'
Abstract   	    iv
List  of Figures	    vi
List  of Tables	    vii
List  of Symbols and  Abbreviations    	    viii
Acknowledgements    	     ix


     1.   Introduction    	    1
     2.   Background    	    3
     3.   Description  of  Experiments  and Results   	    8
          3.1  Direct Interactions with  the  Model Developers    	    8
                 The period  1980 through 1981  	    8
                 The period  1982 through 1983  	   19
                 The period  1984 through 1985  	   26
                 The period  1986 through present    	   32
          3.2  Supplemental  Modeling  of Complex Terrain  	   37
                 Neutral-Flow Wind Tunnel Studies   	   37
                 Stably Stratified Towing-Tank  Studies    	   42
     4.   Summary	   45

References	   47

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                            LIST OF FIGURES
Number                            Title                                Page

1    Oblique view  of  dye  streamers  released from  a  horizontal  rake    11
     upwind  of  the CCB  model  at z//7 = 0.3  under  strongly stratified
     conditions (F=0.2).  Flow is from the left.

2    Top  view of  dye  streamers  impinging  on  CCB  under  strongly    11
     stratified conditions  (z/h = 0.3, F = 0.4).

3    Top view of  CCB under strongly stratified conditions showing vortex    12
     rollup and eddy-shedding in the lee (z/h = 0.6, F = 0.2).  Unretouched
     sequential photographs  cut  and  pieced together.    Building  roof
     shows in background of center  portion.

4    Oblique view of impinging streamers on CCB.  Middle dye  streamer    14
     is released at  the dividing-streamline height;  others at ±1cm (±6m
     full scale).

5    Comparison    of    predicted   dividing-streamline   heights   with    15
     observations  as  functions  of  towing   speed.    Open  symbols:
     predictions using  integral formula; closed symbols: observations.

6    Concentration  distributions measured  during individual  tows of  CCB    17
     with   Hs//7 = 0.31  and HD/h = 0.38; wind direction:  	  117°,	
     122°.

7    Scatter   diagram  comparing    superposition   of   concentration    18
     distributions  from series   of  18  tows  of CCB  model with  field
     distributions.   Dotted  lines denote  factor of  two  on either side of
     perfect fit.

8    Deformation  of vertical  dye line by  upstream  columnar disturbances.    23
     Dye line was formed at a location 16m upstream of starting position
     of fence, at  time when fence was  at x = 12.5m (18.6/7 upstream of
     fence).  Photograph was taken  when  fence was at x = 13.8m (11.6ft
     upstream of fence).  Fence  is  out  of photograph, approaching from
     top  left.

9    Concentration  distributions  measured  on  the   hill  surface  with    28
     HD//7 = 0.5  and  Hs/ft = 0.6.   Top:  fully   submerged;  bottom:  half
     submerged.  Dotted circle indicates half the hill height.

10   Scatter  plot comparing concentrations on fully  immersed  hill with    29
     those on  half-immersed hill  on a  port  by port  basis.   Hs/h = 0.6,
                                     VI

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11   Terrain amplification factors measured upwind of axisymmetric CCB    31
     model.  Heavy  lines divide the region into areas where the  source
     position produced the maximum glc upwind of the hilltop, between
     the  hilltop  and  the separation  point,  and downwind  of the  hill.
     Note that the vertical scale is exaggerated by a factor of 3.

12   Plume cross sections measured in presence (	) and in absence    36
     (	) of  axisymmetric CCB model at x = 0 (hill center).   Hs/h = 0,
     xs/h=-6, ys//7 = 0.

13   Contours  of   constant   terrain   amplification  factors   over  (a)    41
     axisymmetric hill  and  (b)   two-dimensional   ridge.    Note  that
     vertical scale is exaggerated by a factor of  3.
                             LIST OF TABLES
Number                            Title                               Page
1    Summary  of  Terrain  Amplification  Factors  for  Sources  in  the    39
     Vicinity of Hills in Neutral Flow.
                                    VII

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                 LIST OF SYMBOLS AND ABBREVIATIONS
Symbols
A    Terrain amplification factor, Xmx/#mX
F    Froude number, U^/Nh
g    Acceleration  due to gravity
h    Hill height
h0   Height of density  interface from surface
WD  Dividing-streamline height
Hs   Source height
L    Length of ridge
N    Brunt-Vaisala" frequency, [-(c//p)dp/dz]
^oo  Towing speed or free-stream velocity
xs   Source position in along-wind  direction (origin at hill center)
ys   Source position in crosswind direction (origin at hill center)
Ap   Density difference across interface
p    Fluid density
p.,   Density of fluid between interface and surface

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                          ACKNOWLEDGEMENTS
     Many people have contributed to the work described in this report.  I am
particularly  grateful  to   R.E.   Lawson   for   his   untiring  efforts  in  the
day-to-day  operations of  the  laboratory,  to  R.S.  Thompson for  his  unfailing
support and many enlightening  discussions,  to J.C.R.  Hunt for his continual
encouragement, unending infusion  of  new  ideas,  and  enduring patience  in
teaching me so much about  stratified flow over obstacles,  to R.E. Britter and
I.P.  Castro for  their many  contributions,  to  G.L  Marsh  for  his  dogged
persistence  in operating  the towing-tank  experiments, to M.S. Shipman for his
quiet but solid  computer  support, to J.C. Smith  for his many  hours at the
filling station,  to G.C. Holzworth for his  insistence upon FMF  involvement  in
the  CTMD  Program and his  acceptance  of  different viewpoints,  and to  F.A.
Schiermeier for letting us "do our  thing".   Finally, I wish  to  express thanks
to the entire FMF  staff, past and  present,  who do the  real work day in  and day
out,  and whose efforts too often go unrecognized and unrewarded.
                                     IX

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                            1. INTRODUCTION

     In  the  late 1970's  the  Office  of  Air Quality  Planning and Standards
(OAQPS) of the Environmental Protection Agency (EPA)  identified  a  crucial need
to develop  an improved mathematical model  that dealt with plume impaction from
large  sources located  in  mountainous terrain  under stable  flow conditions.  A
workshop was  convened (Hovind ef  a/., 1979) to focus  on complex  terrain
modeling problems and to develop recommendations to  EPA with  respect to the
design  of  a   program  of  experiments   and  model  development  efforts.
Subsequently, Holzworth (1980)  outlined  the EPA plan to achieve the  objective
through an integrated program of model development, fluid modeling  experiments
and   field  studies   of  plume-terrain  interactions  on   hills  of  progressively
increasing  size  and  complexity.   This  multi-year, multi-faceted  program is
known as the Complex Terrain Model Development Program (CTMDP). The prime
contractor  for  this  effort is Environmental  Research  and  Technology  (ERT),
which has  produced a comprehensive series of annual reports, called Milestone
Reports,  that describe all  phases  of the  research program.   The  specific
references   are:  (1) Lavery  ef  a/  (1982),   (2)  Strimaitis  ef a/  (1983),  (3)
Lavery ef  a/ (1983),  (4) Strimaitis  ef a/   (1985),  and  (5)  DiCristofaro  ef a/
(1986); a final report is to be  completed  in  1987.
     The  Fluid  Modeling  Facility  (FMF)   interacted vigorously  with  various
groups participating in  the CTMDP, and provided direct support and guidance in
many different ways.  Whereas the field work and model development  effort up
to the present  time has  been specifically focused on  plume impaction  under
stable conditions, the work at the FMF has taken a much broader view. The FMF
research program has ranged from the development  of broad guidelines (e.g.,
terrain  amplification  factors)  and  physical concepts  (e.g.,  dividing-stream-
line  height)  to modeling of specific  sites  (e.g.,  Cinder  Cone  Butte)   and
regulatory  applications  (e.g.,  good-engineering-practice  stack  height).    The
FMF  has  provided  laboratory  data  to  "fill in  the  gaps"  in  the field  data
(e.g.,  measurements of plume deformations over  hills)  and tested the validity
of convenient modeling assumptions (e.g., cut-off hill  approach).

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     This report  summarizes the  contributions,  both  direct  and  indirect,  of
the FMF to the CTMDP.  The discussion provides a historical perspective and a
comprehensive list of FMF's accomplishments  with  respect to furthering the
physical  understanding of flow  and  diffusion  in  complex  terrain.   In many
cases the early research  results  were first published as  internal documents  or
project reports or presented at workshops or conferences in order to speed the
flow of  information to the model  developers.   In  most  cases,  these results
have  been published  in  peer-reviewed  journals (which  took, in one extreme
case, 8 years to appear in print).  For completeness and to provide the proper
perspective,  both   references  are  cited  at  first mention  in  the  text  that
follows;  thereafter,  only the journal publication is cited.

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                             2. BACKGROUND

     Research work conducted  at the FMF prior to the inception of the CTMDP
had  a strong influence on the directions to be taken in the field  work and  on
the type  of model  (i.e.,  physical  concepts)  to be  developed.   The stratified
towing tank was commissioned in 1976 (Thompson and Snyder, 1976)  and rather
fundamental  studies  were  begun  immediately  on  the  structure  of  stably
stratified flow  over  idealized  three-dimensional  hills  and on diffusion from  a
point source within a stably stratified field  of turbulence.
     The  first published  reports  on  this  work  (Hunt ef a/, 1978;  Hunt and
Snyder, 1980)  described the flow structure  observed over  a  bell-shaped  hill
under neutral  and  stably  stratified  conditions.   Earlier theoretical  work  by
Drazin (1961),  model  experiments by Brighton  (1978) and  Riley  ef  a/  (1976),
and  observations  (e.g.,  Queney ef  a/,  1960)  all  indicated that,   when  the
stratification  is  strong  enough,  the  air flows  in  approximately  horizontal
planes around the topography.  And this observation had been used  by EPA in
estimating  the  surface concentrations on hills caused by  upwind sources of
pollution (Burt  and Slater, 1977).  Up to that time, however, there  had been
little  firm  laboratory  or field data  as to  how  strong  the  stratification must
be for any given streamline  starting below the hill top to pass round the side
rather than over  the  top of  the hill.   The  Hunt  and  Snyder  (1980)  paper
suggested a  criterion  for this  change-over  to occur  on  the   basis  of  the
low-Froude-number theory  of  Drazin  (1961),  and  confirmed that  criterion with
experimental data.
     The   Drazin   (1961)  theory  is  applicable to  strongly  stratified  flows
around  three-dimensional  hills;   indeed,   it   is   asymptotically   valid   at
zero-Froude-number.    In  simplistic  terms,  the  theory  suggests   that  the
stratification   inhibits    vertical   motions,    so  that   fluid   parcels   are
constrained to move in horizontal planes.  Hence, the flow may be described in
terms  of  two-dimensional flow  around  a  cylinder  which  is  not necessarily
circular but,  in  fact,  has  the  cross-sectional shape of  the intersection of  a
horizontal  plane  with  the three-dimensional  hill.   Hunt and  Snyder (1980)

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verified  that,  for  a bell-shaped  hill,  a  linearly  stratified  environment,  and
an  effectively  uniform  approach-flow  velocity  profile,  Drazin's  theory  was
applicable in the range F<0.4, where F is the Froude number (^UJNh, U^ being
the towing speed,  A/ the Brunt-Vaisala frequency, and h the hill height).
     More importantly, Hunt and Snyder (1980) showed evidence for a dividing
streamline  (on the centerplane  determined  by the flow and  the axis  of the
axisymmetric hill) of height Hs  such that streamlines  below Hs would impinge
on  the  hill  surface and  follow the  surface  around the  sides,   whereas
streamlines above Hs would go over the top.  They suggested the simple formula
                              Hs = h (1  - F)                            (1)
as the criterion to determine whether a plume embedded in the  flow approaching
the hill would  impact on the surface or surmount the top, for 0
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     The implications of this flow structure with regard to plume impingement
and  resulting surface  concentrations were amplified  by  Hunt  ef  a/ (1979).
This primarily theoretical work  described two  conceptual models for dealing
with the problem  of  plume impingement.  The first  model  was  applicable  to
strongly stratified  flow  around  three-dimensional  hills, where vertical motion
and  vertical  diffusion  is  negligible.   The  advective-diffusive equation around
a  three-dimensional  hill which is  axisymmetric  about  a  vertical  axis  was
solved (using an eddy diffusivity) to show how source  positions on and off the
centerline affect the  trajectories and splitting  of  impinging  plumes  and the
value and  position of  the  maximum  surface  concentration  on the hill.   The
results  showed that the  plume  behavior  is  very  sensitive to  quite  small
changes in  wind  direction  away from the direction  that  transports  the  plume
onto a  stagnation  point, and the model  provided a simple way to estimate the
effect  of  these  changes.    This   model  also  allowed  the  computation  of
concentrations  within  the  separated,  horizontally  recirculating  wake of the
hill (source upwind of hill).
     In  the  second model,  a  plume in a neutrally stable  potential flow around
a  hemisphere was analyzed, also  using  the diffusion equation.   The  solutions
showed how, because streamlines  approach the surface of a three-dimensional
hill much more  closely  than  that of  a two-dimensional  hill,  the  maximum
surface  concentration on the  hill can become  very  much greater than  in the
absence of the hill (but only for  a  limited range of source heights).
     Prior to the inception  of the CTMDP, another complex  terrain model was
developed by ERT under contract to EPA.  The algorithm developed at that stage
was generally  applicable  to  plume  behavior  in  stability  conditions  ranging
from neutral  to  slightly stable.   The general approach followed  the  theory  of
turbulent plumes embedded in  potential  flow fields as developed  by  Hunt and
Mulhearn (1973), Snyder and Hunt  (1984  - original  manuscript made available to
ERT in  1978),  and  Hunt  ef  a/  (1979).   This  theory  was applied to the
calculation  of ground-level  concentrations  using  a Gaussian form of solution
to the  diffusion equation.   Stream  functions appropriate  to  the  potential flow
over a  cylinder  (aspect ratio,  h/L = 0) and  to the  potential flow over a sphere
(ft/L = 1) form the  cornerstones of the model.  These solutions were extended to
describe flows  over terrain  features of intermediate crosswind aspect ratio  by
a  weighting  of the  two  limiting  stream  functions.   The  derivation  of this
weighting scheme relied heavily  on wind-tunnel experiments  of flows over hills

