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June 1998

A turbulent summer ahead: NCAR to host two major meetings

Over the past year, NCAR-based research on aviation and turbulence has burgeoned (see sidebar, "On the Web," for links to updates). But NCAR's interest in turbulence theory goes back to the center's founding days. From a Turbulence Club in the 1960s to today's Geophysical Turbulence Program (GTP), NCAR has maintained a cadre of scientists deeply committed to the turbulence problem--one so vexing that famed physicist Richard Feynmann called it the last great unsolved problem in classical physics.

High-level winds packing invisible vortex tubes stream east (from top to bottom) in this three-dimensional rendering of the 9 December 1992 weather situation. A vertical cross-section along the Front Range--the rectangle extending from left to right--depicts potential temperature, a height-adjusted index of actual temperature, at intervals of 4degreesC (7degreesF). The cross section is perpendicular to the mean wind flow, which is from the west-northwest. In this image, the viewer is looking down on the bottom of the jet stream, indicated by the stingray-shaped surface cutting through the vertical cross-section. Above this surface, in and near the invisible vortex tubes, winds exceed 123 miles per hour (55 meters per second). (Illustration prepared by Bill Hall, MMM, with the SCD Scientific Visualization Laboratory.)

"Feynmann said that in the 1950s, and it's still true today," says Bob Kerr. He and colleagues Terry Clark and Bill Hall (all in the Mesoscale and Microscale Meteorology Division) have spent the past couple of years scrutinizing an aircraft incident from 1992 as a case study that ties together some of the threads of modern turbulence research. Bob is also the co-coordinator of a major symposium, Developments in Geophysical Turbulence, to be held 16-19 June at the Mesa Lab. The meeting will bring over 100 of the world's leading researchers in turbulence--mathematicians, atmospheric scientists, and oceanographers--to Boulder for four days of discussions and presentations. Another meeting in August will bring a smaller group of researchers to NCAR to focus on the interplay between measurements, experiments, and numerical modeling.

A model points the way

The event being studied by Bob, Terry, and Bill took place on 9 December 1992. It was a moderately windy morning across the Front Range of Colorado, not uncommon for late fall. The high-altitude jet stream and an accompanying front were moving across the Rockies from the west-northwest. Cruising just west of Denver in that river of air was a coast-to-coast DC-8 cargo jet. As the aircraft crossed the Front Range about four miles above the highest peaks, at around 9:00 a.m. local time, without warning, severe updrafts and downdrafts buffeted the DC-8 for two minutes. The clear-air turbulence encounter resulted in major structural damage, with one engine and part of the left wing sheared off. The aircraft hobbled into Stapleton Airport for an emergency landing.

The plane's pilot was lucky to have survived. The event was a lucky one for research meteorology, too, because on this morning an exceptionally wide array of instruments happened to be trained on the skies of Colorado. Dust was in the upper troposphere and lower stratosphere, left over from the 1991 eruption of Mt. Pinatubo. The dust helped NOAA scientists track winds from the ground using a lidar (laser-based radar) at Table Mountain, north of Boulder. The routine morning of sampling produced results that are anything but routine.

"The pilot didn't expect to hit what he hit," says Bob. A satellite photo taken only two minutes after the incident provided few clues as to what had happened up where the cargo plane flew. At lower levels, "There was a huge rotor, but the pilot knew how to avoid that." Rotors--lengthwise circulation tubes formed as flow ascends and descends mountains--are a common feature during windstorms, often made visible through lenticular wave clouds.

The mystery encouraged Terry and Bill to feed reams of data from NOAA and other sources into the Clark-Hall small-scale atmospheric dynamic model. Processing the job took several days of time on one of NCAR's Cray J-9 parallel supercomputers. When the results came back, the investigators were astonished to find tubes of circulation stretching eastward at high altitudes from the mountains near flight level. Were the tubes real, or figments of the model's imagination?

"Bill spent an enormous amount of time trying to figure out what was going on," says Bob. A closer look at the satellite photos revealed traces of the tubes at roughly the same level as the aircraft incident, where the model had depicted them. When the model's output was pushed to its highest resolution (660 feet [200 meters] between points over an 80-mile [50-kilometer] domain), it showed enormous wind shear within the tubes. From one point to the next, the wind speed changed by as much as 100 miles per hour (40 meters per second).

NCAR visitor Yoshifumi Kimura (Nagoya University, Japan) and Bob Kerr (MMM) are convening Developments in Geophysical Turbulence, a symposium at the Mesa Lab 16-19 June.

Given that the actual vortex was probably smaller than the model's limit in resolution, "We have every reason to believe the [circulation] was five times stronger," says Bob. If so, it was as tightly packed and powerful as a modest tornado. But this turbulent phenomenon, called a horizontal vortex tube, occurred outside of any rain or snow, providing nothing visible to warn pilots of its presence.

What might be causing the vortex tubes? Bob and Bill are focusing on two possibilities. One is a dynamic adjustment of the low-level rotor. The other is some sort of circulation anomaly moving in at higher levels, near the tropopause, that happened to reach the Front Range at the same time the low-level rotor developed. The two modelers plan to rerun the case with a grid of even finer resolution at the tropopause level. But it's possible that the tropopause variations were induced by the vortex tubes, rather than the other way around, Bob notes. "Which is the chicken and which is the egg? We're still a long way from understanding this case."

Hiding and seeking

Partly because it hides so well, turbulence remains an enigma to atmospheric science. NCAR's founders recognized that turbulence was central to a better understanding of the atmosphere. Through the 1960s and 1970s, a Turbulence Club founded by NCAR's first associate director, Phil Thompson, met regularly to hash out approaches to the problem. Former NCAR scientists Doug Lilly and James Deardorff came up with a way to depict turbulent eddies in a computer model: instead of trying to model every swirl down to the smallest scales, one could describe statistically how the effects of a group of small eddies might affect larger-scale motions. This technique, called large-eddy simulation (LES), is a cornerstone of turbulence modeling to this day.

