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The effect of the environment on aviation weather safety: Meteorological assessment for the new Denver airport
ENVIRON IMPACT ASSESS REV 1993; 13:63-74
MANAGING
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THE EFFECT OF THE ENVIRONMENT ON AVIATION WEATHER SAFETY: M E T E O R O L O G I C A L ASSESSMENT FOR THE NEW DENVER AIRPORT
S t e v e n L. R h o d e s National Center ]br Atmospheric Research
The new Denver International Airport currently under construction was the subject o f rigorous environmental impact assessment by both the project sponsor and the Federal Aviation Administration (FAA ). Airport design is closely linked to regional (mesoscale) meteorolory, and the Denver region experiences a wide range of extreme weather conditions as a consequence o f proximity to the Rocky Mountains as well as other topographic influences. Therefore, a comprehensive meteorological assessment was conducted to ensure that the new airport would be compatible with the meteorological regime of eastern Colorado. The meteorological assessment process not only assisted in the completion of the EA and EIS for the project, but also illuminated the new airport's capacity to operate efficiently despite the Denver region's variable weather. In addition, the assessment demonstrated the potential value of integrating climate and meteorological information into the development and rebuilding of the nation's infrastructure.
Introduction The newest commercial air carrier airport in the United States is presently under construction near Denver, Colorado (Brown 1991). Denver International Airport (DIA) is scheduled to open in late 1993 and will replace Stapleton International Address requestsfor reprints to: Steven L. RhCxtes. Environmental and Societal Impacts Group. National Center for Atmospheric Research. P.O. Box 3000, Boulder, CO 80307-3000. The National Center h~r Atmc,spheric Research is sponsored by the National Science Foundation.
© 1993 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010
0195-9255/93/$6.00
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STEVEN L. RHODES
Airport, which first opened to aviation activity in 1929. DIA is the product of several years of intense negotiations and cooperation between the City and County of Denver and neighboring Adams County. DIA is presently being built on land annexed to Denver in 1988 following a contentious de-annexation election in Adams County. Figure t illustrates the location of DIA and its environs. The new Denver airport project was subjected to intense environmental impact assessment, which is to be expected with a project of this scope. Denver airport planners worked closely with federal, state, and local agencies to ensure that the environmental assessment (EA)/environmental impact statement (EIS) process was expeditious and thorough. However, in order for the environmental impact assessment to proceed, it was necessary to wait until the location, size, and layout of the new airport and associated access routes were finalized. Yet the size and airfield layout could not be decided without knowledge of the meteorology of the site selected for the new airport, as weather and aviation safety are inextricably related. In eastern Colorado an attribute of the environment itself--topography-is known to significantly influence the weather, even over relatively short distances. Therefore, of critical importance to the new Denver airport environmental impact analysis was an assessment of the likely site's meteorological characteristics. The origins, structure, and findings of this weather impact assessment are described below.
Mesoscale Weather and Airport Capacity Mesoscale meteorology I was an important influence in the early days of the new Denver airport planning process. An important reason for pursuing a new air carrier airport for the Denver metropolitan region was the history of bad weather and its adverse effects on aviation operations at Stapleton International Airport, which has a poor reputation for weather-related flight delays and cancellations nationwide (Coates 1988). The Denver region and Colorado's eastern plains experience snow, high winds, blizzards, and very low temperatures during the spring and winter months, and thunderstorms, hail, and tornadoes during the spring and summer. Stapleton's four main runways are aligned in two parallel pairs, one aligned north-south and the other east-west (see Figure 1). However, the parallel separation of these runways is inadequate for either pair to be used for simultaneous aircraft arrivals under circumstances where pilots must operate on aircraft instrumentation to land. 2 In situations where aircraft pilots are instructed to operate under "instrument meteorological conditions" (IMC) at Stapleton air traffic controllers must close runways because of their insufficient parallel I Mesoscale meteorology refers to conditions that (x:cur on a scale from 4 km x 4 km to 400 km × 400 km (Eujila 1985 ). 2 FAA regulations require a minimum separation o f - I , 3 1 0 m (4,300 It) center line to center line fi)r parallel runways to be used for simultaneous arrivals in IMC. The separation of Stapleton's parallel north-south runways is only -488 m (1.600 fl), and for the parallel east-west runways only -275 m (900 It). FAA's required separation for a third simultaneous arrival stream is -4830 m (3 mi) from the closest parallel runway.
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separation. As a result, Denver's variable and diverse weather often disrupts the nation's air transportation system (City and County of Denver 1988). While severe weather disrupts operations at other airports throughout the United States and elsewhere, Denver's situation is directly tied to inadequate runway capacity. Additional runways cannot be added to Stapleton's airfield because it is constrained by surrounding land uses. The Federal Aviation Administration (FAA) also found Stapleton's airfield inadequate to meet the nation's future future air traffic requirements (FAA 1989). The principal motivating factors behind the new Denver airport project were the desire of both the city and the FAA to relieve air traffic delays attributable to Stapleton Airport and provide for airfield expansion (i.e., new runways) as needed in the event that the nation's commercial air traffic were to grow as projected in the mid-1980s by the FAA. This required not only a large, unconstrained parcel of land but also an airfield design that could be optimized under the FAA's existing runway separation regulations. The eventual airport would unavoidably be significantly larger than any existing commercial air carrier facility in the United States. Because of its very size and the substantial distances between runways (at ultimate buildout up to 10 km [6 mi]), DIA would be a "mesoscale" size airport. This presented an additional reason to thoroughly assess the weather conditions of the new airport site and its surroundings. In order to understand the meteorology of the new airport environs and any possible differences from the weather experienced at Stapleton Airport, new airport planners initiated a multi-year assessment of the mesoscale meteorology present in the vicinity of what eventually was chosen as the site of the 116 km 2 (45 mi 2) DIA. 3
Meteorological Analyses for the New Denver Airport Beginning in mid-1985, as the Adams County-Denver negotiations on siting and configuring a new airport proceeded, a three-pronged meteorological assessment was initiated. The assessment drew upon existing weather records and initiated field data collection in central Adams County to the east of the U.S. Army's Rocky Mountain Arsenal (see Figure I ). This assessment was important not only to the ongoing city-county negotiations but to the FAA as well. Meteorological research was critical to the FAA's responsibilities for airfield operations, airfield instrumentation, and airspace and air traffic control requirements.
