May 2006

Notes from the field: Turbulence and pollution

It’s been a busy couple of months in the field for NCAR. The HIAPER aircraft in its maiden campaign performed flawlessly over California while, in the Mexico City area, scientists gathered vital information about air pollution.

Soaring over the Owens Valley in March and April, HIAPER (High-performance Instrumented Airborne Platform for Environmental Research) flew through areas of turbulence known as mountain waves and aimed its instruments at treacherous atmospheric whirlwinds, called rotors. Its mission was part of a large, multidivisional and multinational field campaign known as the Terrain-Induced Rotor Experiment (T-REX), which collected data about rotors and mountain waves, as well as atmospheric behavior closer to the valley floor.

Meanwhile, ACD researchers and collaborators from around the world studied air pollution in and around Mexico City for the much-anticipated Megacity Impacts on Regional and Global Environments (MIRAGE) field campaign. Their objective was to determine how the pollution affects regional and global air quality, climate, and ecosystems, with an eye toward applying what they learn to other megacities around the world.

Here’s a look at these two important studies.

High and low

HIAPER won rave reviews from the T-REX team almost from the moment that it took off from Jeffco on March 2 for the first of 13 flights to California. “It’s an excellent platform that’s able to get up to high altitudes, cruise for a long time, and adjust to changes in flight conditions created by strong mountain waves,” says EOL’s Jorgen Jensen. “As far as the instrumentation on board, it has been very reliable. I’m really impressed with how things worked.”

The HIAPER flights were an important component of T-REX, which brought 60 scientists, technicians, and students from across the United States and Europe to California’s sparsely settled Owens Valley from March 1 to April 30. Researchers also flew two other aircraft at lower altitudes and set up a large ensemble of ground instruments.

The research team, led by Vanda Grubišić of the Desert Research Institute, picked the region because it has the steepest topography in the continental United States, with Owens Valley sitting some 9,000 feet (about 3,000 meters) directly below the highest peaks of the adjacent Sierra Nevada mountains. The mountains spawn atmospheric waves that propagate upward and can “break” into the stratosphere, creating clear-air turbulence. The strong wind shear and turbulence in these waves and lower- altitude rotors play havoc with aircraft.

HIAPER flew through the waves in the upper troposphere and lower stratosphere at altitudes of up to 45,000 feet (13,700 meters). It joined aircraft from the University of Wyoming and the United Kingdom, which flew at lower elevations and gave scientists additional views of the mountain waves, rotors, and valley phenomena.

Researchers on HIAPER released dropsondes to collect data on the rotors. The aircraft also carried an array of atmospheric chemistry samplers to provide insights into the ways that mountain waves moved air masses between the stratosphere and the underlying troposphere. Heightened ozone levels indicated that air was coming down from the stratosphere, where ozone concentrations tend to be higher; heightened carbon monoxide levels, on the other hand, indicated that air was rising from the lower troposphere. “Ozone and other tracers provided us with nice correlative signatures of mountain waves,” says ACD’s Laura Pan.

To study airflow, the research team also used an array of ground-based instruments, including radars, lidars, automated weather stations, wind profilers, and balloons.

The project didn’t focus just on higher altitudes. On days when HIAPER wasn’t flying, researchers examined turbulent eddies of air just above the levels of trees and bushes. Among the instruments they used was a suite of three hot-film anemometers, developed at NCAR, that took 2,000 measurements per second of winds in three dimensions.

“We’re looking at turbulence on all scales, from mountain waves that reach up to the stratosphere down to very small-scale turbulence near the surface,” explains EOL’s Greg Poulos, one of the project’s principal investigators and the ground-based instrument coordinator. “It’s a very complex study.”

The weather generally cooperated, bringing in high winds with a train of potent Pacific storms. Researchers didn’t always know the exact timing and strength of the airflow features in advance—which is part of the reason for the experiment in the first place—but their location, at least, was somewhat predictable. “Here we know where the turbulence is more likely to occur, relative to the mountain barrier. In severe storms, it sneaks up on you,” says EOL’s Dick Dirks, the field operations director.