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of  various  aspect  ratios  (Snyder  and  Britter,  1987;  data  reports  made
available to ERT  in 1979).   Strictly  speaking,  this algorithm was  applicable
to  neutral flows,  but  an empirical  approximation  was  included  to  define
streamline  lowering  caused  by  an   imposed  stable  stratification.    This
empirical  scheme was derived on  the  basis  of  the stratified towing-tank
experiments of Hunt and  Snyder (1980).    Extensive  comparisons of  model
predictions with  FMF laboratory  data were made for both neutral and weakly
stable  conditions.  A full account of these model-development efforts  and the
essential physics  of the model are provided by  Bass  et  al  (1981).    These
algorithms  were  subsequently  incorporated  into a  routine operational  model
called  COMPLEX/PFM  (Potential  Flow  Model;  Strimaitis  ef  a/, 1982).    In
COMPLEX/PFM,  potential flow calculations are performed whenever the plume lies
above  the dividing-streamline  height and the stability  is  between neutral  and
slightly  stable;  when the plume is  below the  dividing-streamline  height, the
model  reverts to  the  standard COMPLEX  I computation  (see  Wackter  and
Londergan, 1984). The COMPLEX I computation makes the level-plume assumption,
with an effective doubling of surface concentration above the plume centerline
concentration  (Burt and Slater,  1977).   This particular  aspect  of the  complex
terrain  diffusion  problem was  one  of the  hotly contested issues that  provided
the impetus for the CTMDP.
     A "strawman" was proposed by Holzworth and Snyder (1979) for discussion
at the 1979 workshop convened by  EPA to make recommendations with regard  to
the directions to be taken under the CTMDP. This strawman was hotly debated
at  the  workshop  and,  in  the  end, was largely  accepted  by  the workshop
participants (Hovind et al, 1979).   The plan that emerged  (Holzworth, 1980)
called  for  an  enlargement of some of  the  major concepts  arising from the
previous work at  the FMF,  and for a verification  of these concepts through the
conduct of a  series of field  studies  on hills of progressively increasing  size
and complexity.
     Prior  to  the  request  for  bids  on the  CTMD  contract,  a   preliminary
one-week  field study of the nighttime  flow patterns  at Cinder Cone Butte was
organized and conducted primarily by FMF personnel (Snyder et al.  1980); the
primary purpose was to assess  the suitability of Cinder Cone Butte as the site
for  the  first  small  hill  study  (identified  in  the ERT  Milestone  reports   as
Small Hill Impaction Study # 1).  Numerous observations were made  of the flow
structure  and plume  behavior  around the  hill,  including (1) plumes spread

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broadly  in  the  lateral  direction  but  very  thinly  in  the  vertical  direction
over   the  hill   in   neutral   conditions,   (2)   lee-side  separation   under
high-wind-speed, neutral conditions,  (3)  plume impingement  under light wind,
strongly  stable  approach  flows,  and  (4)  katabatic  winds under light-wind,
neutral approach flow conditions  with clear night sky.  Cinder  Cone Butte  was
judged  as  ideally  suited for the  first  small hill  study  in several  respects.
Finally, numerous suggestions were offered as an aid to the  design  and conduct
of future  field  studies at this site  (most  of  which were adopted  in  the later
studies).
     To  recap  the "state  of  the science" immediately  prior  to  the  contract
award,  the dividing-streamline  concept had been  shown  to be  a  useful
conceptual  framework  to use in describing  the  structure of  strongly stratified
flow around three-dimensional  hills.    It had only been shown  to  be valid,
however,  for   quite  a  limited  number  of   hill  shapes,  all  of   which were
axisymmetric.    It  had  only  been  verified under  uniform  stratification  (linear
density gradient) or  under a  step  inversion (sharp density  interface), under a
uniform   approach-flow   velocity  profile,    and,   of  course,   only   under
steady-state,   small-scale  laboratory   conditions   (although   the   preliminary
field study provided reassurances of the validity of  the  concept).

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           3.  DESCRIPTIONS OF EXPERIMENTS AND RESULTS

3.1 Direct Interactions with the Model Developers

The period 1980 through 1981
     The major CTMDP contract was awarded in June,  1980, and the work plan
called for the  small hill impaction  study to begin at Cinder Cone  Butte  (CCB)
in September.   Almost  immediately, the  FMF  was  called upon to  conduct
towing-tank experiments  to aid in  the detailed planning  and  design of the
field  experiments.   The first request was to provide guidance  with regard to
the location of the  main  meteorological  tower.  The second  request was to
provide  guidance  for smoke-  and  tracer-release  strategies,  for  preselecting
locations  for  samplers  and  cameras, and for  choosing  in  advance several
different  sampler  strategies  to  account  for variations  in  flow regimes  and
wind  fields.   The  third   request  was  to test  the  validity  of  an  integral
formula  for predicting the dividing-streamline height.
     At  an early July  meeting at ERT headquarters in Boston, MA,  the  question
arose as to  how  to  predict the  dividing-streamline  height  when the  wind
profile was not uniform  and the density gradient was  not  linear.  This was of
paramount  importance  in   planning  the  release scenarios,  as  the  release
locations  and  heights were to  be  chosen in  real time  during  the field  study
based   upon  the  incoming  real-time  meteorological  data.     J.C.R.   Hunt
immediately sketched the now well-known  integral formula as
                                                                        (3)
     This formula  had,  in  fact,  been published 24 years earlier by Sheppard
(1956) as a small  note, actually  in answer to his own  question which  arose  at
a  meeting of the  Royal  Meteorological Society,  although Sheppard's note was
virtually  unknown  to the  modeling community  at  that time.   This  integral
formula is based upon simple energy arguments.  Sheppard asked the question:
"in a  strongly  stratified flow  approaching  a   hill,  does   a particular  fluid
parcel at some height upstream possess sufficient  kinetic energy to overcome

                                     8

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the  potential   energy  required  to  lift  itself  through  the  potential   density
gradient  from  its upstream  elevation to the  hill top?"  The left-hand side may
be  interpreted  as the  kinetic  energy of the  parcel far upstream  at  elevation
Hs, and  the  right-hand side  as the potential energy  gained by the parcel  in
being  lifted from the  dividing-streamline height Hs to the  hill top h  through
the density gradient  dp/dz.  This integral formula was presumably  applicable
to a fluid with any  shape  of stable density  profile and,  presumably,  with  any
shape  of  approach-flow  velocity  profile.    In practice,  it  must  be  solved
'rteratively,  because  the  unknown  Hs  is the lower  limit  of integration;  the
formula can easily be reduced to the simpler formulae (1) and (2) by using the
boundary conditions  applicable to  those special  cases.   The third request to
the FMF was thus to verify this integral formula.
     Three studies were  conducted in the  summer of 1980,  and three  reports
were  prepared  in  response  to  these  requests.    In  the first  study  (Snyder,
1980a), twenty-six separate tows of a model of CCB were made through the tank
in a two-week period.  The objective was  to  assess  the suitability  of  the
particular site  chosen  for  the  main  (150m)  meteorological tower,  i.e,  was  it
close enough to CCB  (s100m high and  1km in diameter) to be representative of
the flow  approaching the hill,  yet far enough away that the  measurements were
unaffected by  the hill itself?    It  was impossible,  of course,  to  meet  both
these  criteria   for  all  wind  directions;  the  question   addressed   in  the
towing-tank studies,  then,  was whether the flow  field at the proposed tower
site  would  be perturbed  by  the  hill,  given the  climatological  ranges  of
prevailing wind directions for  light,  nighttime winds.  Measurements were made
of  surface  flow  patterns,  deformation   of  material  lines   (instantaneous
crosswind  releases  of tracer),  velocity profiles and  streamline  patterns over
a  model of  CCB,  and  these  measurements were  compared  favorably with
predictions of  potential flow theory.  The findings from  the study suggested
that  no  significant  perturbations  to  the  approach   wind  field  were  to  be
expected due to the  presence of  the hill when the wind direction was within
the range  of the  prevailing wind directions.   In addition  to the  main tower,  a
shorter (20m) tower was recommended, to  be erected at the trailer site (3km
ESE of  the hill).  This recommendation was indeed  implemented in the field
study.
     This  was,  in  fact,  the  first   real-terrain   model   (i.e.,   non-idealized
shape)  to be  studied  in the  FMF  stratified  towing tank.   As  a side  benefit,

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therefore,  the study provided reassurances that the basic flow features and,
of course, the same  physical principles applied  to more realistically shaped
hills.   Some  examples  are  provided  here to  illustrate the  point.    Figure  1
shows an oblique view of the hill, where neutrally buoyant dye  streamers were
released from a  horizontal  rake  upstream of the hill under  strongly stratified
conditions.   The  horizontal  nature  of  the flow field is immediately obvious.
Figure 2  shows a  top or plan view of  the CCB  model  under roughly  similar
conditions.   One of  the dye streamers  obviously  impinged  on  the upwind
shoulder  of  CCB.    Flow separation  in  the  lee of the  hill  is  dramatically
illustrated  in  Figure 3, where  vortex roll-up and eddy-shedding in the lee are
quite vivid.  The Karman vortex street was a common occurrence at  low Froude
number;  a   street   appeared   to   form   at  all  elevations   below  the
dividing-streamline  height  (at  least  for  small  Froude  numbers),  but  the
shedding  frequency seemed  to  vary  with  elevation and  motions  at  different
elevations were seemingly uncorrelated with one another.
     In the second phase of this  summer series, eleven tows of the CCB  model
were made during which two  of the model developers from ERT participated as
observers.   In this case, vertical rakes  of  tubes  emitted  neutrally  buoyant
dye  at up to 6  elevations, with different  colors  of  dye being emitted  at the
different levels.  Each  tow was filmed from the side using a camera  that moved
with the hill, and from directly below using a fixed  camera  pointed  upward at
the  (inverted) model  hill.  The films were viewed with  an  analyst's  projector,
and  the plume paths  and envelopes were sketched.  These results corroborated
the  previous  results  of Hunt and Snyder  (1980)  on idealized hills, i.e.,  that
plumes below the dividing-streamline height Hs  and on a stagnation streamline
would impinge on the upwind side of the butte  and flow around the  sides, and
that   plumes  released  just  above  Hs  may  produce  maximum  ground-level
concentrations on  the upwind side as  they  pass over the top.   The  results
further emphasized that  plumes travelling in  a  direction only  slightly  away
from that  of  the stagnation streamline  would tend to pass around CCB without
significant impact, and that plumes released somewhat higher above Hs may be
caught in  strong  downslope   flows  and  produce  maximum  ground-level
concentrations on  the lee  side  of  the  hill.   The results were also used, of
course, for  the originally  intended purpose as a guide for planning  of  release
and  sampler strategies and  selection  of sampler and camera  locations.   The
results are described by Bass (1980).
                                     10

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Figure 1.   Oblique view  of dye  streamers  released  from  a horizontal  rake
upwind of the CCB model  at z//? = 0.3 under strongly stratified conditions (F =
0.2).  Flow is from the left.
                                                                                 N
Figure  2.   Top  view of  dye  streamers  impinging  on  CCB  under  strongly
stratified conditions  (z/rj = 0.3,  F = 0.4).
                                      11

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     In  the third  phase of this summer  series, the  goal was to  test  the
validity  of the  integral  formula  for  the  height  of  the  dividing  streamline
under density profiles similar to those expected  at  CCB.   A typical nighttime
temperature profile  in  the  Snake River Basin  (site of  CCB)  was found  to
consist of a strong, surface-based inversion of depth 50 to 100m and a weaker
inversion  above  extending  to  several  hill  heights.    Hence,  the  stratified
towing tank was filled with a  strong density gradient  near the surface  and  a
weaker gradient above.  A vertical rake of 3 tubes was positioned well upwind
of the  hill, and neutrally  buoyant dye was  emitted from  each tube.   For each
tow, a  particular  stack  height  (center tube)  was  chosen and the  general
formula was  integrated  numerically  using  the  measured density  profile  to
predict the towing speed required such that the center streamer would  rise to
the elevation of the  saddle  point of  CCB, i.e.,  the minimum height of the draw
between  the two peaks.  If the formula were correct, then, the lower  streamer
should go around the side  of  the hill,  the  upper streamer should go  over  the
top, and the center one should split.  The  height of the break-point  between
the two gradients was then  reduced and the  process  repeated.  In all,  twelve
tows   were   made,   varying   the   height   of  the   break-point  or   the
dividing-streamline height (release height) each time.
     Figure 4 shows a  side view of the impinging  streamers  during  a  typical
tow, i.e.,  the  upper streamer going through the draw, the lower streamer going
round the side, and  the middle one splitting.  Figure 5  shows  the results in
quantitative fashion.   The density profiles were integrated in accordance  with
Equation  (3)  to find  the dividing-streamline heights (based on  the height of
the saddle point)  as  functions of the towing  speed.   These predictions  are
shown  in Figure  5  as the   continuous  lines.    The  observations  of  the
dividing-streamline  heights  made during the twelve tows are also plotted in
the figure; the  agreement between the  predictions and  observations is  regarded
as  excellent.    The  error bars  result because of some fluctuating  behavior of
the streamers, especially at the higher  speeds;  occasionally,  an  intermittent
vortex  at  the  top  windward side of  the hill  would  engulf  all  three streamers
and they  would all go round the sides temporarily;  on other occasions, parts
of the lower streamer could be observed passing through the draw.  The  results
of this set of experiments (Snyder,  I980b) provided confidence  in  the  validity
of  the  general integral formula for  predicting  the  height  of  the  dividing
streamline for  a wide range of  shapes of stable density profiles.
                                     13