Airfield Analysis: Climatological Records Because no long-term climatological records were available for central Adams County, the first element in the new airport meteorological assessment involved 3 By comparison, Dallas-Fort Worth International Airport is the second largest air carrier facility in the United States ;.it 72 km 2 (28 mi2). Chicago's O'Hare International Airport, the nation's busiest commercial air~)rt, is 28 km ~ ( I I mi2).
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analysis and comparison of weather data from Stapleton International Airport and Buckley Air National Guard Base. Both of these airfields are relatively near the new Denver airport site, yet their airfield alignments differ (see Figure 1). Therefore, the comparative assessment of these two airport weather records was intended to determine if one of these airfield orientations might be more suitable for the layout of the new Denver airport. In addition, the comparative assessment would illuminate the capacity of the Stapleton and Buckley runway alignments to accommodate air traffic under different weather conditions (e.g., wind speed and direction, visibility). Stapleton data for 1955-1964 were available from the National Weather Service and from the FAA air traffic control, and Buckley data for 1961-1970 were available from the Colorado Air National Guard. Although these two data sets only overlapped for a period of four years because of interruptions in collection, these records permitted sufficient analysis to indicate that a north-south, east-west airfield alignment with runways separated according to FAA regulations would provide optimum year-round operational capability.
Field Observations and Analysis Actual weather data collection in central Adams County began during summer 1985 at two locations. Meteorological stations were deployed in an area that was believed to be the eventual site of Denver's new airport; the stations were identified as NADOI and NADO2, named for Denver's New Airport Development Office. The locations of these stations are illustrated in Figure 1. These stations monitored and collected data on wind speed and direction, precipitation, minimum and maximum temperatures, and relative humidity. The locations of NADOI and NADO2 were intentionally selected to provide contrasting topographic settings. Central Adams County is sparsely populated, with most of the land historically used for dryland farming. Several creek drainages traverse the region, which is characterized by rolling terrain that is virtually devoid of trees except along some stretches of the intermittent creeks. A high ridgeline separates Box Elder Creek from the land immediately east of Rocky Mountain Arsenal (see Figure 1). NADOI was positioned near the drainage of an intermittent creek at an elevation 1,591 m (5,220 ft) above mean sea level. NADO2 was deployed on a hill 82 m (270 ft) higher in elevation than the site of NADOI. Both stations were free of the influence of surrounding vegetation, trees or structures so that wind speed and direction and precipitation patterns are governed by both regional climatology and local topography (Woodward-Clyde Consultants 1987). Therefore, comparison of data collected at these two locations would help determine which station's weather conditions could be considered more representative of the meteorology at the new airport site. In addition, data from these stations could be compared with similar information being recorded during the same period at Stapleton and Buckley Air National Guard Base.
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Analysis of the first year's data from NADO1 and NADO2 indicated the following: (1) wind speeds at NADOI and NADO2 on average were higher and more variable than those recorded at Stapleton and Buckley for the same year, which may have been due in part to different data recording methods by the automated weather stations and by human observers at the two airports; 4 (2) the predominant wind direction at all four sites (NADOI and NADO2, Stapleton and Buckley) was southerly, and the secondary wind direction was northerly for Stapleton and Buckley but easterly for both weather stations; and (3) all four sites had dominant southerly winds during the winter, spring, and summer. It was concluded from this comparative analysis that the airfield alignment of either Stapleton or Buckley would be acceptable for a new regional commercial airport (Greiner Engineering 1986). As the Adams County-Denver negotiations proceeded into early 1988, the new airport site was migrating further east and north--away from its originally agreed upon location immediately adjacent to Rocky Mountain Arsenal. As a result, new airport planners adapted the meteorological data collection to the changing circumstances. A third weather station (NADO3) was deployed north and east of the first two station locations, and NADO1 was relocated during summer 1988 within the agreed final new airport site and future airfield (see Figure 1). Thus, data collection and analysis reflected both changes in the negotiated new airport site and in the locations of the automated weather sta.tions. Despite addition and relocation of weather stations, wind speeds and directions have remained consistent with the findings of the first year's comparative data analysis. Additional instrumentation began collecting data on visibility and cloud base during summer 1988. NCAR Aviation Weather Review To supplement both the historical climatological comparisons and the field observations/analysis, the City and County of Denver contracted with the National Center for Atmospheric Research (NCAR) and its parent, the University Corporation for Atmospheric Research (UCAR), to examine meteorological conditions that interfere with aviation activity at Stapleton Airport and that may occur at DIA once it is operational. NCAR, located in Boulder, Colorado, is widely recognized for its aviation weather research activities. In addition, NCAR scientists had participated in or were familiar with several large meteorological observation and analysis experiments that coincidentally included the eventual DIA site. The research projects in question were: (1) the PROFS study (Program for Regional Observing and Forecasting Services); (2) the JAWS (Joint Airport 4 The winds recorded by the weather stations were averaged digitally and might include the influence of gusts. Weather observations taken at Stapleton and Buckley are recorded manually, with wind gusts intentionally excluded from the observed average (Greiner Engineering 1986).
