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UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

RESEARCH TRIANGLE PARK, NC 27711

 

September 26, 2008 

OFFICE OF 

AIR QUALITY PLANNING 

AND STANDARDS

MEMORANDUM

SUBJECT: Technical Issues Related to CALPUFF Near-field Applications 

FROM: Roger W. Brode, Physical Scientist 

 U. S. EPA, OAQPS, Air Quality Modeling Group (C439-01) 

 Bret Anderson, Environmental Scientist 

 U. S. EPA, Region 7 

TO: Tyler Fox, Leader 

 U. S. EPA, OAQPS, Air Quality Modeling Group (C439-01) 

 

The purpose of this memorandum is to provide a discussion of technical issues related to the use 

of the CALPUFF modeling system for near-field regulatory applications, under EPA’s Guideline 

on Air Quality Models, 40 CFR Part 51, Appendix W. The information provided in Attachment 

A is intended to supplement the recent Clarification Memo related to near-field regulatory 

applications of CALPUFF under Appendix W, dated August 13, 2008. As noted in the 

attachment, the discussion is not intended to be exhaustive, and may be updated as additional 

relevant information comes to light. 

Attachment 

cc: Regional Modeling Contacts 

Richard Wayland 

Bill Harnett 

Michael Ling 

Raj Rao 

Todd Hawes 

Tim Allen, FWS 

John Notar, NPS 

John Vimont, NPS 

Rick Graw, FS 

Dirk Herkhof, MMS 
ATTACHMENT A 

Technical Issues Related to Use of the 

CALPUFF Modeling System for Near-field Applications 

Roger W. Brode 

U. S. EPA, OAQPS 

Air Quality Modeling Group 

Bret Anderson 

U. S. EPA, Region 7 

September 26, 2008
ii

CONTENTS 

1. INTRODUCTION.................................................................................................................. 1

2. COMPLEX WIND DETERMINATION............................................................................. 2

3. APPROPRIATENESS OF THE PREFERRED MODEL ................................................. 4

4. ALTERNATIVE MODEL CRITERIA ............................................................................... 5

4.1. SCIENTIFIC PEER REVIEW................................................................................................... 5 

4.2. APPLICABILITY TO THE PROBLEM....................................................................................... 6 

4.3. AVAILABILITY OF NECESSARY DATA BASES...................................................................... 9 

4.4. APPROPRIATE PERFORMANCE EVALUATIONS ................................................................... 11 

4.5. MODELING PROTOCOL...................................................................................................... 12 

5. REFERENCES..................................................................................................................... 12
1

1. INTRODUCTION 

EPA has recently issued a memorandum providing clarification of the regulatory status of the 

CALPUFF modeling system for near-field applications,1

 with transport distance up to 50 

kilometers, based on guidance provided in EPA’s Guideline on Air Quality Models

(“Guideline”), published as Appendix W to 40 CFR Part 51.2

 This document discusses technical 

issues related to the use of the CALPUFF modeling system for near-field applications. The use 

of CALPUFF for near-field regulatory applications involving “complex winds” is addressed in 

Section 7.2.8 of the Guideline, which states that “the purpose of choosing a modeling system like 

CALPUFF is to fully treat the time and space variations of meteorology effects on transport and 

dispersion.” The basic requirements for justifying use of CALPUFF for near-field regulatory 

applications consist of three main components: 

1) a determination that treatment of complex winds is critical to estimating 

design concentrations; 

2) a determination that the preferred model (AERMOD) is not appropriate or less 

appropriate than CALPUFF; and 

3) a demonstration that the five criteria listed in paragraph 3.2.2(e) of the 

Guideline for use of CALPUFF as an alternative model are adequately 

addressed. 

Each of these steps involves case-specific considerations. The criteria listed in paragraph 

3.2.2(e) of the Guideline for use of an alternative model are as follows: 

“e. Finally, for condition (3) in paragraph (b) of this subsection [preferred model is 

less appropriate for the specific application, or there is no preferred model], an 

alternative refined model may be used provided that: 

i. The model has received a scientific peer review; 

ii. The model can be demonstrated to be applicable to the problem on a 

theoretical basis; 

iii. The data bases which are necessary to perform the analysis are available and 

adequate; 

iv. Appropriate performance evaluations of the model have shown that the model 

is not biased toward underestimates; and 

v. A protocol on methods and procedures to be followed has been established.” 

