An atmospheric tracer investigation of transport and diffusion around a large, isolated hill

,#t,,,&wic Environnorr Vol. 18. No. IO, pp. 2003-2021, 1984 ooo4-6981/84 13.00 + 0.M

Pnnted in Great Britain. Q 1984 Pequmw Press Ltd.

AN AT~OSP~ERIC TRACER INVESTIGATION OF TRANSPORT AND DIFFUSION AROUND A LARGE,

ISOLATED HILL

WILLIAM RYAN, BRIAN LAMB* and ELMER ROBINSON

Laboratory for Atmospheric Research, College of Engineering, Washington State University, Pullman, WA 99164-2730. U.S.A.

Abstract-Twenty-one single and dual tracer tests were conducted during May and June 1981, at Steptoe Butte (335m), an isolated hill located in eastern Washington state. SF6 and CBrFs tracer releases from upwind of the hill at heights ranging from near the surface to approximately 190 m were used to study plume transport over and around the hill during unstable, neutral, and stable atmospheric conditions.

Plumes released upwind during daytime near-neutral conditions traveled up and over the hill with maximum concentrations on the windward side at receptor elevations between 0.5 and 1.5 times the source height. Maximum concentrations were in the same range as calculated from models for neutral flow over a hemisphere. Plumes released upwind during transitional day-to-night conditions exhibited maximum concentrations similar to the daytime releases, but the tracer isopieths indicated some plume bifurcation.

The concept of a critical streamline height to predict whether plumes impinge upon the hill and pass round it or travel up and over the hill was found to be valid at Steptoe Butte. The two largest 15min average concentrations occurred on the windward side, but more than half of the concentration maxima were observed on the leeward half of the hill predominately as the result of transport up and over the hill. The position of the maximum concentrations tended to shift toward the side of the hill as the horizontal displacement of the source from the flow centerline increased. The largest maximum concentration was in a range calculated previously with an impingement model of a turbulent plume embedded in a potential flow around a cylinder, while the remainder of the maxima were in a range predicted with a similar model of potential flow over a hemisphere.

Key word index: Bromot~uoromethane, complex terrain, tracer study, dilutor around a hill, drainage winds, model, modeRing, comparison of potential flow around a hill with tracer data, plume, transport near a hilt, sulfur hexafluoride, tracer study.

1. INTRODUCl’ION source investigations of EPRI and DOE. In a com-

The present large uncertainty in modeling atmospheric panion paper the field data from Steptoe Butte will be

transport and diffusion over complex terrain can be compared to a number of available and recently

measured in some degree by the number and developed diffusion models for complex terrain.

scope of recent complex terrain field studies. These As Hunt et al. (1978) have noted, there are three

efforts include the EPA Complex Terrain Model aspects of Aow around a hill to consider

Development program (Lavery et al., 1982), the DOE (1) whether streamlines originating upwind will

Atmospheric Studies in Complex Terrain (ASCOT) impinge upon the hill and go round the hill or be lifted

(Dickerson and Gudiksen, 1980), and the EPRI Plume up over the hill summit;

(2) the size and nature of the recirculation zone in Model Validation study (EPRI, 1978). The DOE and the lee of the hii1 and EPRI work is directed toward real sources located in moun~inous terrain, while the EPA effort has initially

(3) the effects of surface heating and cooling upon

involved physiml modeling and tracer studies of flow ptwne transport and diffusion near the surface.

around small, isolated surface features located on flat Observations (e.g. Lavery et al., 1982; Rowe et al.,

terrain (Cinder Cone Butte, 1OOm; Hogback Ridge, 1982; Hunt and Snyder, 1980) and theory (Drazin,

1OOm). In this paper we report results from 21 tracer 1961) show that the nature of the flow is highly

tests of flow near a large, isolated hill situated in low dependent upon the degree of stratification of the flow

rolling terrain (Stcptoe Butte, 335m). In terms of the field. Under very stable conditions, elevated stream- 1’

terrain scale and complexity, this work marks a mes tend to move horizontally round the hill and the

situation with greater scale and complexity than the recirculation zone is mainly horizontal. During neutral

sma&sc&e EPA studies, but with less COntpkated and unstable situations, approaching streamtines, even

topography and source ~~gu~tions than the real those o~~nating ntaf the surface, f,ass over the hiI1

and can become entrained in a three-dimensional recirculation zone in the lee of the hill. In terms of elevated plume impact upon the hill surface, maximum

*Author to whom correspondence should be addressed. concentrations during stable conditions will result

2003

2004 WILLIAM R\4& t’f a/

from direct transport of the central plume streamline to very near the surface; during neutral or unstable conditions, high concentrations will occur because of turbulent diffusion down to the surface from an elevated plume centerline. In the lee of the hill, maximum concentrations will result from either turbulent diffusion from the plume centerline closely approaching the surface or by direct transport via recirculatory streamlines.

The Froude number* is a quantitative measure of flow stratification. Previous fluid modeling studies (e.g. Hunt et al., 1978) as well as the EPA Cinder Cone Butte field study have confirmed Drazin’s early work which indicated that for small Froude numbers and a constant density gradient, a critical height exists (H, = h (1 - Fr)) such that streamlines originating below H, impinge upon the hill and go round it while streamlines above H, pass over the hill summit.

Hunt et al. (1978) also found that as Fr becomes small (Fr c 0.3), the plume shape is compressed vertically with wide lateral fluctuations. These fluid modeling measurements showed that for an elevated plume released directly upwind of the hill, maximum surface concentrations (C,) ranged between 0.5 and 1.2 times the centerline concentration (C,) measured in the absence of the hill. Values of C, greater than Co can occur because of reflection of the plume from the surface. Hunt et al. (1979)calculated a CmnX/CO value of 1.12 for diffusion from a line source around a cylinder. In the Valley model (Burt, 1977), the corresponding ratio is 2, and in wind tunnel measurements of flow over a cylindrical hill, Hunt and Mulhearn (1973) found a value of C,,/CO equal to approximately 1. In tow tank studies by Snyder et al. (1979) the ratio

l See Table 2.

C,,/CO equaled 0.8 on the side of the hill when an elevated source was slightly displaced from the flow centerline.

Present descriptions of flow phenomena near terrain obstacles are based upon physical fluid modeling, the small scale tracer studies conducted at Cinder Cone Butte by EPA, and theoretical calculations. The pur- pose of the work presented here was to examine plume transport around a large, isolated hill in order to verify and improve our present understanding of flow In complex terrain. In this respect the discussion is directed toward qualitative and quantitative descriptions of plume behavior compared to our current expectations.

2. EXPERIMENTAL PROCEDURES

A photograph and topographical map of Steptoe Butte are shown in Figs 1 and 2. Vertical cross-sections of the hill are diagrammed in Fig. 3. Steptoe Butte is a symmetrical hill isolated from other significant surface features by more than 25 km. The hill rises 335 m above the surrounding roiling wheat fields. Steptoe Butte has an aspect ratio (base radius/hill height) equal to 5.0 with a slope of approximately 20” along the upper half of the hill. Surface vegetation is limited to range grass with some low brush on the lowest edge of the hill and a grove of coniferous trees on the lower northeast slope. A paved road spirals to the summit where a number of c~mmu&ation tow&s are located.

Sulfur hexafluoride ISF,) and bromotrifluoromethane (CBrF,) were used as inert, nontoxic tracers in this study. In a typical experiment, the gases were released continuously at a steady rate through polyethylene tubing hoisted aloft with a tethered balloon located upwind of the hill. The release heights ranged from very near the surface up to approximately 19Om as indicated in Table 1. Also, duringa number of tests, tracer was released through tubing supported approximately 4m above the surface of the hill. In all tests release rates were monitored with calibrated rotameters and the total mass of tracer released was determined by weighing

Fig. 1. Photograph of Steptoe Butte from the southeast.