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Weather Studies) and CLAWS (Classify, Locate, Avoid Wind Shear) programs; and (3) the CINDE project (Convective Initiation and Downburst Experiment). These experiments provided an unintended but valuable long-term perspective on the mesoscale weather at Denver's new airport. PROFS was initiated in 1979 and was conducted by scientists from the National Oceanic and Atmospheric Administration (NOAA). The project was designed to assess new technologies that could help improve short-term mesoscale weather forecasting (Reynolds 1983). The PROFS instrumentation network was deployed around and within metropolitan Denver, extending approximately 150 km (93 mi) east to west and 180 km (112 mi) north to south. PROFS and its monitoring network produced a substantial data base that has been used by atmospheric scientists to study several mesoscale phenomena, including convective storms (e.g., Einaudi et al. 1987) and tornadoes (Szoke et al. 1984). The JAWS field program, begun in 1982, and CLAWS, which was conducted during summer 1984, were designed by NCAR scientists to develop windshear detection and warning methods that could be directly used in FAA air traffic control operations (McCarthy et al. 1982; NCAR 1985). Using Stapleton as their research airport, scientists worked to improve their understanding of microburst and gust front events. JAWS facilities included a network of three Doppler radars deployed at Stapleton and north and northeast of the airport. The Doppler radar 23 km (14 mi) northeast of Stapleton has provided an unintended but valuable view of microburst activity over what is now the location of Denver Intemational Airport. 5 Microbursts are short-lived but intense downdrafts that can produce dangerous winds on or near the ground and are considered to be a serious hazard to aviation (NRC 1983). 6 CINDE, conducted during summer 1987 by NCAR and other scientists on the eastern plains of Colorado, addressed the formation of convective summer storms and the physics of downbursts (including microbursts). The new Denver airport site was located within the 85 km × 85 km CINDE instrument network. At least 25 tornadoes occurred within the CINDE instrument network during the experiment, giving scientists an unexpected opportunity to study the formation and behavior of this mesoscale weather hazard (Wilson et al. 1988). With limited meteorological data available from the new airport site area when their assignment commenced, NCAR scientists called upon the research data from PROFS, JAWS/CLAWS, and CINDE, as well as climatological information collected in Denver since the late 1800s and FAA and National Weather Service data. NCAR completed its study of Stapleton and the new Denver airport site area in mid-1988. 5 Microbursts may spread damaging winds on a scale of 4 km (2.5 mi) or less (Fujita 1985). Microbursts can tx:cur with or without accompanying precipitation, making the dry downburst an especially serious aviation hazard. ~'Microbursts can disrupt takeoffs and I',mdings if aircraft pass through the downdrafts without warning. Because of the manner in which microbursts spread as they meet the ground, an aircraft passing through the downdraft and ground turbulence may experience brief but sudden changes in headwinds, tailwinds and flight speed (NRC 1983).
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According to NCAR's final report to Denver, there are minor differences in precipitation and temperatures at Stapleton and the new airport site (Mahoney 1988). NCAR also reported that, on average, aviation operations at Stapleton are affected by weather events more than 140 days each year, with more than one adverse weather condition often occurring on the same day (e.g., high winds and snow) (Mahoney 1988). The NCAR report specifically addressed the role of topography and its influence on mesoscale weather to be expected at the new Denver airport site. According to the NCAR report (Mahoney 1988, p. 1): The Rocky Mountains have a large effect on the weather along the eastern plains of Colorado. Climatological studies of eastern Colorado indicate that the weather can change dramatically over small distances due to orographical influences. Another topographic feature besides the Rocky Mountains plays an influential role in regional meteorology. The Palmer Divide is an elevated, southwest-tonortheast ridge between Denver and Colorado Springs which contributes to an atmospheric phenomenon that is called the Denver Convergence Zone (DCZ). The DCZ has been identified as playing an important part in day-to-day mesoscale weather, particularly during the convective storm season from May through August (Szoke et al. 1984; NOAA 1987; Wilson et al. 1988). The DCZ is an area where two distinctly different air masses meet: cool, moist air moving from the northwest (across the Rocky Mountains) and drier, warmer air moving northward from the southeast. When these air masses converge during spring and summer days, they may form a line along which there is an increased probability of thunderstorms, tornadoes, and other forms of severe weather (Mahoney 1988; Wilson et al. 1988). However, the occurrence of such weather conditions is influenced by several variables, and the formation of a convergence line between the two air masses does not always occur. Figure 2 illustrates seven observed cases of the DCZ where severe weather occurred. Mesoscale Weather Impacts on DIA
With data assembled from the three components of the new airport meteorological assessment, the following conclusions have been drawn about weather impacts on operations at DIA once the new facility opens. Whereas many of the important findings pertain to the convective storm season, it should be noted that snowstorms also occur in Colorado's eastern plains. Thus, snow will be part of the meteorological regime at DIA as it has been at Stapleton. In addition, winds accompanying snowstorms may be of higher speed at DIA because its elevation is higher than that of Stapleton. During the convective storm season, metropolitan Denver and the eastern plains of Colorado also experience microbursts, hail, and gust fronts associated with thunderstorms. Microburst activity is known to occur at Stapleton Airport
METEOROLOGICAL ASSESSMENT FOR NEW DENVER AIRPORT
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mostly during the spring and summer (Mahoney 1988). Microburst frequency at DIA is expected to be similar to that at Stapleton. Gust fronts often include wind gusts that exceed 60 km (37 mi) per hour. Tornado hazard at DIA is not considered significant because of the known frequency and intensity of tornadoes over Colorado's eastern plains as well as the causes of tornadic activity in the region. Tornadoes in the Denver metropolitan region and Adams County may be associated with the DCZ, in which the converging air masses described above initiate a counterclockwise vortex.