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                                 '   /   I    \    \
Figure 4.  Oblique view of impinging  streamers on CCB.  Middle dye streamer is
released at the dividing-streamline height; others at ±1cm (±6m full  scale).
                                     14

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                    5       10       15      20      25


                         TOWING SPEED, CM/S
30
Figure  5.    Comparison  of  predicted  dividing-streamline   heights   with

observations as functions of towing speed.   Open symbols:  predictions  using

integral formula; closed symbols: observations.
                                 15

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     During the six-week field study at CCB, detailed measurements were made
of wind, turbulence,  and  temperature  profiles in the  approach  flow  and  at
other positions on  the hill.  Sulfur hexafluoride (as a tracer) and smoke (for
flow visualization)  were  released  from a  platform  suspended from  a mobile
crane   that  allowed   flexibility   in   positioning   the   source   (height   and
location).    One hundred   samplers on  the  hill  collected  data  on  surface
concentrations,  and   lidar  was   used  to  obtain  plume   trajectories  and
dimensions.
     One particular  hour from the  field study  was  selected  for simulation  in
the towing tank (Snyder and Lawson, 1981).  That hour  was 0500 to 0600, 24
October 1980  (Case  206), which may  be  characterized as  very  stable,  i.e.,
light winds and strong stable temperature gradients.  Measurements made during
the towing-tank experiments included ground-level concentrations under various
stabilities   and  wind  directions,   vertical  distributions  of  concentration  at
selected points,  plume  distributions in the  absence of the  hill, and visual
observations of plume characteristics and trajectories.
     This series of tows  showed that the surface-concentration distributions
were extremely sensitive to changes in wind  direction.   For example,  Figure  6
shows  that  the  distribution shifted  from  the  north  side of the hill to the
south  side  with  a  shift  of only  5° in  wind  direction.    Comparisons  of
individual distributions with field  results  showed very  much  larger  maximum
surface concentrations and much narrower distributions in  the  model results.
To account for the large variability  in the winds measured during the  hour,  a
matrix of 18 tows (three wind directions x six  wind speeds) was conducted, and
the concentration patterns  were superimposed.  A scatter plot of superimposed
model concentrations  versus field concentrations (Figure 7)  shows a  marked
improvement over the single-tow comparisons.  The largest model concentrations
were within a factor of two of the highest  field values,  and 70% of the model
concentrations  were within a factor of two of  the observed field values.
     It  was  interesting to  learn  that, whereas the  location  of  the  maximum
shifted  dramatically with  small   shifts  in  wind  direction,  the  value  of the
maximum changed very little with changes  in wind  direction  or wind speed.
Maximum surface concentrations approached those at the plume centerline in the
absence  of  the  hill  during individual  tows,  but  because  of  the   extreme
sensitivity of the location  to wind direction, the plume was  "smeared" broadly
across  the hill surface as  the  wind direction  changed through  only a few
                                     16

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                                                           600 M
          600 M
        SCALE
              200M
SOURCE
Figure 6.  Concentration distributions measured  during individual tows of CCB
with Hs//7 = 0.31 and H0//? = 0.38; wind direction :	117°,	122°.
                                    17

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  10000
                              I	1—I—I  I I I III
                            100
1000
10000
                           FIELD CONCENTRATION
Figure  7.    Scatter  diagram  comparing  superposition  of  concentration
distributions  measured  over Cinder  Cone  Butte  with  field  distributions.
Dotted lines denote factor of two on either side of perfect fit.
                               18

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degrees.  Therefore, short-term averages (*5min) in the field  may be  expected
to approach plume-centerline concentrations; longer-term averages (*1h) may be
expected to be reduced by factors of five to ten (or more depending  upon the
magnitudes of the fluctuations  in wind speed and direction).

The period  1982 through 1983
     Although considerable work  had been done concerning the validity  and
limits of  applicability  of  the  dividing-streamline  concept,  several  questions
still  remained.    One  question   concerned   the   effects of  shear in  the
approach-flow velocity  profile.   Another concerned  the effects  of the  aspect
ratio  (ratio  of  crosswind  length   of the   hill   to   its   height)   and,  in
particular,   its   applicability   in   strongly   stable   flows   to    a    truly
two-dimensional  ridge.   A third  questioned the effects of  the slope  of the
hill, and a  fourth, the effect of  wind angle on a long ridge.
     A few earlier studies  had  shed  light on some  of these problems.  Baines
(1979), for example, had conducted towing-tank studies of low-Froude-number
flows around a barrier with a gap.  His  results suggested
                            HJh = 1-2F                                 (4)
for barriers with  very  small gaps, tending  toward Hs/rt = 1-F  (Equation  1)  for
those with wider  gaps.    Weil  ef  a/  (1981)  conducted similar  towing-tank
studies,  extending  the work   of Baines,   and  found quite  similar results.
However,  data from a field study  by Rowe ef a/  (1982) of stable air  flow over
a  "long"  ridge showed  much better  agreement with the data  for axisymmetric
hills  (Equation 1) than for  ridges with gaps  (Equation 4).
     In  the early  1980's,   a  series  of  experiments  was  done  by numerous
investigators at the  FMF  and  for  a  variety of different  purposes.  The  overall
objective was to gain fundamental  understanding of flow and  diffusion  under
stably  stratified   conditions  in complex terrain,  but  the individual  projects
were  designed   with   very   specific   and   limited   objectives  in   mind.
Nevertheless, one aspect of each  of the  projects was to examine the concept of
the   dividing-streamline   height,    as   it   obviously   had   very    important
consequences with respect to the  CTMDP. The results of most of these projects
were published separately  and independently,  as will be referenced below, but
the   results    concerning   the    validity    and     applicability    of    the
dividing-streamline  concept were extracted  and  published  as  an   appendix
(Snyder et  a/,  1983) to the Second Milestone  Report   (Strimaitis ef  a/,  1983)
                                     19

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in order  to  provide timely  support  and  guidance  to  (1)  the  mathematical

modelers  attempting to  expand  their models to  include  a  wide variety  of

terrain shapes  and approach flows  and (2) planners  of  the  Second Small Hill

Impaction Study, which was to take  place at the Hogback Ridge in northwestern

New Mexico.  This paper was subsequently published  in a journal (Snyder ef a/,

1985).  The individual  laboratory experiments included:


 1.  Towing-tank  studies on  truncated,  steep-sided  ridges of various
     crosswind aspect  ratios.  These included examination  of upstream
     "blockage"  regions,  surface  flow  patterns  and  lee-wave  structure
     and  were reported  by  Castro  ef a/  (1983); those  aspects dealing
     specifically with the  dividing-streamline concept  were  reported  by
     Snyder ef a/  (1983)   and Snyder  ef   a/ (1985).

 2.  Stratified  wind-tunnel  studies  (in   Japan)   on  shear  flow  over
     vertical fences of  various crosswind aspect ratios and over a  model
     of Cinder Cone Butte.   (Snyder and   Ogawa,  1982; Snyder  ef  a/,
     1985).

 3.  Towing-tank  studies on a truncated sinusoidal  ridge with a maximum
     slope of 40° positioned  perpendicular and  at other  angles to the
     approach  wind direction (Lee ef a/, 1984a, 1984b).

 4.  Towing-tank  studies on an "infinite"  triangular  ridge  and a long
     sinusoidal   ridge   to   test   the   validity  of   the   "steady-state"
     assumption  of flow  upwind  of  an  obstacle  under  strongly stratified
     conditions (Castro and  Snyder, 1987c).


     The  conclusion from the studies with truncated triangular and sinusoidal

ridges perpendicular to the  wind  was that  the aspect  ratio  per se, does not

have a significant  influence  on the  dividing-streamline height Hs.  Deviations

from  the  Hs/h = ~\-F rule  were attributed to the  combination  of  shear in the
approach flow  and  the  very  steep  slope of  the  triangular  ridges,  which

resulted in the formation  of  an upwind vortex with  downward flow on  the front
faces  of  the  ridges.   The  "1-F"  rule was validated for the  sinusoidal  ridge

with  a length-to-height  ratio  greater than 16:1;   in  this  case,  the  shear in

the  approach  flow  was  much  less pronounced,  and the  upwind  slope was

substantially  smaller.   Note  that  these  deviations to the  "1-F"  rule  did not

invalidate Sheppard's  concept,  but  required a reinterpretation of  the rule  as

a  necessary  but   not  sufficient  condition,  i.e.,  a  fluid  parcel  may   possess

sufficient  kinetic  energy  to  surmount  a  hill,  but  it  does not necessarily  do
so.
                                     20

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     In the  stratified  wind-tunnel  studies,  reasonably  strong  shear  layers
with depths  more than twice  the hill  heights  were developed in conjunction
with strong  stable  temperature gradients.   These approach  flows  provided
dividing-streamline  heights  as  large as 0.75/7.   In  the vertical fence studies
with a stratified  approach flow,  the  shear  was found to have an overwhelming
influence.   The conclusions were:  (a) as in  the triangular  ridge  studies,  the
aspect  ratio  was  relatively   unimportant;  the  basic  flow  structure  was
independent  of aspect ratio;  (b)  the shear,  in  conjunction with the  steep
slope,  created an upwind vortex such that plumes were downwashed on the front
faces;  and  (c)  under  strong  enough  stratification,  there  was a  limit to  the
downward  penetration  of elevated  streamlines; the  extent  of this penetration
appeared  to  be  predictable  as  a  balance  between  kinetic  and  potential
energies.  However, when these same  shear  flows approached the much lower
sloped CCB model,  there was  no evidence of upwind vortex formation.  Limited
concentration measurements on the CCB model suggested that Sheppard's integral
formula correctly predicted the  height of the dividing streamline.
     From  the sinusoidal ridge studies with wind angles  at  other  than 90°,  it
was concluded  that the effect of  deviations  in wind direction (from  90°)  are
relatively insignificant  until  the  wind  direction is in  the  vicinity  of  45°  to
the  ridge  axis.   At  30°,  significant  departures  from the   "1-P rule  were
observed; the  fluid  had sufficient kinetic energy to surmount the ridge,  but
found  a path requiring  less  potential energy round the end of the ridge.  When
the dye  streamers were moved closer to the upstream  stagnation streamline
(upwind of the upstream end of the ridge), they behaved according to the "1-P1
rule.
     The two-dimensional ridge studies showed that  steady-state conditions are
not  established  in  strongly   stratified   flows  (say  F<1).     Two   different
physical mechanisms give  rise  to this  unsteadiness; one  is called "squashing",
the other,  upstream wave  propagation.   Brief explanations will be given here;
the interested reader should consult the cited references.
     The squashing  phenomenon is most easily described in terms of the simple
energy arguments  as used in  deriving Sheppard's  formula  (Equation  3).  As
discussed  there,  a  fluid parcel with   insufficient  kinetic  energy  to overcome
the potential  energy requirement to surmount the  hill must  pass round  the
sides  of the hill.   But a two-dimensional hill  has no  sides  around which to
pass  (in a towing-tank, a  "two-dimensional"  hill  is  one that  spans the  entire
                                     21

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width of the tank).  Hence,  the fluid  parcel must be brought  to  rest.   In  the
towing tank, since the  fluid  is  generally at rest and  the hill is towed,  this
means that  the fluid ahead (upstream) of the hill must be pushed  ahead of the
hill,  instead of  being  allowed  to  surmount  the  hill  top.    However,  the
upstream endwall  of  the towing  tank,  of course, prohibits  this  fluid from
being pushed.   Hence, the fluid between the hill and the  upstream endwall is
"squashed"  as  the   hill  approaches  the  endwall;  because  the  fluid  is
incompressible,  it must rise  and  spill  over the top of  the  hill, just  as  the
water  in  a  bucket will  rise  and  spill  over  the  top  when the  sides  are
"squashed".
     This squashing phenomenon seems to have no counterpart in the atmosphere.
If  true blocking occurred  upwind  of  an "infinite" ridge  in  the atmosphere,  it
seems that the flow  would  be blocked to  infinity  upwind (i.e.,  there  is no
"endwall"  forcing  the  flow  toward  the  ridge).    In   more   practical  terms,
"blocking" upstream of a very long ridge would  imply  "upstream  influence" to
very large  distances,  possibly  through  an  upstream-propagating front,  which
would  imply non-steady-state behavior.   From  another viewpoint,  there are no
infinite  ridges   in  the  real  world,   so  that  fluid  parcels  can always  be
diverted  around the obstacles without  changing their elevation.
     The results leading  to  the  "1-2F" formula (Equation 4) by Baines (1979)
and Weil ef al  (1981)  for two-dimensional  ridges and  ridges  with gaps were
surprising because they suggested that  fluid parcels could surmount the  hills
even though  they had insufficient kinetic  energy  to  do  so.   Snyder  ef al
(1983,  1985)  suggested  that these earlier  results  were  erroneous;  that they
were largely due to the squashing  phenomenon, i.e., the  gaps in their ridges
were insufficiently  large to allow a "relief valve" to avoid the  squashing.
     Upstream wave  propagation  is  also   possible in  stratified  flows.    The
introduction of an  obstacle in a stratified flow on  which lee waves  can form
will  result in "columnar" disturbances  extending upstream  (see Turner, 1973);
if  such  motions are present,  they will  modify the  approaching  flow.   These
columnar disturbances take  a sinusoidal form  in the vertical,  with the "mode"
(number of  oscillations) being dependent upon the Froude number  based on the
depth of the tank.  An example of an upstream columnar disturbance is shown in
Figure 8.   Dye  crystals were  dropped  into the stratified  tank  at  a  position
16m upstream of the  starting  position of an obstacle  (in  this case,  a vertical
fence)  which was  to  be towed  along the  water surface.   These  crystals
                                     22