An atmospkzk tracer investigation of transport and diffusion around a large, isolated hill 2005

STEPTOE BUY’TE I

Fig. 2. Topographical map of Steptoe Butte with tracer sampling points. Locations are designated by radiai; SW6 indicates point 6 on the southwest radial. Contour lines in feet.

STEPTOE BUTTE VERTICAL CROSS-SECTIONS

1200 t

E aa Z E

IIOO-

1000 -

COO-

800

i

I200 r

2400

meters

I ,

3600 4800

Fig. 3. Vertical cross-sections of Steptoe Butte with hill radius (to), radius at half-height (L), and hill height (It) indicated. Vertical scale is expanded.

2006 WILLIAM RYAN PI al.

Table 1. Tracer release data --

Test Date Time

(pdt)

Location tr. )‘t

(km, deg) Height Rate tmrll tgs-11

Afternoon tests

1 19/5/81 2 22/5/g 1 3 22/5/81 4 2315181

Transition period tests 9 30/5/g 1 5 26/5/g 1

14a 1 O/6/8 1 14b

1539-1630 130&1400 1605-1700 130&1400

073&0830 19W2C0O 1830-1930

Night-time tests

6 2715181 2135-2230 8 2915181 23554100

10 l/6/81 2300-2400 1’ i 15a 15b 18a 18b 19a 19b 20a 20b 2la 21b

4/6/S 1 1 O/6/8 1

13/6/g 1

1516182

17/6/g 1

181618 1

Hill surface and summit tests 11 2/6/g 1 13a 9/6/g 1 13b 16a 1 l/6/81 16b 17a 1 l/6/81 17b

2200-2300 220&2300

2200-2230 2230-2300 224G2330

2241-2330

023M330

200&2100 1615-1700

190&2000

2220-2330

1.8. 268 161 0.653 2.2, 227.5 124 0.653 1.8, 214 186 0.653 2.1. 167.5 -lR* 0.65 3

1.8. 214 149 1.8, 301 151 2.2. 228 122 2.2. 228 9

2.1, 250 149 1.8. 270 86 1.6, 46 123 1.9, 201 150 1.9, 194 115 1.9, 194 30 1.8.297 187 1.9, 286 0 1.9, 18.5 146 1.9. 185 81 1.9, 194 156 1.9, 194 91 2.0, 218 166 2.0, 218 96

summit 339: 0.179 0.44.207 197* 0.217 summit 3398 0.202 0.55. 270 142t 0.282 0.55, 270 134* 0.504 summit 339s 0.220 summit 3471 0.216

1.500 1.55 0.631 0.416

1.46 1.46 1.22 1.26 0.631 0.416 0.952 0.952 0.952 1.14 1.24 1.14 1.24 1.14

* 4 m above ground level. t 12m above ground level. Test ? was aborted due to an abrupt change in wind direction.

the gas cylinders before and after each experiment. Tracer releases typically lasted 75 min. The height and location of the release point were determined from double-theodolite sight- ings and range finder readings on the tethered balloon. The length and angle of the tetherline were also used to estimate the release position.

All locations (y, r, L) are given in polar coordinates (north = 0”) with the hill summit as the origin. Heights are given in meters above a reference level (mrl) of 766 meters above sea level, which approximates the average height of the terrain surrounding Steptoe Butte.

A ground-level sampling grid of 73 points is shown in Fig. 2. This array consisted of five or six points along each of sixteen radials extending down the sides of the hill. In the remainder of the paper, a sampk point will be identified by the direction of the radial and a number; SW4 represents sample point 4 on the southwest radial. The locations were surveyed prior to the field sudy using a precision altimeter and transit. The coordinates are estimated to be accurate to within 0.5” azimuth and 3m elevation. Radial distances were determined using the survey data and a topographic map.

During each test, automatic air samplers @Sk Pullman, WA) were deployed at 30 of the potential sample points. Each sampler collected four sequential IS-min average samples of 20cm3 each in polyethylene syringes. An alarm clock on each

sampler was used to start the sampler at a predetermined time. Samplers were deployed just before each test and retrieved as soon as possible after the test.

Instantaneous (- 1 s) tracer samples were collected in syringes along automobile traverses on the road to the summit during each test. Typically, four traverses were conducted during a test. Sample locations were determined from the sample array markers along the road. In some tests, samples were also collected up to 10 m above the road using a tethered balloon, tubing, and a small pump. During a limited number of experiments a continuous SF, analyzer was also operated in the traverse vehicle.

Tracer samples were analyzed using electron capture gas chromatography. Three tracer gas chromatographs (designated Al, A3, and A4) constructed at WSU and two commercial HP 57OOinstruments (designated HP1 and HP2) were used for the analyses. All samples were analyzed for SF, using Al, A3, or A4; dual tracer samples were also analyzed using HP1 or HP2. Chromatograph peaks were integrated using an electronic integrator.

The analytical system was calibrated periodically during the study period using an exponential dilution calibration system (Lamb, 1978). Results from these calibrations indicated that response. for the SF6 instruments changed less than 7 % during the period. Response for the CBrFs instru-

An atmospheric tracer investigation of transport and diffusion around a large, isolated hill 2007

ments apparently varied by as much as 29 %. Approximately 10 % of the samples were re-analyzed; the average absolute difference between the repeated analyses was 117; for SF6 samples and 20 y0 for CBrFl samples.

During each test day, surface meteorological data were collected continuously for several hours at two levels on a 30m tower located southeast of the hill, at a 3m station positioned on the hill summit, and also with a portable 2 m system usually located on the upwind surface of the hill. Data from the 30m tower were electronically scanned each 10s and averaged over 15-min and I-h periods. Directly observed data included wind speed, wind direction, standard deviation of wind direction, and temperature at 30m above the surface, and wind speed, temperature, and solar radiation at 2 m above the surface. The vertical temperature gradient (AT) was also recorded directly using a differential temperature sensor. All sensors and signal translators were from Climet, Inc. For each averaging period these data were used to calculate the standard deviation of wind speed at 30m, the Monin- Obukhov length, and the Richardson number. At the 3 m and 2m hill sites, wind speed and direction were recorded on portable strip-chart recorders.

Upper air data were obtained during each test period by using a tethersonde system (Air Research, Inc.) which provided wind speed, direction, temperature, relative humidity, potential temperature, and height above the surface. This tethersonde system was typically operated from the tracer release point upwind of the hill. At the start of each experiment vertical profiles of the meteorological variables were obtained by raising the sonde to approximately the summit height and then lowering the sonde to the approxi- mate tracer release elevation. Data were recorded continuously during the test at the release height, and at the conclusion of the tracer release, a second ascent to the summit height and descent to the surface were performed. Telemetered data scans were recorded on cassette tape every 30s and were recorded as printed output every 60s.

Due to the limited applicability of the meteorological data from the 30m tower for describing the flow over the entire hill, the meteorological data collected with the tethersonde data system were used to determine stability parameters for describing the approach flow field. The system used is similar to the system described by Morris et al. (1975). Richardson numbers (RI’) and Froude numbers (Fr) generated from the data obtained by this system must be used with caution since the observed profiles were not instantaneously measured. Errors will result to the extent that conditions change during the ascent or descent. However, we believe that the values obtained utilizing the tethersonde data provide a generally good description of the Row field. Results from an experimen- tak uncertainty analysis indicated that Froude numbers are accurate to within + 25 g,

During each of the experiments, weighted, slow-rise balloons were periodically released from the surface at the tracer release point and tracked either visually or with a double- theodolite system. These balloon releases provided an excellent means of estimating the tracer plume trajectory for properly locating the release point and for directing sampling efforts. A limited amount of flow visualization measurements was also performed on the hill surface using 5-min smoke candles and 35 mm photography. These efforts were used to better define the flow patterns and diffusion conditions near the surface during the experiments.