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However, tornadoes in the region are typically weak and short-lived (Wilson et al. 1988). Moreover, when compared to a broad expanse of the midwestern and southern United States, eastern Colorado is a mild tornado impact region (Fujita 1980; McCarthy 1988). Yet numerous airports operate in locales where more frequent and severe tornadoes occur than in Colorado. With respect to the DCZ and its influence on mesoscale weather events, this phenomenon has now been observed over the entire Denver metropolitan region and over less densely populated eastern Colorado. As Figure 2 illustrates, the DCZ has even been observed over Stapleton Airport as well as further west near the eastern foothills of the Rocky Mountains. Moreover, despite the higher probability that the DCZ may influence the weather at DIA more than at Stapleton, NCAR scientists expressed confidence that the site selected for Denver's new airport is acceptable from an aviation weather safety perspective. A UCAR/NCAR report (McCarthy, 1988) states: the total adverse impact of the new airport site is only slightly greater than for Stapleton. The conclusion of this study is that the weather differences are sufficiently small to dispel any suggestion that the new site is flawed with respect to weather... For those who would focus on the weather event differences as a cause for alarm, we would merely point out that if any new site were to be selected for the Front Range area (other than keeping the new airport at Stapleton), we would expect an even greater increase in adverse weather. This posture was supported by both the FAA and the aviation weather committee of the Airline Pilots Association (ALPA), which reviewed the studies of DIA's meteorology. The chairman of this ALPA committee is quoted in a recent General Accounting Office report (GAO 1991, p. 20) that DIA would provide "greater operational flexibility in dealing with microburst events and tornado activity," and that the new airport's additional distance from the mountains reduces the maneuvering risks that confront aviation in the Denver metropolitan region. Aviation activity at D I A should also benefit from wind shear forecast and air traffic control improvements in recent years. More broadly, advances in severe weather forecasting will benefit operations at DIA. As the N C A R report (Mahoney 1988, pp. 17-18) states: New technologies have recently been developed to aid forecasters... The airlines and the FAA are actively involved in developing [capabilities] to make accurate, short-term forecasts of thunderstorms, surface and upper level winds, icing snowstorms, tornadoes, etc. The inclusion of such information will greatly reduce the effect of disruptive weather on the aviation system. Doppler and other ground-based radar warning systems have already been successfully tested by air traffic controllers and pilots at Stapleton Airport since
METEOROLOGICAL ASSESSMENT FOR NEW DENVER AIRPORT
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1982. This equipment is to be installed by the FAA at major commercial airports throughout the United States, including DIA (McCarthy 1988). In addition, FAA-sponsored research is in progress on airborne wind shear detection during aircraft final approach to landing (Aviation Week & Space Technology 1991). DIA's airfield alignment and runway spacing will provide additional flexibility for airport operations and aviation safety as compared to Stapleton. DIA will not only be able to accept two simultaneous arrival streams because of the runway separations, but will also be able to accommodate a third and fourth simultaneous arrival streams with the construction of future planned runways (City and County of Denver 1988). Because D1A's airfield was designed with the Denver region's varied meteorology explicitly in mind, the new airport should operate more safely and efficiently than Stapleton. This will help to improve the nation's air traffic system once DIA is operational. Conclusion
The meteorological assessment for the new Denver airport was an integral part of the entire ENEIS and technical planning process. In addition to the weather assessment's utility for environmental impact assessment purposes (i.e., finalizing the "preferred alternative" airfield and location), it served to illuminate meteorological impacts to, and mitigation measures for, the new airport. Because of the Denver region's varied and often severe year-round weather, the new airport meteorological assessment helped planners develop an increased awareness of specific aviation weather safety opportunities. The DIA weather assessment also provides a constructive lesson in the usefulness of integrating climate and meteorology into the EA/EIS process where appropriate, particularly as the nation's infrastructure is rebuilt, replaced and expanded in the coming years. The integration of climate/meteorology impact assessment with the established environmental impact assessment process may yield important insights into the relationship between society and atmospheric processes.
The author thanks Dr. John McCarthy and Dr. William P. Mahoney IIl--both of NCAR, and Mr. Richard F. Veazey of the City and County of Denver for their assistance and support.
References Aviation Week & Space Technology. 1991. Aircraft radar to be tested in detection of wind
shear. 134(20): 45. Brown, D.A. 1991, Denver aims for global hub status with new airport under construction. Aviation Week & Space Technology 134(10)" 42-45. City and County of Denver. 1988. New Denver Airport Environmental Assessment, Final Report, vol. 1. New Airport Office, Stapleton International Airport, Denver, CO 80207.
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Coates, J. 1988. Denver's past clouds hopes for new airport. Chicago Tribune, May I 1, Section I: 15. Einaudi, F., Clark, W.L., Fua, D., Green, J.L., and VanZandt, T.E. 1987. Gravity waves and convection in Colorado during July 1983. Journal of the Atmospheric Sciences 44(11): 1534-1553. FAA (Federal Aviation Administration) 1989. Final Environmental Impact Statement, New Denver Airport, vol. 1. Federal Aviation Administration, Denver International Airport Liaison Office, Stapleton International Airport, Denver, CO 80207. Fujita, T.T. 1980. U.S.Tornadoes, 1930-78. Chicago: University of Chicago, map printed for National Weather Service/NOAA, March. Fujita, T.T. 1985. The Downburst: Microburst and Macroburst, Chicago: University of Chicago, Department of Geophysical Sciences. GAO (General Accounting Office). 1991. New Denver Airport: Safety, Construction, Capacity, and Financing Considerations. Washington, D.C.: GAO/RCED-91-240, September. Greiner Engineering, 1986. New Denver Airport Wind Data Analysis,August 1985 to July 1986. Prepared for New Airport Development Office, Stapleton International Airport, Denver, Colorado 80207. Mahoney, W.P. III. 1988. A Study of Weather Factors Influencing Stapleton and the New Airport Site. Boulder, CO: University Corporation for Atmospheric Research, May. McCarthy, J. 1988. A Weather System for the Future of America's Airports, Airport of the Future: Final Project Report. Boulder, CO: University Corporation for Atmospheric Research, November. McCarthy, J., Wilson, J.W, and Fujita, T.T. 1982. The Joint Airport Weather Studies project. Bulletin of the American Meteorological Society 63(1): 15-22. NCAR (National Center for Atmospheric Research). 1985. Annual Report Fiscal Year 1984. Boulder, CO 80307. NOAA (National Oceanic and Atmospheric Administration). 1987. Colorado "tornado alley" claimed by NOAA scientist. NOAA Press Release L 87-224, August 17. NRC (National Research Council). 1983. Low-Altitude Wind Shear and its Hazards to Aviation. Washington, D.C.: National Academy Press. Reynolds, D.W. 1983. Prototype Workstation for Mesoscale Forecasting. Bulletin of the American Meteorological Society 64(3): 264-273. Szoke, E.J., Weisman, J.L., Brown, J.M., Caracena, F., and Schlatter, T.W 1984. A Subsynoptic Analysis of the Denver Tornadoes of 3 June 198 I. Monthly Weather Review 112(April): 790-808. Wilson, J.W., Moore, J.A., Foote, G.B., Martner, B., Rodi, A.R., Uttal, T., and Wilczak, J.W 1988. Convection Initiation and Downburst Experiment (CINDE). Bulletin of the American Meteorological Society 69( 11): 1328-1348. Woodward-Clyde Consultants, 1987. Meteorological Data Comparison of Two PAM Sites in the Vicinity of the New Airport. Prepared for New Airport Development Office, Stapleton International Airport, Denver, CO 80207.