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Figure 8.  Deformation of vertical dye line  by  upstream columnar disturbances.
Dye line was formed  at a location 16m upstream  of starting position of fence,
at time when fence was at x = 12.5m (18.6/7 upstream of fence).  Photograph was
taken when  fence  was at x = 13.8m (11.6/7  upstream of fence).  Fence is out of
photograph,  approaching from top left.
                                    23

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dissolved as  they sank to the bottom, leaving  behind  a vertical dye line.  The
dye line was formed after the commencement of the tow, and the photograph was
taken well  before the  obstacle  reached the  dye-line  position, i.e.,  the dye
line was deformed by the upstream columnar disturbance into the sinuous curve
shown in Figure 8.
     These columnar disturbances, unlike the squashing phenomenon, do have
counterparts  in the real atmosphere.   They result  in  "blocking" and  "upstream
influence".    However,  in  the  laboratory tank, these upstream  waves  are
reflected from the upstream endwall of the tank  and return to modify the flow
locally around the model hill; this reflection from  the upstream endwall does
not have  a  counterpart in  the  real  atmosphere.  Baines  (1979)  argued that
valid observations could be made of  the  flow over and  around  the obstacle in
isolation (in  the  absence  of end effects)  by  making  the observations after
steady  state  was   reached  (estimated  by  direct  observation),  but  before
reflected upstream  motions  arrived.    Evidently,  he  believed that  a local
steady  state  was achieved  in that, at some not-too-distant point  upstream of
the  obstacle,  steady-state  velocity   and  density  profiles were  established
before the  reflected motions  returned to modify them.
     Snyder  et al (1983;  1985)  showed that  steady-state conditions  are  not
established   in  strongly  stratified  flows (say  F<1)  over  two-dimensional
ridges.   The squashing phenomenon and  reflections  of  upstream columnar
disturbances  continuously changed the shapes of  the "approach flow" velocity
and  density  profiles.   Thus, these experiments  have  no analogue in  the  real
atmosphere.   Further, because long  ridges  cut by periodic small gaps require
very long tow distances in order for  steady state to  be established,  Snyder ef
al concluded that the previous  laboratory  studies were  not  valid models of
atmospheric flows; specifically, the Hs/ft = 1-2F formula proposed for flow about
ridges with small gaps is not expected to apply to  the real  atmosphere.
     Further  work was done to better  understand  the nature  and  causes  of these
upstream motions and lee waves by Thompson  and  Snyder (1984), Castro (1987)
and  Castro and Snyder (1987b,  1987c),  but the interpretation of these results
is somewhat controversial.   More work  is  required  to establish  the  precise
relationships  between model  size  and shape,  stability, and  tank  size,  shape
and  configuration in  order  to   determine  the  limits  of  applicability  of fluid
modeling and ranges of transferability  to the  atmosphere.
                                     24

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     The second Small Hill Impaction Study was conducted during October 1982
at Hogback Ridge (HBR) (s100m high) near Farmington,  NM.  In  providing input
to the experimental  design,  the  FMF  conducted  a series  of wind-tunnel  and
towing-tank  flow-visualization  experiments   prior   to  the  field  study.     The
laboratory studies were  designed to investigate

      • plume height above the surface  over the hill crest and at
        the upwind edge of the hill,
      • apparent size of any plume deformation upwind  of the hill,
      • lee wave importance and structure, and
      • sensitivity of the plume trajectory to  "wind angle".

This information  was subsequently used by the  field designers to  guide the
design of the smoke and tracer-gas  release  protocols at HBR, and  to help
select sampler and camera locations.
     Two tests were  made  in the wind  tunnel. One test was done with the ridge
perpendicular to the flow, the  other  with the  ridge  rotated by 30°.    These
tests  suggested that  in  neutral  conditions   the  streamline   patterns  were
similar to those  expected  from potential flow  theory; a plume  released at  a
given height upwind of the ridge should traverse the crest at an  elevation of
one-half its  initial height.   The  test  with the  ridge at  an  angle  to the flow
showed only a  very  small (<4°) deflection  of  the plume  path  as the  plume
traversed the ridge.
     Eight  individual  tows of  the  HBR  model were  done  in  the stratified towing
tank, varying the Froude number and wind  direction,  and each  time releasing
dye  at eleven different  elevations upstream.   Heights of these  dye  streamers
were  measured  at the  upstream base and at  the crest of the  ridge.    These
experiments showed that, during weakly  stratified  conditions, plumes  rose near
the upwind base and fell  over the  crest to near or slightly lower  than their
upstream  heights.   Low-level  releases  experienced extensive  mixing.    More
detailed  results  are  contained  in the Third Milestone  Report  (Lavery  ef  a/,
1983, p.  117-123).
     Around this same time period, the FMF  undertook two separate  laboratory
experiments  that  attempted  to  simulate two  specific  one-hour  periods  as
observed  in the field at  Cinder Cone  Butte.   The first simulated  a  neutral
stability period in the Meteorological Wind Tunnel   (Thompson  et  a/,  1983).
                                    25

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The  second simulated a moderately stable  period in the stratified towing tank
(Eskridge  ef a/,  1983).   [Recall  that  the  simulation  of  a strongly   stable
period was described earlier (Snyder and Lawson, 1981).]
     In the summer of 1983, Ben Greene and colleagues from ERT, in cooperation
with  the FMF staff, conducted experiments in the Meteorological Wind Tunnel to
characterize the response of the Climatronics UVW propeller anemometers. The
primary objectives of the experiments were  to determine the  calibration  curves
and  the non-cosine response corrections, especially at  low wind speeds.  The
results  of  these  tests   are  contained  in  the  Fourth  Milestone   Report
(Strimaitis ef a/,  1985,  p. 85-93).   The  calibration factors  and non-cosine
response  correction  factors were applied to the HBR data base  in forming the
Modeler's  Data  Archive.
     In late  summer  of  1983,  discussions  were  held  with  ERT concerning
possible  contributions  of FMF to the Full-Scale Plume Study planned for the
following year  at the  Tracy  Power  Plant near  Reno, NV.    Considerations of
scaling the  site  for  towing-tank  studies  revealed  that,   at  any  reasonable
scale,  the  model would appear as a two-dimensional ridge  with a  small gap
(river valley)  running through it.   Recent work at the FMF as  discussed  above
had  shown that  this  situation could  not  be modeled  under strongly  stable
conditions.  Hence, specific  site modeling at the Tracy Power  Plant was not
undertaken at the FMF.  Instead, other studies  in direct  support  of the model-
development effort were undertaken as described below.

The period 1984 through 1985
     In September  1983, A.  Venkatram, D.  Strimaitis and  R.  Britter  from ERT
requested   that  the   FMF   conduct   two  studies  in   support  of   their
model-development  efforts.  The first  study  attempted  to shed light  on the
question  of  the  validity  of the  assumption   of  a  flat  dividing-streamline
surface, a key  assumption  in the model under development.  The second study
was  to provide a complete set of data on neutral flow and diffusion around a
three-dimensional hill  with a shape  and slope approximating that  of  Cinder
Cone  Butte.  These data were to help  ERT to evaluate the separate effects of
plume deformation  kinematics and those of  increased turbulence around the
hill.
     The  first   study,  testing  the  validity  of  the  flat  dividing-streamline
assumption, consisted of a series of 26 tows of a model hill in the stratified
                                    26

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towing tank.   The  model  hill was the fourth-order polynomial (45° maximum
slope)  used by Hunt  et al (1978), except that it was in this case instrumented
with 100  sampling  ports  located  along 8  radial  lines.   The  density  gradient
was linear  and  the  dividing-streamline  height  was   fixed  at  half  the  hill
height.      Effluent   was   released   at   three   elevations    above   the
dividing-streamline  height.   Pairs  of  tows were made  such that,  in one tow,
the hill (upside down) was fully immersed in the  water and the towing  speed
was adjusted  to  provide  a  "natural"  dividing-streamline  surface.    In  the
second tow of the  pair, the  model (baseplate, hill, and  source, as a unit) was
raised  out of  the water to the  point  where only  the top half  of  the  hill was
immersed,  thus,  forcing  a  flat  dividing-streamline  surface,  while all  other
conditions  remained identical.   Concentration distributions  were measured  on
the  hill  surface  (and   in  the  absence  of  the   hill).     Concentration
distributions  from  each  pair  of  tows  were  compared  to  ascertain any
differences between the "natural"  dividing-streamline  surface  and  the  (forced)
flat  dividing-streamline  surface.    A  comparison  of  surface-concentration
patterns from  a typical pair of tows  is shown in  Figure 9, and a scatter plot
comparing concentrations  on a port by port basis is shown in Figure 10.  These
results showed that the assumption  of a flat dividing-streamline surface is a
reasonable  assumption  to  make,  at least with  regard to  predicting  the
locations  and values  of  the maximum surface  concentrations  and  areas  of
coverage  on the windward side of the  hill.   The results are  contained  in  an
appendix to the Fourth Milestone Report (Snyder and Lawson, 1985a) and were
presented at the  Third International Symposium on Stratified Flows  (Snyder and
Lawson, 1987).
     The second study, providing  a relatively complete  set of data  on flow and
diffusion around  a  three-dimensional  hill, was conducted in the  Meteorological
Wind Tunnel.  The  primary  objective  was to determine  the influence of the  hill
on the maximum ground-level  concentration (glc)  and  to  locate  the source
positions where this influence was greatest.  All measurements were made with
an approach  flow  that  simulated  the  neutral   atmospheric  boundary  layer
measured at Cinder  Cone Butte. However, the nearly axisymmetric CCB shape was
replaced by  a truly  axisymmetric  hill  represented  by   a simple  mathematical
formula, and having  a maximum slope of 24° (the same as CCB).
     The  measure  of the hill's influence on  the maximum glc was the "terrain
amplification factor" A.   This factor  is defined  as the ratio  of  the maximum
                                    27

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Figure  9.    Concentration  distributions measured  on the  hill surface  with
HD//i = 0.5 and Hs/h = O.Q.  Top: fully submerged; bottom: half submerged.  Dotted
circle indicates half the hill height.
                                     28

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 100
      .1
10
100
            CONCENTRATION, HALF SUBMERGED
Figure 10.  Scatter plot comparing  concentrations  on fully immersed  hill with
those on half-immersed hill on a port by port basis.  Hg//? = 0.6, H0/h = 0.5.
                              29

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glc observed  in  the presence  of  the hill to the  maximum  observed  in  the
absence of the  hill.  The  locations of the maxima are not considered  in this
evaluation;  the  maxima may  be  found  at  entirely  different  places   in  the
presence  and in the absence of the hill.
     A  matrix of  source locations was used  covering  the  range from 4  to  16
hill heights (h) upstream of the hill center and  to 1.25r» in the  vertical.   A
map of terrain amplification factors is  shown in Figure 11.   The presence of
the hill  was found  to influence the transport  and dispersion  of  the  plume and
to increase the maximum  glc in  three ways.   For low sources at moderate
distances  from the hill,  the reduction in  mean wind  speed and  increase in
turbulence allow the plume to reach the ground surface closer to  the  source,
thus producing higher concentrations  than  in the absence  of the hill.   Plumes
from higher sources may  be thought of as being  intercepted by the hill, that
is, the  hill penetrates the  plume to where  the concentrations are greater than
those that would  occur at ground-level farther downstream over flat  terrain.
For yet higher sources,  the streamline convergence over the hill top and  the
corresponding downward flow and much enhanced  turbulence in the lee of  the
hill again  bring  the plume to  the  ground  more rapidly  than over flat  terrain.
Terrain  amplification factors  ranged from  near  1.0  to  3.63,  and the range of
source  locations   that   produced  an  amplification factor  greater  than  1.4
extended  to  an  upwind  distance of  14  hill  heights.   These results were
reported in an appendix to the Fourth Milestone Report (Thompson and  Snyder,
1985b).
     In  the fall of  1984,  ERT requested  a list of data  sets  available from
previous complex terrain studies that had been conducted at the FMF.  A report
was prepared by  Thompson  et al  (1985)  listing 24 separate complex terrain
studies.   Each project was synopsized with a brief description of  the  project,
the name of the  principal  investigator(s), the  facilities used,  types  of data
collected,  names of data  reports  available,  major  conclusions  reached,  listing
of  published  results from  the  project, and  a listing  and description  of  the
data files  available.
     An earlier request (prior to summer 1983) from the modelers  at ERT had
been  to provide data on  streamline  trajectories  in neutral  and stratified flow
over  a three-dimensional  hill, i.e.,   to  provide  data  to  use  in  developing
algorithms for predicting  lateral and  vertical  streamline  displacements   over  a
hill as  functions of source location  and  stratification.  Earlier work  on  this
                                     30

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project had  been set aside because of experimental difficulties and because  of
the more urgent requests described immediately above.  Having completed those
studies, work commenced  again on the streamline trajectories.
     In attempting to predict the maximum  glc from a  source upwind of a hill,
the  most important  feature of the  flow is the  displacement of  the mean
streamline through the source,  because that displacement determines how near
to the surface  the "centerline"  of the plume will  reach.   The exact path taken
by the plume in  circumventing the hill and the plume's closeness of approach
to the hill surface are  critical in determining the location  and  magnitude  of
the glc's.  These displacements are known  to be  strongly affected  by  the  hill
shape and especially by the stratification in the approach flow.  The purpose
of  this  study  was  thus  to  characterize  the  effects  of  stability   on  the
horizontal and  vertical  deflections around  an  isolated  hill.   A large  set  of
streamline trajectories over  the axisymmetric CCB  model was measured using the
stratified  towing tank.   Three-dimensional  coordinates  of the  streamlines (86
independent  trajectories) were  determined  through  stereographic  analysis  of
photographs of  dye streak lines  released at  a  matrix of source positions
(heights   and   lateral   offsets   from   the   hill/flow   centerline),  and   at
stabilities  ranging from  strongly  stable  to  neutral (Froude numbers  of  0.6,
1.0,  2.0,  and <»).  These measurements provided a relatively  complete data set
for testing mathematical  models  and algorithms of the  detailed structure  of
stratified  flow over hills.   The results were presented  in an appendix to the
Fifth Milestone  Report (Snyder  ef a/, 1986).
     As  an  example use  of the data set,  a mathematicai  model using linear
theory  and  a  Fast  Fourier  Transform  (FFT)  technique   to  predict these
streamline trajectories was evaluated  and described in the above appendix  by
Snyder  et at (1986) and,  with  some additional work and computations,   by
Thompson and Shipman (1986).  The calculated results  agreed well with the
experimental results  for neutral flow.   In the stable  flow  (Fr=2.0), however,
lateral  deflections   were  underpredicted   and  vertical   deflections  were
overpredicted using the FFT model.