Two basic flow parameters used to describe the approach flow are the critical dividing streamline height (If,) and the hill Froude number (Fr). The dividing streamline height is defined as the height where the kinetic energy in the approach flow equals the potential energy required to surmount the summit of the hill. Hence, H, is determined in a iterative approach from

where u is the wind speed at height H, h is the hill height. p is a reference ambient density, g is the acceleration of gravity, and iiP

z IS the local density gradient. The Froude number is a

measure of the ratio of inertial to gravitational forces

where u(h) is the free stream wind speed at the height of the summit and N is the Brunt-Vaisiila frequency

(3)

By squaring (2) and substituting into (l), it can be shown that H, = h (I- Fr) for constant winds and a constant density profile. For our purposes, we calculate H, from (1) using available tethersonde data. We define a single layer or local Froude number for upper level flow as

F+-h,)=U(h) (h-hi)N’

where the potential temperature gradient is taken as the best- fit gradient from the top of the surface inversion (hi) to the hill summit. Similarly, for plumes released within the surface inversion, the local Froude number is

u(hi) “(hi) = (hiIN’

In both cases, plumes are expected to be lifted above the scale height (h or hi) if the local Froude number is greater than 1. Further discussion of methods for calculating H, and Fr is available elsewhere (Ryan and Lamb, 1984).

3. PRESENTATION AND DISCUSSION OF RESULTS

Twenty-one tracer tests were completed between 19 May and 18 June 198 1. The dates, times, and release locations are listed in Table 1, and meteorological conditions for each test are given in Table 2. For discussion purposes, the results from the field study will be classified in four groups: (1) daytime tests with upwind releases during near neutral stability (Tests 1, 2, 3 and 4); (2) transition period tests with upwind releases from night-time to daytime (Test 9) and from daytime to night-time (Tests 5 and 14); (3) night-time tests with upwind releases during neutral to stable conditions (Tests 6, 8, 10, 12, 15 and 18-21); and (4) afternoon, evening, and night-time tests with hillside and summit surface releases (Tests 11,13,16 and 17). In the remainder of this paper, case studies and results from the first three groups of experiments are presented. Additional details and discussion of this field study have been presented by Ryan (1983). Descriptions and discussions of the releases from the hill surface will be presented in a later publication along with the results from a second series of field measurements conducted at Steptoe Butte during July 1983.

Daytime upwind release

Test 1 153&1630 PDT 19 May 1981. Test 1 was carried out under conditions of moderate westerly winds (292”-14.2ms-’ at 16Omrl), sunny sky with a

2008 WILLIAM RYAN PI ui

Table 2. Tracer test meteorological condinons ~-_____-__~.. -_ .Ix

Test Time (PDT)

u(h) WD 3 Hc

(1) III (11 (0 (ms-‘1 Cd?g,

ii, (de&

Prlh -11,l Imrl) (mrl) (mrl) (1) (fJ

____-I_~ -_-l_ll

Afternoon tests t 1539-1630 2 1300-1400 3 1605-1700 4 130&1400

Transition period tests 9 0730830 5 1900-2000

14a 1830-1930 14b

4.6 3.1 1.5 3.2

8.2 2.7 6.8

Night-time tests 6 2135-2230 8 2355-0100

10 23CC2400 12 2200-2300 i5a 22OG2300 15a 18a 2200-2230 18b 2230-2300 19a 2240-2330 19b 2Oa 2241-2330 20b 21a 0230-0330 21b

4.5 5.8 2.7 7.4 8.3

4.0 4.0 9.5

6.7

7.2

Will surface and summit tests 11 2000-2100 5.9 13a 1615-1700 5.6 13b 16a 190%2000 2.7 16b t?a 222C-2330 2.0 17b

294 8 161 0 0 0 0 226 26 124 0 0 0 0 192 27 186 0 0 0 0 198 27 -18’ 0 0 0 0

260 308 239

258 266 268 195 220

8 10 8

3 - 12 4

-

- -

5

3

10

-...

3

4

149 0 0 12s 69 7.h 3.6 151 0 100 0 100 % 1.4 122 0 38 0 30 * 2.1

9 0 38 0 30 I 5.0 f

149 86

123 150 115 30

187 0

146 81

156 91

166 96

40 - 186

0

100 186 206 123 29 29 50 50 97 97

150 150 72 72

37 _._

223 89

297 300 174

189

215

-_ -

193 193 146 146 72 72

_”

s4 54

124 124 61 61

70 1.9 2.5 117 1.1+ 181 0.1 + 0.5+ 89 1.8 2.2

110 1.2 110 1.2 85 1.7 85 1.2+ 71 0.-J+ 3.0 71 0.7 + 1.4+ 66 t.3 4.5 66 0.8+ 1.4+ 73 1.0 1.6 73 10 1.6

165 228

12 -.

13

295

339f 197’ 339’ 142* i34* 339* 347+

0 -_

0 0

100 loo

0 0

> 30 >30 100 100

_.. 0 0

257 257

0 0

- L97 L97

* 4 m above ground level. i 12 m above ground level.

Terms

(i) from tethersonde data collected before or during the initial portion of the test. (f) from tethersonde data collected during or after the final portion of the test. u(h) wind speed at summit. WD wind direction at summit. 0.9 standard deviation of wind direction fluctuations based upon 60s instantaneous tethersonde observations near the

source height.

h, source height.

h, top of surface inversion. dividing streamline height iterated from ipu(H,)* = g

= gravitational acceleration.

Fr (h -hi) single layer Froude number Fr = g; h’ = h - h,.

Fr(h - h,)+ inversion layer Froude number Fr = 2.

N Brunt-Vaitila frequency N = [52]“zz [$$p: g = potential temperature gradient.

few cumulus clouds, and a superadiabatic SF6 tracer was released due west of the hill at 161 mrl (1.4”C/lOOm) lapse rate in the lowest 300 m above the from 1539 to 1630 PDT. Isopleths of 45-min average surface (Fig. 4a). The 45-min average standard devi- SF6 concentrations are shown in Figure 4. Trajectories ation (g@) in 60 s instantaneous wind direction observa- from four weighted balloons released at the surface tions recorded near the release with the tethersonde from the tracer emission site are atso shown and are in was 8’. good agreement with the SF6 isopleths. Patterns of

An atmospherk tracer investigation of transport and diffusion around a large, isolated hill 2009

12 13 14 15 f6 T PC)

B 300

z ZOO

r N 100

0 1. - 160 360

ii (deg)

Fig. 4. Isopleths of 4%min average SF, concentrations during Test 1, 1545-1630 PDT from a source (*) at 161 mrl. Balloon trajectories f+) Inset: vertical meteorological measurements at I533-1.545 PDT from the release location.

SF6 ~ncentration averaged over individual 1%min periods were each similar to the overall 45-min average pattern.

Maximum concentrations during the three 15min sampling periods in Test 1 occurred on the west and southwest flank of the hill between 77 and 174mrl elevation, either below or approximately at the release height of 161 mrl. Maximum concentrations measured during the automobile traverses also occurred on the southwest face. Maximum time-averaged concentrations during the three 15-min sampling periods were 295, 222 and 733 ppt. The (highest (_ 1 s) instantaneous value measured during the automobile traverses was 1772 ppt. In the lee of the hill SF, concentrations were less than 63 ppt. AI1 of these plume features are consistent with a broad plume moving from the release point generally around the south flank of Steptoe Butte.