The discussion provided below is structured to address each of the three main components, and 

includes specific examples to illustrate some of the issues and concerns. This discussion is not 

intended to be exhaustive in relation to the range of issues and possible scenarios that may be 

encountered, since each application includes case-specific considerations, but to provide an 

indication of some of the issues that should be addressed in assessing the appropriateness of 

CALPUFF for use in near-field applications. Some of the information presented in this 

document is preliminary in nature, based upon current understanding of the CALPUFF modeling 

system from available documentation. 
2

2. COMPLEX WIND DETERMINATION 

Paragraph 7.2.8(a) of the Guideline provides the following examples of complex wind situations: 

“a. Inhomogeneous Local Winds. In many parts of the United States, the ground is 

neither flat nor is the ground cover (or land use) uniform. These geographical 

variations can generate local winds and circulations, and modify the prevailing 

ambient winds and circulations. Geographic effects are most apparent when the 

ambient winds are light or calm. In general these geographically induced wind 

circulation effects are named after the source location of the winds, e.g., lake and 

sea breezes, and mountain and valley winds. In very rugged hilly or mountainous 

terrain, along coastlines, or near large land use variations, the characterization of 

the winds is a balance of various forces, such that the assumptions of steady-state 

straight-line transport both in time and space are inappropriate.” 

An assessment of the potential influence of complex winds on design concentrations should be 

based on examining the source characteristics (release height and plume buoyancy) in relation to 

the local topography to determine whether the design concentrations would be adequately 

represented by a steady-state model. Any available information documenting typical flow 

patterns at plume height level(s) may also be used to inform that determination. However, use of 

CALMET-generated wind fields as “evidence” of the importance of complex winds involves 

circular reasoning, and is not sufficient justification. 

For most situations involving elevated plumes with relatively nearby terrain at or near plume 

height, the “line-of-sight” plume impaction scenario will likely drive the design concentrations, 

for which the AERMOD model is considered appropriate. Complex winds are also not likely to 

play a significant role for applications involving low-level plumes or plumes dominated by 

building downwash influences, where the design concentrations would likely occur in the 

vicinity of the source. Applications where the controlling design concentrations are likely to be 

strongly influenced by valley stagnation and/or recirculation under persistent light wind 

conditions, and where that likelihood can be documented and justified, may be appropriate for 

consideration as a CALPUFF near-field application based on the criterion of the appropriateness 

of the preferred model. However, in these cases a clarification is needed regarding the relative 

appropriateness for these applications of the preferred model at the time of CALPUFF’s 

promulgation, ISCST3, as compared to the current preferred model, AERMOD. Since 

AERMOD has been designed to handle lighter wind conditions than ISCST3 (less than 1 m/s), 

and includes a horizontal meander algorithm to account for increased lateral plume spread under 

such light wind conditions that includes upwind dispersion, it will generally be more appropriate 

for these conditions than ISCST3. 

For low- to mid-level releases, with plume heights below the height of adjacent terrain features, 

but elevated enough to be transported beyond the immediate vicinity of the source, concern for 

valley channeling of winds and their importance relative to estimating design concentrations may 

be a factor for consideration of CALPUFF for a near-field application. If valley channeling or 

other complex valley circulations dominate plume transport enough that the design concentration 

is likely to be controlled by phenomena other than line-of-sight plume impaction, then 
3

consideration of CALPUFF for near-field application may be appropriate. Dominant valley 

channeling may also result in significant persistence of wind directions leading to elevated 24-

hour average concentrations that could be underestimated by a steady-state plume model driven 

by single station meteorological inputs that do not reflect that persistence. However, some 

caution is needed regarding this line of reasoning. First, it is important to recognize that the 

appropriateness of AERMOD in this situation may depend upon whether meteorological data 

representative of plume transport are available. The lack of such representative meteorological 

data may be one of the justifications given for use of CALPUFF in these situations, based on the 

presumption that CALMET can simulate the important features of the wind field in the absence 

of representative data. However, justification of CALPUFF for this situation is dependent on the 

ability of CALMET to provide realistic non-steady-state meteorological fields, which may in 

turn also depend on the availability of representative meteorological measurements as inputs. 

These considerations highlight the importance of addressing item iii from paragraph 3.2.2(e) to 

ensure that “[T]he data bases which are necessary to perform the analysis are available and 

adequate.” Issues related to that item are discussed in more detail in Section 4.3. 

Another category of complex winds cited in paragraph 7.2.8(a) of the Guideline involves coastal 

influences, including lake or sea breezes. As with the complex terrain cases discussed above, the 

importance of complex wind influences for coastal applications may vary based on source 

characteristics and proximity to the coastline. The two major effects of coastal influences that 

are most relevant to this discussion are the land/sea-breeze circulation patterns, driven by 

differential heating of the land and water, and the localized effects of enhanced vertical mixing 

within the thermal internal boundary layer (TIBL), which forms during the daytime with onshore 

flow. The land/sea-breeze cycle typically includes a sea (or lake) breeze (onshore flow at the 

surface) during the daytime, as the land area responds faster to solar heating, followed by a 

weaker land breeze (offshore flow) at night. The TIBL is characterized by a convective 

boundary that grows with distance inland from the coast, with the layer above the TIBL 

reflecting the stably stratified air of the marine boundary layer. The TIBL can result in more 

limited vertical mixing for plumes released below the TIBL than would occur without the coastal 

influence, or in fumigation (rapid downward dispersion) for elevated plumes released into the 

stable layer above the TIBL. The potential importance of non-steady-state coastal influences on 

design concentrations is probably greater for elevated releases near the coastline than for lowlevel releases. The magnitude of the impact of non-steady-state effects will generally decrease 

with distance from the coastline. 

