Washington State Regional Haze State Implementation Plan 

Appendix I 

Use of 0.5-km Grid Spacing to Evaluate the Impacts of Best 

Available Retrofit Technology-Eligible Units at Alcoa Wenatchee 

Works on the Alpine Lakes Wilderness

Final December 2010 I-1
Use of 0.5-km Grid Spacing to Evaluate the Impacts of Best Available Retrofit TechnologyEligible Units at Alcoa Wenatchee Works on the Alpine Lakes Wilderness 

As discussed in section 11.4.1, Alcoa Wenatchee Works proposed, and the Washington State 

Department of Ecology (Ecology) accepted, refinements to the Washington-Oregon-Idaho BART 

Modeling Protocol1

. Specifically, Ecology accepted an alternative meteorological data file, 

which used a finer grid size than the default grid size specified by the protocol, and the use of an 

alternate version of CALPUFF. The reasons for accepting these refinements are discussed 

below. 

 

Alcoa Wenatchee Works is an aluminum smelter located within the Columbia River Valley 

(Figure 1). About 20 kilometers (km) upstream from the smelter, the valley forks with one 

branch heading northwest toward the Alpine Lakes Wilderness while the other branch continues 

north along the Columbia River Valley. The topography allows southerly flow to be channeled 

from the smelter up the valley to the fork and then to the north and the northwest. 

 

Terrain in this region is complex. Elevations vary from 200 meters (m) elevation mean sea level 

(MSL) in the vicinity of the smelter to 2500 m elevation at some peaks within the Alpine Lakes 

Wilderness (see Figure 6). 

Alcoa Wenatchee Works initially followed the Washington-Oregon-Idaho BART Modeling 

Protocol and used 4-km meteorology to model the impacts from the aluminum smelter on nearby 

Class I Areas. However, close examination of the surface wind fields (for example, Figure 2) 

showed numerous locations where the modeled wind directions (indicated by the blue wind barb 

in the lower left corner of the 4-km grids) did not reflect the effects of the topography. Figure 3 

shows a portion of Figure 2 in greater detail. It can be seen that the 4-km wind field attempts to 

cut across the terrain and fails to model the down-slope, down-valley flow normally observed 

early in the morning. 

 

Alcoa Wenatchee Works believed that the apparent errors in the wind field were due to 

unresolved features of the complex terrain. Figure 4 shows terrain corresponding to a 4-km grid 

spacing. Alcoa Wenatchee Works proposed doing additional modeling at a higher grid resolution 

to improve the wind field for BART exemption modeling. 

 

Ecology, through its membership in the Northwest Regional Modeling Center, has over a decade 

of daily experience with mesoscale meteorological models with grid spacing ranging from 36-km 

down to 1.3- km and occasionally in special studies down to 0.44-km. Ecology staff have seen 

the improvement in model performance that comes with refined grids and understand the 

conditions that require the smallest grid spacing. 

 

Based on accumulated experience, Ecology was inclined to entertain a proposal to model with a 

finer grid. The characteristic scale of the terrain affecting transport of emissions from the Alcoa 

Wenatchee Works smelter to the Alpine Lakes Wilderness is on the same order as that 

encountered in the Columbia River Gorge. Additionally, like the Columbia River Gorge the 

 

1

 The Washington-Oregon-Idaho Modeling Protocol may be found in Appendix H. 

Final December 2010 I-2
maximum impact occurs during the winter. Modeling wintertime flow through the Columbia 

River Gorge required the smallest grid spacing of 0.44 km to correctly characterize the flow2

. 

After finding numerous references to analyses with CALPUFF using similar grid spacing in 

equally complex terrain between 1999 and 2007 and finding no mention of any lower limit to the 

acceptable meteorological grid spacing3

, Ecology concurred with Alcoa Wenatchee Works’ 

proposal and notified the affected Federal Land Managers (FLMs). The horizontal grid spacing 

of the fine grid CALMET/CALPUFF modeling domain was 0.5-km. The domain covered an 

area of 209-km by 143-km (Figure 4). The domain includes the Alcoa Wenatchee Works, the 

Alpine Lakes Wilderness, and at least a 50-km buffer zone in each direction from the aluminum 

smelter and this Class I Area. 

The revised BART exemption modeling followed the Washington-Oregon-Idaho BART 

modeling protocol except for a finer grid spacing of 0.5-km in the meteorological pre-processor 

and the use of CALPUFF version 5.8. EPA approved CALPUFF version 5.8 as a replacement 

for version 6.112 on June 28, 2007. The modeling used the same 3 years of 12-km mesoscale 

model wind fields as specified by the protocol. 

The two years, 2007 and 2008, saw numerous changes in CALPUFF as one version after another 

became accepted as a guideline model only to be replaced by another. At the time Ecology made 

our decision to accept the 0.5-km grid modeling there were two model versions used for 

regulatory analyses: Version 5.8 for permitting (the Guideline model) and Version 6.112, 

accepted for BART analyses. Each analysis approved by Ecology was conducted according to 

the guideline or protocol in effect at the time. 

