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July 1997

Boundary issues: NCAR heads to the plains to study earth-air exchange

Come late spring, the winter wheat grows quickly in Oklahoma and Kansas. The rolling prairie becomes a sea of emerald-green stalks that brings to mind an Irish hillside.

Peggy LeMone and Julie Lundquist at the CASES-97 control center. (Photo by Bob Henson.)

As the grasslands and wheat fields bloomed this April and May, a crop of NCAR scientists and technicians was on hand watching the growth of something less obvious: the atmospheric boundary layer. The air's lowest kilometer cools down each night as heat escapes from the surface and atmosphere. Then, each morning, the layer "grows," as Peggy LeMone (Mesoscale and Microscale Meteorology Division) puts it. The sandwich of air heats up on both sides: from below, as sunlight hits the earth, and from above, as warmer air mixes down.

Peggy was one of the key scientists tracking the boundary layer's waxing and waning for CASES-97, the Cooperative Atmosphere-Surface Exchange Study's 1997 field program. She and Bob Grossman (University of Colorado) were the coordinators for this spring's experiment.

The goal of CASES is to analyze the links between air, plants, and ground water as they interact on time scales of minutes to years. CASES will take particular advantage of the U.S. Department of Energy's Atmospheric Radiation Measurement field site, centered in north-central Oklahoma. This spring's measurements also relied heavily on instruments deployed by the Argonne National Laboratory for its Argonne Boundary Layer Experiment (ABLE), expected to run for the next 10 to 15 years.

Where does the water go?

Study areas for the two major boundary-layer experiments held in Kansas and Oklahoma this spring and summer. (Illustration by Liesel Brunson.)

Unlike the rectangular study areas favored by many experiments, CASES-97 featured natural boundaries. The focus was the lower two-thirds of the Walnut River watershed, which covers about 1500 square kilometers (600 square miles) in a wedge-shaped area east of Wichita. The watershed is a relatively tight one, meaning that most rainfall either evaporates or emerges as runoff rather than seeping through the area's limestone substrate. That makes it well suited for analyzing a hydrological budget. With tools that included NCAR's S-Pol radar (see below) and a rain-gauge network deployed by Oregon State University, CASES-97 may provide the most exhaustive hydrologic/atmospheric data set to date across a watershed this large.

To pull it all together in the field, Peggy worked with CU graduate student Julie Lundquist, Argonne's Jerry Klazura, and other Argonne staff at the operations center for CASES: a rented office complex and former beauty parlor in Augusta, Kansas, just east of Wichita. (Peggy reports that the computers were installed along the old hair-drier wall.)

The control center was at the heart of an equilateral triangle, 60 km (37 miles) on each side, formed by the observing network's linchpins: three vertically pointing systems from ABLE that provided hourly profiles of wind and temperature. The systems included minisodars (sonic backscattering profilers) for the lowest 200 meters (650 feet) and 915-megahertz profilers for heights from 200 meters to several kilometers.

NCAR's Atmospheric Technology Division filled in the research area with six portable automated mesonet stations and two atmosphere-surface turbulence exchange research (ASTER) sites. The network was installed and maintained by ATD's Tony Delaney, Tom Horst, Cathy Jirak, Kurt Knudsen, Errol Korn, Matt Michaelis, John Militzer, Steve Oncley, and Steve Semmer. At times, the crew had to work in "some pretty miserable weather," notes Peggy. Additional surface flux stations and other equipment were provided by NOAA, ABLE, and CU. Instruments were deployed across a variety of vegetation types--bluestem, Indian grass, milo, corn, and soybeans, along with the ubiquitous winter wheat.

The University of Wyoming's King Air flies over the Walnut River watershed during CASES-97. (Photo courtesy Bob McMillen, NOAA.)

There were six intensive observing periods (IOPs) during CASES, most of them spanning an entire day and night. During each one, ATD's Ned Chamberlain and Larry Murphy marshalled area residents to launch radionsondes every 90 minutes from the three ABLE sites for cross-comparison with the profiler and aircraft data. During daylight hours, the University of Wyoming's King Air and a NOAA Twin Otter aircraft passed over the countryside at heights from 30 meters to 3 km (100 to 10,000 feet), taking measurements through the heart of the boundary layer. (Scientists alerted residents about the flyovers via local media to minimize any undue concern, and the planes kept their distance from houses and cattle.)

