The edge of weather:
Charting the boundary
between atmospheric zones

The ozone hole is just one of the surprises to emerge from the little-explored atmosphere at the upper end of balloon and aircraft range. Upcoming satellites and a high-altitude jet promise to yield comprehensive detail on the chemistry and dynamics that rule our planet's atmosphere above five miles and influence life at the surface.

Pilots and astronauts fly through it, but few people spend much time in the shallow, bitterly cold slice of air known as the tropopause. Located anywhere from about 8 to 16 kilometers high (5–10 miles), depending on the season and region, the tropopause is the zone that separates the lowest two atmospheric layers. In the troposphere below, roiling air masses create thunderstorms, blizzards, and other familiar kinds of weather (the Latin root "trop" means "to turn"). In the more stable stratosphere above, ozone quietly shields us from solar harm.

Although it's defined by its upper and lower neighbors, the tropopause is an interesting place in its own right.

hand for IHOP.

When strong winds bump against mountains, or when thunderstorms push upward, they can send pulses of energy—gravity waves—propagating upward and outward. Occasionally, part of the tropopause gets folded into developing weather systems, a process that brings frigid air well below its usual province and can even help energize ground-level storms. Ozone, water vapor, and other greenhouse gases sift through the troposphere, yet their journeys are only dimly understood.

For many years, observing the tropopause has been a daunting ordeal. Weather balloons and some aircraft can penetrate the region, especially in midlatitude winter, but vast parts of the tropopause in the tropics and elsewhere go unsampled. By mid-decade, a new NSF/NCAR aircraft will be equipped to probe sections of the tropopause for hours on end. Meanwhile, in space, an infrared sensor developed largely at NCAR in collaboration with the University of Oxford will peer down at the tropopause, collecting the densest picture yet of its complex chemistry.

Computer modelers at NCAR are staying ahead of the instrument curve. Already, they're gaining new insights into the tropopause through a powerful blend of software that links the region's chemistry to winds, temperatures, and sunlight.

HIRDLS to come

To satellite instrument developers like NCAR's John Gille, patience is more than a virtue—it's practically a job requirement. The atmospheric chemist and his colleagues at NCAR and Oxford are more than a decade along in their laborious quest to get a breakthrough instrument in orbit.

At the core of HIRDLS is an infrared radiometer, a device that senses light in the wavelengths just beyond those that humans can see. Using 21 different infrared channels, HIRDLS will distinguish and measure various aerosols (airborne particles), along with each component in a goulash of ten chemical compounds. The sensor will also infer temperature based on emissions from carbon dioxide, which is well mixed throughout the atmosphere, and it will track cirrus clouds that are often too thin and faint to assess fully by traditional satellites.

Of all the chemicals HIRDLS will measure, it's an ordinary one—H2O—that interests Gille as much as any. "Water vapor is the most important greenhouse gas. We don't know how it's distributed in the upper troposphere and stratosphere and what maintains that distribution." Puzzling patterns have emerged from the scanty observations collected to date. For instance, water vapor seems to be increasing in the lower stratosphere, where it has a cooling effect. Most likely as a result of this increase, along with the well-publicized depletion of another greenhouse gas, ozone (which warms by absorbing sunlight), temperatures have fallen to record lows in the lower stratosphere.

HIRDLS is a limb scanner: instead of looking directly down, it peers over its shoulder, with its field of view slicing through the atmosphere just above Earth's receding horizon. Scanning both up-and-down and across, HIRDLS will divide its global domain into over 100 layers vertically and more than 1,000 boxes horizontally. This unprecedented detail in all three directions will produce more than 8,000 profiles of temperature, trace gases, and aerosols each day, allowing for region-by-region analyses of chemistry and dynamics. "It's a very flexible instrument," says Gille, "so it gives us the ability to tune what we're doing as questions or concerns evolve."