MANAGING
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THE EFFECT OF THE ENVIRONMENT ON AVIATION WEATHER SAFETY: M E T E O R O L O G I C A L ASSESSMENT FOR THE NEW DENVER AIRPORT
S t e v e n L. R h o d e s National Center ]br Atmospheric Research
The new Denver International Airport currently under construction was the subject o f rigorous environmental impact assessment by both the project sponsor and the Federal Aviation Administration (FAA ). Airport design is closely linked to regional (mesoscale) meteorolory, and the Denver region experiences a wide range of extreme weather conditions as a consequence o f proximity to the Rocky Mountains as well as other topographic influences. Therefore, a comprehensive meteorological assessment was conducted to ensure that the new airport would be compatible with the meteorological regime of eastern Colorado. The meteorological assessment process not only assisted in the completion of the EA and EIS for the project, but also illuminated the new airport's capacity to operate efficiently despite the Denver region's variable weather. In addition, the assessment demonstrated the potential value of integrating climate and meteorological information into the development and rebuilding of the nation's infrastructure.
Introduction The newest commercial air carrier airport in the United States is presently under construction near Denver, Colorado (Brown 1991). Denver International Airport (DIA) is scheduled to open in late 1993 and will replace Stapleton International Address requestsfor reprints to: Steven L. RhCxtes. Environmental and Societal Impacts Group. National Center for Atmospheric Research. P.O. Box 3000, Boulder, CO 80307-3000. The National Center h~r Atmc,spheric Research is sponsored by the National Science Foundation.
© 1993 Elsevier Science Publishing Co., Inc. 655 Avenue of the Americas, New York, NY 10010
0195-9255/93/$6.00
64
STEVEN L. RHODES
Airport, which first opened to aviation activity in 1929. DIA is the product of several years of intense negotiations and cooperation between the City and County of Denver and neighboring Adams County. DIA is presently being built on land annexed to Denver in 1988 following a contentious de-annexation election in Adams County. Figure t illustrates the location of DIA and its environs. The new Denver airport project was subjected to intense environmental impact assessment, which is to be expected with a project of this scope. Denver airport planners worked closely with federal, state, and local agencies to ensure that the environmental assessment (EA)/environmental impact statement (EIS) process was expeditious and thorough. However, in order for the environmental impact assessment to proceed, it was necessary to wait until the location, size, and layout of the new airport and associated access routes were finalized. Yet the size and airfield layout could not be decided without knowledge of the meteorology of the site selected for the new airport, as weather and aviation safety are inextricably related. In eastern Colorado an attribute of the environment itself--topography-is known to significantly influence the weather, even over relatively short distances. Therefore, of critical importance to the new Denver airport environmental impact analysis was an assessment of the likely site's meteorological characteristics. The origins, structure, and findings of this weather impact assessment are described below.
Mesoscale Weather and Airport Capacity Mesoscale meteorology I was an important influence in the early days of the new Denver airport planning process. An important reason for pursuing a new air carrier airport for the Denver metropolitan region was the history of bad weather and its adverse effects on aviation operations at Stapleton International Airport, which has a poor reputation for weather-related flight delays and cancellations nationwide (Coates 1988). The Denver region and Colorado's eastern plains experience snow, high winds, blizzards, and very low temperatures during the spring and winter months, and thunderstorms, hail, and tornadoes during the spring and summer. Stapleton's four main runways are aligned in two parallel pairs, one aligned north-south and the other east-west (see Figure 1). However, the parallel separation of these runways is inadequate for either pair to be used for simultaneous aircraft arrivals under circumstances where pilots must operate on aircraft instrumentation to land. 2 In situations where aircraft pilots are instructed to operate under "instrument meteorological conditions" (IMC) at Stapleton air traffic controllers must close runways because of their insufficient parallel I Mesoscale meteorology refers to conditions that (x:cur on a scale from 4 km x 4 km to 400 km × 400 km (Eujila 1985 ). 2 FAA regulations require a minimum separation o f - I , 3 1 0 m (4,300 It) center line to center line fi)r parallel runways to be used for simultaneous arrivals in IMC. The separation of Stapleton's parallel north-south runways is only -488 m (1.600 fl), and for the parallel east-west runways only -275 m (900 It). FAA's required separation for a third simultaneous arrival stream is -4830 m (3 mi) from the closest parallel runway.
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separation. As a result, Denver's variable and diverse weather often disrupts the nation's air transportation system (City and County of Denver 1988). While severe weather disrupts operations at other airports throughout the United States and elsewhere, Denver's situation is directly tied to inadequate runway capacity. Additional runways cannot be added to Stapleton's airfield because it is constrained by surrounding land uses. The Federal Aviation Administration (FAA) also found Stapleton's airfield inadequate to meet the nation's future future air traffic requirements (FAA 1989). The principal motivating factors behind the new Denver airport project were the desire of both the city and the FAA to relieve air traffic delays attributable to Stapleton Airport and provide for airfield expansion (i.e., new runways) as needed in the event that the nation's commercial air traffic were to grow as projected in the mid-1980s by the FAA. This required not only a large, unconstrained parcel of land but also an airfield design that could be optimized under the FAA's existing runway separation regulations. The eventual airport would unavoidably be significantly larger than any existing commercial air carrier facility in the United States. Because of its very size and the substantial distances between runways (at ultimate buildout up to 10 km [6 mi]), DIA would be a "mesoscale" size airport. This presented an additional reason to thoroughly assess the weather conditions of the new airport site and its surroundings. In order to understand the meteorology of the new airport environs and any possible differences from the weather experienced at Stapleton Airport, new airport planners initiated a multi-year assessment of the mesoscale meteorology present in the vicinity of what eventually was chosen as the site of the 116 km 2 (45 mi 2) DIA. 3
Meteorological Analyses for the New Denver Airport Beginning in mid-1985, as the Adams County-Denver negotiations on siting and configuring a new airport proceeded, a three-pronged meteorological assessment was initiated. The assessment drew upon existing weather records and initiated field data collection in central Adams County to the east of the U.S. Army's Rocky Mountain Arsenal (see Figure I ). This assessment was important not only to the ongoing city-county negotiations but to the FAA as well. Meteorological research was critical to the FAA's responsibilities for airfield operations, airfield instrumentation, and airspace and air traffic control requirements.