The other daytime experiments, Tests 2, 3, 4, had maximum concentrations relative to 1, that were 3 to 10 times lower and much less variable. While wind speeds during Tests 2, 3 and 4 were about half those during 1, the standard deviation of the wind direction at the release height was about 3 times larger: 26” in Tests 2, 3,4 compared to 8” in Test 1. The rn~irn~ concentrations ~no~alized by source strength)

were inversely correlated to a small degree with a, (r = -0.5). The maximum concentrations showed a positive correlation with wind speed (r = 0.5) in contrast to thecommonly expected increased dilution with increasing wind speed. Theoretically, as the wind speed increases, plume trajectories will approach the hill more closely and horizontal wind direction fluctuations will decrease. Either or both of these changes would result in increased concentrations on the hill.

Although there were differences in some of the details of the tracer patterns between daytime tests, in general the four tests showed similar characteristics. Plume widths were of the same scale as the hill radius and tended to encompass a large portion of the upwind hill surface. Although some small portions of the plume passed over the top and around the opposite side of the hill from the main plume axis no major bifurcation of the plume was observed. Maximum concentrations tended to occur at elevations on the lower to middle section of the hill both below and above the tracer release height. There was significant vertical variation in the elevation of the maximum receptors during the release periods.

Transjtiun period Unix release

Test 14 1815-1930 PDT, lOJune 1981. Thisevening

2010 WILLIAM RYAN PI al.

test was conducted during a period when the stability was changing from slightly unstable near 1830 PDT to a deepening surface inversion at 1930 PDT. The winds were from tL southwest throughout the period with little change except in the lowest 50m. Wind speed increased through the profile from about 5 m s- ’ at 50m to about 8ms-’ at 2OOm, but there was little change in speed at a given height during the period.

The development of a stable nocturnal situation was shown clearly by the temperature profiles, Fig. 5a. At about 1800 PDT the layer from the surface to 150 m was nearly neutral (- 2.2”C 2OOm-‘) while at 1920 PDT the higher layer from 150 to 260mrl was already slightly stable (5 O.S”C 100 m- I). After the conclusion of the tracer release, between 2007 and 2030 PDT, the layer to 230 mrl was stable (lapse rate + 0.5”C lOOm-’ above 35 m) with a surface inversion below 35m.

Two tracers were released between 1815 and 1930 PDT from a point 2.2 km from the summit on the 228’ azimuth. SF6 was released from 122mrl and CBrF, from 9 mrl. Figures 5 and 6 show the SF6 and CBrF, hourly average concentration isopleths. The patterns indicate that both plumes passed over the top of the hill. The maximum 15-min average CBrF, concentration from the low release point exceeded 1000 ppt and occurred immediately downwind of the release.

However, for the elevated release point the maxtmum SF, concentrations occurred on the lee side of the hill. i.e. on the northeast face as shown in Fig. 5. These lee side concentrations were in the range of 6@-90 ppt. The general directions of both tracer plume axes were III

agreement with the observed winds. Details of the surface released CBrF, tracer plume

are shown in Fig. 7 in the form of isopleths on a cross section of the hill for the four successive 15min sampling periods. These plots show that the plume tended to move around the upper south side of the hill in the first, second, and fourth periods and around the north side in the third period (190&1915 PDT). The SF6 plume showed similar oscillations. These detailed patterns for individual sampling periods fail to show clearly that the axes of either tracer plume pass directly over the top of the hill, even though this could be inferred from the longer term average patterns shown in Figs 5 and 6.

The cause of the side-to-side swings by the tracer plumes shown in Fig. 7 is not readily apparent. There were some small shifts in the 15-minute average winds at the release height during the release: 240’, 241’. 240’ during sample periods 1, 2 and 4 and 237’ during period 3. Although these are consistent with the plume shift to the north face of the hill from 1900 to 1915 PDT. it is recognized that a 4-5 shift in average

c

\

\

= 2oo r t P E lO,c

i c

ck ‘\p 0 ‘\.

12 14 16

2 4 6 610

U(ms-')

c 3ooL

L !b

90 160 270

WD (deg)

Fig. 5. lsopleths of hourly average SF6 concentrations during Test 14 183&1930PDT from a source at 122mrl. Inset: vertical meteorological measurements from the release location southwest of the hill (a)

1757-l 805 PDT, (b) 1921.-1934 PDT and (c) 2007-2030 PDT

An atmosphe~c tracer investigation of transport and diffusion around a large, isolated hill

Fig. 6. Isopieths of hourly average CBrF, con~ntrations during Test 14,183~1930 PDT, from a source at 9 mrl.

I 1830-1845 , 1845- 1900

400

t

Lzk

300

200 1 IOO~yyJJ \

I 2400

NORTHWEST

L””

400

100

500

200 & 1150

METERS

I -?.

2400 3600 SOUTHEAST

Fig. 7. Fift~n-minute average CBrF3 isopleths for Test 14. Vertical scale is expanded.

wind direction is very small and within the noise signal of the sensor. It may also be likely that the plume movements were due to oscillations in the flow field near the obstacle. Hunt and Snyder (1980) (hereafter referred to as HS) reported irregular low frequency oscillations of a plume as well as an intermittent windward horseshoe vortex at low Froude number. These authors estimated a Strouhaf number (a meas-

ure of the frequency of vortex shedding) for these oscillations of approximately 0.5. For the radius of Steptoe Butte at the source height and an oscillation period of lSmin, the local Strouhal number is 0.13.

The concentration patterns shown in Figs 5-7 show anomalous areas of high and low concentrations within the genera1 plume zones. These may be due to combinations of meander, lifting, and channeling of

2012 WILLIAM

the plume. Examples of the results of these effects include the high CBrF, concentrations shown near the summit in Fig. 7 for the 2nd and 4th sampling periods. From the maximum tracer concentrations observed during the several periods of this test it is apparent that concentrations increased during the test period. This would be consistent with an increase in stabifity during the time period.

The plumes released during Tests 5,9 and 14 during the transition periods were similar to the plumes observed during the daytime in that the observed plume widths were similar to the scale of the hill. Fifteen-minute averaged non-dimensional maximum concentrations (x = Cuh’Q; where C is the observed maximum concentration, u is the wind speed at the tracer release point, h is the height of the hi11 and Q is the tracer release rate) were between 0.1 and 1.7 for the elevated plumes and between 5.4 and 8.5 for the low- level plume in Test 14. The wind speeds in Test 5 were quite different from those in Tests 9 and 14, but the c@ values were near lo” in all 3 tests. The normalized maximum concentrations for the elevated plumes were not significantly correlated with cg or U.

Plume trajectories in the transition tests exhibited some curvature around the hill implying stability- induced horizontal motion. For Test 5 there was also evidence of plume bifurcation, and in Test 14 areas of

RYAN YI (11.

elevated concentrations were observed away from thr general plume centerline. In general, the plume isopleths from these transition periods appeared more complicated than in the daytime cases.

Might -time upwind reiruscs

There were nine tracer tests carried out at night with upwind releases. During neutral to stable night-time conditions, flow conditions were complicated by the interaction of surface inversions, local and regional drainage winds, and the existing synoptic winds. Significant wind speed and directional shears were associated with the existence of the different flow regimes. The following discussion describes four of the nine night-time experiments.

Test 15 2200-23oOPDT lOJune 1981. Thisdual tracer experiment was conducted several hours after transition Test 14 discussed above. Winds above 120mrl were from the south southwest at 7ms-‘. A strong surface inversion and southeasterly winds at l-2 m s- i were present below about 25mrl (Figs 8a-c). The dividing streamline height was calculated to be 1lOmrl while the local Froude number for flow above the inversion was 1.2.