The previous paragraphs focused on determining the importance of complex winds on design 

concentrations for different types of sources in various settings. However, many applications 

will involve a range of source types at different locations within the modeling domain, raising 

additional considerations. In these cases, the determination should take into account the relative 

importance of each source to the overall design concentration, based on emissions and other 

source characteristics. If a single source or single type of source will clearly dominate the design 

value, then the determination may be based primarily on an assessment of that source type. An 

additional consideration that may need to be addressed in cases involving multiple sources is 

whether plumes from different sources may effectively merge as a result of complex winds, 

resulting in higher impacts than would occur based on a steady-state modeling assessment. An 

example of this case would be multiple low-level releases within a complex of valleys or at 
4

different locations within the same valley that would likely merge in the bottom of the valley due 

to drainage flows under light wind, stable conditions. 

Another situation where application of CALPUFF for the near field may be considered is a case 

where concentration estimates are needed in a Class I area that is located within the near-field 

domain, perhaps 10 kilometers from the source, and where the fetch from the source to the Class 

I area is characterized by a winding valley. While the source-specific considerations discussed 

above may still apply in this situation, additional factors may need to be considered. If the 

plume trajectory is expected to be channeled by the winding valley, one might conclude that 

getting the trajectory correct is the most important consideration in determining the Class I 

design concentration. However, such a finding might not be justified, for the same source, if it 

were a Class II application in which it was clear that the design concentration would occur on the 

side of the closest hill to the source (i.e., a line-of-sight source receptor relationship). This 

situation also highlights the issue of whether the non-steady-state capabilities of the CALPUFF 

modeling system can more appropriately address the temporal and spatial pairing of predicted vs. 

observed concentrations called for in such an application. Given the nature of the complex wind 

phenomena that might justify use of CALPUFF for near-field applications, the issue of temporalspatial pairing of impacts is likely to be a consideration to some degree for all near-field 

applications of CALPUFF. This issue is discussed in more detail in the following sections. 

It is important to recognize that while CALMET can generate spatially varying threedimensional wind fields, this does not guarantee that the wind fields generated by CALMET will 

provide a more appropriate treatment of plume transport and dispersion, or result in an improved 

estimate of design concentrations compared to AERMOD. Furthermore, the mere presence of 

“complex winds” within a domain is not sufficient justification for use of CALPUFF for nearfield applications. In a very real sense, every modeling application involves complex winds to 

some degree since the atmosphere is inherently inhomogeneous. The burden is in showing 

clearly that accounting for some aspect(s) of the complex winds is critical to an adequate 

determination of design concentrations for the source(s) of concern, and then demonstrating that 

CALPUFF is more appropriate than the preferred model and is capable of simulating those 

important aspects with an acceptable degree of confidence given the data available for the 

application. 

3. APPROPRIATENESS OF THE PREFERRED MODEL 

Once a credible determination has been made that treatment of complex winds is critical to 

estimating design concentrations, a separate determination should be made that the preferred 

model (AERMOD) is not appropriate for the application, or that CALPUFF is clearly more 

appropriate than the preferred model, based on condition (3) in paragraph 3.2.2(b) of the 

Guideline that “the preferred model is less appropriate for the specific application, or there is no 

preferred model”2

. As noted in the EPA clarification memo1, the promulgation of AERMOD as 

the preferred model for regulatory modeling in all terrain settings affects the applicability of this 

criterion for justifying the use of CALPUFF for such applications, due to the fact that AERMOD 

is considered to be appropriate for a wider range of applications involving terrain effects than 

was the case for ISCST3, the preferred model at the time of CALPUFF’s promulgation. 

AERMOD’s performance for near-field regulatory modeling applications in simple and complex 
5

terrain, with and without downwash, has been well-documented based on a total of 17 field study 

evaluation databases,3,4 including several field studies in complex terrain settings. In contrast, 

there has been no comprehensive demonstration made that the CALPUFF modeling system, 

including CALMET-generated wind fields, performs as well or better than AERMOD for nearfield regulatory applications in complex wind situations based on field study data. More 

information related to this point is provided in Section 4.2. 

