UCAR > Communications > Quarterly > Fall 1996

STERAO storms into action

A field program designed to examine the effects of thunderstorms on the chemical composition of the troposphere and lower stratosphere found all the action it wanted last summer in northeastern Colorado. The Stratosphere-Troposphere Experiment: Radiation, Aerosols, and Ozone--a.k.a. STERAO--Deep Convection field program was the first installment of a multiyear, multisite experiment designed to examine physical and chemical processes that occur near the tropopause.

"We were exceptionally fortunate in terms of weather," said co-principal investigator James Dye (NCAR). "In a one-week period in mid-July we had four excellent storm cases, and that's more than you can even dream of in the weather business."

The interdisciplinary experiment was led by principal investigators Dye, Steve Rutledge of Colorado State University (CSU), and Adrian Tuck of the NOAA Aeronomy Laboratory and joined by other investigators from five universities and six research institutions. Equipment included two aircraft, a Doppler radar, NCAR's mobile cross-chain Loran atmospheric sounding system (CLASS) unit, and some specialized instruments for lightning detection and chemical analysis. According to Dye, "This is the first major project that includes comprehensive, coordinated chemical, electrical, and dynamical measurements in and around thunderstorms."

STERAO focused on what happens when thunderstorms suck air from the boundary layer and transport it to the upper troposphere and lower stratosphere, up to about 15 kilometers. Dye described the main effects of this convection:

Storms redistribute trace gases and aerosols from both anthropogenic and natural sources into the upper troposphere on short time scales. Often the lifetime of these gases is longer in the upper troposphere than at a lower altitude, and thus they can play a larger chemical role there.
Lightning is a dominant source of nitrogen oxides (NOx) for the free troposphere, but there are factor-of-ten uncertainties as to the strength of this source. NOx species are key players in determining the ozone balance in the troposphere.
Storms also redistribute water vapor, which, along with ice particles from storm anvils, plays a significant role in the earth's radiation balance.

To determine what effects thunderstorms have it is necessary to obtain measurements before, during, and after storms occur and also from the boundary layer up into the upper troposphere and lower stratosphere. The original experiment design called for three aircraft to fly together: a NOAA P-3 with tail-mounted Doppler radar and extensive chemistry instrumentation, to obtain air-motion and chemical measurements in the boundary layer and midtroposphere; the University of North Dakota Citation, to make measurements in and around the anvil of the storms; and the newly acquired NCAR/NSF WB-57, to fly over and downwind of the storms. The B-57 was grounded by a series of problems (see p. X), but the other two aircraft flew below, around, and in the anvils of clouds and sometimes (for the Citation) even over thunderclouds, making in situ measurements and taking some samples for later chemical analysis.

The ground-based CSU/University of Chicago/Illinois State Water Survey Radar (CHILL) multiparameter Doppler radar provided remote measurements of microphysical conditions and air motions within the storms. The CSU/CHILL radar site near Greeley also was the operations center for coordinating the experiment and communicating with aircraft. A unique part of the experiment was the French ONERA (Office National d'Etudes et de Récherches Aerospatiales) lightning interferometer, which mapped three-dimensional lightning channels and obtained counts of intracloud and cloud-to-ground flashes. These measurements were supplemented by cloud-to-ground lightning measurements from the National Lightning Detection Network and three electric-field-change meters, one mobile and two fixed, that could detect both intracloud and cloud-to-ground lightning. The NCAR mobile CLASS unit was used to make both morning soundings for forecasts and afternoon soundings to characterize the environment in which the storms grew. Weather forecasting for the project was done by the NOAA Forecast Systems Laboratory with supplementary forecasts from CSU.

The combination of lightning and chemical data will allow researchers to quantify how much NOx was produced by lightning in these storms and how the resulting output varied with altitude. In 1989 Brian Ridley of NCAR and Dye, in an exploratory study in New Mexico, found that most of the NOx produced by lightning was in the upper part of the anvil. Although STERAO's data are just beginning to be analyzed, "that seems to hold true here as well," says Dye.

For further information, contact Dye (303-497-8944 or dye@ucar.edu) or Ridley (303-497-1420 or ridley@ucar.edu).

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 10:18:15 MDT 2000