Tracers were released from 1.9 km south-southwest of the hill. SF, was released From 115 mrt and C&F,

U tms“)

C 200

t60

c g 120

N 80

WD (deg)

Fig. 8. lsopleths of lS-min average SF, concentrations measured during Test 15 between 2200 and 2215 PDT from a source at 115 mrl. Inset: vertical meteorological measurements from upwind of the hill,

(a) 20X-21 11 PDT and (b) 2226-2245 PDT.


from 30mrl. Because CBrFs was released below the dividing streamline height within the surface inversion layer with light southeast winds, no significant concentrations were measured on the hill. However, concentrations of CBrF, at the detection limit were observed at several sample points on the southwest slope. This behavior sharply contrasts with that observed earlier in Test 14 when the plume released from 9mrl swept directly up the hill with maximum concentrations exceeding 1OOOppt on the windward slope.

The SF6 plume released just above H,, at 115 mrl, impinged upon the hill near 150mrl and bifurcated round the hill. Isopleths of 15-min average SF6 concentrations from the first sample period are shown in Fig. 8. The major portion of the plume traveled up the southern ridge and curvedaround the eastern side of the hill. Maximum concentrations along this trajectory exceeded 5OOppt and occurred between 200 and 300mrl. A smaller part of the plume split at the southern ridge and traveled across the bowl of the southwest slope to impact low upon the western ridge. Maximum concentrations on the west side varied from 78 to 121 ppt during the four sample periods. A finger of tracer also curled west around the hill near 230mrl during each sample period as shown by the 50ppt isopleth enclosing the hill summit.

The closeness of the source height (II,) to the estimated dividing streamline height may explain why one trajectory climbed up and over the hill while the second trajectory curved horizontally around the side of the hill. This dual character of the plume is illustrated in 1 h-averaged isopleths drawn on a vertical cross-section of the hill in Fig. 9. For h, > H,, we expect plume transport up and over the hill while for h, c H, we expect horizontal motion with plume impingement and bifurcation. These kinds of behavior are predicted from the definition of H, as well as from observations of flow over a hill in a tow tank by HS.

Test 19 2230-2330PDT 15 June 1981. Conditions for this test included lOms- ’ southerly winds and a

strong surface inversion extending above lOOmr1 (Figs 1Oa-c). Light easterly winds near the surface were evidence of drainage flow down the terrain which rises eastward from Steptoe Butte. The dividing streamline height increased from approx. 50 to 70mrl during the test, and the local Froude number for flow above the inversion was 3.0.

Dual tracers were released from 1.9 km south of the summit; the source heights for SF, and CBrF, were 146 and 81 mrl. As a result of the south-easterly wind below lUOmr1, very little CBrF3 was observed within the sampler array. Low concentrations were measured in samples collected along automobile traverses across the southern ridge. At the same time, however, SF6 tracer released above the top of the inversion and above H, traveled over the hill and down to the lee surface. Isopleths of 15-min average concentrations from the second sample period are shown in Fig. 10. Maximum concentrations exceeded 1OOOppt on the northwest radial at approximately 0.5 h. During the first half of the test a secondary zone of high concentrations (20&500 ppt) covered the summit. The highest concentrations measured on the windward face were approximately 1OOppt. These occurred at approximately 0.5 h on the south and southwest radials.

This experiment is an excellent example of a plume released at less than half the height of the hill passing over the upwind slope with minimal impact and into the lee recirculation zone where the maximum surface concentrations occurred. Lavery er ol. (1982) reported a similar plume trajectory in a case where h, > H, with 5.5 m s- ’ winds and Fr equal to 2. However, in their test the concentrations increased down the lee slope to the edge of the sampler array. This trend was only observed in the first sample period during Test 19; the other periods were similar to that shown in Fig. 10.

Test 20 2230-2330 PDT 17 June 1981. The bi-level flow observed in all of the night-time experiments was particularly evident in Test 20 where a strong surface inversion reached to almost 0.5 h (Figs 1 la-c). Winds

300 -

$

F w200-

5

100;

- 0

1200 2400 WEST

3600 4000 EAST

METERS

Fig. 9. Isopleths of 1 h-average SF6 concentrations during Test 15 (22CC23tMPDT) on a W-E vertical cross-section of Steptoe Butte. Vertical scale is expanded. Projected source

location indicated by*.

2014 WILLIAM RYAN YI al

0 so 180 270

WD (deg)

Fig. 10. Isopleths of 15-min average SF, concentrations during Test 19 between 2245 and 2300 PDT from a source (*) at 146mrl. Inset: vertical meteorological measurements from the release point upwind of the hilt, (a) 2211-2221 PDT and

(b) 2316-2352PDT.

within the inversion veered from easterly near the surface to southerly at 4 m s- ’ near 100 mrl. Above the inversion, the winds moved to the south-southwest and increased to 7ms- ‘. The dividing streamline height changed from 124 to 71 mrl during the test as the winds increased. The local Froude number for transport above the inversion increased from 1.3 to 4.5 while the local Froude number for flow in the surface layer varied between 0.8 and 1.4.

Dual tracers were released from south-southwest of the summit. SF6 was emitted above the inversion at 156 mrl, and CBrF3 was released within the inversion at 91 mrl. The differences between plume impingement for h, -Z H, and plume transport over the hill for h, > H, are obvious in the tracer isopleths from 2245 to 2300 in Fig. 11. Only a faint trace of a plume was apparent in the SF6 data. The maximum SF6 concentrations during the last three sample periods were 39, 110 and 41 ppt. The overall appearance was that of a plume passing over the hill with minimal diffusion to the surface. Unlike Test 19, significant entrainment into a recirculation zone on the lee side of the hill was not observed.

The CBrFX plume, released within the inversion, came very close to the surface of the hill as it skirted

round the western flank (Fig. 11). Maximum 15-min average concentrations between 2245 and 2330 were 2715,1244and 1309ppt at the lowest points on the SW and WSW radials. The receptor elevations for these maximum concentrations were 77 and 9Omrl which are very near the CBrF, source height. These results indicate horizontal flow of the tracer plume with probable impingement on the upwind side.

Test 21023CHI330 PDT 18 June 1981. The final test was conducted with overcast skies and southwesterly winds. During the experiment as the winds increased to 8 m s- ‘, the surface inversion weakened, and very light southeasterly flow at the surface changed to light westerly flow (Figs 12c-e). The dividing streamline height calculated before and after the tracer release decreased from 125 to 75mrl. Both tracers were released above the top of the inversion where the local Froude number increased from 1.0 to 1.6. The SF6 and CBrF, source heights were 166 and 96 mrl at 2 km southwest of the summit.

The differences in the release heights coupled with the vertical variation in flow conditions produced significant differences in plume impact. Isopleths of 15- min average tracer concentrations are diagrammed in

TPC)

201s

300 b

300 b L

0

f 200

.E : . ..f

N 100

~ > t

: .’

*’ :... 1:

!.’

0 2 4 6 a10 U hS-‘1

300 F b

C 2 N i

2oo ~ .a

100 ,I I

,

0 90 I80 270

WD (deg)

Fig. 11. Isopleths of 15-mitt average SF, (- ) and CBrF, (. . _ . ) concentrations during Test 20 between 2245 and 2300 PDT from an SF, source (*)at 156 mrl and a CBrF, source (*) at 91 mrl. Inset: vertical meteorological measurements

from the release point (*), (a) 2203-2229 PDT and (b) 2356-0011 PDT.

