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9th Conference on Air Qual

Modeling – A&WMA AB-3 

Comments on Building an

Terrain Downwash Issues

Presented by Ron Petersen, CPP Inc.

1415 Blue Spruce Drive

Fort Collins CO 80524

970 221 3371

rpetersen@cppwind.com
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Comment Areas

• Problems with BPIP• AERMOD/PRIME Problem for Shor

Buildings

• AERMOD/PRIME Underestimation F

Corner Vortex

• Terrain Wake Effects
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Ultimately, PRIME Needs the Building Shape and Position that Places Stack in the Correct Snyder/Lawson Data Base Flow Region (i.e., Data Base Used to Develop Downwash Algorithms)

Sampling of Snyd

Streamline figures from: Snyder, W.H. and R.E. Lawson, Jr.: Wind Tunnel Measurements of Flow Fields in the Vicinity 

of Buildings; 8th Joint Conference on Appl. of Air Poll. Met. With A&WMA; AMS, Boston, MA, 1994; pp. 244-250
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Other considerations

• Building downwash algorithms in AERMOD are designe

rectangular buildings.

• Building downwash algorithms in AERMOD only approp

certain building aspect ratios

• Use of wind tunnel testing to determine Equivalent Build

Dimensions (EBD) has been used to help solve the prob

– EBD guidance provided in Tikvart July 1994 Memorandum 

analysis is viewed as a source characterization study whichas been considered under the purview of the Regional Of

– All testing to determine EBD under neutral stratification, sim

assumptions in Prime Algorithms.

– With AERMOD/PRIME building location is also a variable a

methods may be appropriate and has been used on recent 
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Picking Dominant Structure Example

BPIP Input Information
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BPIP Input for Entire Si
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BPIP Input without upwind To
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Input needed to model disper

based on EBD Study
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Mirant Power Station – Recent Example

• AERMOD with BPIP predicting high 

concentrations.

• AERMOD with Equivalent Building 

Dimensions gave lower concentratiand ones that agreed better with fieobservations.

• Petersen, R. L., J. Reifschneider, D. Shea, D. Cramer, and L. Labrie, “

building Dimension Inputs for AERMOD Modeling of the Mirant PotomGenerating Station,” 100th Annual A&WMA Conference, Pittsburgh, P

• Shea,D., O. Kostrova, A. MacNutt, R. Paine, D. Cramer, L. Labrie, “ M

Study of AERMOD Using Wind Tunnel and Ambient Measurements at Locations,” 100th Annual A&WMA Conference, Pittsburgh, PA, June 2
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AERMOD/PRIME Problem for 

Short/Large Buildings

• The wake algorithms have only 

developed/tested for limited buiaspect ratios

• Short/large industrial facilities fa

outside this range.
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Problems with BPIP

Short/Large Industrial Facil

BPIP Building Dimensions:

H = 17 m

L/H = 23

H/W = 0.02

See Red Footprint

Limitations in 

PRIME Equations:

0.3<L/H<3.0

Building dimensions used 

to develop PRIME cavity 

and wake dimensions: 

W = H and L/H = 0 - 4

W = L and H/W = 1 - 3

stack
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Problems with BPIP

BPIP Building Dimensions:

H = 17 m

L/H = 53

H/W = 0.03

See Red Footprint

stack
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AERMOD/BPIP Problem fo

Short/Large Buildings

310.9302.4310.8315.2294.1

Stack Exit 

Temperature 

Ts (K)

85.70.67.65.15.

Flow R

(m^3

4 25.91 2.24 17.9 

1 23.16 1.07 17.8 

2 26.33 2.74 11.1 

17.811.4

Stack Exit 

Velocity 

V (m/s)

2.462.74

Stack Inside 

Diameter 

d (m)

5 21.34 

3 26.33 

Source 

Height 

Hs (m)

Source 

Number

Source Input Parameters:

Other Model Input:• Meteorology: one year of surface and upper air data from nearby Airport• Flat terrain • Uniform Cartesian grid with 3600 receptors• Regulatory default dispersion options
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AERMOD/BPIP Problem fo

Short/Large Buildings 

24 hr maximum ground-level concentrations 

BPIP 

Max = 173.7 ug/m^3 

EBD

Max = 43.7 ug/m^3 
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AERMOD Corner Vortex Issue

• Current building wake 

equations do not account for corner vortex

• Corner vortex causes 

higher concentrations than currently predicted in 

AERMOD.
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Corner Vortex Issue

Hb = 39 m 

(1:2:1)

Hb = 39 m 

(1:4:1) Hb 

AERMOD/

PRIME

Maximum Ground-Level Concentration

02468

10121416

0 100 200 300 400 500 600 700 800 900

Distance (m)

C/Q (ug/m^3 per g/s) 

Stack Height: 

Hs = 48 m

Stack Top Wind Speed: 

Us = 12.5 m/s

Stack Inside Diameter: 

d = 2.4 m
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Corner Vortex Issue

Hb = 39 m 

(1:2:1)

Hb = 39 m 

(1:4:1) Hb 

Maximum Ground-Level Concentration

0

102030405060

0 100 200 300 400 500 600 700

Distance (m)

C/Q (ug/m^3 per g/s) Hb

HbHbro

Stack Height: 

Hs = 48 m

Stack Top Wind Speed: 

Us = 12.5 m/s

Stack Inside Diameter: 

d = 2.4 m

Wind Tunnel 

Measurements
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Terrain Wake Effects Neglec

• The GEP stack height regulation defines 

nearby terrain for the purpose of limiting stack heights.

• Past EPA research shows that the effect 

of upwind terrain can be significant. Currently this effect is neglected.

• Recent study1) showed concentrations 

increased by a nearly a factor of two when terrain wake effect is accounted for using Equivalent Building Dimensions in 

AERMOD.

• A method should be developed to 

determine when upwind terrain wake effects should be considered. 

1) Petersen, R., J. Reifschneider, R. Paine, K Schmidt, “Use 

of Equivalent Building Dimensions (EBD) to Characterize Upwind Terrain Wake Effects for AERMOD,” 100th Annual A&WMA Conference, 

Pittsburgh, PA, June 2007.
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Terrain Wake Effects Neglec

Variation of terrain amplsources at or downwind reattachment point (CasLawson, (Atmospheric EVolume 22, 1988) 

Contours of constant terrain amplification factors over a two-dimensional ridge (Snyder Atmospheric Environment, Volume 24, 1990) 

EPA Research: Terrain Wake Amplification Factor, A, Ranges from 1.2 to 8.
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Recommendations

• Continue research on ways to impro

BPIP so input dimensions match assumptions in algorithms.

• If needed, update guidance on use o

in place of BPIP for AERMOD/PRIM

• Develop algorithms for the corner vo

situation.

• Develop method for accounting for u

terrain wake effects.