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Fall 1997

What's new in boundary-layer experiments?


by Bob Henson and
Carol Rasmussen,
UCAR Communications

If you're from the Midwest, you know about the importance of water. Farmers and ranchers there spend most of their time suffering from either too little moisture, too much, or the right amount at the wrong time. Even if you've only flown over the plains, the gentle green of watercourses and the sharper emerald of center-pivot circles studded across the soft, brown landscape are object lessons in water's transforming power.

The SGP-97 study area. Black dots indicate the preexisting ARM CART Extended Facility sites; gray dots are Oklahoma mesonet sites; white dots are mesonet sites with soil moisture measurements. The irregular circle toward the bottom of the area is the Little Washita watershed. Inset, Paul Houser (left, NASA) and Tom Jackson (U.S. Department of Agriculture) taking soil samples at a site near Kingfisher, Oklahoma. (Photo by A. Hsu.)

But it's not only agriculturalists who'd like to know more about water in the Great Plains. This summer, scientists from around the country converged on the plains for two experiments. One examined with interactions of water and soil, and the other looked at water's effects on the air above the land in the atmospheric boundary layer--the kilometer of air closest to the earth.

This year's efforts were "part of a continued evolution of boundary-layer experiments," says NCAR's Donald Lenschow (Mesoscale and Microscale Meteorology Division), who participated in one of the studies. He explains that as new instrumentation and new techniques have evolved, field experiments have changed as well. Instead of studying relatively idealized land forms and simple weather patterns, scientists are trying their tools on more complex situations: variations in topography and land use, sharp horizontal gradients in air temperature and moisture, and the like. "We're trying to understand what happens when things aren't homogeneous," Lenschow sums up.

Donald Lenschow

Like the observing instruments, climate models have also become able to handle land-atmosphere interactions, such as heat and moisture fluxes, in regions of large variability. To bring these phenomena into the virtual atmosphere, modelers need more data on how they unfold in the real atmosphere in a variety of settings.

The Oklahoma soil The Southern Great Plains 1997 (SGP-97) Hydrology Experiment examined surface soil moisture across a tall, narrow swath of central Oklahoma (see map). One of its goals was to take high-quality, kilometer-scale measurements that could test the validity of coarser measurements from instruments soon to be satellite-borne over an area an order of magnitude larger than anything previously studied.

The experiment materialized last year under the guidance of NASA hydrologic program manager Ming-Ying Wei, who saw the opportunity to link a number of existing projects and proposals with NASA seed money. "NASA is especially interested in relating ground-scale measurements to satellite data," notes Lenschow, NCAR's main participant in SGP-97. "It all came together pretty rapidly."

Soil moisture, an important variable in the earth's water budget and an influence on the evolution of the boundary layer and clouds, is difficult to measure. Properties of the soil itself affect its ability to retain moisture, as do plant roots, topography, and so on. Soil-moisture measurements have been made by sensors on trucks and aircraft, but scientists weren't certain that the computer programs that process these measurements (called the retrieval algorithms) would work on the much larger-scale satellite data that are or soon will be available. Besides getting the scale of data needed to answer that question, the experiment took other measurements that were chosen at a workshop last summer by hydrologists, soil scientists, ecologists, and meteorologists, making the experiment's data set a more useful interdisciplinary tool.

The study area had the advantage of being already heavily instrumented. The U.S. Department of Agriculture's Agricultural Research Service has facilities near the towns of Chickasha and El Reno and has been pursuing extensive hydrologic research in the watershed of the Little Washita, on the southern edge of the study area, for more than 35 years. The U.S. Department of Energy's Atmospheric Radiation Measurement program Clouds and Radiation Testband (ARM CART), near Lamont, was also included, as were stations of the Oklahoma Mesonet.

The core of the project was daily flights from 18 June through 18 July that mapped soil moisture over an area of about 10,000 km2 (6,200 square miles) This was accomplished by a microwave mapping instrument called ESTAR (electronically scanned thinned array radar) and two other radiometers on a NASA P-3 aircraft on north-south traverses of more than 1,000 km (600 miles). A truck-based microwave radiometer system made continuous measurements from the ARM CART facility to complement the aircraft measurements taken overhead once a day.

To help with the issue of making the retrieval algorithms compatible with satellite measurements, the experiment also flew two instruments whose later generations will be satellite-borne:

the thermal infrared multispectral scanner (TIMS), a simulator for the advanced spaceborne thermal emission and reflection radiometer (ASTER). TIMS flew twice on a DOE Cessna Citation, with funding by NASA's Earth Observing System. ASTER will fly on the EOS AM-1 platform next summer.

LASE, the lidar atmospheric sensing experiment, measured atmospheric water vapor and aerosols and charted the locations of cloud tops. LASE is a precursor to a space-based differential absorption lidar (DIAL) system.

Also during the month-long experiment, observations for the study area were obtained from four satellites on about ten days.