Airfield Analysis: Climatological Records Because no long-term climatological records were available for central Adams County, the first element in the new airport meteorological assessment involved 3 By comparison, Dallas-Fort Worth International Airport is the second largest air carrier facility in the United States ;.it 72 km 2 (28 mi2). Chicago's O'Hare International Airport, the nation's busiest commercial air~)rt, is 28 km ~ ( I I mi2).
METEOROLOGICAL ASSESSMENT FOR NEW DENVER AIRPORT
67
analysis and comparison of weather data from Stapleton International Airport and Buckley Air National Guard Base. Both of these airfields are relatively near the new Denver airport site, yet their airfield alignments differ (see Figure 1). Therefore, the comparative assessment of these two airport weather records was intended to determine if one of these airfield orientations might be more suitable for the layout of the new Denver airport. In addition, the comparative assessment would illuminate the capacity of the Stapleton and Buckley runway alignments to accommodate air traffic under different weather conditions (e.g., wind speed and direction, visibility). Stapleton data for 1955-1964 were available from the National Weather Service and from the FAA air traffic control, and Buckley data for 1961-1970 were available from the Colorado Air National Guard. Although these two data sets only overlapped for a period of four years because of interruptions in collection, these records permitted sufficient analysis to indicate that a north-south, east-west airfield alignment with runways separated according to FAA regulations would provide optimum year-round operational capability.
Field Observations and Analysis Actual weather data collection in central Adams County began during summer 1985 at two locations. Meteorological stations were deployed in an area that was believed to be the eventual site of Denver's new airport; the stations were identified as NADOI and NADO2, named for Denver's New Airport Development Office. The locations of these stations are illustrated in Figure 1. These stations monitored and collected data on wind speed and direction, precipitation, minimum and maximum temperatures, and relative humidity. The locations of NADOI and NADO2 were intentionally selected to provide contrasting topographic settings. Central Adams County is sparsely populated, with most of the land historically used for dryland farming. Several creek drainages traverse the region, which is characterized by rolling terrain that is virtually devoid of trees except along some stretches of the intermittent creeks. A high ridgeline separates Box Elder Creek from the land immediately east of Rocky Mountain Arsenal (see Figure 1). NADOI was positioned near the drainage of an intermittent creek at an elevation 1,591 m (5,220 ft) above mean sea level. NADO2 was deployed on a hill 82 m (270 ft) higher in elevation than the site of NADOI. Both stations were free of the influence of surrounding vegetation, trees or structures so that wind speed and direction and precipitation patterns are governed by both regional climatology and local topography (Woodward-Clyde Consultants 1987). Therefore, comparison of data collected at these two locations would help determine which station's weather conditions could be considered more representative of the meteorology at the new airport site. In addition, data from these stations could be compared with similar information being recorded during the same period at Stapleton and Buckley Air National Guard Base.
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Analysis of the first year's data from NADO1 and NADO2 indicated the following: (1) wind speeds at NADOI and NADO2 on average were higher and more variable than those recorded at Stapleton and Buckley for the same year, which may have been due in part to different data recording methods by the automated weather stations and by human observers at the two airports; 4 (2) the predominant wind direction at all four sites (NADOI and NADO2, Stapleton and Buckley) was southerly, and the secondary wind direction was northerly for Stapleton and Buckley but easterly for both weather stations; and (3) all four sites had dominant southerly winds during the winter, spring, and summer. It was concluded from this comparative analysis that the airfield alignment of either Stapleton or Buckley would be acceptable for a new regional commercial airport (Greiner Engineering 1986). As the Adams County-Denver negotiations proceeded into early 1988, the new airport site was migrating further east and north--away from its originally agreed upon location immediately adjacent to Rocky Mountain Arsenal. As a result, new airport planners adapted the meteorological data collection to the changing circumstances. A third weather station (NADO3) was deployed north and east of the first two station locations, and NADO1 was relocated during summer 1988 within the agreed final new airport site and future airfield (see Figure 1). Thus, data collection and analysis reflected both changes in the negotiated new airport site and in the locations of the automated weather sta.tions. Despite addition and relocation of weather stations, wind speeds and directions have remained consistent with the findings of the first year's comparative data analysis. Additional instrumentation began collecting data on visibility and cloud base during summer 1988. NCAR Aviation Weather Review To supplement both the historical climatological comparisons and the field observations/analysis, the City and County of Denver contracted with the National Center for Atmospheric Research (NCAR) and its parent, the University Corporation for Atmospheric Research (UCAR), to examine meteorological conditions that interfere with aviation activity at Stapleton Airport and that may occur at DIA once it is operational. NCAR, located in Boulder, Colorado, is widely recognized for its aviation weather research activities. In addition, NCAR scientists had participated in or were familiar with several large meteorological observation and analysis experiments that coincidentally included the eventual DIA site. The research projects in question were: (1) the PROFS study (Program for Regional Observing and Forecasting Services); (2) the JAWS (Joint Airport 4 The winds recorded by the weather stations were averaged digitally and might include the influence of gusts. Weather observations taken at Stapleton and Buckley are recorded manually, with wind gusts intentionally excluded from the observed average (Greiner Engineering 1986).