The case cited in Section 2 regarding a Class I area located in the near-field highlights the issue 

of whether the non-steady-state capabilities of the CALPUFF modeling system can more 

appropriately address the temporal and spatial pairing of predicted vs. observed concentrations 

called for in such an application. Unfortunately, this is a very difficult question to answer due to 

limitations of adequate field-study data bases, and due to the difficulty in generalizing model 

performance based on existing studies given the highly complex and site-specific nature of the 

problem. Further complicating the determination of appropriateness of CALPUFF for near-field 

applications is the fact that the limited evaluation studies documented thus far have not evaluated 

the skill of the modeling system to accurately simulate plume impacts under non-steady-state 

meteorological conditions paired in time and space over the domain. The question of appropriate 

model performance methods and metrics to support the determination of appropriateness of 

CALPUFF for near-field applications is addressed further in a separate document.5

While the need for temporally and spatially more realistic concentration fields provided by a 

non-steady-state model may arise in regulatory applications as described above, this issue may 

also be brought up in non-regulatory applications as a possible justification for use of CALPUFF 

in near-field applications. An example of the latter would be for risk assessment applications 

where the full spatial field of impacts may contribute to the determination of total risk or 

exposure, requiring additional skill from the dispersion model beyond that required for typical 

near-field regulatory applications, where peak concentrations unpaired in time and space are the 

primary metric for model performance. Many of the same concerns expressed regarding 

regulatory applications of CALPUFF in near-field settings would apply in these non-regulatory 

cases, but the added significance of temporal and spatial pairing of concentration fields with 

population distributions to determine exposures further increases the demand on model skill 

beyond what has thus far been demonstrated. 

4. ALTERNATIVE MODEL CRITERIA 

This section provides more details regarding the technical considerations involved in assessing 

the appropriateness of CALPUFF relative to the preferred model for potential near-field 

applications, based on the criteria for use of an alternative model listed in paragraph 3.2.2(e) of 

the Guideline. 

4.1. Scientific Peer Review

The CALPUFF modeling system was subjected to a scientific peer review6

 to support the process 

of promulgating CALPUFF as a preferred model in the Guideline. While the primary regulatory 

niche for CALPUFF is for long range transport (LRT) applications, with transport distances 

beyond 50 kilometers, the scope of the scientific peer review also included the potential 
6

application of the model for near-field dispersion (≤ 50 kilometers). The assessment of 

CALPUFF’s appropriateness for near-field applications in the peer review comments is very 

general and limited. Only one reviewer explicitly addressed the Peer Review Charge question 

related to the adequacy of model performance evaluations and sensitivity studies to “recommend 

use of the model.” That reviewer’s comments were that it is “a very difficult set of questions to 

answer,” but that the answer is “probably yes, because CALPUFF incorporates a basic formalism 

that is well understood and numerous algorithms, each of which has been reasonably well 

characterized individually.” This reviewer also supports the response by noting that “the 

mesoscale and DWM [diagnostic wind model] modeling approaches used in CALMET have 

undergone a history of more than 20 years of test and evaluation in the meteorological and wind 

power communities.” However, no specific examples of CALMET evaluation are cited in this 

peer review. 

Given the reference in the peer review comments to tests and evaluations of diagnostic wind 

models for wind power applications, it is important to note that the requirements of wind field 

modeling for estimating wind power potential are very different from the requirements for nearfield air quality impact assessments. Wind field modeling for wind power is typically designed 

to identify areas of high wind power potential and to provide a quantitative estimate of that 

potential for planning purposes. However, actual siting and installation of wind turbines would 

typically be further supported by more detailed site assessments. On the other hand, wind field 

modeling for near-field air quality assessments may determine whether or not an emission source 

will be constructed and permitted to operate at a given site, without any additional means of 

assessing potential impacts prior to operation. 

It should also be noted that while model evaluations for wind power applications may be relevant 

to some near-field applications of CALMET, for the most part the meteorological conditions 

associated with high wind energy potential, i.e., high-wind/neutral conditions, are less 

technically challenging to simulate with acceptable accuracy than the meteorological conditions 

of most concern for air quality applications, i.e., light wind, stable conditions. These highwind/neutral conditions will also be less subject to significant spatial variability in the wind field, 

thus making it more likely the peak concentrations will be through line-of-sight plume impaction 

on nearby terrain. As noted in paragraph 7.2.8(a) of the Guideline regarding the complex winds 

of interest for CALPUFF near-field applications, “geographic effects are most apparent when the 

ambient winds are light or calm.” 

The conclusion from this assessment is that the EPA-sponsored scientific peer review of the 

CALPUFF modeling system for near-field applications has been very limited in scope. 

4.2. Applicability to the Problem

Since the stated goal of using a non-steady-state modeling system for a near-field complex wind 

situation is “to fully treat the time and space variations of meteorology effects on transport and 

dispersion,” a significant part of the focus for addressing the applicability of the CALPUFF 

modeling system will be on the ability of CALMET to adequately simulate the non-steady-state 

meteorology. Given the very case-specific nature of near-field complex wind modeling 

applications, the criterion of applicability to the problem should be determined based on some of 
7

the case-specific considerations discussed in previous sections. The applicability determination 

should also be supported by relevant model performance demonstrations. As noted above, 

AERMOD’s performance for near-field regulatory modeling applications has been welldocumented based on a total of 17 field study evaluation databases3,4, whereas there has been no 

comprehensive demonstration made that the CALPUFF modeling system performs as well or 

better than AERMOD for near-field regulatory applications in complex terrain based on field 

study data. 