The six IOPs gave CASES researchers the range of conditions they sought, says Peggy--with one exception. "We never had an IOP with cumulus clouds. Isn't that weird? I never would have guessed it. That's good in a way, because it keeps things simpler. Cumulus make things in the boundary layer more complicated."

Peggy looks forward to comparing the radiosonde and profiler data. "We're trying to develop some confidence in the sort of data gathered in the ABLE array. We hope to use it again and again, and other people will be using it, too." She's interested in analyzing the morning growth period for the boundary layer, especially the brief window in late morning when the near-ground temperature normally gets warm enough for air near the surface to shoot past the early-morning temperature inversion, leading to rapid boundary-layer growth.

Although the initial CASES grant didn't include funding for computer simulations using the project data, a few of the principals hope to get support for post-event mesoscale modeling. Data produced by the Regional Mesoscale Atmospheric Modeling System could help to evaluate heat, moisture, and momentum budgets from the study period. CASES data may also be used to evaluate the Biosphere-Atmosphere Transfer Scheme--which connects earth and atmosphere in the latest NCAR community climate model--and to link it with a planetary boundary layer scheme.

Rain supreme

Storms prowled the CASES region through the spring, most dramatically on 25 May, when tornado-producing supercells crossed the area and dumped hail larger than golf balls. ATD's S-Pol dual-polarization radar was on site for an adjunct CASES project, examining rain and hail within storms. The goal was to correlate the radar returns with the extensive hydrological network on the ground in order to improve rainfall estimates. On the night of 12-13 June, unofficial reports of more than 15 centimeters (6 inches) of rain came in from parts of the study region, saturating some of the ground-based rain gauges.

"We collected data from about 10 good storm systems," says Ed Brandes (Research Applications Program), who was on hand for the project along with fellow scientists Jim Wilson and Jothiram Vivekanandan (RAP/ATD) and ATD radar operators Joe Vinson, Tim Rucker, Mike Strong, and Al Phinney. CASES will serve as a useful follow-up to the S-Pol testing done east of Denver last year. "The Kansas storms tend to be much larger and produce a lot more rain than Colorado storms, so we're comparing the two," says Ed.

Planes over the plains

Another boundary-layer field study was launched across the prairie not long after CASES-97 ended on 22 May. The Southern Great Plains 1997 Experiment (SPG-97) examined surface soil moisture across west-central Oklahoma. The goal was to take high-quality, kilometer-scale measurements as benchmarks for testing the validity of coarser measurements from instruments soon to be satellite-borne. The project sent a NASA P-3 on daily north-south traverses of more than 1000 kilometers (600 miles) from 18 June through 18 July. Two other aircraft conducted smaller-scale flights to gauge heat flux above several sites within the SPG-97 region.

Don Lenschow (MMM) was the only NCAR investigator in SPG-97. His subproject, which included former NCAR postdoctoral researcher Ken Davis (University of Minnesota), is aimed at learning how varying soil moisture across an otherwise uniform surface affects the daily growth of the boundary layer, including any clouds and storms that might develop.

"We're going to compare notes afterward," says Peggy of the CASES and SPG-97 endeavors, "particularly on what we found about boundary-layer growth in the morning and the effect of surface moisture on boundary-layer depth."•BH

Cooperative Atmosphere-Surface Exchange Study (CASES-97)

Where: The upper Walnut River watershed, north of Winfield, Kansas

Who: NCAR, NOAA Air Resources Laboratory, Argonne National Laboratory, National Severe Storms Laboratory, U.S. Weather Research Program, Colorado State University (CSU), University of Colorado. Collaborators in data exchange include the U.S. Department of Energy, Global Energy and Water Cycle Experiment's Continental-scale International Project, CSU, and the U.S. Geological Survey. Daily weather briefings were provided by the National Weather Service in Wichita.

When: 22 April-22 May 1997

Why: To observe, understand, and model links among the atmosphere, hydrosphere, and terrestrial biosphere

How: Using instrumented towers, profilers, soil-moisture sensors, and rain gauges over the entire period, supplemented by low-flying aircraft, enhanced radiosonde launches, and radar scans during intensive observing periods

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Edited by Bob Henson, bhenson@ucar.edu