The period  1986 through present
     In February  1986, ERT conducted a Complex Terrain Workshop at Research
Triangle  Park,  NC  (Lavery ef  a/, 1986).   Each  participant was  beforehand
provided a  diskette  containing the Complex Terrain  Dispersion  Model  (CTDM)
                                     32

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code and a draft User's Guide and was asked to exercise the  model to assess
its overall effectiveness and  validity  in whatever way he  chose.  The  purpose
of the workshop,  then, was to exchange information on the  results of these
exercises and to make recommendations to the model  developers concerning
further refinements of the CTDM model.
     The present author exercised the CTDM by comparing is  predictions with
previous laboratory measurements of flow and diffusion  over hills  made in the
FMF.  This was accomplished in four phases.  In  phase  1, CTDM  calculations
were compared with wind-tunnel simulations of plumes released upwind of two-
and  three-dimensional hills in a neutral  atmospheric boundary layer.   Terrain
amplification factors  were  compared  for a matrix of source locations upwind of
the hills.  This phase was intended to test the LIFT module of CTDM, where the
stratification was neutral  and  the  potential  flow  calculations  of  LIFT  should
be most applicable.  In phase 2, CTDM calculations were compared with stably
stratified towing-tank  observations,  where  plumes were released above  the
dividing-streamline height  upwind of a three-dimensional  hill.  This phase was
intended  to  again  test   the   LIFT  module,  but this  time   under  strongly
stratified conditions.   In   phase 3,  CTDM  calculations  were  compared with
strongly  stratified  towing-tank  observations  wherein  plumes  were  released
below the  dividing-streamline height upwind of the Cinder Cone Butte model.
This phase was intended to test the WRAP module exclusively. In phase 4, CTDM
calculations were  made for  one selected hour of field  conditions, and  were
compared with  results  of towing-tank observations.  This phase was intended to
exercise both the LIFT and WRAP modules of CTDM.
     The results, made available in a  detailed report  that  was distributed to
the workshop participants  (Snyder,  1986),  may be summarized as follows:
 1.   From the  neutral flow simulations  (phase  1),  the hill  effects  (as
     exemplified  through   computations   of  terrain  amplification   factors)
     appeared  to be  much  too  small.    Reasons  speculated  for  this
     discrepancy included: (a)  plume  trajectories were  too far from the
     hill   surface,    (b)  potential  flow   calculations  did   not   properly
     handle the deep  boundary-layer flow approaching the  hill, or,  more
     likely  (c)   the  plume  centerline  did   approach  the  hill   surface
     closely enough, but the plume did not mix to the the surface through
     the hill-surface  boundary layer.
 2.   From  the  stable   flow  simulations   with   releases  above  the
     dividing-streamline height  (phase 2), it appeared  that  the  plume
     trajectories were   again too  far  from  the  hill  surface.    Vertical
     deflections of streamlines appeared  to be strongly overestimated and
     lateral deflections appeared to be  strongly underestimated.    In the
                                    33

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     towing tank,  plumes  released  slightly above the dividing-streamline
     height spread  broadly  but thinly  to cover  the entire  hill  surface
     above the dividing-streamline  height, whereas the CTDM plume was
     apparently deformed only slightly - it "hung together"  in going  over
     the top of the hill.   An apparent  shortcoming  of LIFT at that  time
     was  its  lack  of  appropriate  treatment  of  the  stratification  effects
     in  the  flow  that  surmounted  the  hill,  i.e.,  streamline  (hence,
     plume)  deformations  under quite  strongly    stratified  flows   (Frs1)
     were treated the same as those for neutral  flow (Fr=w).

 3.   From   the  stable   flow  simulations   with   releases  below   the
     dividing-streamline height (phase 3), the  WRAP module yielded rather
     poor  results  except when the input parameters  (primarily 
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an  upstream source with height such as  to  obtain a  plume that just "grazed"
the hill top.  The postulate  here was that  the roughness on the  surface would
maximize  the  effects   of  a  rapid-mixing  layer  near  the  hill  surface,  thus
mixing material from this elevated  plume  to the surface,  whereas the  smooth
surface  would  minimize the effects of this mixing layer.  The results showed
that the postulate  of   an  "inner  hill-surface  boundary layer"  was  untenable;
extremely  steep concentration gradients  remained near the  hill  surface,  even
when the hill was  roughened,  so  that rapid mixing was not  induced  by the
hill-surface boundary layer.
     To satisfy the  second  goal, a  series of measurements was made of plume
characteristics  in  flat   terrain  and over  a  three-dimensional  hill.    Effluent
was released  at a number  of elevations, upwind  distances,   and positions
laterally  offset from the centerplane determined by the wind direction and the
center of the hill.  Sufficient concentration measurements were  made to enable
the construction of plume cross sections  at the downwind  position of  the  hill
center and, in  a few cases, at  the upwind base of the hill.  These data were
analyzed to provide the desired information  on horizontal and vertical  plume
deflections and deformations  effected  by  the hill.  One of the more dramatic
examples  is shown in  Figure  12.   In  this  case,  the  source was  on the
centerplane at ground  level,  6 hill heights  upwind  of  the  hill center  (the
skirt of the  hill extended  to  5h).   Plume cross sections  measured  at the
position of the center  of the hill, both in  the presence and in the absence  of
the hill, are  shown.   The  hill  effected  a  91%  increase  in  the lateral  plume
width.  In  this case, the maximum surface  concentration (at the  same downwind
distance) was decreased by a factor of 2  but, of  course, the area of coverage
by  large  concentrations was greatly  increased.   Detailed  data reports  were
provided to ERT in  March 1986,  and the results were  published  by Snyder and
Lawson(1986).
     Subsequent  to the CTMD  Workshop (and as a  result  of  the  rather poor
comparisons of the  CTDM  predictions   of terrain  amplification  factors with
wind-tunnel data),  refinements  were made to CTDM.   Specifically,  the strain
inferred  or  measured  over  the crests  of  two- and  three-dimensional  hills  in
the wind  tunnel  were  used  in  the  calculations,  i.e.,  the  T-factors  in the
model  were  adjusted  in  accordance  with  wind-tunnel  data.     Substantial
improvements  in  the  CTDM  predictions  of terrain amplification factors  were
obtained, as described  by Strimaitis and Snyder (1986).
                                     35

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    2  r
Z

H
     1  -
             -2
                                                               .05
                      -1
                                        Y/H
                                                              -) and in absence
Figure 12. Plume cross sections measured in presence (—
(	) of axisymmetric CCB model at x = 0 (hill center).  Hs/h = 0, xs/rt = -6,
/.//) = 0.
                                        36

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3.2 Supplemental Modeling of Complex Terrain

     In addition to the modeling done in direct support of the CTMDP, numerous
other complex terrain studies were conducted at the FMF, primarily in response
to envisioned needs of and direct requests from the OAQPS, the regulatory arm
of EPA.   These ranged  from generic studies  attempting to  understand the
fundamental physics of flow  and diffusion in neutral and  stable environments
to a practical  demonstration  to determine the good-engineering-practice  stack
height for  a specific power  plant  located  in complex terrain.   Whereas the
studies  done  in direct support of the  CTMDP were primarily  concerned  with
plume impingement  from  upwind sources and focussed  primarily on strongly
stable conditions, the supplemental  studies were  broader ranging,  for example,
including sources on the  tops and  lee  sides of  hills, perhaps a broader range
in stability  from  strongly stable  to  neutral,  and  investigation  of similarity
criteria -  rules  to ensure  that the  behavior of the  flow  in  the  laboratory
simulates  that in the real world.   An  example  of the latter  is the  Guideline
for Fluid  Modeling of Atmospheric Diffusion,  prepared by  Snyder  (1981)  in
response  to a request from  the OAQPS.   In several cases,  studies that were
initiated through the CTDM developers  were subsequently enlarged  upon and
expanded so as to be useful to the  modeling community at large.  Hence,  in
many cases, studies could have been described as supplemental (this section)
or in direct support of the  CTMDP  (Section 3.1).   The  choices have  been
somewhat arbitrary.
     One  of the important overall  goals  in  this  effort  was to  ascertain what
circumstances  lead to   the  largest  ground-level  concentrations,  i.e.,  are
larger glc's expected when the plume from an upwind source impinges on a hill
or when the source  is  downwind of that hill such that the plume is caught in a
recirculation region  and downwashed to the  surface?  Which are likely  to lead
to   larger   glc's,   two-dimensional  or  three-dimensional   hills?      Stable
conditions or neutral conditions?  In  each of these  circumstances, what  order
of magnitude of surface concentrations may be expected?

Neutral-Flow Wind-Tunnel Studies
     A simple  method  used to intercompare  effects of terrain on the  maximum
glc  and   to   determine   worst-case  conditions  is   through  the   terrain
amplification  factor, as  mentioned  in  Section  3.1.    Again,  the   terrain

                                    37

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amplification factor, A, is defined  as the ratio  of  the  maximum  ground-level
concentration  occurring  in the  presence of the terrain  feature,  xmx,  to the
maximum that would  occur from the same  source  located  in flat terrain, x£x,
i.e.,  A=Xmx/Xnx-   Tnis  definition  is  useful only  for  elevated  sources,   of
course,  because for ground-level sources, the maximum surface concentration
occurs at the source  itself.
     Numerous neutral-flow wind-tunnel  studies have been conducted at the FMF
on  diffusion over two-dimensional terrain features:   (a) a  ramp with a  slope
of 14° followed by a  plateau  (Snyder and Pendergrass,  1980;  Pendergrass and
Arya, 1983; Pendergrass and  Snyder,  1987),   (b)  a  bell-shaped hill  with  a
maximum slope of 12° (Courtney,  1979; Courtney and  Arya, 1980), (c) a  steep
triangular ridge with  a slope of 63° (Arya  and Shipman,  1981;  Arya  et   al,
1981), (d) a series of smooth shaped hills  of various  slopes  (Khurshudyan  et
al, 1981; Capuano,  1983;  and Lawson and Snyder, 1985, 1987) and (e) a valley
formed  between  two  ridges of  sinusoidal  cross  section  (Lee  et al,  1981).
Three studies  have been performed  to  determine the effects  of the crosswind
aspect  ratio of  a triangular  ridge on  dispersion from  nearby sources.   As
mentioned in  Section  2,  Snyder  and  Britter (1987)  investigated surface
concentrations on the ridges from upwind sources.   (Note  that the work was
done in  1979, much  earlier than the publication date, so that the results were
available,  indeed, used in the development of a forerunner  to CTDM.)  Castro
and  Snyder (1982) extended the  study by measuring the sizes and shapes of the
recirculation regions  downwind  of  these  hills  of  various crosswind  aspect
ratio, and by measuring  the  concentration fields resulting from sources placed
at various downwind locations.    Recently,  Castro  and Snyder  (1987a)  have
further extended this  work to include the case  when the approaching wind  is
not  perpendicular  to  the long  axis  of  the  hill.   This allows  one to use the
wind-tunnel  data  to  estimate the effects of long-time-scale wind  meander.
Other generic  three-dimensional  hill studies included:  (a)  conical  hills with
slopes  of 26.5°  and  17.5°  with  sources located  at  the  hill top or at the
downwind base (Gadiyaram, 1984;  Arya and Gadiyaram,  1986)  and (b) the
axisymmetric CCB model with downwind sources (Lawson and Snyder, 1985,1987).
These various studies were summarized through publications at various stages
by Thompson and Snyder (1981, proceedings published 1985a) and Snyder (1983a,
1983b,  1984).    Only a  broad  overview and  a few  typical results will   be
presented here.
                                    38

