10/18/00

Dawn Levy, News Service (650) 725-1944; e-mail: dawnlevy@stanford.edu

Stanford collaborates in Utah balloon study of weather and smog

If Utah officials get more than their fair share of UFO sightings this October, just blame it on the weather.

On Oct. 5, scientists at government laboratories, private institutions and 14 universities including Stanford released hundreds of balloons -- tethered and free-floating, dangling thermometers, anemometers, transponders and other instruments -- over the Salt Lake Valley as part of a month-long study to better understand how and where air pollution moves. Sponsored by the Department of Energy, the study focuses on poorly understood air mixing in the nighttime atmosphere and is part of a four-year, $12 million research effort.

While most participants are using data they themselves are collecting, Stanford researchers Frank Ludwig and Bob Street are using a mass of data collected by others to develop analytic techniques and software tools that will help them build better computer models to predict air quality and weather. Ground, atmosphere and satellite data will come from about 40 sources including the U.S. Forest Service, the National Weather Service, the U.S. Department of Agriculture, the Federal Aviation Administration (FAA) and even a skiing organization. By "mining" huge data sets, the Stanford researchers hope to strike research gold by uncovering previously unknown patterns.

"Sometimes two and two add up to five or six," says Ludwig, a consulting professor in civil and environmental engineering who will analyze data collected hourly by the University of Utah. He is a retired meteorologist who went back to school later in life, turning in his doctoral dissertation in 1993 on his 62nd birthday.

Ludwig and Street, the William Alden and Martha Campbell Professor in the School of Engineering, are especially interested in describing and modeling atmospheric turbulence. With graduate students Ying Chen, Fotini Katopodes and Shwetha Reddy, they will develop and apply analytic techniques and numerical tools that eventually could benefit other groups as well.

Sixty researchers are using hundreds of balloons, dozens of meteorological ground stations, nine major instrument sites, six radars, three laser systems, gaseous tracers and a plane to make observations throughout an area of 1,200 square kilometers (about 460 square miles). The data will help scientists test the ability of computer models to describe wind, temperature and moisture patterns.

"It's a monumental task to coordinate an experiment such as this," Ludwig says. "One of the big problems is you can't put the balloons just anywhere because there's the Salt Lake City airport, and you have to coordinate with the FAA."

Due to its complex topography, the Salt Lake Valley provides a good setting for studying temperature inversions that trap pollutants. Normally, the atmosphere is colder at higher altitudes. But often on clear nights, air near the Earth's surface cools more rapidly than the air above. This colder, denser surface air doesn't mix much, and pollutants become trapped under a blanket of warmer, less dense air. Frequently, surface winds are calm and disconnected from winds aloft. This stagnation further contributes to the accumulation of pollutants.

Such a study could not be done as effectively in the San Francisco Bay Area, Ludwig says: "The Bay Area is not as rough a region as Salt Lake City. Even more important, the air over the bay does not cool so rapidly at night because the water retains heat, so the cold, stable layers of air don't form and there is usually more mixing."

To incorporate data from other routine measurement sites, the Stanford researchers are focusing on a 10,000-square-kilometer region that extends beyond the area where most of the special measurements are being made. The Salt Lake urban area, the mountains to the east and west of the valley and the large open basin produce a variety of effects. In one experiment, the researchers will look at data from inert, nontoxic tracer gases released to track cold, nighttime air as it flows down mountains and collects in the basin.

"Researchers understand daytime convective mixing better than they do the nighttime cases, and it certainly can be modeled better," says Ludwig. That has to do with the size of eddies ­ smaller motions superimposed on the general flow that mix the air. In the daytime, when atmospheric eddies are large, "the models can do a reasonably good job of reproducing what's observed," Ludwig says.

But they don't do a very good job of reproducing nighttime conditions, he says: "At night, most of the eddies tend to be damped because the atmosphere is very stable. The eddies are small, and they fall between the points on the grid used for the calculations." As part of a National Science Foundation project, Street and Katopodes are creating ways to model small-scale motions for better simulation of the nighttime atmosphere. The larger data sets will allow researchers to understand the nature of the smaller eddies better, which should help the modeling efforts.

In addition to Stanford, other study participants are the Department of Energy's Argonne, Brookhaven, Los Alamos and Pacific Northwest National Laboratories; the National Oceanic and Atmospheric Administration's Environmental Technology Laboratory and Atmospheric Turbulence and Diffusion Division; the National Center for Atmospheric Research; Colorado Research Associates; Desert Research Institute; Arizona and Oregon State Universities and the Universities of Massachusetts and Utah.

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By Dawn Levy