METEOROLOGICAL ASSESSMENT FOR NEW DENVER AIRPORT
69
Weather Studies) and CLAWS (Classify, Locate, Avoid Wind Shear) programs; and (3) the CINDE project (Convective Initiation and Downburst Experiment). These experiments provided an unintended but valuable long-term perspective on the mesoscale weather at Denver's new airport. PROFS was initiated in 1979 and was conducted by scientists from the National Oceanic and Atmospheric Administration (NOAA). The project was designed to assess new technologies that could help improve short-term mesoscale weather forecasting (Reynolds 1983). The PROFS instrumentation network was deployed around and within metropolitan Denver, extending approximately 150 km (93 mi) east to west and 180 km (112 mi) north to south. PROFS and its monitoring network produced a substantial data base that has been used by atmospheric scientists to study several mesoscale phenomena, including convective storms (e.g., Einaudi et al. 1987) and tornadoes (Szoke et al. 1984). The JAWS field program, begun in 1982, and CLAWS, which was conducted during summer 1984, were designed by NCAR scientists to develop windshear detection and warning methods that could be directly used in FAA air traffic control operations (McCarthy et al. 1982; NCAR 1985). Using Stapleton as their research airport, scientists worked to improve their understanding of microburst and gust front events. JAWS facilities included a network of three Doppler radars deployed at Stapleton and north and northeast of the airport. The Doppler radar 23 km (14 mi) northeast of Stapleton has provided an unintended but valuable view of microburst activity over what is now the location of Denver Intemational Airport. 5 Microbursts are short-lived but intense downdrafts that can produce dangerous winds on or near the ground and are considered to be a serious hazard to aviation (NRC 1983). 6 CINDE, conducted during summer 1987 by NCAR and other scientists on the eastern plains of Colorado, addressed the formation of convective summer storms and the physics of downbursts (including microbursts). The new Denver airport site was located within the 85 km × 85 km CINDE instrument network. At least 25 tornadoes occurred within the CINDE instrument network during the experiment, giving scientists an unexpected opportunity to study the formation and behavior of this mesoscale weather hazard (Wilson et al. 1988). With limited meteorological data available from the new airport site area when their assignment commenced, NCAR scientists called upon the research data from PROFS, JAWS/CLAWS, and CINDE, as well as climatological information collected in Denver since the late 1800s and FAA and National Weather Service data. NCAR completed its study of Stapleton and the new Denver airport site area in mid-1988. 5 Microbursts may spread damaging winds on a scale of 4 km (2.5 mi) or less (Fujita 1985). Microbursts can tx:cur with or without accompanying precipitation, making the dry downburst an especially serious aviation hazard. ~'Microbursts can disrupt takeoffs and I',mdings if aircraft pass through the downdrafts without warning. Because of the manner in which microbursts spread as they meet the ground, an aircraft passing through the downdraft and ground turbulence may experience brief but sudden changes in headwinds, tailwinds and flight speed (NRC 1983).
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According to NCAR's final report to Denver, there are minor differences in precipitation and temperatures at Stapleton and the new airport site (Mahoney 1988). NCAR also reported that, on average, aviation operations at Stapleton are affected by weather events more than 140 days each year, with more than one adverse weather condition often occurring on the same day (e.g., high winds and snow) (Mahoney 1988). The NCAR report specifically addressed the role of topography and its influence on mesoscale weather to be expected at the new Denver airport site. According to the NCAR report (Mahoney 1988, p. 1): The Rocky Mountains have a large effect on the weather along the eastern plains of Colorado. Climatological studies of eastern Colorado indicate that the weather can change dramatically over small distances due to orographical influences. Another topographic feature besides the Rocky Mountains plays an influential role in regional meteorology. The Palmer Divide is an elevated, southwest-tonortheast ridge between Denver and Colorado Springs which contributes to an atmospheric phenomenon that is called the Denver Convergence Zone (DCZ). The DCZ has been identified as playing an important part in day-to-day mesoscale weather, particularly during the convective storm season from May through August (Szoke et al. 1984; NOAA 1987; Wilson et al. 1988). The DCZ is an area where two distinctly different air masses meet: cool, moist air moving from the northwest (across the Rocky Mountains) and drier, warmer air moving northward from the southeast. When these air masses converge during spring and summer days, they may form a line along which there is an increased probability of thunderstorms, tornadoes, and other forms of severe weather (Mahoney 1988; Wilson et al. 1988). However, the occurrence of such weather conditions is influenced by several variables, and the formation of a convergence line between the two air masses does not always occur. Figure 2 illustrates seven observed cases of the DCZ where severe weather occurred. Mesoscale Weather Impacts on DIA
With data assembled from the three components of the new airport meteorological assessment, the following conclusions have been drawn about weather impacts on operations at DIA once the new facility opens. Whereas many of the important findings pertain to the convective storm season, it should be noted that snowstorms also occur in Colorado's eastern plains. Thus, snow will be part of the meteorological regime at DIA as it has been at Stapleton. In addition, winds accompanying snowstorms may be of higher speed at DIA because its elevation is higher than that of Stapleton. During the convective storm season, metropolitan Denver and the eastern plains of Colorado also experience microbursts, hail, and gust fronts associated with thunderstorms. Microburst activity is known to occur at Stapleton Airport
METEOROLOGICAL ASSESSMENT FOR NEW DENVER AIRPORT
J
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FIGURE 2. Location of Denver Convergence Zone in seven cases where severe weather occurred. Source: Mahoney 1988.
mostly during the spring and summer (Mahoney 1988). Microburst frequency at DIA is expected to be similar to that at Stapleton. Gust fronts often include wind gusts that exceed 60 km (37 mi) per hour. Tornado hazard at DIA is not considered significant because of the known frequency and intensity of tornadoes over Colorado's eastern plains as well as the causes of tornadic activity in the region. Tornadoes in the Denver metropolitan region and Adams County may be associated with the DCZ, in which the converging air masses described above initiate a counterclockwise vortex.