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     Table 1  shows the  terrain  amplification  factors  for  the  cases  listed
above, in  order of decreasing A.   From the standpoint of a fixed stack height,
the  worst location   for  a  source  appears  to  be  just  downwind  of  a
two-dimensional  ridge.    Downwind sources  generally  result  in  larger  glc's
because of the excess  turbulence  generated  by the hills  and  because  the
effluent is  generally  emitted  into  a  low speed  region  where  the  streamlines
are descending toward the surface.  Maximum As are considerably larger than
those downwind of three-dimensional hills.   A probable cause  of this  effect is
that,  in  three-dimensional  flows,   lateral  and  vertical  turbulence  intensities
are enhanced by roughly equal factors, whereas  in two-dimensional flows,  the
lateral  turbulence  intensities  are  not enhanced as  much  as  are the vertical
turbulence intensities  (because of the two-dimensionality).  Since the maximum
glc depends  upon the  ratio 
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They  do  not  provide  practical  estimates  for  use  by,  say, an  air  pollution
meteorologist in determining the  maximum glc resulting from a particular power
plant  or for  determining  the best location for that  plant.   For that  purpose,
the concept of a "window"  of excess concentrations, as  introduced by Hunt ef
a/ (1979)  is  more  useful.   For  any given plant  location (say,  upwind of the
hill), there is  a  limited  range  of  stack  heights Hs  for  which  a significant
amplification  of the  glc  will occur.   (For sake of argument, we   will  here
define  significant  as  a factor  of 2.)   This  amplification  can  occur only  if
the position  of the  maximum  glc lies  on or near the hill surface.   For small
Hs, xmx will  occur  upwind of the hill and  thus be  little  influenced by  the
hill,  so that  A  (s*mx/*mx)  wi" approach  unity.    If  Hs  is  too large  (for
example,  Hs»/7, the hill height), xmx  will  lie well beyond the  hill and A will
again  approach  unity.   In either  case,  there  is  little  amplification.   These
"windows" of critical Hs values have been measured by Lawson and Snyder (1985,
1987)  for two  typical  hill shapes that  might be  found in the real world,  one
axisymmetric,  the  other two-dimensional. The results are shown in Figure 13.
The 1.4-window, for example, extends to about 14/7 upstream, 10/7 downstream,
and as  high  as  1.8/7  in  the  vertical for  the  axisymmetric  hill.    For  the
two-dimensional  hill,   this   1.4-window  extends about  Bh   upstream,   15/7
downstream,  and as high as 2.2/7 in the vertical.
     Such contour maps as provided in  Figure  13 can  be very useful for the
practitioner.    Once   an  acceptable  terrain  amplification  factor  (or "excess
concentration") is decided upon, it  is  a simple matter  to trace  the window on
the contour map to determine the area (plant location and/or stack  height) to
be avoided.    Conversely,  from  such maps, the  likely maximum glc  for  a
potential  site  and  stack  height  can  be  estimated.    The  use of terrain
amplification  factors   simplifies  the  application   of  these   data  to   full-scale
situations.    The  expected  maximum  glc  in flat terrain  is calculated  (from
mathematical  models  or   standard curves),  then   the  concentration  in  the
presence  of  the  hill is simply  the product  of this quantity and  the TAF.  This
study  was  initiated through a  request  from the  EPA Office of Air  Quality
Planning and Standards  (OAQPS) to  aid in the  decision-making  process  with
regard to the promulgation  of the Stack Height Regulations  under the  Clean Air
Act, and the data were provided to OAQPS much earlier than the publication
dates shown.
                                     40

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             41

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     Subsequent to the idealized study described above, the OAQPS requested
the FMF to conduct a study demonstrating the application of the fluid modeling
approach to the determination of good-engineering-practice (GEP) stack height
for a power plant  in  complex terrain, i.e.,  to  provide an example study/report
for industry to follow  in the conduct of a GEP determination.  The site chosen
for this  demonstration was  the  Clinch  River  Power  Plant in  southwestern
Virginia,  and a  1:1920 scale model of the surrounding terrain was constructed.
Measurements were presented  (Snyder and Lawson, 1985b) that described  the
simulated   atmospheric    boundary    layer    structure,    plume-dispersion
characteristics  in  that boundary layer,   and  the  maximum  glc of  effluent
downstream from  the plant,  both in the  presence of  all significant  terrain
surrounding the plant and  in the absence of "nearby" upwind terrain.   Analysis
of the maximum glc showed that, in this  case, a stack height of 326m met  the
GEP  criteria under   50%  load  conditions,  i.e.,  the  nearby  upwind  terrain
effected  an increase of 40% in the maximum  ground-level  concentration. This
study  followed   the  general  guidance  set forth  in the  Guideline  for Fluid
Modeling  of   Atmospheric   Diffusion   (Snyder,   1981)   and   the   specific
recommendations  set  forth in the  Guideline  for  Use  of  Fluid  Modeling  to
Determine Good Engineering Practice Stack Height (EPA, 1981) and the Guideline
for Determination of Good Engineering Practice Stack Height (Technical Support
Document for the Stack Height Regulations, Revised Draft) (EPA, 1985).

Stably Stratified Towing-Tank Studies
     Lamb and  Britter (1984)  conducted  a combined numerical and laboratory
study of so-called shallow  water flow over an isolated hill.  They  showed how
certain geometrical and flow parameters  affect the tendency of a fluid to flow
around rather than over an obstacle in the  case of a homogeneous single layer
fluid,   i.e.,  simulating  the   atmospheric   condition  of  an   elevated  step
inversion.   A   series  of  numerical  experiments  was  conducted   using   a
finite-difference  model.  Measures were  suggested for  quantitative assessment
of the tendency of the fluid to  flow around the obstacle as a  function of  the
relative  hill  height and the  Froude  number.    The  laboratory  experiments
examined the motions of two superposed  homogeneous layers of fluid  past  a
conical hill in the towing  tank.  The resulting motions were found to  agree
with the  results  of  the numerical experiments and extended  the  understanding
                                    42

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gained from them.  Flow visualization techniques were used to demonstrate the
impingement of the interface on the obstacle, and its dependence on flow speed
and hill height.
     Another numerical model was acquired and  implemented at the FMF for
comparison  with  laboratory  results.   The  numerical  model  was  originally
developed by Mason and Sykes (1979); it integrates the Navier-^Stokes equations
for  incompressible stratified  flow  using  a  finite-difference  scheme.   Direct
comparisons were made between the  results  of this  model  and  laboratory
experiments for  density-stratified  flow around the  idealized  axisymmetric  CCB
model by Rottman ef a/  (1987)  for three specific experimental  arrangements.
First,  a small  towing  tank was used  in which both the  Reynolds number and
Froude number were matched exactly with the numerical model.  This provided an
overall assessment of the accuracy of the approximations made in the numerical
model.   Second, the  large  towing  tank  was  used in  which mean  plume
trajectories were measured and compared with particle paths computed through
the numerical  model.  Third,  some comparisons  were made with  wind-tunnel
measurements of  the flow structure  over  the hill.   In general,  the numerical
model qualitatively  reproduced the experimental results  on the  flow structure.
     Whereas  the  following  area  of  investigation  is not directly  related  to
complex terrain,  it  is  included  here because it played  an  important  (and
somewhat controversial)  role  in  the  CTDM  formulation.    This is the area of
describing  the effects  of stable  stratification on turbulent  diffusion or,  put
another way, estimating vertical plume growth in the  nighttime stable boundary
layer.  Experiments were conducted  (Britter et a/, 1983) in which  a  grid was
towed horizontally along  the stratified  towing  tank.    The  vertical  velocity
fluctuations  produced near  the  grid  were  reduced  under strong  stratification
by up to 30%, but the decay rates of the turbulent velocity fluctuations  were
found  to be  unaffected  by  the  stratification  over  a  considerable  distance
downstream.  Turbulent diffusion from a point source located downstream of the
grid  was also measured.   The lateral plume widths  were found to be  largely
unaffected  by the  stratification and grew  with  the 1/2-power  of  time.   The
vertical plume  growth, however, was found to reach an asymptotic limit.  These
results  were  largely  in  agreement with the theoretical  models of  Csanady
(1964)  and  Pearson ef  a/  (1983),  but  in contradiction  to  the  theory  and
limited data of Venkatram ef a/ (1984).  The latter data suggest a  continuous
                                    43

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vertical  plume  growth  (for large times),  but  the  measurements  did  not,  in
fact, extend very far downwind (maximum downwind distance of about 1 km).
     Further grid-turbulence studies were done in the towing tank with the aim
of  investigating   internal  wave  effects  and  providing  guidance  on  the
partitioning  of  wave  and  turbulence  energies  in   stably  stratified  flows
(Rottman  and  Britter,  1986).    The  results  suggested  that  the fraction  of
available  turbulent  kinetic energy that  is   converted  to  potential  energy,
commonly  referred to  as  the mixing efficiency,  increases  monotonically  with
increasing  stability,  and there  is  some indication  that  this  mixing  efficiency
approaches a constant as the flow becomes strongly stable.
     A  cooperative  project  was  completed  with  the  Los  Alamos  National
Laboratory to examine the conditions under which flushing of  a valley between
two  ridges will  occur, i.e.,  to  answer the question of  when  a stable crosswind
will sweep the  valley clean and when  the flow will separate from  the  top lee
side of the  first  ridge, reattach  at the  top  windward  side  of the  second
ridge, and thus form a  nearly stagnant region in the valley beneath.  In this
series  of  towing-tank studies,  three  experimental parameters  were varied: the
steepness  of   the ridge/valley  slopes  (40°,  27°  and  13°),  the  separation
distance between  the ridges,  and the  Froude number  that characterizes the
stability of the crosswind.   In  broad  terms,  the  characteristics of the  flow
between  the   ridges  may  be  explained using  criteria  for  boundary-layer
separation from the  lee side of a single ridge.  The downstream  ridge appears
to induce separation from the lee side  of the upstream ridge only when  it is
steep-sided (Lee  er  a/,  1984a,b, 1986, 1987).   As an  offshoot of this work,
the conditions  conducive to the onset of  severe downslope winds on  the lee
sides of mountains  was investigated (Rottman and  Smith,  1987).   The results
showed that an intrusion  (breaking wave  -  associated  with severe  downslope
winds) existed  when the Froude number based on the ridge height was in the
range 0.2 ^ F < 0.6 for  a steep-sloped ridge (maximum slope 40°) and 0.2 ^ F ^
1.1 for a low-sloped  ridge (13°).
     An  overview  of  fluid modeling  of  pollutant  transport  and  diffusion  in
stably  stratified flows over complex terrain was provided for  Annual Review  of
Fluid Mechanics by Snyder  (1985).
                                     44

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                               4.  SUMMARY

     The EPA Fluid Modeling Facility  has conducted a wide range of laboratory
studies  and a  limited  amount  of  numerical  modeling of flow and  diffusion  in
association with the Complex Terrain  Model Development Program.   The goal  of
the CTMDP is the development of a  dispersion  model valid in complex terrain,
with emphasis   on  plume impaction  on nearby  hills during  nighttime  stable
conditions.  Work at the FMF  prior to the inception of the  program provided
the basic framework for the model -  the dividing-streamline concept - and the
focal point around which to design the field program.
     Throughout the course of the CTMDP, the FMF interacted vigorously with
the  model  developers by  providing  support  in  various  ways.    Early  work
provided direct  support  in planning  the details  and  strategies of  the field
experiments  and  solidifying and  testing  the  limits of  applicability  of  the
dividing-streamline  concept.   Later  work  included  exercises  of "filling in the
gaps"  in   the   field   data,  furthering  the   understanding   of  the   physical
mechanisms  important  to plume  impaction  in  complex terrain  and  in  stably
stratified flows   in general,  and  testing  the  ability  of the  laboratory  models
to simulate full-scale  field  conditions.   And,  as the  needs  arose,  the FMF
tested various modeling assumptions,  concepts, and hypotheses and provided
data for "calibration" of various  parameters within the CTDM model.
     Simultaneously, the FMF responded to the  needs of the  regulatory arm  of
EPA, the Office of  Air  Quality  Planning and Standards,  by providing  guidance
concerning expected terrain effects and by providing a  demonstration study -
an  example for industries to  follow  in conducting good-engineering-practice
stack height determinations in  complex terrain.   Also,   a   broad  range   of
supplemental studies was conducted,  expanding  and enlarging upon  the specific
requests of the  OAQPS and the CTDM model developers to provide information  of
general  use to  the  scientific and air pollution modeling communities.  Many  of
the data sets generated in the  course of this program have  been  provided  to
and   used   by  various  groups   (nationally  and  internationally)   in  the
development, testing and  evaluation of complex terrain dispersion models.
                                    45

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     The  most significant  contributions  included  (1) the conceptual  framework
for  the  mathematical  model  (i.e.,  the  division  of  the  flow-field  into  two
regimes,  a  lower layer below  the  dividing-streamline height which flows  in
essentially horizontal planes around the  hill,  and  an  upper layer  above the
dividing-streamline  height  which is treated as modified potential  flow  over a
cut-off  hill)  and  the  detailed  experimental  validation  and  establishment  of
limits  of  applicability  of   these   concepts,   (2)  verification  of   the   integral
formula for  the height  of  the  dividing streamline - this allowed  computations
of  the  dividing-streamline height  under  arbitrary  approach-flow  conditions,
including  shear  in  the   approaching   wind-speed   profile  and   nonlinear
temperature  gradients,  (3)  demonstration  of the  extreme sensitivity  of  surface
concentration patterns  to   wind  direction  under strongly  stratified  conditions,
(4)  measurements of plume deflections and deformations  over hills in  neutral
flow - these permitted  adjustment of  the T-factors in CTDM and resulted  in
substantial improvements in the CTDM  predictions, and (5) the introduction  of
the concept  of "windows of excess concentration" and measurements of terrain
amplification  factors - these  provided  simple  and  practical  methods  for
estimation and  intercomparison of  effects of  terrain  and  source  locations on
maximum ground-level  concentrations that may result from sources placed in the
vicinities of  hills.
                                     46

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                               REFERENCES
* Arya,  S.P.S. & Gadiyaram, P.S. 1986  An  Experimental  Study of Flow and
      Dispersion in the Wakes  of Three-Dimensional Low Hills.  Atmos. Envir.,
      20, 729-40.
* Arya,  S.P.S. & Shipman,  M.S.  1981   An Experimental Investigation  of  Flow
       and Diffusion  in the  Disturbed  Boundary  Layer  over a  Ridge,  Part  I:
      Mean Flow and Turbulence  Structure.  Atmos. Envir., 15, 1173-84.

* Arya,  S.P.S.,  Shipman,  M.S.  &  Courtney,  LY.  1981    An  Experimental
      Investigation of Flow and Diffusion in  the  Disturbed Boundary Layer over
      a Ridge,  Part  II: Diffusion from  a Continuous Point  Source.    Atmos.
      Envir., 15, 1185-94.

  Baines,  P.O.  1979   Observations  of  Stratified Flow Past  Three-Dimensional
      Barriers.  J.  Geophys. Res., 84, no. C12,  7834-8.
* Bass, A.  1980 Towing Tank Studies in Support of Field Experiments at Cinder
      Cone Butte,  Idaho,  Part II: Plume Behavior with Froude  Number and
      Incident Wind Direction.    Rpt. by  Envir.  Res.  & Tech.  on  cooperative
      work with  Fluid Mod. Facility,  Envir. Prot. Agcy.,  Res. Tri. Pk.,  NC.