Lenschow's subproject, which included former NCAR postdoctoral researcher Ken Davis (University of Minnesota), was 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 develop. The LASE instrument was used for this project, flying on a Twin Otter from the National Research Council (Canada) or a NOAA Long-EZ. Lenschow notes, "ESTAR was very important--the fact that it was available and could be used in the presence of the other tools. The Twin Otter and the Long-EZ aircraft gave us the in situ measurements to interpret what ESTAR was seeing."

CASES

This May, as the grasslands and wheat fields of Kansas bloomed after the spring rains, a crop of NCAR scientists and technicians was on hand watching the growth of something less obvious: the boundary layer. The air's lowest kilometer cools down each night as heat escapes the atmosphere. Then, each morning, the layer "grows," as Margaret 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 LeMone and Julie Lundquist at the CASES-97 control center. (Photo by Bob Henson.)

LeMone was one of the key scientists who tracked 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. Other field efforts may follow, pending support.

The goal of CASES is to analyze the links between air, plants, and ground water as they interact on the time scales of minutes to years. CASES took particular advantage of the ARM site. This spring's measurements also relied heavily on instruments deployed by Argonne National Laboratory for its Argonne Boundary Layer Experiment (ABLE), expected to run for the next 10 to 15 years.

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 2,000 km2 (800 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.

To pull it all together in the field, LeMone worked with University of Colorado (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.

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 m to several kilometers.

NCAR's Atmospheric Technology Division filled in the research area with six portable automated mesonet (PAM) stations and two atmosphere-surface turbulence exchange research (ASTER) sites. Additional surface flux stations and other equipment were provided by NOAA, ABLE, and the University of Colorado. Instruments were deployed across a variety of vegetation types--bluestem grass, Indian grass, milo, corn, and soybeans, along with the ubiquitous winter wheat.

There were six intensive observing periods (IOPs) during CASES. Most of them spanned an entire night, and several were close to 24 hours long. During each of these, ATD's Edward Chamberlain and Larry Murphy launched radiosondes every 90 minutes from the three ABLE sites for cross-comparison with the profiler data. During daylight hours, the University of Wyoming's King Air and the NOAA Twin Otter aircraft that later was used in SGP-97 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.

LeMone 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.

Lundquist has her eye on the other end of the daily cycle. For her CU dissertation, Lundquist plans to analyze the effect of surface variations on the evening collapse of the boundary layer, when it shifts from a turbulent, unstable profile to a more stable mode. Her goal is to understand how the rapid evening changes in surface flux affect the resulting boundary-layer structure. "Can we use the behavior of the evening transition," asks Lundquist, "to predict what will happen at night?" The various crops and grasses across the CASES study area cause variations in the ratio of sensible to latent heat flux, which could affect the evolution of the evening transition.

Storms prowled the CASES region through the course of the experiment, most dramatically on 25 May, when tornado-producing supercells crossed the area and dumped hail larger than golf balls. "We collected data from about ten good storm systems," says Ed Brandes (NCAR Research Applications Program). 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 in scale and produce a lot more rain than Colorado storms, so we're comparing the two," says Brandes.


Southern Great Plains 1997 Hydrology Experiment

Where:
Central Oklahoma, from the Kansas border to south of the Little Washita watershed

Who:
More than 60 investigators from Alabama A&M University, Boston University, Georgia Institute of Technology, Massachusetts Institute of Technology, Oklahoma State University, Oregon State University, Pennsylvania State University, Princeton University, Texas A&M University, University of Arizona, University of Cincinnatti, University of Colorado, University of Guelph (Canada), University of Illinois, University of Massachusetts, University of Michigan, University of Minnesota, University of Oklahoma, University of Texas at Austin, University of Wisconsin, Illinois State Water Survey, Institute of Hydrology (U.K.), NASA Jet Propulsion Laboratory, NASA Goddard Space Flight Center, NASA Langley Research Center, NASA Wallops Flight Facility, National Research Council (Canada), NCAR, NOAA Air Resources Laboratory, US. Department of Agriculture Agricultural Research Service, U.S. Department of Energy, and the Netherlands Department of Water Resources

When:
18 June-18 July 1997

Why:
To determine whether small-scale soil moisture measurements will extrapolate to satellite scale, and to study the effect of soil moisture on the evolution of the atmospheric boundary layer and clouds

How:
Daily aircraft flights, ground-level and subsurface measurements, and satellite measurements by a wide variety of sensors

Cooperative Atmosphere-Surface Exchange Study

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 U.S. Department of Energy, Global Energy and Water Cycle Experiment's Continental-scale International Project, CSU, and 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, along with low-flying aircraft and enhanced radiosonde launches during intensive observing periods


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Edited by Carol Rasmussen, carolr@ucar.edu
Prepared for the Web by Jacque Marshall
Last revised: Tue Apr 4 14:38:45 MDT 2000