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STEVEN L. RHODES
However, tornadoes in the region are typically weak and short-lived (Wilson et al. 1988). Moreover, when compared to a broad expanse of the midwestern and southern United States, eastern Colorado is a mild tornado impact region (Fujita 1980; McCarthy 1988). Yet numerous airports operate in locales where more frequent and severe tornadoes occur than in Colorado. With respect to the DCZ and its influence on mesoscale weather events, this phenomenon has now been observed over the entire Denver metropolitan region and over less densely populated eastern Colorado. As Figure 2 illustrates, the DCZ has even been observed over Stapleton Airport as well as further west near the eastern foothills of the Rocky Mountains. Moreover, despite the higher probability that the DCZ may influence the weather at DIA more than at Stapleton, NCAR scientists expressed confidence that the site selected for Denver's new airport is acceptable from an aviation weather safety perspective. A UCAR/NCAR report (McCarthy, 1988) states: the total adverse impact of the new airport site is only slightly greater than for Stapleton. The conclusion of this study is that the weather differences are sufficiently small to dispel any suggestion that the new site is flawed with respect to weather... For those who would focus on the weather event differences as a cause for alarm, we would merely point out that if any new site were to be selected for the Front Range area (other than keeping the new airport at Stapleton), we would expect an even greater increase in adverse weather. This posture was supported by both the FAA and the aviation weather committee of the Airline Pilots Association (ALPA), which reviewed the studies of DIA's meteorology. The chairman of this ALPA committee is quoted in a recent General Accounting Office report (GAO 1991, p. 20) that DIA would provide "greater operational flexibility in dealing with microburst events and tornado activity," and that the new airport's additional distance from the mountains reduces the maneuvering risks that confront aviation in the Denver metropolitan region. Aviation activity at D I A should also benefit from wind shear forecast and air traffic control improvements in recent years. More broadly, advances in severe weather forecasting will benefit operations at DIA. As the N C A R report (Mahoney 1988, pp. 17-18) states: New technologies have recently been developed to aid forecasters... The airlines and the FAA are actively involved in developing [capabilities] to make accurate, short-term forecasts of thunderstorms, surface and upper level winds, icing snowstorms, tornadoes, etc. The inclusion of such information will greatly reduce the effect of disruptive weather on the aviation system. Doppler and other ground-based radar warning systems have already been successfully tested by air traffic controllers and pilots at Stapleton Airport since
METEOROLOGICAL ASSESSMENT FOR NEW DENVER AIRPORT
73
1982. This equipment is to be installed by the FAA at major commercial airports throughout the United States, including DIA (McCarthy 1988). In addition, FAA-sponsored research is in progress on airborne wind shear detection during aircraft final approach to landing (Aviation Week & Space Technology 1991). DIA's airfield alignment and runway spacing will provide additional flexibility for airport operations and aviation safety as compared to Stapleton. DIA will not only be able to accept two simultaneous arrival streams because of the runway separations, but will also be able to accommodate a third and fourth simultaneous arrival streams with the construction of future planned runways (City and County of Denver 1988). Because D1A's airfield was designed with the Denver region's varied meteorology explicitly in mind, the new airport should operate more safely and efficiently than Stapleton. This will help to improve the nation's air traffic system once DIA is operational. Conclusion
The meteorological assessment for the new Denver airport was an integral part of the entire ENEIS and technical planning process. In addition to the weather assessment's utility for environmental impact assessment purposes (i.e., finalizing the "preferred alternative" airfield and location), it served to illuminate meteorological impacts to, and mitigation measures for, the new airport. Because of the Denver region's varied and often severe year-round weather, the new airport meteorological assessment helped planners develop an increased awareness of specific aviation weather safety opportunities. The DIA weather assessment also provides a constructive lesson in the usefulness of integrating climate and meteorology into the EA/EIS process where appropriate, particularly as the nation's infrastructure is rebuilt, replaced and expanded in the coming years. The integration of climate/meteorology impact assessment with the established environmental impact assessment process may yield important insights into the relationship between society and atmospheric processes.
The author thanks Dr. John McCarthy and Dr. William P. Mahoney IIl--both of NCAR, and Mr. Richard F. Veazey of the City and County of Denver for their assistance and support.
References Aviation Week & Space Technology. 1991. Aircraft radar to be tested in detection of wind
shear. 134(20): 45. Brown, D.A. 1991, Denver aims for global hub status with new airport under construction. Aviation Week & Space Technology 134(10)" 42-45. City and County of Denver. 1988. New Denver Airport Environmental Assessment, Final Report, vol. 1. New Airport Office, Stapleton International Airport, Denver, CO 80207.
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Coates, J. 1988. Denver's past clouds hopes for new airport. Chicago Tribune, May I 1, Section I: 15. Einaudi, F., Clark, W.L., Fua, D., Green, J.L., and VanZandt, T.E. 1987. Gravity waves and convection in Colorado during July 1983. Journal of the Atmospheric Sciences 44(11): 1534-1553. FAA (Federal Aviation Administration) 1989. Final Environmental Impact Statement, New Denver Airport, vol. 1. Federal Aviation Administration, Denver International Airport Liaison Office, Stapleton International Airport, Denver, CO 80207. Fujita, T.T. 1980. U.S.Tornadoes, 1930-78. Chicago: University of Chicago, map printed for National Weather Service/NOAA, March. Fujita, T.T. 1985. The Downburst: Microburst and Macroburst, Chicago: University of Chicago, Department of Geophysical Sciences. GAO (General Accounting Office). 1991. New Denver Airport: Safety, Construction, Capacity, and Financing Considerations. Washington, D.C.: GAO/RCED-91-240, September. Greiner Engineering, 1986. New Denver Airport Wind Data Analysis,August 1985 to July 1986. Prepared for New Airport Development Office, Stapleton International Airport, Denver, Colorado 80207. Mahoney, W.P. III. 1988. A Study of Weather Factors Influencing Stapleton and the New Airport Site. Boulder, CO: University Corporation for Atmospheric Research, May. McCarthy, J. 1988. A Weather System for the Future of America's Airports, Airport of the Future: Final Project Report. Boulder, CO: University Corporation for Atmospheric Research, November. McCarthy, J., Wilson, J.W, and Fujita, T.T. 1982. The Joint Airport Weather Studies project. Bulletin of the American Meteorological Society 63(1): 15-22. NCAR (National Center for Atmospheric Research). 1985. Annual Report Fiscal Year 1984. Boulder, CO 80307. NOAA (National Oceanic and Atmospheric Administration). 1987. Colorado "tornado alley" claimed by NOAA scientist. NOAA Press Release L 87-224, August 17. NRC (National Research Council). 1983. Low-Altitude Wind Shear and its Hazards to Aviation. Washington, D.C.: National Academy Press. Reynolds, D.W. 1983. Prototype Workstation for Mesoscale Forecasting. Bulletin of the American Meteorological Society 64(3): 264-273. Szoke, E.J., Weisman, J.L., Brown, J.M., Caracena, F., and Schlatter, T.W 1984. A Subsynoptic Analysis of the Denver Tornadoes of 3 June 198 I. Monthly Weather Review 112(April): 790-808. Wilson, J.W., Moore, J.A., Foote, G.B., Martner, B., Rodi, A.R., Uttal, T., and Wilczak, J.W 1988. Convection Initiation and Downburst Experiment (CINDE). Bulletin of the American Meteorological Society 69( 11): 1328-1348. Woodward-Clyde Consultants, 1987. Meteorological Data Comparison of Two PAM Sites in the Vicinity of the New Airport. Prepared for New Airport Development Office, Stapleton International Airport, Denver, CO 80207.