  Bass, A., Strimaitis,  D.G.  &  Egan,  B.A.  1981   Potential  Flow Model for
      Gaussian  Plume  Interaction with  Simple  Terrain  Features.   Rpt.  under
      Contract No.  68-02-2759,  Envir.  Prot. Agcy.,  Res. Tri. Pk., NC, 201 p.

  Brighton, P.W.M.  1978   Strongly Stratified  Flow  Past  Three-Dimensional
      Obstacles.  Quart. J. Roy. Meteorol. Soc., 104, 289-307.
* Britter, R.E., Hunt, J.C.R., Marsh, G.L &  Snyder,  W.H. 1983  The Effects of
      Stable  Stratification  on  Turbulent Diffusion  and  the  Decay   of  Grid
      Turbulence.  J. Fluid Mech., 127,  27-44.

  Burt, E.W. & Slater,  H.H.  1977   Evaluation of the Valley Model.   AMS-APCA
      Joint Conf. on Appl.  of  Air Poll.  Meteorol.,  Salt Lake  City,  UT,    Amer.
      Meteorol. Soc., Boston, MA.

* Capuano, M.E.  1983  The Effects of  Hill  Slope  on Flow and  Dispersion over
      Two-dimensional  Hills  -  A Wind Tunnel Study.   M.S.  Thesis,   Dept.
      Marine, Earth,  Atmos.  Sci.,  NC State Univ., Raleigh, NC, 153p.
* Castro,  I.P  .  1987  A Note on Lee Wave Structures  in  Stratified Flow over
      Three-Dimensional Obstacles.  Tellus,  39A, 72-81.
* Castro,  I.P. &  Snyder, W.H. 1982   A  Wind  Tunnel Study of Dispersion from
      Sources  Downwind  of  Three-Dimensional   Hills.     Atmos.   Envir.,  16,
      1869-87.

* Castro, I.P.  &  Snyder, W.H. 1987a   Wind  Direction Effects on  Dispersion from
      Sources Downwind of  Steep  Hills.   Atmos.  Envir. (to be submitted).
* Publications generated  from research conducted within  the  Fluid  Modeling
  Facility.
                                    47

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* Castro, IP. & Snyder, W.H. 1987b  Obstacle Drag and Upstream Motions in
      Stratified  Flow.    Proc.   Third  Int.  Symp.  Stratified  Flows,  Cal.  Inst.
      Tech., Pasadena, CA, Feb. 3-5 (general session).

* Castro, IP. & Snyder, W.H.  1987c  Upstream Motions in  Stratified Flow.  J.
      Fluid Mech. (submitted).

* Castro, IP.,  Snyder,  W.H. &  Marsh,  G.L  1983  Stratified Flow  over  Three-
      Dimensional Ridges.  J. Fluid Mech., 135, 261-82.

* Courtney,  LY. 1979  A Wind Tunnel Study of Flow and Diffusion  over a Two-
      Dimensional Low Hill.  M.S. Thesis,   Dept.  of Meteorol., NC State Univ.,
      Raleigh,  NC, 134p.

* Courtney,  LY. & Arya, SP.S. 1980  Boundary Layer Flow and Diffusion over a
      Two-dimensional Low Hill.  Preprints  Vol.,  2nd Jt. Conf.  Appl. Air Poll.
      Meteorol., Mar. 24-28, New Orleans, LA, 551-8.  Amer.  Meteorol. Soc.,
      Boston, MA.

  Csanady,  G.T. 1964   Turbulent Diffusion in a Stratified Fluid.   Atmos. Sci.,
      21,439-47.
  DiCristofaro,   D.C.,   Strimaitis,   D.G.,   Greene,   B.R.,    Yamartino,   R.J.,
      Venkatram A., Godden, D.A., Lavery, T.F. & Egan, B.A.  1986 EPA Complex
      Terrain Model Development  Fifth Milestone Report - 1985.  Rpt.  No.
      EPA/600/3-85/069,   Envir. Prot. Agcy., Res. Tri.  Pk.,  NC, 277p.

  Drazin, P.G.  1961   On the Steady Flow of a Fluid of Variable Density  Past an
      Obstacle.  Te//us, 13, 239-51.

  EPA 1981  Guideline for Use of Fluid Modeling to Determine Good Engineering
      Practice  Stack Height.  Rpt. No.  EPA-450/4-81-003,   Envir. Prot. Agcy.,
      Res. Tri. Pk.,  NC, 47p.

  EPA 1985  Guideline for Determination  of  Good Engineering  Practice Stack
      Height (Technical Support Document for the Stack Height  Regulations).
      Rpt. No.  EPA-450/4-80-023R  (Revised June 1985),   Envir.  Prot. Agcy.,
      Res. Tri. Pk.,  NC, 102p.

* Eskridge,  R.E.,  Lawson,  R.E.  Jr.  &  Marsh,  G.L  1983    Simulation  of  an
      Atmospheric  Tracer  Experiment  in  Complex Terrain  Using a Stratified
      Towing Tank: A  Case Study.   6th Symp. Turb.  & Diffusion,  Boston,  MA,
      Mar. 22-25, Amer. Meteorol. Soc.,  Boston, MA.
* Gadiyaram,  P.S.   1984    Flow  and  Dispersion  over   Three-Dimensional
      Axisymmetric  Hills: A Wind Tunnel Study.   M.S.  Thesis,  Dept.  Marine,
      Earth, Atmos. Sci.,  NC State Univ., Raleigh, NC, 126p.

  Holzworth, G.C. 1980  The EPA Program for Dispersion Model Development for
      Sources in Complex Terrain.   2nd  Jt.  Conf.  Appl.  Air  Poll.  Meteorol.,
      March 24-27,  New Orleans, LA, Amer. Meteorol. Soc., Boston, MA.

* Holzworth, G.C. & Snyder,  W.H. 1979  Program Plan for Development of a
      Mathematical  Air Quality  Assessment System for Use  in Complex Terrain.
      Rpt. No.  EPA-600/9-79-041, Workshop on Atmos. Disp. Models in Complex
      Terrain,  137- 50.  Envir. Prot. Agcy.,  Res. Tri. Pk., NC.

  Hovind, E.L,  Edelstein,  M.W.  & Sutherland,   V.C.  1979    Workshop  on
      Atmospheric  Dispersion   Models   in  Complex  Terrain.     Rpt.   No.
      EPA-600/9-79-041,  Envir. Prot.  Agcy., Res. Tri.  Pk.,  NC, 213p.


                                    48

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  Hunt,  J.C.R. & Mulhearn, P.J.  1973  Turbulent Dispersion from Sources Near
      Two-Dimensional Obstacles.  J. Fluid Mech.,  61, 245-74.

* Hunt,  J.C.R.,  Puttock, U.S.  & Snyder, W.H. 1979  Turbulent  Diffusion from  a
      Point Source in Stratified and  Neutral Flows around a Three-Dimensional
      Hill: Part  I: Diffusion Equation Analysis.  Atmos. Envir., 13, 1227-39.

* Hunt,  J.C.R.  &  Snyder,  W.H. 1980   Experiments on  Stably  and  Neutrally
      Stratified  Flow over  a Model  Three-Dimensional Hill.   J.  Fluid Mech.,
      96, 671-  704.

* Hunt,  J.C.R.,  Snyder, W.H. & Lawson,  R.E. Jr. 1978   Flow Structure and
      Turbulent  Diffusion  around  a Three-Dimensional  Hill:  Fluid  Modeling
      Study  on Effects  of  Stratification;  Part  I:   Flow Structure.    Rpt. No.
      EPA-600/4-78-041,   Envir. Prot. Agcy., Res. Tri. Pk., NC..

* Khurshudyan, L.H., Snyder, W.H. & Nekrasov, I.V. 1981   Flow and Dispersion
      of Pollutants over  Two-Dimensional Hills:  Summary Report  on  Joint
      Soviet-American  Study.  Rpt. No. EPA-600/4-81-067,  Envir. Prot. Agcy.,
      Res.  Tri.  Pk., NC, 143p.

* Lamb,  V.R. &  Britter,  R.E.  1984   Shallow  Water  Flow over  an  Isolated
      Obstacle.  J. Fluid Mech., 147, 291-313.

  Lavery,  T.F.,  Bass,  A.,   Strimaitis,  D.G.,  Venkatram,   A.,  Greene,  B.R.,
      Drivas,  P.J. & Egan, B.A. 1982 EPA Complex Terrain Modeling  Program:
      First  Milestone Report - 1981.   Rpt.  No. EPA-600/3-82-036,   Envir. Prot.
      Agcy.,  Res. Tri. Pk., NC,  304p.

  Lavery, T.F.,  Strimaitis,  D.G. & Egan,  B.A. 1986   A Workshop Report on  the
      Complex  Terrain Model Development  Project (February 4-6, 1986).   Rpt.
      under Contract 68-02-3421,  Envir. Prot. Agcy., Res. Tri.  Pk.,  NC, 75p.

  Lavery,  T.F.,   Strimaitis,  D.G., Venkatram,  A.,   Greene, B.R.,   DiCristofaro,
      D.C.  and  Egan, B.A.  1983  EPA Complex Terrain Model Development: Third
      Milestone  Report -  1983.  Rpt.  No. EPA-600/3-83-101,   Envir.  Prot.
      Agcy.,  Res. Tri. Pk., NC,  271 p.

* Lawson, R.E.  Jr.  & Snyder, W.H. 1985  Stack  Heights and Locations in Complex
      Terrain.  Preprints  Vol:  7th Symp. Turb. Diff., Nov.  12-15,  Boulder, CO,
      223-6.  Amer. Meteorol. Soc., Boston, MA.

* Lawson,  R.E.  Jr. & Snyder, W.H. 1987  Estimation  of Pollutant Concentration
      from  Sources Near  Complex Terrain in  Neutral Flow.  Atmos. Envir., (to
      be submitted).

* Lee, J.T.,  Barr,  S.,  Lawson,  R.E., Jr., Snyder,  W.H.  &  Marsh,  G.L  1984a
      Towing Tank Studies of Stratified Flow over  Ridges and Valleys.   Rpt.
      No. LA-UR-84-1314,  Los Alamos National Laboratory, Los Alamos, NM, 29p.

* Lee, J.T.,  Barr,  S.,  Lawson,  R.E., Jr., Snyder,  W.H.  &  Marsh,  G.L.  1984b
      Towing  Tank  Studies  of Stratified Flow   over  Ridges   and  Valleys.
      Preprints  Vol.  3rd  Conf.  Mtn. Meteorol.,  Portland,  OR,  37-41.  Amer.
      Meteorol.  Soc., Boston, MA.

* Lee, J.T., Barr,  S.,  Snyder,  W.H.   & Lawson,  R.E.  Jr.  1981   Wind Tunnel
      Studies of  Flow Channeling   in Valleys.  Preprint  Vol.  2nd  Conf.  Mtn.
      Meteorol., Nov.  9-12,  Steamboat  Springs,  CO,  Amer.  Meteorol.  Soc.,
      Boston, MA.
                                    49

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* Lee,  J.T.,  Lawson,  R.E.  Jr.  &  Marsh,  G.L   1986    Flow  Visualization
      Experiments on  Stably Stratified Flow  over  Ridges and Valleys.   Proc.
      3rd Int. Workshop on Wind and Water Tunnel  Modeling of Atmos. Flow and
      Dispersion,  Sept., Lausanne,  Switzerland.

* Lee,  J.T.,  Lawson,  R.E. Jr.  &   Marsh,  G.L.  1987    Flow  Visualization
      Experiments on  Stably  Stratified  Flow Over Ridges  and Valleys:  Final
      Report. Rpt. No. LA-UR-87-127,   Los Alamos National  Laboratory, Los
      Alamos, NM.
  Mason, P.J. & Sykes, R.I. 1979  Three-Dimensional Numerical Integrations  of
      the Navier-Stokes Equations  for Flow Over Surface-Mounted Obstacles.  J.
      Fluid Mech., 91,  433-50.

  Pasquill,  F. 1974    Atmospheric  Diffusion.   2nd  Ed.,    Chichester,  Ellis
      Horwood Ltd., John Wiley & Sons, NY, NY, 429p.

  Pearson,  H.J.,  Puttock,  J.S.  &  Hunt,  J.C.R.  1983   A Statistical  Model  of
      Fluid-Element Motions and Vertical Diffusion  in a Homogeneous  Stratified
      Turbulent Flow.   J. Fluid Mech., 129, 219-49.

* Pendergrass, W.R. &  Arya, S.P.S. 1983  Vortex Development in Boundary  Layer
      Flows over Two-Dimensional Ramps.   Preprint Vol.  6th Symp.  Turb.  &
      Diff.,  Mar. 22-25, Boston, MA,  Amer. Meteorol.  Soc., Boston, MA.

* Pendergrass, W.R. &  Snyder, W.H. 1987  Wind Tunnel Measurements of Terrain
      Amplification Factors for Sources  Upwind  of Two-Dimensional Ramps  of
      Various Slopes.  Atmos. Envir. (to be submitted).

* Queney,  P., Corby, G.A., Gerbier, N., Koschmieder, H. & Zierep, J. 1960  The
      Airflow over  Mountains.   World  Meteorol.  Org.,  Tech.  Note  No. 34,
      Geneva, Switz.

  Riley, J.J., Liu,  H.T. &  Geller,  E.W.  1976   A  Numerical  and  Experimental
      Study  of  Stably Stratified  Flow  Around  Complex Terrain.    Rpt. No.
      EPA-600/4-76-021, Envir. Prot. Agcy., Res. Tri.  Pk., NC, 41 p.