New heights for NCAR-university field work

As HIRDLS goes into orbit aboard Aqua, engineers at a Lockheed Martin plant in Greenville, South Carolina, will be putting the final touches on a Gulfstream V jet. It will be the first new NSF/NCAR aircraft in years and, at just over $80 million, the single biggest item ever in the NCAR budget. The High-performance Instrumented Airborne Platform for Environmental Research (HIAPER) has already stimulated planning among university scientists. Starting in 2005, researchers will use this sleek, swift vehicle to address the most vexing questions of the upper troposphere, tropopause, and lower stratosphere.

More than 30 nations have shuttled heads of state and other VIPs aboard Gulfstream aircraft. Without the wine-glass holders and other corporate accoutrements, the aircraft makes for a sturdy, resilient research platform. A Gulfstream IV flown by NOAA since the mid-1990s has probed the upper troposphere around hurricanes in never-before-seen detail.

Such sorties are made possible by the Gulfstream's combination of altitude (the G-V is certified up to 51,000 feet) and range (more than 5,600 mi or 9,000 km). According to David Carlson, head of NCAR's Atmospheric Technology Division, "A G-V can get above 45,000 feet in less than an hour and can easily cross major continents or oceans in a single flight. No current research aircraft has that combination of capabilities."

The HIAPER advisory committee, made up of scientists from NCAR, NSF, NOAA, NASA, and universities, opted to limit the amount of airborne weight in order to maximize the aircraft's reach. Thus, according to NSF program official James Huning, "instrument developers will be encouraged to partner among themselves and incorporate emerging technology to reduce mass, size, and power needs."

Ronald Smith (Yale University) led community planning for a midsized jet for several years. "I'm certainly excited about the role HIAPER will play in university-based research on the upper troposphere and lower stratosphere," says Smith. He's interested in the dynamics of the region, including gravity waves, and hopes to use HIAPER to monitor stable isotopes of oxygen that can trace the path of water vapor up to high altitudes and down again as rainfall or snowfall. "The HIAPER capabilities will open many new opportunities for me," says Smith.

A meeting of the models

As if screening a preview of what HIRDLS and HIAPER may uncover, scientists at NCAR and elsewhere are taking a close look at jewel-toned maps created from computer models. A melding of three different software packages, each created largely on its own in the 1990s, has produced a new and entirely different tool. This hybrid approach could help match up cyclic aspects of the ocean and lower atmosphere with the ebb and flow of other features far overhead.

"People have talked about the linkages for a long time. It's just that we've never had the tools to study them," says Douglas Kinnison. He joined NCAR in 1999 to work on the Model for Ozone and Related Chemical Tracers. The first incarnation of MOZART, conceived by Guy Brasseur (now at the Max Planck Institute for Meteorology), tracked 56 chemicals at 25 different heights across the global troposphere. A second version, featuring a host of improvements, was made available to university scientists in 2002.

Now Kinnison is helping yoke MOZART to WACCM, the Whole Atmosphere Community Climate Model, itself a blend of NCAR's main tropospheric model and an upper-atmosphere model developed by NCAR's Raymond Roble to simulate conditions above about 50 km (30 mi). Even in its primitive one-way stage—with sea-surface temperatures and WACCM data influencing chemistry, but not vice versa—this meeting of the models holds promise. One analysis by NCAR's Fabrizio Sassi has shown that El Niño and La Niña may affect the intensity of the Arctic's seasonal ozone depletion. This depletion is typically much less than that of the Antarctic's famed ozone hole, but it has intensified in recent years, perhaps due to the stratosphere's record cold.

Steps like this may become leaps in understanding once a fully interactive blend of WACCM and MOZART is running. Thanks to the lengthened time frame made possible through WACCM, "multi-year runs of chemical transport are now possible," according to NCAR's Peter Hess. In 2002 NCAR launched several simulations designed to chart the locations of ozone and other chemicals over a 40-year period. These and other results are expected to feed into the next major report from the Intergovernmental Panel on Climate Change, due in 2007—just after a new generation of sensors will have set its sights in and around the tropopause.