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November 2000

Pollution at its roots: ATD probes the stable air behind smog

NCAR's Tethered Atmospheric Observing System is a balloon 27 feet (8 meters) long that can hover up to 3,200 feet (1 km) above the ground. Instruments attached at several heights along its string provide information about once a second on winds, air pressure, and other atmospheric conditions. (Photo by Charlie Martin.)

Members of the TAOS team include, left to right: Bill Brown, Ned Chamberlain, Dave Parsons, James Pinto, Steve Cohn, and Charlie Martin. Not pictured are Mike Susedik and Tim Lim, who spent all of October at the project, as well as Bruce Morley and Craig Walther. (Photo by Carlye Calvin.)

Measuring a couple of square meters across, NCAR's Multiple Antenna Radar Profiler is a 915-megahertz unit modified to rapidly sample wind in three dimensions by pointing in a single direction. ATD hopes to build a second unit soon with spare parts. (Photo by Charlie Martin.)

A new type of tethered weather balloon that can hover more than half a mile high made its maiden vorage last month at a research project last month in Utah. NCAR scientists launched radiosondes and pointed radar, lidar, sodar, and wind profilers into the skies of the Salt Lake Valley from 1 to 31 October. The U.S. Department of Energy sponsored the Vertical Transport and Mixing Experiment, an intensive study of how air moves in the valley, especially overnight during colder months. Salt Lake City, Denver, and other cities prone to pollution-trapping temperature inversions could benefit from the results, which will be used to improve computer models employed in forecasting weather and tracking air quality.

"The U.S. population's been growing in the West, particularly in urban basins. Predicting the weather and air quality in these areas, where calm, stable air interacts with mountain and valley circulations, is a fairly difficult proposition," says Dave Parsons (ATD). Through October, Dave and 16 NCAR colleagues shared duties at Bluffdale, about 25 miles (40 kilometers) south of downtown Salt Lake City. The NCAR site was one of nine in the experiment, which involved 60 personnel from 14 research institutions. Principal investigators from NCAR are Dave, Bill Brown, Steve Cohn, and Tom Horst.

Making its debut at the project was the Tethered Atmospheric Observing System (see photo). TAOS was developed by Ned Chamberlain and colleagues to take measurements every second or so of winds, air pressure, and other conditions needed to understand and predict turbulence. Its vertical range is just over half a mile (about 1 km), with measurements taken at eight different levels up to that height. For the Salt Lake Valley experiment, which took place below a major flight path, the balloon sat at about 2,000 feet (600 meters), and sent back rapid readings from five or six heights.

Also at the site was NCAR's Multiple Antenna Profiler Radar, in its first field use after recent hardware and software upgrades headed by Bill Brown, Charlie Martin, and Mike Susedik. MAPR looks upward to take rapid measurements of winds up to about a mile above the instrument (almost 2 km) in clear air. Sampling is possible at far higher altitudes when rain or snow is around to provide radar targets. While most wind profilers report measurements every 15 minutes, MAPR's new transmitter—about ten times more powerful than before—is capable of sensing wind behavior every 60 seconds. "MAPR's designed to sample a smaller volume and do it more rapidly," says Dave.

Deciphering the subtle mixing of stable air overnight is key to forecasting the smog-trapping inversions—cold, polluted urban air trapped under a layer of warmer air—that greet residents of Denver and other western U.S. cities on winter mornings. Data from the experiment will be analyzed to see how models might be able to better handle inversions. James Pinto (CU/ATD) will be running case studies from the project on the Penn State/NCAR Mesoscale Model, version 5, "and, we hope, the new [Weather Research and Forecasting] model," says Dave (see http://wrf-model.org). Without high-resolution models, "you really can't get the drainage flows [cool air that descends from higher elevations at night] and predict the air quality within the urban basin."

While other weather projects focus on dramatic events, "for this project we wanted cold, clear, dry, stable conditions," says Parsons. Autumn was picked because the nightly inversions have a better chance to mix out and then reform, rather than settling in for days or weeks as they often do in midwinter. The atmosphere obliged in October with plenty of single-day events that will make good case studies, says Dave. "It's been a pretty rich environment for mesoscale winds. We can see the lake breeze and the drainage flows."

• Zhenya Gallon and Bob Henson

On the Web:
Vertical Transport and Mixing Experiment

MAPR site, including data from Utah


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Edited by Bob Henson, bhenson@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Mon Oct 30 15:12:44 MST 2000