5_/ (,_j, “::k 8 12 0 2 4 6 8-

Utmi’)

.L 0 10 90 180 210

WD (drg)

Fig 12. Isopleths of U-mitt average SF6 (A)and CBrF, (B)concentrations during Test 21 between 0300 and 0315PDT from sources (*) at 166 (SF,) and 96mrI (CBrF,). Inset: vertical meteorologicat

measurements from the release location, (a) 0233-0246 PDT and (b) 0332-0344 PDT.

20f6 WILLIAM RYAN el al.

Fig. 12 for the third sample period. The SF6 plume passed up the windward face and bifurcated round the top of the hill. Maximum concentrations between 230 and 298 ppt were measured above 0.7 h on the SE, E, and NW radials. Concentrations greater than 1OOppt were also observed in the lee of the hill near 0.3 h.

The trajectory of the CBrF, plume closely matched the SF6 trajectory up the windward face and round the summit into the lee of the hill. However, the centerline of the CBrFj plume clearly passed much closer to the surface than did the SF, plume. Maximum C&F, con~ntrations occurred near 0.6 h and exceeded 4OOUppt. The CBrF, plume also diverged over a larger area of the hill than the SF, plume. Concentrations from the CBrF, release were greater than 1OOOppt in the lee of the hill. This behavior of the two plumes is in general agreement with the expectation that a plume will pass up and over the hill when Fr > 1 and h, > H,.

These several examples iilustrate the conditions that were observed under common night-time meteorological situations. It is also apparent that with accurate evaluations of Fr and H, the behavior of plumes relative to the hill obstacle can be predicted. In Test 15. the slight variations in the source height coupled with small changes in the approach flow could have caused intermittent plume transport above and below N, to produce the two different trajectories evident in Figs 8 and 9. The portion of the SF, plume which traveled up the hill agrees with the first expectation for h, > H,, and the part of the SF6 plume which bifurcated around the west side of the hiti agrees with the second expectation for h, < H,. This type. of behavior is quite similar to observations reported by Lavery et al. (1982) for a test conducted during the EPA small hill impaction study. In their case No. 206, S-min. averaged values of H, varied between 29 and 45 m while tracer was released from 35 m. Tracer isopleths from this test showed transport hor~ontally round the hili as well as vertically up the hill. Together the Steptoe and Cinder Cone Butte tests emphasize the need to carefully monitor changes in the approach flow in order to properly predict and, hence, model plume behavior. The data also suggest the need for a composite mode1 capable of treating both horizontat and vertical flow.

The example of pronounced lee side concentrations in Test 19 (Fig. 10) is similar to fluid modeling tests reported by HS that explored theconditions when flow separation on the lee slope existed and those when separation was suppressed to a point further down-

wind by strong flow stratifi~tion. The results from the HS experiments for large Froude numbers (similar to Test 19) show a large region of recirculating flow beginning very near the summit. The reader is referred to HS Fig. 15 for Fr = 1.7 and Fr = cc. These diagrams of the flow observed in the tow tank should be used with caution since HS reported that the size, location, and existence of the recirculation zones were widely irregular. However, the HS flow patterns do suggest that maximum concentrations will occur at the mid-level of the lee face if the plume streamline is

entrained into the recircuiation zone. The exact location of the maximum will depend upon the relative positions of the plume streamline and the separation zone.

in contrast to the other night-time examples. in Test 21 both tracer plumes could be identi~ed clearly and their behavior showed reasonably good agreement with the expectation that for Fr > 1 and /I, > H, a plume will pass up and over the hill. Fluid modeling measurements of streamline displacements above the crest of a hill (see HS Fig. 1.8) predict that for Fr = 1 .h the tracer plume with source height equal to 0.3 h will be displaced by less than IOm (0.03 h) above the summit of Steptoe Butte. For a tracer plume released from 0.5 h with Fr = 1.6, the displacement is predicted to be twice as large, or approximately 25 m (0.07 h) above the crest of the butte. At the same time, convergence of streamhnes passing over thecrest will cause the vertical diffusion coefficient to be small and comparable to the predicted displacement heights as shown by Bass ez al. (1981). In this case, it is quite feasible to have an order of magnitude difference in the surface concentrations of the two different plumes such as observed in Test 21.

The occurrence of horizontal flow with impingement when the local Froude number exceeds 1 is a situation unique to the real world and is not represen- ted in the wind tunnel or tow tank studies. This does not mean that fluid modeling results are not applicable to these situations, but it does require care to use the conclusions from HS. Because uniform density gradients and constant flow speeds were used in the tow tank tests, H, could be calculated directly from the local Froude number as given previously in (2). However, with a strong surface inversion and significant wind speed shear as occurred in field tests, H, must be obtained iteratively from (1). For example, in Test 15, tracer was released at or below N, with resultant horizontal flow while the governing local Froude number was 1.2-a number suggestive of vertical flow over the hill based upon the results in HS. We thus emphasize as others have (e.g. Lavery er al., 1982) that in shear flows H, is the parameter which properly accounts for the ratio of inertiai and gravitational forces while the local Froude number becomes an indicator of stability useful for dispersion modeling purposes, but not for predicting the streamline dividing height (see also, Ryan and Lamb, 1984).

From these several examples of the night-time tests at Steptoe Butte. it is clear that the key to modeling these plumes is to correctiy specify the release height, the dividing streamhne height, and the winds at the release height. For real sources with buoyant plumes, plume rise calculations (or measurements) will be critical in determining how to model subsequent plume behavior as well as in determining the appropriate wind direction. The data presented here should be strong motivation to improve and validate plume rise calculations for situations with complicated atmospheric vertical structure. A recent presentation by Glendening et al. (1982) addresses this problem.


Maximum concentrations

Maximum concentrations on an isolated hill occur

on the windward face due to impingement or by diffusion from a plume passing close to the surface. High concentrations can also occur on the lee slope if a plume is entrained in the lee recirculation zone or if the plume streamline is brought down close to the lee surface. For the elevated plumes observed during the night at Steptoe Butte, the maximum non-dimensional concentrations (x = C,, uh2/Q; h = hill height, C,, = maximum 1%min average) ranged between 0.1 and 16.1. For comparison, the non-dimensional maximum concentrations observed during the daytime tests ranged from 0.01 to 3.2 while the corresponding values from the transition experiments were between 0.1 and 1.7 for elevated plumes and between 5.4 and 8.5 for a plume released near the surface.

Although the number of 15-min average periods is small, the frequency distributions of x graphed in Figs 13(a)-(b) highlight the differences between the night-time plumes and those from the daytime and transition periods. Values of x greater than 0.5 com- prised approximately 70 % of the maxima at night, but the same range accounted for less than 45% of the daytime and transition maxima. At night, although the two largest concentrations occurred on the windward side of the hill, more than half of the maxima greater than 0.5 occurred on the leeward half of the hill. During the daytime and early evening, all but one maxima greater than 0.5 were measured on the windward side.

We can learn more about where the maximum concentrations occurred by examining the frequency distributions of the maxima receptors classified by sectors around the hill: upwind front, upwind sides, lee sides, and lee back. These sectors are defined by the mean wind direction during each test period as indicated in Fig. 14 where the frequency distributions are graphed. During the night-time tests, the locations of the maximum concentrations were approximately evenly distributed around the hill. Slightly more high concentrations occurred on the lee side than on the other areas and slightly fewer maxima were observed

A. 50 I

40

Freq (%)a

20

IO

0

UIGHT

on the upwind side than on the other sectors. It is significant that maxima observed on the lee back at night accounted for more than 20 yi of the occurrences. This was not true during the daytime and transition periods where no maximum concentrations were measured on the lee back. The difference probably reflects greater vertical dispersion rates during the day than at night; daytime plumes touch down closer to the source than do night-time plumes even though stable plumes pass more closely to the surface of the hill. It is very important, therefore, to specify correctly plume dispersion rates relative to the displacement of the plume trajectory above the surface. Bass et al. (1981) addressed this problem by developing a method to account for effects of streamline deformation over the hill upon plume dispersion rates.