Despite HIAPER’s excellent performance, researchers did encounter some challenges. On the ground, team members found themselves contending with gusty winds that pushed around instruments and researchers alike. And on HIAPER, the team had to make adjustments to sensors and overly noisy amplifiers. “These are normal problems when you initially instrument a new aircraft,” Jorgen explained.

But perhaps the greatest challenge was coordinating the aircraft. As Greg puts it: “If you can envision three airplanes stacked on top of each other flying around in coordinated fashion, with two of them releasing dropsondes and trying to avoid the other airplanes, while working around the sensitive national park lands in this area, that was a really significant challenge to overcome.”

Notorious air

On March 1, several dozen staffers, most from ACD and EOL, descended upon Mexico City for the MIRAGE field campaign. Working with collaborators from around the world, they took a close look at the chemistry of the city’s notorious air pollution.

The researchers didn’t have to search far to find the justification for their study. “Overall, we found an amazing haze everywhere, especially over the city but also outside it,” says ACD’s Sasha Madronich, one of the principal investigators. “It was a great mixture of different pollution conditions, including urban pollution, smoke from regional forest fires, and dust events.”

The field campaign included air and ground components, with researchers measuring both aerosols (airborne particles of dust, soot, and other pollutants) and gaseous pollutants (including ozone, nitrogen oxides, carbon monoxide, sulfur dioxide, and hydrocarbons and their oxidation products).

One of the biggest surprises during the campaign was the extent of active particle production the team observed in the atmosphere—that is, gases condensing to form particles. The especially tiny particles common in highly polluted areas can damage human lung tissue and affect global climate.

“You’d expect in a very polluted location not to have particle production,” explains ACD’s Alex Guenther, who was stationed at a ground site outside Mexico City. “What happens in a polluted atmosphere is that you already have so many particles in the air that the gases attach to existing particles. But we observed particle production going on at the ground site, and that was something unexpected.”

One of the researchers’ goals will be to determine if the city’s exceptionally high level of pollution actually changes the underlying chemistry of particle ­production.

ACD’s Jim Smith ran an experiment at the ground site to look at how quickly particles of different sizes take on water and turn into cloud droplets, since the formation of clouds can impact climate. “The experiments worked out as well if not better than expected,” he says.

From the air, researchers aboard the NSF/NCAR C-130 aircraft made multiple flights to transect the plume of air pollution that blows out of Mexico City, usually spreading northeast due to prevailing winds.

“We didn’t see the flow to the northeast as often as we had thought, but we had a much more varied situation,” says MMM’s Bill Skamarock, who ran the Weather Research and Forecasting model (WRF) during the campaign to predict the plume’s movement. “But the model performed well and we were able to find the plume, more often than not, where we expected.”

The aircraft team logged 88 hours of flight time in all, on some occasions following the plume all the way to the Texas coastline and the Yucatan peninsula. Five other research aircraft were also in the air as part of MILAGRO, an umbrella campaign of which MIRAGE was one component. “Aircraft controllers in Mexico were extremely helpful in coordinating six airplanes buzzing through their airspace,” Sasha says.

The logistical complications of transporting equipment across the U.S.-Mexico border proved to be the biggest challenge during the field campaign. “Things were a little slow getting started because we had issues with shipments arriving late, but eventually everything made it,” Sasha says.

From Mexico City, the C-130 flew directly to Seattle for INTEX-B (Intercontinental Chemical Transport Experiment-B), another of the four MILAGRO field campaigns. The main goal of INTEX-B was to quantify the transport and evolution of air pollution from Asia across the Pacific Ocean to North America, and assess its implications for regional air quality and climate.

• by David Hosansky and Nicole Gordon

On the Web

More about T-REX

More about MIRAGE

In this issue...

Notes from the field: Turbulence and pollution

Terrorism and climate change

New program is a star

Peter Gilman wins Hale Prize

At the helm of ESSL

New Digital Image Library

Just One Look

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