Figures 2 and 3 compare the wind field computed at the 4 kilometer spacing with the wind field 

computed at the 0.5-km spacing. A measure of the frequency of misdiagnosed wind directions is 

depicted in Figure 2. The figure shows that ten of the 70 points on the 4-km grid shown in the 

figure had wind directions that failed to conform to the topography. These ten points are 

correctly modeled at the 0.5-km spacing. 

Figure 3 is an enlargement of the area of Figure 2 outlined by a black box. Figure 3 shows in 

greater detail that the 4-km spacing produces a defective wind field that ignores the influence of 

the terrain or produces convergence on the slope of the valleys rather than at the bottom. The 

former is illustrated by the 4-km grid indicated by a blue barb inside a black box on the right side 

of the figure. The latter can be seen in two 4-km grids in the center of the figure. The 0.5-km 

wind field seems to accurately reflect the influence of topographic features. 

A vertical cross section of the terrain elevation along the east-west line in Figure 5 is shown in 

Figure 6. The terrain elevation is shown for 4 different resolutions corresponding to grid 

spacings of 12, 4, 0.5, and 0.1-km. The 12-km grid spacing corresponds to that of the mesoscale 

 

2

 Sharp, J and C Mass, The Mesoscale Meteorology of the Columbia River Gorge, Ninth Conference on Mesoscale 

Processes, 2001. 

3 Chang, et al, 2003 used 0.25 km. State and Federal agencies and regional planning organizations have used the 

following grid spacing: Colorado - 0.5 km, VISTAS – 1 km, Georgia -1.5 km, EPA - 0.4 km, and California - 0.1 to 

0.5 km. 

Final December 2010 I-3
model used for the input meteorological wind fields, the 4-km terrain corresponds to the 

CALMET grid used in the initial runs, and the 0.5-km terrain was used in the final runs. Terrain 

at the high resolution 0.1 km grid spacing is provided for comparison. 

The cross section at 4-km (plotted in cyan) smoothes out the peaks and the valleys drastically. 

The highest elevation of 4-km terrain is only 1820 m, well below the maximum of 2340 m for 

0.5 km terrain (plotted in magenta). Many ridges and valleys are not resolved at 4-km 

resolution. The effects of smoothing are even more pronounced at the 12-km spacing (plotted in 

red). 

The similarity of the 0.5-km and 0.1-km profiles confirms that the 0.5-km spacing adequately 

represents the terrain for modeling purposes. 

Figure 3 clearly demonstrates that the 0.5-km resolved terrain provides a much better 

representation of terrain features and the wind field than does the 4-km resolution. Figures 2 and 

3 show that the finer resolution modeling provides a much more detailed spatial pattern in the 

winds which conforms more accurately to the terrain when compared with the wind fields from 

the lower resolution runs. It is clear from these plots that the lower resolution runs will miss 

much of the terrain channeling within the modeling domain. More importantly, the failure of the 

4-km grid to accurately represent peaks will allow air to flow unimpeded through regions that 

would otherwise deflect it. 

The Environmental Protection Agency (EPA) has commented on the use of finer grid spacing in 

BART exemption modeling for the Otter Tail Power generating facility in Big Stone, SD4

. The 

Clearinghouse memo set forth two components that an argument must address to run the 

CALPUFF meteorological pre-processor at a smaller grid spacing than the input meteorological 

wind fields: 

(1) The resolution of the mesoscale model is insufficient to correctly characterize the 

transport of emissions between their source and the receptors of concern. 

(2) The diagnostic wind model, CALMET, can enhance the mesoscale model data 

sufficiently to adequately replicate the key meteorological features governing the 

transport of emissions between their source and the receptors of concern. 

The first has been addressed above. The 4-km wind field, produced to meet the WashingtonOregon-Idaho Modeling Protocol, was strongly influenced by the input meteorological wind 

fields and has been shown to have a significant fraction of misaligned wind directions. The 

terrain analysis shown in Figure 6 satisfies the implied requirement of demonstrable terrain 

complexity stated in the Model Clearinghouse review of May 15, 2009. 

The second is more difficult to satisfy directly in this analysis. There are no observations at sites 

along the path between the Alcoa Wenatchee Works and the Alpine Lakes Wilderness. 

Additionally, there were errors in the observations at Pangborn field in Wenatchee. Therefore 

there is no direct way to assess the improvement in the wind field by using the finer grid. An 

 

4

 EPA, Model Clearinghouse Review of CALPUFF Modeling Protocol for BART, Memorandum from Tyler Fox, 

EPA-OAQPS, Research Triangle Park, NC, May 15, 2009. 

Final December 2010 I-4
indirect argument must be made by combining experience with close scrutiny of the wind fields 

computed at the 0.5-km grid spacing. Figure 3 seems to indicate that the wind field computed on 

the 0.5-km grid responds to the topography in a reasonable manner. 