The effects of source displacement (yJ from the flow centerline over the hill are clearly indicated in Fig. 14. For all of the test periods, the position of the maximum concentration shifted toward the lee side as the source displacement increased. For displacements greater than the hill radius at half-height (y, > r), the frequency of occurrence increased from less than a third of the maxima on the upwind front to more than two-thirds of the maxima on the lee side. This shift in the receptor location with a change in source position has been examined previously for stable flow in a tow tank by Snyder et a!. (1979) and also in calculations by Hunt et al. (1979). These authors found that in stable flow only small increases in y, caused the maximum concentration to shift to the side of the hill.

In the 30 night-time sample periods, we found that the elevated tracer plumes generally traveled up and over the hill. This is clearly evident in Fig. 15 where the distribution of sample period maxima is graphed vs the elevation of the maxima positions (Z,) relative to the source height. At night almost 70% of the maximum concentrations were measured at receptors above h,. Only 10 “/;, of the maxima occurred near h, ( f 0.1 II,), while slightly more than 20 y0 of the maximum concentrations were observed at Z,, < h,. In each category, the receptors were predominately on the lee of the hill and source heights were predominately greater than the dividing streamline height. The results in Fig. 15

LJ. - DAY

r 40

Fteq (.Z)~

20

IO

0

Fig. 13. Frequency of Occurrence of maximum nondimensional concentrations during ni@t-time tests (A, N = 30) and during daytime and transition period tests (B, N = 28). Shaded areas represent occurrences on the lee

half of the hill.

2018 WILLIAM RYAN et al

a

Freq (X) 20

IO

0

3-J -

Freq (2) m

IO L I

/

0 __ ! IIf Lff uwm IPWIKI lff Lff Mlrn MlKi Em Slrf Slof Fml Bux EKE SEE FKW

Fig. 14. Frequency of occurrence of maximum nondimensional concentrations on the lee back, lee sides, upwind sides, and upwind front of the hill (a) night-time tests, N = 30 and (b) daytime and transition wriod tests. N = 28.

Shaded areas represent occurrences ‘whdre y, > r.

A. ’ NIGHT

m-

YI-

Freq (%)a -

30 -

ZQ-

10 -

O- i! m.‘hs &nr=hs h,x’h,

B. 70

60

w

Freq (I)~

a

20

IO

0

Fig. 15. Frequency of occurrence of maximum nondimensional concentrations for receptor elevations less than, equal to, and greater than the source height. (a) night-time tests, N = 30and (b) daytime and transition period tests,

N = 28. Shaded areas represent occurrences on the lee half of the hill.

indicate that in most cases the night-time plumes traveled up the hill and produced maximum surface concentrations near the top on the lee of the hill. The occurrence of maximum concentrations on the lee of the hill at elevations below the source height as observed in about 20% of our data, indicates entrainment of the plume into the recirculation region with subsequent transport to the surface on the lower edge of the lee slope. For the weak to moderate stratification conditions existing in the upper level of flow during these tests, results from HS imply that separation of flow occurred very near the summit and that a zone of recirculating flow covered a large portion of the lee slope.

The pattern of the elevations of maximum concentrations is more variable in the daytime and transition data than in the night-time periods. Maxima were observed in approximately equal numbers at elevations above and below h,. Unlike the night-time tests,

the maximum concentrations occurred predominately on the windward half of the hill (Fig. 14b). The greater variation in Figure 15b than evident in Fig, 15a probably results from higher turbulence during the daytime than at night. Large fluctuations in plume trajectories coupled with large vertical dispersion rates can cause maximum concentrations short distances from the source at terrain elevations less than the source height.

Nondimensional concentrations on the surface of a hill have been calculated by Hunt et al. (1979) using a potential flow model for flow over a hemisphere. With a diffusivity corresponding to neutral conditions, a source. 4.5 h upwind of the hill, and a source height of 0.4 h, the nondimensional surface concentrations increased from approximately 0.7 upwind of the hill to approximately 5.0 on the upwind face. This range agrees reasonably well with the maximum concentrations observed for elevated plumes in the daytime


and transition period tests at Steptoe Butte. The non- dimensional concentrations calculated by Hunt et al. for neutral conditions also agree closely with approximately 50% of the maximum non-dimensional concentrations recorded in the night-time tests at Steptoe Butte.

Other calculations by Hunt et al. using a smaller diffusivity to represent stable conditions yielded a maximum non-dimensional surface concentration near 8. In comparison, only the highest maximum observed at Steptoe Butte exceeded 8. This is an important point since the vertical potential flow model treats only vertical diffusion to the surface from an elevated streamline and does not provide for impingement of an elevated plume. The conclusion suggested by this comparison is that the model designed for large Froude number behavior performs very well in comparison to our observations of elevated plumes released into flows where Fr > 1.

With regard to the highest concentration observed at Steptoe Butte, it is worth considering calculations by Hunt ef al. (1979) using a horizontal potential flow model to account for impingement and with eddy diffusivities for stable conditions. With no source offset from the flow centerline, the maximum non- dimensional concentration calculated with the impingement model equaled approximately 21 and occurred on the flow centerline. For slight source displacements the calculated maximum concentrations decreased to approximately 10 and occurred on the side of the hill. This range between 10 and 21 encompasses the highest non-dimensional concentration observed (16) which also occurred on the side of the hill with a relatively large source displacement. It should be noted again, however, that this high concentration occurred at an elevation above the source height. This comparison with the impingement model suggests that some impingement may have occurred on Steptoe Butte even though the plume lifted partially up the hill.

Calculations by Bass et al. (1981) are also similar to our observations. They used a potential flow model with dispersion rates calculated as a function of transport time along the plume streamline over a hemispherical hill. For neutral conditions with a source 2000m upwind of the hill at heights between 0.4 and 0.5 h, the maximum non-dimensional concentrations on the hill were in the range 2.0 to 4.0 which is essentially the same as our observations. The good agreement between the theoretical modeling results and our near neutral observations is an important verification of current assumptions about neutral flow over a hill.

Application of the common bivariate Gaussian plume model for this situation with no plume height adjustment yields non-dimensional surface concentrations equal to 0.7 for C stability and 0.07 for D stability. This range underestimates the observations and illustrates the problem associated with ‘fix-ups’ to the common Gaussian plume model.

Surface drainage winds

A final aspect of our observations at Steptoe Butte is the effect of surface drainage winds upon plume transport near the hill. The strong surface inversions which developed during the night were accompanied by apparently katabatic winds from the Idaho moun- tains to the east and from Steptoe Butte itself. At 28 km east of the hill the terrain reaches 450 mrl and at 55 km east the highest points are approximately 86Omrl. AS shown in Figs 11 and 12, the light easterly surface winds increased in speed and changed direction with increasing height. However, the effects of the easterly surface flow upon plumes released above the inversion were not apparent in the surface concentration patterns. For plumes released at lower levels within the inversion layer, the directional shear apparently caused plumes to follow trajectories outside the sampler array.

Downslope flow on the side of the hill was observed on several occasions. These drainage winds were estimated to be l-2m s- 1 from smoke and soap bubble releases and from portable anemometer measurements. In several case& weighted balloons released from the ground 2 km from the summit drifted away from the hill until reaching lO-20m above the ground and then reversed direction to travel directly toward the summit. In Test 18 where this flow reversal was observed, concentrations exceeding 500 ppt occurred at and just beyond the summit, while concentrations between 20 and 70 ppt were observed on the windward side of the hill. This occurrence of significant tracer concentrations down the windward face can be inter- preted as evidence of plume entrainment from aloft into downslope flow. However, the lack of similar behavior in other tests prevents any firm conclusion regarding plume entrainment near the surface.

