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NCAR chemists and physicists are sampling storms in STERAO

What feature in the sky could be more solid-looking than a wall of cumulonimbus clouds? Actually, a thunderstorm is less a solid body than it is a process--a continuous up-and-out flow of air, moisture, chemicals, and particles.

A new field program will be sampling that flow in detail this summer. Scientists from the Atmospheric Chemistry Division, Atmospheric Technology Division, and Mesoscale and Microscale Meteorology Division are joining the NOAA Aeronomy Laboratory, Colorado State University (CSU), University of North Dakota (UND), and several other institutions to mount the first of three experiments in STERAO, the Stratosphere-Troposphere Experiment: Radiation, Aerosols, and Ozone.

STERAO will range over several years and several sites. Its aim is to profile the chemical and dynamical interchange between the atmosphere's lowest two layers. This summer the focus is on STERAO's first field program, subtitled Deep Convection and the Composition of the Upper Troposphere and Lower Stratosphere. The site is northeast Colorado; the period is late June to late July. The tools include three aircraft, a Doppler radar, NCAR's mobile sounding unit, and some specialized equipment for lightning detection and chemical analysis.

Jim Dye, MMM
Photo: Carlye Calvin
Much as hurricanes bring heat and moisture from the tropics to more polar latitudes, thunderstorms pull huge volumes of mass from the boundary layer (the atmosphere's lowest kilometer) to the upper troposphere and lower stratosphere (heights of 10-20 kilometers). According to Jim Dye, MMM senior scientist and co-principal investigator of this summer's project with Brian Ridley (ACD), the convection has multiple effects:

• Storms redistribute water vapor by transporting it directly and by forming rain and hail.

• Lightning produces large amounts of nitrogen oxides (NOX)--perhaps 10-15% of the global budget--to be distributed by the storms' circulation.

• Ice particles in storm anvils play a major role in the earth's radiation balance.

• Thunderstorms help maintain the global electric circuit.

Until now, the interests of chemists and physicists in studying thunderstorms hadn't converged to produce a unified field program. "This is the first major project that includes comprehensive, coordinated chemical, electrical, and dynamic measurements," says Jim.

One of the main goals of STERAO is to cast an observational net in three dimensions around cumulonimbus. To do this, the project will rely on three aircraft circulating in and near storms: a NOAA P-3 "hurricane hunter," a Citation from UND, and the NSF-owned WB-57 in its first NCAR research expedition. The first two planes are now on hand, while the WB-57 should be ready to join the project in July (see sidebar).

As they make their rounds, the aircraft will make in-place measurements and take samples for later chemical analysis, while obtaining remote measurements, such as wind fields from the P-3's Doppler radar. A ground-based multiparameter radar operated by CSU will work in concert with the P-3 to provide remote microphysical measurements while allowing for multiple Doppler analyses later on.

Brian Ridley, ACD
Photo: Carlye Calvin
Lightning is high on the list of phenomena targeted by STERAO. The nitrogen oxides it generates are an important precursor for ozone formation in the lower atmosphere, says STERAO investigator Brian Ridley (ACD). "Not only is it important to measure the total production of nitrogen oxides by lightning activity, it is equally important to understand how this production varies with altitude in the troposphere."

Dye and colleagues plan to use data from a French ground-based lightning interferometer to map three-dimensional lightning channels and obtain counts of intracloud and cloud-to-ground flashes. The interferometer readings will be supplemented by three electric-field-change meters, one mobile and two fixed, that also can detect intracloud and cloud-to-ground flashes. The lightning measurements will be correlated with chemical measurements to see how the production of nitric oxide varies with electric activity.

The operations center will be based at the CSU radar site near Greeley to coordinate the airborne and ground-based teams. The latter include ATD's mobile cross-chain Loran atmospheric sounding system (CLASS) and a van to shuttle the mobile electric-field sensor.

Among the other institutions participating in STERAO are the National Severe Storms Laboratory, NASA/Marshall Space Flight Center, the Universities of Washington and Maryland, and the Jet Propulsion Laboratory. •BH

Sparring with the WB-57

The X-rays were in, and the news was not good. From the WB-57's point of view, it was the equivalent of going in for a routine checkup and having the doctor say, "Uh-oh."

The NSF-owned aircraft had sprouted a serious set of cracks along two of the spars that support its right wing. The problem was discovered earlier this spring, forcing a speedy repair and the postponement of the WB-57's use in STERAO (its NCAR debut) from June until July.

"We knew that the cracks were there in the wing," says Bruce Gandrud, a scientist in ATD's Research Aviation Facility. "What we didn't know was that they'd grow. We'd X-rayed the wings when we got the plane [in 1994] as part of the routine purchase inspection." Although the tiny cracks along the spar were noted then, their extent fell below the threshhold that mandated immediate repair or replacement. The next X-rays were taken in March, revealing the bad news.

RAF lucked out by finding one spar in the aviation warehouse at Kelly Air Force Base, near San Antonio, Texas. The other had to be manufactured at McClelland AFB, near Sacramento, California. "It had to be made of a certain aircraft-grade aluminum," says Bruce, "and it had to be 167 inches long. Most chunks of aluminum are in 12-foot billets, or 144 inches. Once they had the material in hand, the machining was relatively straightforward."

Still, the timeline was tight, and the relief was palpable around RAF as the repair neared completion. Bruce reports--understatedly--"It's nice to see this problem solved." •BH

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Prepared by Jacque Marshall, jacque@ucar.edu, 303-497-8616