by Bob Henson
This aerial shot reveals the stark geometry of northeast Arizona’s
kilometer-wide Meteor Crater, the site of the METCRAX study. (Photo ©John Shelton.)
Many atmospheric field projects are epic in scope, centered on highly dramatic events like tornadoes or tropical cyclones. By these standards, the Meteor Crater Experiment (METCRAX) was a low-key campaign—but quite a successful one.
The goal of the NSF-funded METCRAX was to gather data on the subtle and occasionally puzzling microclimate produced in Arizona by a kilometer-wide, meteorite-carved depression, aptly named Meteor Crater. During the month of October 2006, a group of about 20 scientists and technicians planted themselves in and near the crater, located about 40 kilometers (25 miles) east of Flagstaff. This year, the science is coming to fruition, with four journal articles in the works and more than a half-dozen conference presentations planned for this spring and summer.
Principal investigators in METCRAX were David Whiteman (University of Utah), Andreas Muschinski (University
of Massachusetts Amherst), Sharon Zhong (Michigan State University), and David Fritts (NorthWest Research Associates).
When it comes to studying the boundary layer—the atmosphere’s lowest kilometer or so—scientists were hard-pressed to find a better natural laboratory than Meteor Crater. “We’re very interested in stable boundary layers and how they evolve over time,” says Whiteman. These stable layers often include inversions, where cool air sits below warm air, inhibiting the usual vertical mixing. High-altitude cities that lie in river valleys, such as Denver and Salt Lake City, are especially prone to the smog that wintertime inversions can trap.
A crater made to order
Whiteman and colleagues have spent years conducting field work in other natural depressions across North America and Europe, hoping that sheltered and simplified geography can shed light on basic aspects of the boundary layer. The problem is that most basins aren’t very symmetric. Valleys or gullies along their rims allow surface air to enter and disrupt the inversions.
Pictured during a pre-METCRAX crater visit are Brad Andes, president of Meteor Crater Enterprises; Drew Barringer, president of Barringer Crater Corporation; Andreas Muschinski, University of Massachusetts Amherst; Sharon Zhong, Michigan State University; David Whiteman, University of Utah; and David Kring, Lunar and Planetary Institute. (Photo by Craig Clements.)
Meteor Crater is much closer to
the mark. Created by a house-sized meteorite that struck about 49,000 years ago, the crater is about 1.2 kilometers (0.75 miles) wide and 165 meters (535 feet) deep. Two geologic faults crisscross it, lending a slightly squared-off aspect
to the crater’s circular geometry. The crater is also encircled by a rim about
50 m (160 ft) higher than the surrounding plain. Aside from some windblown dust at the bottom, the site is remarkably intact, says Whiteman. He calls it “the best-preserved meteor crater on Earth.”
Not only is Meteor Crater nearly ideal for inversion research, but it’s also tailor-made for producing seiches: standing waves, or back-and-forth oscillations of air driven by wind and gravity (think of water sloshing from one end of a bathtub to the other). Hydrological seiches are common in lakes and oceans, but METCRAX was the first major study of their atmospheric counterparts.
The idea for METCRAX arose after Muschinski visited Meteor Crater on a family vacation in 2001 and wrote NSF a letter of intent about using the site to study atmospheric seiches in a cold-air pool. Though his letter was well received, the idea remained dormant until 2004, when Whiteman gave a seminar at the NOAA lab in Boulder where Muschinski then worked. “I’d met Dave years before and was familiar with his research,” says Muschinski. “I asked him right after the seminar whether he would be interested in a collaboration.”
A variety of instruments were deployed in and around Meteor Crater during METCRAX. (Image courtesy David Whiteman.)
As it happens, Whiteman had been thinking along similar lines for years—even keeping a photo of Meteor Crater on his bulletin board in Utah. “Because of our convergence of interest, we joined forces and put together a project team,” says Whiteman. He oversaw the field effort, with Muschinski as lead principal investigator.
The remoteness of Meteor Crater made it a pristine but challenging location. The site is owned by a private foundation called the Barringer Crater Company, which has dedicated the crater and its environs to science and education and kept development minimal. With no electrical outlets in the crater, the METCRAX technicians drew on solar power and generators. Participants had to hike in and out on a trail used by astronauts training in the crater in the 1960s. Most of the equipment was deposited and removed via helicopter.
Not your usual inversions
Arizona’s dry climate provides the large day-to-night temperature spread needed for strong inversions. To get a sense of how the inversions vary by season, METCRAX deployed two recording thermometers called HOBOs, one on the rim and one on the crater floor. Temperatures were sampled every 10 minutes over a year-long period. During the month-long field phase, about 50 other HOBOs were laid along north-south and east-west axes that crossed the crater and extended just beyond its rim.
METCRAX opened with almost two weeks of unusually windy and showery weather. Later in the month, the researchers got more of the sunny days and clear, calm nights they sought. There were seven intensive observing periods, each extending from one afternoon to the next morning. With the help of equipment from NCAR’s Earth Observing Laboratory and other partners, the team mapped out the nighttime inversions they expected, plus some variations they didn’t.