The one evaluation study often cited to support the use of CALPUFF for near-field applications 

is the Lovett power plant complex terrain field study7

. While CALPUFF shows good 

performance for the Lovett evaluation, as documented in the IWAQM Phase 2 Report8, the 

AERMOD model exhibits comparable performance results for that data set3

. However, the 

published CALPUFF performance evaluation results for Lovett are not well-suited as a 

demonstration of CALPUFF modeling system performance for near-field complex flow 

applications for two important reasons. First, the Lovett field study consists of an elevated stack 

located in the Hudson River valley, with SO2 monitors located along the adjacent ridges. This 

situation would not qualify as a complex flow application for CALPUFF since the effects of 

complex winds as defined in paragraph 7.2.8 of the Guideline are not expected to contribute 

significantly to the design concentration, which will clearly be dominated by the elevated plume 

impacting the adjacent terrain through a “line-of-sight” trajectory. Secondly, the published 

CALPUFF evaluation results for Lovett are based on use of the CTDM surface and profile 

meteorological inputs and use of the Complex Terrain algorithm for Sub-Grid-scale features 

(CTSG) option in CALPUFF, options that essentially emulate the CTDMPLUS model and 

bypass the CALMET meteorological processor completely9. Therefore, the published Lovett 

evaluation results provide no information on the performance of CALMET in simulating nonsteady-state winds in this near-field setting. 

The diagnostic wind field model in CALMET has some limitations that are important to 

recognize and understand in relation to the question of applicability for near-field applications. 

Some of these limitations are generic to the use of any gridded meteorological model, while 

other limitations stem from specific formulations within CALMET. A generic limitation of 

gridded models is that their ability to simulate terrain responding wind fields may be severely 

limited by the horizontal resolution of the input terrain and land use data as represented within 

the model grid. For example, a river valley that is about 1 kilometer wide from peak to peak and 

about 500 meters deep would not be adequately resolved by a 250 meter grid spacing, which has 

been a typical minimum grid resolution for near-field modeling. A single grid cell could span 

the entire valley wall from ridge top to river level, such that the slopes of the valley walls 

represented by gridded terrain elevations could be reduced for 50 percent or more, significantly 

affecting the gravity driven slope flows and other diagnostic wind field adjustments in 

CALMET. Vertical grid resolution will also be a significant consideration for near-field 

applications, especially in valley locations given the complex flow structures and significant 

vertical gradients that may occur in such situations. 

Limitations that are inherent to CALMET formulations are largely due to its inability to ensure 

dynamical consistency in the simulated wind field. An example of the potential importance of 

this limitation is given by the phenomenon of drainage flows that often occur in valley situations 
8

under light-wind stable conditions. The three-dimensional structure of gravity-driven wind fields 

within a valley can be very complex, including significant discontinuities in wind direction with 

height. These wind fields are often associated with complex thermal structures within the valley 

that develop as cold air drains down from the ridge tops and accumulates within the valley. A 

transition from down-slope to down-valley flows will typically develop over time and with 

distance from the ridge, creating significant lateral and vertical gradients of wind and 

temperature. While limitations due to grid resolution may be important in these cases, a more 

fundamental limitation is CALMET’s inability to simulate the thermal structures within the 

valley that are associated with these complex flows. The three-dimensional temperature fields 

computed within CALMET are based on either available upper air soundings and surface 

measurements or gridded prognostic model inputs, depending on user-specified options. The 

three-dimensional temperature fields are not adjusted to reflect the influence of these drainage 

flows. Furthermore, the terrain blocking effects in CALMET are determined based on a single 

domain-wide average lapse rate, typically computed across a layer from the surface up to 200 

meters. Unless gridded meteorological inputs of sufficient resolution to capture these thermal 

structures within the valley are input to CALMET, they will not be reflected in the gridded 

CALMET outputs for use by CALPUFF. A potential consequence of this limitation is that the 

lapse rate used to compute plume rise in CALPUFF would not reflect the stable stratification 

generated by drainage flows, which could lead to an overestimation of plume height for buoyant 

releases and possible underestimation of ground-level concentrations. Even if the simulated 

wind fields within the valley are realistic, placement of the plume within the wrong grid layer 

due to these limitations in characterizing the thermal structure could result in significant errors in 

plume trajectory leading to impact estimates that reflect spatially-varying wind fields, but bear 

little or no resemblance to reality. 