LES relies on statistics to describe turbulent behavior, but thanks to ever-more-powerful computers, it's now possible for some simulations (like the one created by Bill and Bob) to depict turbulence more directly. Until now, colorful names--pancakes, worms, spaghetti, and noodles--have denoted structures found only in mathematical theory or in idealized simulations less intense than actual turbulence in the atmosphere. These phenomena are "very intermittent, and for atmospheric conditions they've been at much smaller scale than we could represent on our computers," says NCAR senior scientist Jack Herring (MMM).

Jack arrived at NCAR in 1972 full of hope. "At that time, I think we were more optimistic than now that we'd find a general theory of turbulence," he recalls. Despite the lack of an overarching theory, approaches such as LES have proven useful, especially in studies of the planetary boundary layer and other regions where turbulence is a key player.

Jack Herring.

One of the most important variables in turbulence work is the Reynolds number. This index of flow is a guide to how much nonlinearity can occur in a system. For a slowly moving flow or one where viscosity plays a dominant role (think of molasses), the Reynolds number is low and the motion is more predictable. If the flow is fast, as in the atmosphere, the Reynolds number is high and the motion is dominated by strongly nonlinear events (like explosive thunderstorm development) that are exceedingly hard to model or to reproduce in a laboratory setting. Some lab experiments are now using low-viscosity fluids such as low-temperature helium to better simulate the actual atmosphere.

According to Jack, the challenge that still faces theorists and LES modelers is "to construct a theory that represents--on a statistical level--a mixture of coherent structures in a near-random background flow." Jack offers a caution for LES modelers: "You may come out with a solution that looks very nice, but you have to be sure that the structures that are so small that their presence can be represented only statistically still have the proper effect on the larger scales that transfer momentum and heat."

One summer, two meetings

Given that turbulence research is, in Jack's words, "a difficult field with different people doing different things," the need for collaboration and communication is great. Turbulence work went through a downturn in interest at NCAR in the late 1970s, but Jack (along with NCAR alumni John Wyngaard and Jim McWilliams) helped spark a revitalization in the mid-1980s. The result was the Geophysical Turbulence Program (GTP), a small but vital activity that brings scientists from virtually every part of NCAR together with each other and with colleagues from around the world.

"We felt the need to reform the Turbulence Club into something more significant and permanent," recalls Jack. Officially based in the Advanced Study Program, GTP receives a small amount of base funding provided, since 1991, by the NCAR Director's Office. The program holds frequent seminars, sponsors one or two scientific visitors each year, and hosts a major symposium every year or two.

The Mesa Lab symposium this June will be one of the largest to date. It is cosponsored by GTP, the International Union for Theoretical and Applied Mechanics, and the International Union of Geodesy and Geophysics. "It's a big meeting by NCAR standards and a very important meeting by international standards," says Bob. The symposium is dedicated to Jack Herring and his decades of work in turbulence. Jack will deliver an opening address on the history of GTP and its predecessors at NCAR.

One of the hottest topics on the agenda is small-scale rotation events. The vortex tubes of 9 December 1992 will be discussed, as will other phenomena--for instance, oceanic eddies that entrain surface waters down to the lowest depths, where the waters feed into the global conveyor belt and can directly modify climate. "I had to create a whole subcategory [of presentations] on rotation," says Bob.

Later this summer, another GTP meeting will bring turbulence researchers to both ML and FL. Like its June counterpart, Observations, Experiments, and LES 1998 (OEL98) will draw from the modeling, observational, and lab-experiment communities to see what each has to offer. The meeting, sponsored by GTP with special NSF support, is expected to draw 60 to 80 invited participants, according to organizer Don Lenschow (MMM). "It will be a more informal gathering than the one in June," says Don.

As modelers explore other approaches to turbulence, LES modeling itself is changing, Don explains. It is now used not only to make statistical statements but also to address fundamental problems of structure. Yet, according to Don, "Questions remain as to how closely some LES simulations represent nature. . . . Furthermore, there are different implementation schemes for LES, and often no basis for evaluating their relative merits."

Don hopes the meeting will serve to bring LES specialists together with observationalists who are using the latest lidars and radars to track turbulence in clouds and in the low-level boundary layer, as well as with people doing new lab work with low-viscosity fluids. "As the technology advances in all these areas, the questions become more subtle and difficult to address," notes Don. •BH

On the Web

The Geophysical Turbulence Program Web site includes links to the Developments in Geophysical Turbulence symposium and the OEL98 workshop. (The latter link will be accessible only to participants.)

Geophysical Turbulence Program of NCAR

A NASA-supported study in March and April successfully sent the NSF/NCAR Electra and an onboard lidar into bumpy skies to sense clear-air turbulence up to a few kilometers ahead of its flight path.

"NSF/NCAR Aircraft Tests NASA Clear-Air Turbulence Sensor through Colorado Skies"

Using lidar and profilers, RAP scientists are working to map out turbulence in and near Juneau, Alaska, for the FAA in the first of what could be a series of customized detection and warning systems for airport turbulence.

"FAA and NCAR Chart Juneau's Turbulent Skies"

RAP's effort to use aircraft as turbulence sensing platforms continues to expand. Over 200 United Airlines planes will be participating by the end of 1998, and American and Northwest will join the system, providing about 450 more participating aircraft. The project was profiled in Staff Notes Monthly's November 1997 issue.

"Fasten your seat belts: RAP turns aircraft into turbulence sensors"

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Edited by Bob Henson, bhenson@ucar.edu

Prepared for the Web by Jacque Marshall