A tethered weather balloon carrying instruments to measure humidity,
temperature, and windspeed is launched.
World Weather: The Soil Has a Role
This past summer, two planes flew over Oklahoma into the border between wet
and dry weather fronts. This atmospheric border zone spawns air turbulence that
can affect local and, possibly, global weather. The boundary grows to more than
a mile above ground during the heat of the day. At night, as land temperatures
drop, it shrinks.
One of the two planes was a Twin Otter flown for the National Research
Council of Canada; the other, a Long EZ experimental plane flown by the
National Oceanic and Atmospheric Administration (NOAA). Both planes were part
of an international fleet comprising 6 planes and 15 satellites, including the
Russian Mir space station.
The fleet's mission: a joint USDA-NASA hydrology project called the Southern
Great Plains 97 project (SGP97). Its purpose: to map soil moisture daily by
airplane, using sensors that are prototypes for future satellites, with
intermittent back-up from satellites. Together, the fleet mapped a 24-mile-wide
rectangular path stretching 150 miles and bisecting the middle of Oklahoma,
almost from its border with Texas right up to its border with Kansas. [More
about the Southern Great
Plains 97 project.]
"During droughts, the air gets very warm and dry over winter wheat
fields that have been harvested and left bare," says ARS hydrologist Tom
Jackson. "But air becomes cool and wet when it passes over irrigated
fields, or over pasture or prairie grasses after a rain.
"This work," he adds, "should help us find out how large an
area has to be in order to affect weather, producing a thunderstorm, for
example. Knowing this, in turn, would help us make global climate change models
more accurate, while at the same time helping farmers plan their
At the aircraft mission control center in Oklahoma City, NASA's Ann Hsu checks
statewide weather conditions for Oklahoma.
Jackson is based at the Hydrology Laboratory run by USDA's
Agricultural Research Service in
Beltsville, Maryland. He was chief researcher for the entire SGP97 project.
The project, proposed by Jackson, was greatly expanded when NASA made it
part of its climate-monitoring Mission to Planet Earth project.
"Ultimately, we want to be able to predict soil moisture anywhere in
the world on any day of the year," says Jackson.
From June 18 to July 17 last summer, planes and satellites monitored how
torrential rain and its rapid evaporation in 100oF heat affected air
turbulence and weather, including tornadoes.
A Lockheed P-3B Orion flew in from NASA's Wallops Flight Facility in
southern Maryland. With a soil microwave sensor in its bomb bay, the P-3B
mapped soil moisture and temperature from 22,000 feet. It flew almost daily,
passing over the entire study area between 9:30 and 11:30 in the morning.
Jackson says the P-3B also sent out laser beams to map a vertical profile of
water vapor from plane to ground.
Other aircraft monitoring the area were the National Science Foundation's
Cessna Citation based in North Dakota, a Piper Navaho Chieftain flown by the
Ontario (Canada) Provincial Remote Sensing Office, and a U.S. Department of
Energy (DOE) Cessna Citation.
Microwave radiometers mounted on this boom truck can quickly measure soil
DOE's Cessna, flown at 16,000 feet, carried a prototype of a
thermal-infrared instrument that maps soil surface temperature. A Mission to
Planet Earth satellite carrying this instrument will soon be launched.
The satellites included U.S. defense and NOAA weather satellites and radar
and other satellites from Japan, Europe, and Canada.
On the ground, about 100 scientists and assistants from various countries
scurried to coordinate ground measurements with the airplanes and satellites.
Hazards abounded: bulls, brambles, heat, humidity, high winds, and critters
that chewed electrical wires.
Jackson says the atmospheric boundary layer study made SGP97 unique compared
to earlier studiesat Arizona's Walnut Gulch in 1990 and in southwestern
Oklahoma's Little Washita River watershed in 1992.
"We're starting to better understand the interactions between this
layer and the soil surface," Jackson says. "It looks like the scope
and distance of their effect on climate could be broader and farther than we
thought. They involve similar shifts in moisture and temperature seen in the
ocean-atmosphere interaction with El Niño."
Everything on the ground was measured three ways: by airplane, satellite,
and ground testing. Airits turbulence, chemistry, temperature, and
moisturewas measured from airplanes and on the ground.
Ground measurements are extremely accurate, but only for a small point on
the land, Jackson says. "Airplanes cover large areas and help us bridge
the gap by verifying extrapolations we make from ground points to the large
areas typically observed by satellites."
The hydrology project built upon the earlier projects. It included the
Little Washita again as one of three on-ground sampling sites, along with the
ARS Grazinglands Research Laboratory at El Reno, Oklahoma, just above the
Little Washita, and the DOE's Central Facility near the Kansas border. The
three sites were used to verify the aerial information and were within the
3,600-square-mile aerial mapping path.
The Central Facility site sits in the middle of a 55,000-square-mile DOE
meteorological study area that stretches from Oklahoma into central Kansas. The
entire DOE area has been described as a weather station without walls. Its
"floor" is more than 350,000 acres of grass- and wheatlands. DOE
technicians launch at least nine weather balloons every day, on the hour,
year-round. Only part of this area was mapped for this project.