Finer grid resolutions may improve the capability of the model to simulate these complex flow 

structures to some degree, and may now be more feasible with the availability of finer resolution 

land cover and terrain data. However, CALMET currently requires that the first (lowest) grid 

level be 20 meters deep, and grid resolution alone cannot overcome other limitations of the 

model formulation. The computational burden will also increase significantly with finer grid 

resolutions unless the overall domain size is decreased, which could limit the applicability of the 

results by excluding important synoptic or mesoscale features that influence the complex winds. 

The sensitivity of model results to grid resolution needs to be investigated in order to assess the 

robustness of the model. Recent studies have shown significant sensitivity to grid resolution, 

with some evidence of a possible bias toward lower concentrations as grid resolution increases. 

These sensitivities to grid resolution are still being examined to determine the key contributing 

factor(s), and whether the results for finer grid resolution reflect improved model performance or 

are indicative of a potential bias toward underprediction. 

While CALMET incorporates terrain-blocking and slope-flow algorithms that may account for 

some of the complex flows that occur in complex terrain settings, cross-valley circulations are 

also common occurrences in some valleys, driven by differential heating that occurs during the 

daytime as the sun heats one side of the valley wall while the other side is shaded. These 

circulation patterns will vary depending upon the orientation of the valley and solar elevation 

angle (based on time of day and season), and may significantly affect plume transport and 

dispersion depending on the location of the source relative to the valley orientation. CALMET 
9

currently does not account for these circulation patterns in the slope flow algorithms since there 

is no mechanism to account for the differential heating that drives the circulation. As a result, if 

these cross-valley circulations are important to the design value determination, then the 

applicability of CALPUFF would be limited. 

As noted above, since “the purpose of choosing a modeling system like CALPUFF is to fully 

treat the time and space variations of meteorology effects on transport and dispersion,” the 

applicability of CALPUFF for near-field situations may depend on the model’s ability to 

estimate air quality impacts with skill in terms of the actual temporal and spatial distribution of 

impacts. If the modeling system lacks demonstrable skill in terms of temporal/spatial pairing of 

impacts, or at least demonstrably better skill than the preferred model, then the argument for 

applicability to the problem is seriously undermined. The lack of a detailed independent 

assessment of the applicability of the CALPUFF modeling system, and in particular the 

CALMET meteorological processor, for near-field applications involving complex winds raises 

serious doubts as to whether the second criterion of paragraph 3.2.2(e) has been adequately met 

in a general sense. As a result, the burden to demonstrate applicability for specific applications 

remains relatively high. 

4.3. Availability of Necessary Data Bases

The appropriateness of a particular model for a given application may depend in part on the 

availability of the necessary data bases to support its use. For near-field applications of 

CALPUFF, the necessary data bases include meteorological data (both surface and aloft), terrain 

elevation data, and land use/land cover data. The quality and representativeness of available 

meteorological data will often be a critical, but difficult issue to address for these applications. 

Due to the very nature of complex wind applications, involving spatially non-uniform wind 

fields, the representativeness of meteorological measurements at particular locations within the 

domain relative to the dominant flow structures across the domain will be difficult to determine. 

The ability of the wind field model to properly account for these influences in its use of such data 

deserves further consideration. 

The assessment of available data bases is further complicated by limitations of CALMET with 

respect to its ability to utilize site-specific meteorological measurements in generating the threedimensional wind fields and thermal structures. The most direct approach for inputting sitespecific meteorological measurements to CALMET is as surface observations. However, all 

surface winds, including National Weather Service (NWS) and site-specific data, are adjusted 

from anemometer height to the midpoint of the first CALMET level, which is hard-wired to 10 

meters above ground. The default value specified for the controlling parameter in CALMET 

(IEXTRP = -4) is to extrapolate winds from anemometer height to 10 meters based on similarity 

theory profiling, including wind speed and direction adjustments. Even the CALMET option 

which is documented as the “no extrapolation” option (IEXTRP = 1) still extrapolates all surface 

wind speeds to 10 meters based on a neutral log profile. Meteorological measurements from 

multi-level towers, which may provide valuable information regarding vertical profiles of wind 

and temperature, can be input to CALMET as separate “surface stations” for each tower level, 

with different anemometer heights to reflect the measurement heights. However, for these cases 

CALMET will extrapolate winds for each tower level to 10 meters, and these collocated wind 
10

measurements will be represented by a single wind “observation” at 10 meters based on the 

average of the u- and v-components of the wind across the levels. This effectively destroys any 

site-specific information on the vertical wind profiles, which could compromise this aspect of the 

applicability determination. 

The treatment of multiple levels of site-specific temperature data may be as important as the 

treatment of site-specific wind data in some cases. Multiple level temperature measurements 

could be used to determine a site-specific lapse rate to more accurately account for terrainblocking effects and to calculate plume rise for buoyant releases. Similar to wind profiles, 

CALMET treats the multi-level temperature measurements as a single surface temperature based 

on an average across the levels. As with multi-level wind measurements, this not only loses any 

information on the vertical temperature structure reflected in the measurements, but replaces it 

with an inaccurate pseudo-observation. 