Figure I-7 shows the dates and changes of 98th percentile impacts at 4-km and 0.5-km grid 

spacings. The highest deciview impact, without regard to location within the Alpine Lakes 

Wilderness Area, was used to characterize the impact on every day at each of the two grid 

spacings used in the modeling. The 22 highest deciview values at each grid spacing constitute 

the upper two percent of the calculated impacts. Each of these 44 values is paired with the 

highest deciview value at the other grid spacing on the same day. 

For example, taking the isolated red point in early 2003, the 0.5-km grid spacing run produced a 

slightly higher maximum impact to the Alpine Lakes Wilderness Area on one of the 22 days that 

make up the highest two percent of impacts than was computed using the 4-km spacing. It can 

also be seen that computed impacts using the 4-km spacing during the first part of 2003 were not 

in the highest two percent. 

Note that the highest impacts are in the winter regardless of the grid spacing used. It can also be 

seen that ten of the 22 highest impacts on the 0.5 km grid (shown by red dots above the black 

horizontal line indicating zero change) were between 0 and 1 deciview larger than the impacts on 

the same days calculated using the 4 km grid. Ecology interprets this as meaning that modeling 

at the smaller grid spacing has also redistributed the plume from the smelter rather than simply 

increasing its dispersion. Changes of less than 1 deciview are generally not noticeable. 

Figure 8 does show an important difference in the spatial location of impacts between the 4-km 

grid and the finer grid. Impacts occur at the eastern and southern boundaries of Alpine Lakes 

Wilderness during the winter for both the 4-km and 0.5-km grid spacings. Impacts occur at the 

western boundary, which is west of the Cascade Crest, only at the 4-km grid spacing. The spatial 

variation and differences in magnitude can be attributed to the different effect that the more 

highly resolved terrain has on the trajectories of emissions from the Alcoa Wenatchee Works 

aluminum smelter. Because the maximum impacts computed using the 0.5-km grid continue to 

occur during winter, we may conclude that the predicted maximum impacts will occur in similar 

meteorological conditions at both grid scales. 

In summary, Ecology’s decision to accept Alcoa Wenatchee Works’ use of 0.5-km grid resolution 

and CALPUFF 5.8 for BART exemption modeling of the Alcoa Wenatchee Works facility was 

based on an analysis showing that the finer grid resolution provided a more accurate result, given 

the complex terrain surrounding the Alcoa Wenatchee Works aluminum smelter. 

Final December 2010 I-5
Figure I-1 Terrain near the Alcoa Wenatchee Works. The green pushpin marks the location of the Alcoa Wenatch

smelter in the Columbia River Valley. The Columbia River flows from north to south along the east side of the Ca

Valley branches upstream from the smelter at Olds with one branch heading northwest to the Alpine Lakes Wildeyellow. The black rectangle outlines the area shown in Figure 2. North is up on all maps in this appendix. 

I-6
Figure I-2 Sample wind field (5 am on Jan 5, 2003. The ten small black squares show points where there is a sign

direction between the 4-km (blue barbs) and 0.5-km (red barbs) wind fields. The large black rectangle outlines thdetail in Figure 3. 

I-7
Figure I-3 Close-up of 0.5-km terrain with 4-km (blue barbs at lower left of each 4-km grid square) and 0.5- km (

Small black squares are drawn to emphasize where there is a significant difference in the wind fields. 

I-8
Figure I-4 Terrain at 4-km resolution with FLM-defined receptors in the Alpine Lakes Wilderness Class I Area. T

Wenatchee Works aluminum smelter is marked by the red dot. Note the relative softness of the terrain compared features that affect the transport of emissions into the Class I Area have been smoothed and are missing. 

I-9
Figure I-5 Terrain at 0.5-km resolution with FLM-defined receptors in Alpine Lakes Wilderness Class I Area. Th

Wenatchee Works aluminum smelter is marked by the red dot. Note the better definition of terrain feature comp

especially valleys and side channels, which are important in defining the transport of emissions into the Class I Ar

I-10
Figure I-6 Terrain profile at four different resolutions along the east-west line in Figure 5. Elevations are in mete

kilometers. The horizontal scale is approximately the same as Figure 5. 

I-11
Figure I-7 Dates and changes of 98th percentile impacts at 4-km and 0.5-km grid spacings. The highest deciview, 

within the Alpine Lakes Wilderness Area, was used to characterize the impact on every day at each of the two gri

modeling. Each of the 44 values constituting the upper two percent of each distribution is paired with the highest grid spacing on the same day. Ten of the 22 highest impacts on the 0.5 km grid (shown by red dots above the black zero change) were between 0 and 1 deciview larger than the impacts on the same days calculated using the 4 km gexplanation) 

I-12
Figure I-8 Locations of the impacts in the highest two percent. The open purple symbols show locations from the 

symbols are locations from the 0.5-km run. The green pushpin shows the location of the Alcoa Wenatchee Works 

Wilderness is outlined in yellow. Note that in addition to the impacted locations along the east and south wildern

spacings, the 4-km run has high impacts along the west wilderness boundary, which is west of the crest of the Cas

I-13