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T-REX: Catching the Sierra’s waves and rotors

 

by Bob Henson

Pollack, Cowan

Postdoctoral student Ilana Pollack confers with NCAR flight technician John Cowan aboard HIAPER. T-REX was the first major field deployment of the new NSF/NCAR aircraft. (Photo by Carlye Calvin.)

Few people outside the military ever get a bird's-eye view of the stark topography around California's Owens Valley. This restricted airspace, which serves as a training ground for U.S. Air Force pilots, also gets some of the most violent mountain-induced airflows in North America. Such was the target of the Terrain-Induced Rotor Experiment (T-REX), which brought 60 scientists, technicians, and students from across the United States and Europe to this sparsely settled area from 1 March to 30 April.

When upper-level winds from the Pacific Ocean slam into the Sierra Nevada, they produce atmospheric waves that can propagate and "break" into the stratosphere at heights of more than 16 kilometers (10 miles). The strong wind shear and turbulence in these waves can play havoc with aircraft. An atmospheric wave may touch ground and produce a rotor, a huge rolling-pin-shaped zone of high turbulence with weak east-to-west winds across the valley floor and howling west-to-east winds higher up. Although the general structure of breaking waves and rotors has been known for decades, the details remain fuzzy.

Led by scientific project director Vanda Grubišić (Desert Research Institute), researchers studied the mountain-generated flow from several perspectives during T-REX. HIAPER joined the University of Wyoming King Air and U.K. Facility for Airborne Atmospheric Measurements Bae146 aircraft. The planes deployed dropsondes (see sidebar) and aimed cloud radars into the rotors and lee waves. On the ground, radars, lidars, automated weather stations, wind profilers, and balloons sampled the airflow. "We've flown through and measured rotors and breaking gravity waves, and we've measured how strong wind shear and turbulence is in them," says Grubišić. "We're discovering what the relation is between the rotors and waves close to the mountains and higher up. While the rotors are the focus of this study, we are learning a whole lot about the waves aloft as well."

The train of potent Pacific storms that gave parts of California the wettest March on record kept T-REX participants busy. The project's aircraft were buffeted by updrafts and downdrafts as strong as 20 meters per second (45 mph). The exact timing and strength of the airflow features weren't always known in advance—which is part of the reason for the experiment in the first place—but their location, at least, was somewhat predictable. "Here [in the Owens Valley] we know where the turbulence is more likely to occur. In severe storms, it sneaks up on you," says Richard Dirks, the NCAR field operations director.

Lidar

Screen
Kuettner
Left: The NCAR Raman-shifted Eye-safe Aerosol Lidar, or REAL (on trailer) scanned clouds. The display above includes the cloud in the photo. Right: Joach Kuettner. (Photos by Carlye Calvin.)

T-REX was the first major deployment for the NSF/NCAR High-performance Instrumented Airborne Platform for Environmental Research. HIAPER's ability to fly at altitudes up to 51,000 feet (about 17,000 meters) made the aircraft ideal for probing the high-level features studied in T-REX, and the plane's cruising range of 11,000 kilometers (7,000 miles) allowed it to commute to work. Apart from a few overnight stays, most of the HIAPER flights were one-day sorties from its home base at Jefferson County Airport (Jeffco), midway between Boulder and Denver. Thanks to improvements in long-distance communication, the HIAPER operations center was also based at Jeffco. This allowed many NCAR scientists to stay involved in Colorado as the project unfolded in California.

The better knowledge of breaking waves and rotors that is likely to result from T-REX could help improve turbulence forecasts for aviation, according to Ronald Smith (Yale University), one of the project's principal investigators. "I believe that within a few years we could see the measurements and forecast-model tests from T-REX translated into improved forecasts of turbulence in the troposphere and stratosphere, which will have a direct impact on commercial aviation," says Smith.

The project will also shed light on chemical processes at altitude, especially the ways in which breaking waves can help foster the mixing of constituents between the troposphere and stratosphere. NCAR postdoctoral researcher Ilana Pollack upgraded and operated NCAR's ozone instrument on HIAPER and assisted with additional chemical tracer measurements set up by NCAR scientist Teresa Campos. "Ozone and other tracers provided us with nice correlative signatures of mountain waves," says NCAR's Laura Pan, another T-REX investigator. HIAPER data will also help illuminate the springtime air chemistry at the troposphere-stratosphere boundary along the plane's flight tracks, which crisscrossed the Great Basin.

It's hard to imagine anyone more gratified by T-REX than principal investigator Joachim Kuettner. The UCAR distinguished scientist first explored mountain waves in Germany in the 1930s as part of his doctoral research. At the helm of an open sailplane, he set world records for glider altitude. In the 1950s, Kuettner led the Sierra Waves Project. Now 96, Kuettner found himself again exploring the region's mountain airflow, this time aboard HIAPER. He's long been familiar with rotors, but this experiment offered the most in-depth sampling of the phenomenon in Kuettner's eight decades of research on mountain flow. "I've always wanted to explore the rotors," he says. "It's taken me this long."

Where did that dropsonde go?

Once a deployment crew launches a radiosonde or dropsonde to monitor the structure of the atmosphere, they don't expect to see the instrument again. The cost of recovering a sonde would likely exceed its value. Besides, it's not easy to find out where the device hit ground—or more likely water, because most of the thousands of Global Positioning System (GPS) dropsonde deployments during the last decade have been over the ocean for hurricane monitoring and other marine-based studies.

Things were different during T-REX. "For the first time in the dropsonde's field history, T-REX gave us a chance to retrieve and examine our instruments after they were deployed," says NCAR's Dean Lauritsen. To gain access to the project area, a restricted military airspace surrounded by national parks and wilderness areas, the T-REX organizers agreed to retrieve as many of the sondes as was practical. The small size of the area helped, but the big assist came from a new high-accuracy GPS receiver installed in the latest-generation dropsondes. T-REX was the second field project to use these new sondes, which debuted last year in the Hurricane Rainband and Intensity Change Experiment (RAINEX).

After each T-REX sonde completed its journey from the NSF/NCAR HIAPER aircraft to the ground, volunteers from the T-REX staff combed the study area to find it. Armed with handheld GPS receivers, they entered the last latitude and longitude reported by the sonde before impact. "Many sondes continue to transmit for a short time after they've landed, which often makes the final position even more accurate," says Lauritsen. Most sondes were found within 10 meters (33 feet) of the locations indicated by the handheld GPS units. Some of the sondes had only minor damage from the impact, so Lauritsen and colleagues may be able to refurbish and reuse them in a future field program.

 

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