Another option that may be considered for some near-field applications of CALPUFF is the use 

of gridded meteorological inputs from a prognostic meteorological model, such as MM5. 

Gridded prognostic meteorological data has been widely used for LRT applications of 

CALPUFF, and several options are available for utilizing such data within the CALMET 

meteorological processor. While prognostic models have been routinely applied for several 

years to simulating non-steady-state wind fields at meso- to synoptic scales (with grid resolutions 

of about 4 kilometers or greater), many complex wind phenomena that might prompt the need for 

a non-steady-state dispersion model will require treatment of smaller scales of motion. 

Advances in computing capabilities have allowed for finer-scale applications of these models in 

recent years. However, the issues of grid resolution discussed in Section 4.2 in relation to 

CALMET would also apply for prognostics models. Until such time as prognostic models have 

been demonstrated to be capable of simulating the necessary non-steady-state features of the 

wind field adequately for the CALPUFF model, effectively bypassing the need for a diagnostic 

model like CALMET, the user will be faced with the challenges associated with blending 

prognostic meteorological fields with observations. 

The blending of prognostic meteorological data with observations is a generic issue related to the 

use of CALPUFF for both near-field and LRT applications, and some problems have been 

encountered with this aspect of the model. CALMET includes a number of options for 

controlling how observations are blended with prognostic model inputs, or with the initial guess 

wind field generated from upper air data in the absence of prognostic data. In general, CALMET 

applies an inverse-distance squared approach for the initial adjustment of gridded met winds to 

observations, and one of the key user-specified parameters is the radius of influence. While 

CALMET currently applies a single user-specified radius of influence for all surface 

observations, other options are available, such as barriers, to isolate the potential impact of some 

observations on certain portions of the domain. This technique may be necessary, for example, 

to restrict an observation taken in a river valley from influencing the wind field on the other side 

of the ridge. 

These adjustments to the wind field to blend with observations lack any physical mechanisms 

that would ensure dynamical consistency of the blended wind fields. This can result in very 

unrealistic flow patterns within portions of the modeling domain if the observation differs from 
11

the initial guess wind field provided by the prognostic model. The blended wind fields are then 

smoothed and further adjusted to minimize divergence in most cases. While these latter steps 

may be reasonable for larger scale domains typical of LRT applications, their appropriateness for 

adjusting wind fields in near-field settings may be questionable. As noted above, the wind and 

temperature fields of importance to near-field complex wind applications may be characterized 

by sharp gradients both vertically and horizontally, and some of the important terrain-responding 

flows may also be inherently divergent. Applying simple techniques, such as inverse-distance 

weighting, smoothing, and divergence minimization may introduce unrealistic features to the 

wind field in near-field applications. 

The other alternative to the treatment of inputting multi-level measurements to CALMET as 

separate surface observations is to construct a pseudo-upper-air sounding from the available 

measurements. However, this is not a very practical alternative and may require manufacturing 

data to extend the profile in some cases. This approach could also result in and may result in the 

site-specific profiles of wind and temperature being applied across portions of the domain for 

which they are not representative. The only option to directly utilize site-specific information on 

vertical wind and temperature profiles from multi-level towers in the CALPUFF modeling 

system is to bypass the CALMET processor and input the data directly to CALPUFF as CTDM 

or AERMET surface and profile files, as was done in the CALPUFF evaluation for Lovett. As 

noted above, the latter approach is not consistent with the intent of the Guideline for near-field 

applications of CALPUFF, which is to “fully treat the time and space variations of meteorology 

effects on transport and dispersion.” 

4.4. Appropriate Performance Evaluations

One of the requirements for the use of an alternative model stated in paragraph 3.2.2(e)(iv) of the 

Guideline is that “appropriate performance evaluations of the model have shown that the model 

is not biased toward underestimates.” This is a somewhat less stringent requirement than that 

imposed for a preferred model, which is to demonstrate generally unbiased model performance 

across a range of evaluation studies. Previous sections have addressed some basic issues related 

to the lack of adequate model performance evaluations to support the use of CALPUFF for nearfield applications, with the Lovett power plant evaluation being the case cited most often for 

near-field performance. Beyond the limitation noted above that the Lovett evaluation for 

CALPUFF did not utilize the CALMET-generated wind fields, the other issue related to 

performance evaluations that should be emphasized is that past model evaluation methods and 

metrics employed for regulatory model evaluations10, which place little or no emphasis on 

temporal or spatial pairing of modeled and observed concentrations, do not adequately address 

the skill implied in the use of CALPUFF for most near-field applications. 