The HOBOs revealed that nighttime temperatures on the crater floor were often 10–15°C (18–27°F) colder than at the rim. However, each one of the nighttime inversions was scoured away by sunshine and wind the next day.
A variety of other instruments monitored the air in, near, and above the crater. NCAR provided seven of its Integrated Surface Flux Facility towers, which sampled wind, temperature, humidity, and turbulent fluxes at five heights, as well as soil heat flux and incoming and outgoing radiation. On the crater’s rim, the University of Massachusetts installed two 6-meter (20-foot) towers with ultrasonic anemometers.
Instrument packages borne by tethersonde sailed in and around Meteor Crater during the METCRAX study period. (Photo by Craig Clements.)
The University of Utah furnished three tethersondes, balloon-borne instrument packages connected to the ground by rope. These were repeatedly lofted and returned from the center of the crater’s floor as well as from its west and east walls, allowing the tethersondes to catch variations produced by the rising and setting Sun as it heated the crater walls in a regular daily sequence.
Among the surprises emerging from METCRAX was the shallowness of the crater’s inversions. The team had expected to find temperatures rising more or less smoothly with altitude from the crater floor to the height of the rim. Instead, very cold air near the crater floor was typically topped by a much deeper layer of isothermal (constant-temperature) air that extended slightly above rim level. This isothermal layer encompassed as much as 60–80% of the total distance from base to rim. “We’re not sure why this is,” says Whiteman. “It’s a surprising result that we haven’t seen in other basins of this size.”
As expected, the daily east-to-west contrasts in heating led to seiches. Winds at the crater base showed a 15-minute oscillation consistent with seiche behavior, says Whiteman. Higher up, the main seiche-hunting tool was a 36-centimeter (14-inch) telescope brought by Muschinski and positioned just below the north rim’s visitor center. This telescope, equipped with a charge-coupled device camera, measured light arriving from an array of four small flashlight bulbs placed by Muschinski and his team just below the southeast rim. As wind and temperature variations crossed the light’s 900-meter (3000-foot) path, they produced fluctuations in its angle of arrival at the telescope. “We hope to be able to extract a seiche-related, path-integrated velocity signal from those data,”
NCAR’s Integrated Sounding System, stationed about 5 km (3 mi) northwest of the crater, kept an eye on the larger weather picture with a radar wind profiler and a radio acoustic sounding system. These data were supplemented by standard radiosonde launches during the intensive observing periods.
The boundary layer outside the crater yielded surprises of its own. On several otherwise-calm nights, southwest winds developed near ground level, sometimes topping 5 meters per second (11 miles per hour). Since the landscape rolls gently northeast from the Mogollon Rim toward Meteor Crater, the flow appears to be a drainage wind that blows downslope at night as higher terrain cools.
For his master’s thesis, Michigan State graduate student Crosby Savage, working with MSU adviser Zhong, is studying the downslope wind by comparing regional METCRAX data to theoretical and numerical models. “The many observations from METCRAX gave me a unique opportunity to characterize this flow,” he says.
What's in a name?
The term “meteorology” dates back to Aristotle’s classic text Meteorologica, written in 340 BCE. At that time, “meteor” referred to any object in the sky, including raindrops and snowflakes. Centuries later, when researchers began to specialize in the study of extraterrestrial objects entering Earth’s atmosphere, the term “meteorology” had already found a home in the context of weather. Thus, the science of what we now call meteors became known as meteoritics, and its practitioners meteoriticists. There’s even a Meteoritical Society, founded in 1933, with nearly 1,000 members.
Other participants, including Fritts, are modeling the atmosphere within the crater itself. With support from the U.S. Army Research Office, Utah postdoctoral researcher Sebastian Hoch is working on a three-dimensional radiative transfer model to try to balance the heat budget of the crater air—which is proving to be quite a challenge, he reports.
Life in a bowl
“It was a wonderful experience working in the crater,” says Craig Clements (San Jose State University). “We’d head in for an all-nighter, flying tethersondes, then hike out the next morning. You could observe the turbulence by watching the balloons going crazy just below the crater rim.”
Although it’s off the beaten path, Meteor Crater does attract a few visitors who are allowed on the north rim but not in the crater itself. As a result, the public had a bird’s eye view of each day’s METRAX operations—including trips to a portable toilet. By nightfall, however, the site was empty except for
a handful of scientists and local wildlife. “We could hear owls calling from
one side of the crater to the other,” Whiteman says.
At times, the METCRAX participants found themselves pondering the cataclysm that formed their study area. “One night there was a meteor shower. To be in the crater on a night like that was just unreal,” recalls Clements. NCAR project scientist William Brown had his own epiphany: “Climbing into the crater felt like hiking into a canyon, except that I found myself visualizing how it was formed. At one point I caught myself looking into the sky and wondering if history could somehow repeat itself.”