* Rottman, J.W.  & Britter, R.E. 1986 The Mixing Efficiency and Decay of  Grid-
      Generated   Turbulence   in   Stably   Stratified   Fluids.     Proc.   9th
      Australasian Fluid Mech. Conf., Dec. 8-12,  Univ. Auckland, Auckland, New
      Zealand.
* Rottman, J.W.,  Lawson,  R.E.  Jr. & Snyder,  W.H.  1987   A Comparison  of
      Numerical  and Laboratory  Experiments on  Density-Stratified Flows around
      a Three-Dimensional Hill.   Proc.  Third  Int.  Symp. Stratified  Flows, Cal.
      Inst.  Tech., Pasadena, CA, Feb. 3-5.

* Rottman,  J.W.  & Smith, R.B. 1987   Tow-Tank Simulations  of  the Severe
      Downslope Wind.  Proc. Third Int.  Symp. on Stratified Flows, Cal. Inst.
      Tech., Pasadena, CA, Feb. 3-5.

  Rowe,  R.D., Benjamin,  S.F., Chung,  K.P., Havlena, J.J.  &  Lee,  C.Z.  1982
      Field  Studies of Stable  Air  Flow  over and around  a  Ridge.   Afmos.
      Envir.,  16, 643-53.

  Sheppard,  P.A. 1956  Airflow over Mountains.  Quart. J. Floy. Meteorol.  Soc.,
      82, 528-9.
                                    50

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* Snyder,  W.H. 1980a  Towing Tank Studies in Support of Field Experiments at
      Cinder  Cone Butte, Idaho, Phase I: Influence  of  Hill  on Wind Field  at
      the  Meteorological  Tower  Site.    Fluid  Modeling   Facility  Internal  Rpt.,
      July 30, 20p.  Envir. Prot.  Agcy., Res. Tri. Pk., NC.
* Snyder,  W.H. 1980b  Towing Tank Studies in Support of Field Experiments at
      Cinder   Cone  Butte,   Idaho,  Phase  III:  Verification  of   Formula  for
      Prediction   of  Dividing  Streamline  Height.     Fluid  Modeling  Facility
      Internal Rpt., Aug. 29, Envir. Prot. Agcy.,  Res. Tri. Pk., NC,  12p.

* Snyder,  W.H.  1981  Guideline for Fluid Modeling  of  Atmospheric Diffusion.
      Rpt. No. EPA-600/8-81-009,  Envir. Prot. Agcy., Res. Tri. Pk.,  NC, 200p.

* Snyder,  W.H.  1983a   Fluid Modeling of Terrain  Aerodynamics and  Plume
      Dispersion - A Perspective View.  Invited  Presentation, AMS Workshop on
      Dispersion in Complex Terrain, Keystone, CO, May 17-20.

* Snyder,  W.H.  1983b   Fluid Modeling of Terrain  Aerodynamics and  Plume
      Dispersion - A Perspective View.  Preprint Vol.  6th Symp. Turb.  & Diff.,
      March 22-25, Boston, MA, 317-20.  Amer.  Meteorol.  Soc., Boston, MA.
* Snyder,  W.H. 1984 Terrain Aerodynamics and Plume  Dispersion: A Perspective
      View Gained from Fluid Modeling Studies.  Proc. Symp. Tibetan Plateau &
      Mtn. Meteorol., Beijing, P.R.C., March.

* Snyder,  W.H.  1985  Fluid Modeling of Pollutant Transport and Diffusion  in
      Stably  Stratified  Flows over Complex Terrain.   Ann. Rev.  Fluid Mech.,
      17, 239-66.

* Snyder,  W.H.  1986  Comparisons of CTDM Calculations with Fluid Modeling
      Observations.  Complex Terrain  Workshop,  Research Triangle Park, NC,
      Feb. 4-6,  45p.

* Snyder,  W.H. & Britter, R.E. 1987 A  Wind Tunnel Study of the Flow Structure
      and Dispersion from Sources Upwind of Three-Dimensional Hills.  Atmos.
      Envir., 21,  735.

* Snyder,  W.H.,  Britter, R.E.  &  Hunt, J.C.R. 1980  A  Fluid Modeling Study  of
      the  Flow Structure and Plume Impingement on a Three-Dimensional Hill in
      Stably  Stratified  Flow.    Proc.  Fifth Int.  Conf.  on  Wind  Engr.  (J.E.
      Cermak, ed.), 1, 319-29. Pergamon Press, NY, NY.

* Snyder,  W.H. & Hunt, J.C.R. 1984  Turbulent  Diffusion from a Point Source in
      Stratified and  Neutral  Flows around  a  Three-Dimensional  Hill,  Part  II:
      Laboratory Measurements  of Surface Concentrations.  Atmos.  Envir.,  18,
      1969-2002.
* Snyder,  W.H. & Lawson, R.E. Jr. 1981  Laboratory Simulation of Stable Plume
      Dispersion  over  Cinder   Cone   Butte:  Comparison  with  Field  Data.
      Appendix:  EPA Complex Terrain Model Development  First Milestone Report -
      1982, Rpt. No. EPA-600/3-82-036, p. 250-304.  Envir.  Prot. Agcy., Res.
      Tri.  Pk., NC.

* Snyder,  W.H.  & Lawson,  R.E.   Jr.  1985a   Stable Plume Dispersion over an
      Isolated Hill: Releases above the  Dividing-Streamline Height.   Appendix
      A: EPA  Complex Terrain Model Development Fourth Milestone Report - 1984,
      Rpt. No. EPA/600/3-84/110, 233-68. Envir. Prot. Agcy., Res. Tri. Pk, NC.
                                    51

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* Snyder, W.H. & Lawson, R.E. Jr. 1985b Fluid Modeling Demonstration of Good-
      Engineering-Practice Stack  Height  in  Complex   Terrain.    Rpt.  No.
      EPA-600/3-85/022,  Envir.  Prot.  Agcy., Res. Tri. Pk., NC, 89p.
* Snyder, W.H.  &  Lawson, R.E.  Jr.  1986   Laboratory Observations  of  Plume
      Deformations  in  Neutral  Flow  over a  Three-Dimensional Hill.   Preprint
      Vol. AMS 5th Jt. Conf.  Appl. Air  Poll. Meteorol.  with APCA, Nov., Chapel
      Hill, NC. Amer. Meteorol. Soc., Boston,  MA.

* Snyder, W.H.  &  Lawson, R.E.  Jr.  1987   Stable Plume Dispersion  over an
      Isolated  Hill:  Releases  above  the Dividing-Streamline  Height.    Proc.
      Third  Int.  Symp. Stratified  Flows,  Cal. Inst. Tech., Pasadena,  CA,  Feb.
      3-5.

* Snyder, W.H.,  Lawson, R.E. Jr.,  Thompson,  R.S.  &  Holzworth, G.C.  1980
      Observations  of Flow  around  Cinder Cone  Butte,  Idaho.    Rpt. No.
      EPA-600/7-80-150,  Envir.  Prot.  Agcy., Res. Tri. Pk., NC, 30p.

* Snyder, W.H. & Ogawa, Y. 1982 Simulation of Flow and Diffusion over Cinder
      Cone  Butte  in  a Stratified Wind Tunnel.   Data Report,   National  Inst.
      for Envir.  Studies, Tsukuba, Japan.

* Snyder, W.H. &  Pendergrass, W.R.  Ill  1980  Ramp Study: Idealized Widows
      Creek.  Unpublished Data  Rpt.,   Fluid  Modeling Facility,  Envir.  Prot.
      Agcy., Res. Tri. Pk., NC.

* Snyder, W.H.,  Thompson,  R.S.,  Eskridge,  R.E.,  Lawson,  R.E.,  Jr.,  Castro,
      I.P.,  Lee,  J.T.,  Hunt,  J.C.R.  &  Ogawa,  Y.  1983   The  Structure  of
      Strongly   Stratified  Flow  over  Hills:   Dividing-Streamline  Concept.
      Appendix: EPA Complex Terrain Model Development Second Milestone Report
      - 1982, Rpt. No. EPA-600/3-83-015, p. 319-75. Envir. Prot. Agcy.,  Res.
      Tri. Pk., NC.

* Snyder, W.H.,  Thompson,  R.S.,  Eskridge,  R.E.,  Lawson,  R.E.,  Jr.,  Castro,
      I.P.,  Lee,  J.T.,  Hunt,  J.C.R.  &  Ogawa,  Y.  1985   The  Structure  of
      Strongly Stratified   Flow over  Hills:  Dividing-Streamline  Concept.    J.
      Fluid Mech.,  152, 249-88.

* Snyder, W.H.,  Thompson, R.S. & Shipman,  M.S.  1986   Streamline Trajectories
      in  Neutral  and Stratified  Flow over a Three-Dimensional  Hill.  Appendix:
      Rpt. No. EPA/600/3-85/069, EPA Complex Terrain Model Development Fifth
      Milestone  Report -  1985,  240-277.    Envir.  Prot.   Agcy.,  Res.  Tri. Pk.,
      NC.
  Strimaitis,  D.G.,   Lavery, T.F.,  Venkatram,  A.,  DiCristofaro,  D.C.,  Greene,
      B.R. & Egan,  B.A. 1985  EPA Complex Terrain Model Development:  Fourth
      Milestone  Report -  1984.   Rpt. No.  EPA-600/3-84-110,   Envir.  Prot.
      Agcy., Res. Tri. Pk., NC.

  Strimaitis,  D.G., Scire, J.S. & Bass, A. 1982  COMPLEX/PFM Air Quality Model
      User's Guide.   Rpt. (awaiting  printing),    Envir.   Prot.  Agcy.,  Res. Tri.
      Pk., NC, 114p.

* Strimaitis,  D.G. & Snyder, W.H. 1986  An Evaluation of the Complex Terrain
      Dispersion Model Against Laboratory Observations:  Neutral Flow over 2-D
      and  3-D  Hills.     Preprint  Vol.  AMS  5th  Jt.   Conf.   Appl.  Air  Poll.
      Meteorol.  with  APCA,  Nov.,  Chapel  Hill,  NC,   Amer.  Meteorol.  Soc.,
      Boston, MA.
                                    52

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  Strimaitis,  D.G.,   Venkatram,  A.,  Greene,  B.R.,  Hanna,  S.,  Heisler,  S.,
      Lavery, T.F.,  Bass,  A., &  Egan,  B.A. 1983  EPA Complex Terrain Model
      Development: Second Milestone Report - 1982. Rpt. No. EPA-600/3-83-015,
      Envir.  Prot. Agcy., Res.  Tri. Pk.,  NC.
* Thompson, R.S. & Shipman, M.S. 1986  Streamlines in Stratified Flow over a
      Three-Dimensional  Hill.   Preprint Vol.  AMS 5th Jt. Conf. Appl. Air  Poll.
      Meteorol. with APCA,  Nov.,  Chapel Hill,  NC,    Amer.  Meteorol.  Soc.,
      Boston, MA.

* Thompson, R.S., Shipman, M.S. & Snyder, W.H. 1985 Synopses of FMF Projects
      in Complex Terrain.  Rpt. to  ERT on CTMD Program,  Envir.  Prot. Agcy.,
      Res. Tri. Pk., NC, 24p.
* Thompson, R.S.  & Snyder,  W.H.  1976  EPA Fluid  Modeling  Facility.   Proc.
      Conf. on Modeling & Simulation, Rpt. No. EPA-600/9-76-016,  Envir. Prot.
      Agcy., Wash. D.C.,  July.

* Thompson, R.S. & Snyder, W.H.  1981  Air Pollution and Terrain Aerodynamics:
      A Review of Fluid  Modeling  Studies at the EPA  Fluid Modeling Facility,
      ASCE Fall  Conv., St. Louis, MO, Oct.

* Thompson, R.S. & Snyder, W.H. 1984 Fluid Modeling of Blocking and Upstream
      Influences  of  Stable  Flow  over  Two-Dimensional  Hills.    Proc.  2nd
      Workshop Wind/Water Tunnel Dispersion Modeling, Oxford, England, Sept.
      26-28, C3.1-3.7.

* Thompson, R.S. & Snyder, W.H. 1985a Air Pollution and Terrain Aerodynamics:
      A Review of Fluid  Modeling  Studies at the EPA  Fluid Modeling Facility.
      J. Wind  Engr. & Indus.  Aerodyn., 21, 1-19.

* Thompson, R.S. & Snyder, W.H.  1985b  Dispersion from a Source Upwind of a
      Three-Dimensional  Hill  of Moderate Slope.   Appendix B: EPA  Complex
      Terrain Model  Development  Fourth Milestone  Report -  1984,  Rpt. No.
      EPA/600/3-84/110, 269-86.   Envir.  Prot. Agcy., Res.  Tri. Pk.,  NC.

* Thompson, R.S.,  Snyder, W.H. &  Lawson, R.E. Jr. 1983 Laboratory Simulation
      of  Neutral  Plume Dispersion  over  Cinder Cone Butte:  Comparison  with
      Field Data.  Appendix:  EPA  Complex  Terrain Model  Development Third
      Milestone Report -1983, Rpt.  No.  EPA-600/3-83-101, p. 212-51.  Envir.
      Prot.  Agcy., Res. Tri.  Pk., NC.

  Turner, J.S.  1973   Buoyancy Effects  in Fluids.    Cambridge  Univ. Press,
      Cambridge, England, 368p.

  Venkatram, A., Strimaitis,  D. & DiCristofaro, D. 1984   A  Semiempirical Model
      to  Estimate  Vertical  Dispersion  of Elevated  Releases  in   the  Stable
      Boundary Layer.  Atmos. Envir.,  18, 923-8.

  Wackter,  D.J. &  Londergan,  R.J. 1984  Evaluation of Complex  Terrain Air
      Quality  Models.   Rpt.  under Contract  No.  68-02-3514,    Envir. Prot.
      Agcy., Res. Tri. Pk., NC, 233p.

  Weil, J.C., Traugott,  S.C.  & Wong,  O.K. 1981   Stack Plume  Interaction and
      Flow Characteristics for a Notched Ridge.   Rpt.  No. PPRP-61,   Martin
      Marietta  Corp., Baltimore,  MD, 92p.
                                    53

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