The lack of appropriate performance evaluations to address this requirement for near-field 

applications of CALPUFF, together with a range of technical issues regarding the applicability of 

model algorithms and availability of adequate data bases, raises serious questions regarding 

whether the model can be applied with confidence that model results are not biased toward 

underestimates. The complexity of the model formulations and the range of options available for 

data input, grid resolution, wind field adjustments, etc., suggests a potentially wide range of 
12

sensitivity of modeled concentrations. These sensitivities need to be more fully documented and 

understood in order to build more confidence in whether and how this criterion can be met. 

Preliminary results from our reassessment of the CALPUFF modeling system performance5

, 

including evaluations of CALPUFF for the Lovett database utilizing CALMET-generated wind 

fields, have documented the sensitivity of the model to some of the technical issues discussed 

above, such as grid resolution and treatment of site-specific meteorological inputs. The 

reassessment of CALPUFF model performance has also raised additional concerns regarding the 

theoretical basis for the applicability of CALPUFF to near-field complex wind situations, which 

are still being analyzed, and will be further documented as appropriate. 

4.5. Modeling Protocol

A modeling protocol establishing the methods and procedures to be followed is one of the 

criteria identified in paragraph 3.2.2(e) of the Guideline for use of CALPUFF as an alternative 

model for near-field applications. Given the complex technical issues and concerns discussed in 

previous sections in relation to use of CALPUFF for these applications, the importance of the 

modeling protocol cannot be overstated. The protocol should address each of the criteria 

discussed above, starting with the determination that treatment of complex winds is critical to 

estimating design concentrations, and providing justification for the determination that 

AERMOD is not appropriate or less appropriate than CALPUFF for that application. 

The modeling protocol should provide an adequate demonstration that CALPUFF is applicable 

on a theoretical basis given the specifics of the particular application. The adequacy of the 

available data bases needed to apply CALPUFF, including the capability of the CALPUFF 

modeling system to effectively utilize the available data, should also be addressed. In addition to 

addressing the criteria in paragraph 3.2.2(e) of the Guideline, the modeling protocol should 

provide detailed information regarding the data sources to be used as input to the model, grid 

resolutions, model option settings, and how the resulting wind fields will be assessed to 

determine their adequacy for the particular application. 

5. REFERENCES

1. EPA, 2008. Clarification of Regulatory Status of CALPUFF for Near-field Applications, 

memorandum from Richard A. Wayland to Regional Air Division Directors, dated 

August 13, 2008. http://www.epa.gov/scram001/clarification of regulatory status of 

calpuff.pdf

2. EPA, 2005. Guideline on Air Quality Models, 40 CFR Part 51, Appendix W. Published 

in the Federal Register, Vol. 70, No. 216, November 9, 2005. 

http://www.epa.gov/scram001/guidance/guide/appw_05.pdf

3. Perry, S. G., A. J. Cimorelli, R. J. Paine, R. W. Brode, J. C. Weil, A. Venkatram, R. B. 

Wilson, R. F. Lee, and W. D. Peters, 2005. AERMOD: A Dispersion Model for Industrial 

Source Applications. Part II: Model Performance against 17 Field Study Databases. J. 

Appl. Meteor., 44, pp. 694-708. 
13

4. EPA, 2003. AERMOD: Latest Features and Evaluation Results. EPA-454/R-03-003. 

U.S. Environmental Protection Agency, Research Triangle Park, NC. 

http://www.epa.gov/scram001/7thconf/aermod/aermod_mep.pdf

5. EPA, 2008. Reassessment of CALPUFF Model Performance (draft report). 

6. Allwine, J.K., W.F. Dabberdt, and L.L. Simmons, 1998. Peer Review of the 

CALMET/CALPUFF Modeling System, 

http://www.epa.gov/scram001/7thconf/calpuff/calpeer.pdf

7. Paumier, J. O., S. G. Perry, and D. J. Burns, 1992. CTDMPLUS: A dispersion model for 

sources near complex topography. Part II: Performance characteristics. J. Appl. Meteor., 

31, pp. 646–660. 

8. EPA, 1998. Interagency Workgroup on Air Quality Modeling (IWAQM) Phase 2 

Summary Report and Recommendations for Modeling Long Range Transport Impacts, 

EPA-454/R-98-019. U.S. Environmental Protection Agency, Research Triangle Park, 

NC. www.epa.gov/scram001/7thconf/calpuff/phase2.pdf

9. Strimaitis, D.G., J.S. Scire and J.C. Chang, 1998. Evaluation of the CALPUFF 

Dispersion Model with Two Power Plant Data Sets. Tenth Joint Conference on the 

Application of Air Pollution Meteorology, Phoenix, Arizona. American Meteorological 

Society, Boston, MA. January 11–16, 1998. 

10. EPA, 1992. Protocol for Determining the Best Performing Model. EPA-454/R-92-025. 

U.S. Environmental Protection Agency, Research Triangle Park, NC. 

http://www.epa.gov/scram001/guidance/guide/modleval.zip