SUMMARY AND CONCLUSIONS

The large size of Steptoe Butte introduces complicated, multiple layer atmospheric structure not significantly present in previous studies of flow around a hill. As a result, the tracer tests completed in our study represent real world situations of plume transport over and around an isolated obstacle that is of a realistic scale and appropriate to many terrain obstacles.

Surface inversions at night reached as high as 0.6 h (2OOmrl) and winds within the inversions typically had an easterly component due to drainage flow at speeds less than about 2 m s- ‘. Above the inversion, the flow was stably stratified from the southwest quadrant at speeds ranging from 1 to lOms_‘. Because of the nearly counter-current flow layers and large differences in speeds within and above the inversion, strong directional and velocity shear was usually present within the flow around the hill. Dividing streamline heights calculated from vertical temperature and wind speed profiles ranged from approximately 0.1 to 0.7 h. Local Froude numbers for the

2020 WILLIAM RYAN PI ul.

plumes released above the inversion (12 releases out of AcLnowlrdgements-We would like to thank Mr. and 17) varied between 1.3 and 4.5, while Froude numbers Mrs. Delbert Moore and the State of Washington Parks

for plumes released within the inversion ranged from Department for providing access to Steptoe Butte. Special

0.5 to 1.4. recognition is also given to the enthusiastic work of the

Plumes released above H, traveled up and over the Steptoe Butte field study team: Mark Anderson. David Harden. John Raudsep. Frank Sutman. Duane Truitt and

hill with maximum impact very near the summit on Don Stearns. either the windward or leeward side. For source heights approximately equal to H,, the plumes also passed up the hill, but the trajectories appeared to be close to the hill surface. Overall, in spite of the complicated atmospheric structure and apparent surface drainage winds, the results from the night-time studies confirm that plumes released above the dividing streamline height tend to travel up and over the hill while plumes released very near or below H, can impinge upon the hill.

The locations of maximum surface concentrations at night were approximately evenly distributed around the hill. As the horizontal displacement of the source from the flow centerline increased, the location of maximum concentration tended to shift toward the siites of the hill. Receptor elevations where maximum concentrations occurred typically were greater than

This work was supported by the U.S. EnvIronmental Protection Agency, Grant R807336-01-0. Thecontents of this paper do not necessarily reflect the views and policies of the U.S. Environmental Protection Agency. nor does mention 01 trade names or commercial products constnute endorsement or recommendauon for use.

REFERENCES

Bass A., Strimaitis D. and Egan B. (198 1) Potential flow model for Gaussian plume interaction with simple terrain features. EPA-600/4-81-008, Research Triangle Park, NC 27711.

Burt E. W. (1977) Valley Model User’s Guide. EPA-450/Z-77- 018. US. Environmental Protection Agency, Research Triangle Park, NC.

Dickerson M. H. and Gudiksen P. H. (1980) The Department of Energy’s Atmospheric Studies on Complex Terrain (ASCOT) Program. Presented at the Second Joint __ _

the source height as expected from the moderate Conference on Applications of Air Pollution Meteorology,

Froude numbers and the number of cases where New Orleans. AMS, Boston, MA, 469-473.

h, > H,. However, a significant number of maximum Drazin P. G. (1961) On the steady flow of a fluid of variable

concentrations were measured at elevations less than density past an obstacle. Tellus 13, 239-251.

Electric Power Research Institute 11978) Plume model valid- h,. Most of these occurrences were on the lee half of the ation. EPRI-EA-917~SY Workshop Summary Report,

h;ll. Palo Alto, CA.

More than SOY& of the maximum non-dimensional Glendening J. W., Businger J. A. and Farber R. J. (1982) A

concentrations were in the range of surface concen- method for predicting plume rise in stable atmospheres with complex vertical structure. 82-29.9. Presented at the

trations calculated previously by Hunt et al. (1979) for 75th Annual Meeting of the Air Pollution Control an ideal hill using a potential flbw model with neutral Association. New Orleans, LA.

diffusivities. Application of the same model with stable Hunt J. C. R. and Mulhearn R. J. (1973) Turbulence

diffusivities produced a range of surface concen- dispersion from sources near two-dimensional obstacles. .I. Fluid Mech. 61. 24s-274.

trations which encompassed all but one of the ob- Hunt J. C. R.. Snyder W. H. and Lawson R. E., Jr. (1978) Flow served maximum concentrations at Steptoe Butte. The structure and turbulent diffusion around a three-

highest concentration did agree with maximum con- dimensional hill. Fluid modeling study on effects of

centrations calculated by Hunt et ul. (1979) using an stratification--I. Flow structure. EPA-600,/4-78-041,

impingement potential flow model with stable dif- Research Triangle Park. NC 27711.

Hunt J. C. R.. Puttock J. S. and Snyder W. H. (1979) fusivities. These comparisons indicate that the theory Turbulent diffusion from a point source in stratified and of a turbulent plume embedded in a potential flow field neutral flows around a three-dimensional hill- 1. Diffusion

is a very feasible approach for modeling flow over and equation analysis. Atmospheric Encironmenf 13,

around a hill. 1227.-1239.

Hunt J. C. R. and Snyder W. H. (1980) Experiments on stably As a closing remark, the complexity of the bi-level and neutrally stratified flow over a model three-

Row around Steptoe Butte is a strong indicator that dimensional hill. J. Fluid Mech. 96, 671690.

simulating these conditions correctly will also be a Lamb B. K. (1978) Development of application of dual

complex process requiring consideration of the time- atmospheric tracer techniques for the characterization of

dependent thermal driving forces and the topo- pollutant transport and dispersion. Ph.D. thesis, California Institute of Technology. Pasadena, CA.

graphical boundaries. Ad hoc fixes to Gaussian ex- Lavery T., Bass A., Strimaitis D., Venkatram A., Greene 8..

pressions or numerical solutions to the diffusion Drivas P. and Egan B. (1982) EPA Complex Terrain Model

equation may both be limited to simple approxima- Development Program-First Milestone Report. EPA-

tions and parameterizations of the actual flow con- 600/3-82-036, Research Triangle Park, NC 2771 I.

Morris A. L.. Call D. B. and McBeth R. B. (1975) A small ditions. For this reason. the demonstrated success of tethered b&oon sounding system. Bull. A& mer: Ser. 56,

the tracer and meteorological techniques applied at 964969.

Steptoe Butte is an excellent reminder of the utility of Rowe R. D., Benjamin S. F., Chung K. P.. Havlena J. J. and

tracer measurements for directly analyzing the impact Lee C. Z. (1982) Field studies of stable air flow over and

of real or proposed sources in regions of complicated around a ridge. Atmospheric Emironmenr 16, 643654.

Ryan W. (1983) An atmospheric tracer investigation of flow terrain. near a large, isolated hill. M. S. thesis, Washington State


University, Pullman, WA. Snyder W. H.. Britter R. and Hunt J. C. R. (1979) A fluid Ryan W. and Lamb B. (1984) Determination of dividing

streamline heights and Froude numbers for predicting modeling study of the flow structure and plume impinge-

plume transport in complex terrain. J. Air Pollur. Control ment on a three-dimensional hill in stably stratitied flow.

Ass. 34, 152-155. Received from W. H. Snyder, Meteorology and Assessment Division, U.S. E.P.A., Research Triangle Park, NC 27711.

Documents

An atmospheric tracer investigation of transport and diffusion around a large, isolated hill