Cloud composition and dynamics take researchers to new heights

By Rachel Hauser
winter, 2002

The NASA WB-57F high-altitude research aircraft taxiing before a CRYSTAL-FACE science flight at the Key West Naval Air Facility.

Members of NOAA-CIRES Climate Modeling and Diagnostics Laboratory and NOAA Aeronomy Lab [ About this Lab ] take part in carefully-orchestrated study

Cirrus clouds, found at high altitudes and formed from ice crystals, often act as gatekeepers for radiation flowing into and emitted from Earth. The clouds' ice crystals scatter incoming sunlight and cause surface cooling. In the opposite direction, they absorb and rereflect long-wave radiation emitted from Earth's surface into the troposphere below the clouds, indirectly resulting in surface warming.

While scientists understand cloud formation basics, many details, particularly with high-altitude clouds such as cirrus, elude them, said David Fahey, a research physicist at NOAA's Aeronomy Laboratory in Boulder.

Without thoroughly understanding the specifics of cloud dynamics, it's difficult to isolate how human activities interfere with or add to cloud processes, he says. And because cirrus clouds, particularly in the tropics, develop near the stratosphere, detailed studies have been difficult at best.

In an effort to take a thorough look at tropical cirrus clouds, CIRES researchers involved in NASA's and NOAA's airborne Cirrus Regional Study of Tropical Anvils and Cirrus Layers-Florida Area Cirrus Experiment (CRYSTAL-FACE) field study worked throughout July 2002 to gather data.

The study, one of the most intensive ever for NOAA and NASA, teamed more than 200 scientists and support staff, six aircraft and ground-based radar stations to observe tropical anvil and cirrus clouds. Data gathered will ultimately contribute to global climate models. The experiment also tracked pollutant transport through the troposphere into the stratosphere.

Intense regional heating creates the setting for both spectacular clouds and storms. Tropical cumulonimbus form as convective forces sweep up parcels of warm, moist air-along with pollutants and particles. At the cloud mass's highest point, sometimes as high as 60,000 feet, anvil shapes form where warmer air flows out and hits colder air.

As convective mixing desists, it's possible to see trails of cirrus clouds blown from the anvil by prevailing winds. When convection-driven thunderstorms disperse, they often leave behind extensive cirrus cloudiness, which sometimes provides the only tangible evidence of a storm's passage.

"Tropical anvils look rather benign and stationary, but they often have a lot of water vapor 'flow through' from thunderstorms," Fahey said. "Water vapor gets swept up from the lower atmosphere along with any particulate matter found near the Earth's surface-the process acts as a kind of surface vacuum."

Scientists propose that driven by intense tropical radiative heating, powerful convective forcing transports chemicals and aerosols from lower tropospheric regions into the stratosphere.

"Tropical anvils are thought to be one of the main mechanisms for transporting tropospheric pollutants into the stratosphere," said James Elkins, a group chief with NOAA-CIRES' Climate Monitoring and Diagnostics Laboratory.

"The tropics have a lot of convective activity, and because of this chemicals and particulates are easily lofted into the atmosphere," said Erik Richard, another CIRES research scientist at the Aeronomy Laboratory. "Tropical convective activity resembles boiling water; warm, moist air boils up at the equator and makes its way to the poles."

Chemical components found in the upper atmosphere, such as ozone, nitric acid, methane and water vapor were some of the compounds measured by instruments loaded onto NASA's WB-57F, an airplane capable of flying above 60,000 feet while carrying more than 4,000 pounds of cargo and instrumentation. "Most of the instruments on board required taking an air sample directly from the atmosphere," said Fahey.

"As the plane flew we got a scalpel cut of the atmosphere, taking measurements of atmospheric methane and ozone as well as other gases and particulate matter about every second," Richard said.

Richard explains that while interesting in themselves, ozone and methane also act as markers identifying the transitional boundary between the troposphere and stratosphere. By comparing levels of methane to ozone, researchers can determine whether air samples are tropospheric (high methane) or stratospheric (high ozone) in origin.

Each scientific team ultimately depends on more than their own instruments' measurements, which provide little information without other points of reference. Interpreting each set of measurements' significance depends on comparison to others.

"Because ozone and methane provide a diagnostic, telling us what air we're looking at, my team's findings will play a big supporting role for many other missions," said Richard. "For instance, groups trying to understand the implications of a given particle's state, its size, and its distribution, need to know where the particle was collected, whether in the stratosphere or troposphere."

Each piece of information builds on the next. For example, Fahey's group studies atmospheric nitric acid, but other researchers who study regional particle densities and distribution aid their research findings. Based on this second group's study results, Fahey's scientists have a better chance of identifying changes in nitric acid signals or, alternately, determining whether airborne particulates form preferentially with nitric acid.

Working on the first round of fine-tuning their data, the researchers seem satisfied that they have looked at clouds in a way never before done. Because of the intricate planning and equipment deployment, discrete information about individual clouds will be augmented by large-scale views provided by ground-, upper atmosphere and space-based instruments.

The hangar used for the CRYSTAL-FACE aircraft is at the Key West Naval Air Facility in Key West, Florida. The aircraft shown left to right are the ER-2, Proteus, WB-57F and Twin Otter. NASA/Bill Ingalls photo NA

Ground-based radar stations provided cloud and weather observations. Simultaneously, above the WB-57F, at altitudes close to 70,000 feet, remote sensing instruments flying on NASA's ER-2 airplane looked at radiation scatter by clouds providing upper altitude information in real-time. Earth-orbiting satellites provided a view from space.

As well as corroborating and enhancing the CRYSTAL-FACE results, mission data will aid validation and calibration of NASA satellites that flew overhead, which enhances the value of such programs and data collected.

With the first round of data processing soon to finish, Fahey is impressed by the mission's initial achievements.

"Deploying six planes, 200 people and this much measurement equipment successfully was a significant technical challenge. A tremendous amount of energy went into figuring out where each plane should go on a particular day.

"As it turned out, CRYSTAL-FACE worked extremely well and serves as a precedent for future programs," Fahey said. See www.etl.noaa.gov/programs/2002/fireface for more information on CRYSTAL- FACE.