Jackson says these sites were chosen for SGP97 because they have the most
extensive networks of soil moisture sensors and meteorological instruments
anywhere in the world.
At ARS' Little Washita watershed near Chickasha, Oklahoma, Teferi Tsegaye of
Alabama A& M University (foreground) demonstrates a soil measurement
technique as other students and Bill Crosson of the Global Hydrology and
Climate Center look on.
Much of the land at the three sites is rangeland, a land-use type that
covers at least 40 percent of the Earth's surface. This is why understanding
what happens when widespread storms strike rangeland is important to making
global predictions of everythingfrom weather to soil moisture to erosion
to pesticide movement to floodingJackson says.
Jackson and his crew found what they were looking for on June 17, the day
they arrived: a torrential downpour over most of the study area. They watched
the P-3B fly every day, mapping the drying out of this zone. This
patternalternating rain and droughtrepeated itself several times
throughout the study. The study ended with a dry-out period following a 6-inch
downpour in the southern part of the flight path.
"The alternation between rain and drought is exactly what we
needed," Jackson says.
Adios MIR, Adios ADEOS
While the weather, airplanes, and ground crews played their parts with near
perfection, the researchers' orbiting "eyes" ran into problems. As
the project began, the P-3B airplane and the Mir station passed over, capturing
soil moisture data across both the mapping area and a wide swath of the United
States. But a week later, on June 25, the researchers and an anxious world
learned that a docking accident had severely damaged MIR, temporarily knocking
out all power. While power for life support was quickly restored, it would be
several months before power for experimental apparatus such as the soil
microwave sensor would be available.
Compounding the problems for researchers, another satellite involved in the
projectJapan's ADEOS satellitespun out of control 10 days later and
was lost in space forever.
Still, the P-3B plane, carrying two types of soil moisture microwave
sensors, flew the flight path 22 out of 30 days. This was exactly what Jackson
and his colleagues had planned for, knowing from experience that it is about as
close to perfection as Nature and airplane logistics allow.
Alabama A&M University students Jimmy Moore (foreground) and Jacques
Surrency weigh soil samples in preparation for moisture measurements.
Meanwhile, Down Below
Students and faculty from Alabama A&M University's NASA-sponsored
remote-sensing center helped gather data. The University benefits from being in
Normal, Alabama, near NASA's Marshall Space Flight Center in Huntsville.
A&M University students joined graduate students from around the country in
making ground measurements on government rangeland. Other ARS participants
included the Grazinglands Research lab at El Reno, the Soil Tilth Laboratory at
Ames, Iowa, and the Salinity Laboratory at Riverside, California.
At the El Reno site, Jackson's Beltsville colleague, hydrologist Bill
Kustas, supervised measurement of meteorological conditions at four locations.
At each site, 10-foot towers supported sophisticated instrumentation for
measuring air turbulence at ground level. One tower was toppled by extreme wind
gusts and had to be repaired.
Kustas kept an eye on weather reports. He often brought his crew out at 5
a.m. to beat a storm, download the previous day's data, and put protective bags
over sensitive instruments to protect them from hail and blowing debris.
The P-3B aircraft unfurled from its bomb bay an experimental soil-moisture
sensora bundle of antennae "sticks" called ESTAR, for
Electronically Scanned Thinned Array Microwave Radiometer.
"The antennae receive natural microwave emissions from the soil. The
weaker the emission, the more the soil moisture," Jackson explains.
Together, the sticks, each about 3 feet long, did the work of what would
otherwise have been a large, heavy antenna. "The ESTAR sensors were the
core of the experiments, and they worked," he says.
ESTAR uses a microwave band frequency that is more suitable for measuring
soil moisture," says Jackson. "A preliminary review of the data shows
a remarkable match between air and ground data, convincing me beyond a shadow
of a doubt that the data will be useful."
NASA is considering using ESTAR on satellites, possibly making a decision as
early as this year. It is an alternative to the AMSR, or Advanced Microwave
Scanning Radiometer, which is similar to the sensors tested aboard Mir and the
radar satellites. The AMSR will be used by a NASA satellite planned for launch
in 2000 and by the Japanese ADEOS II, also planned for launch in 2000.
At the Oklahoma City Airport, Canadian pilot Dave Bluhm and scientist Ian
MacPherson, of the National Research Council of Canada, discuss the grid
patterns they will fly. The boom extending from the nose of their Twin Otter
measures wind direction during flight.
By around 11 o'clock each night, Jackson's NASA colleague Ann Hsu had posted
the day's data on the World Wide Web site created by the Hydrology Laboratory
for the experiment. This site allowed Jackson to communicate with participants
in the experimenteven before they arrived from around the world.
Website Open to All
Once the experiment began, the entire teamon site at various
locations in Oklahomaused the web to review the data each night and make
plans for the next day.
Jackson says that one of the ways the SGP97 project differed from the
smaller, earlier tests at Walnut Gulch and Little Washita was the dramatic
increase in e-mail and World Wide Web use.
In fact, anyone can view various aspects of the experiment on that website,
including preliminary data prepared for the public. (Go to
http://hydrolab.arsusda.gov/sgp97) The experiment's results will be presented
at the American Geophysical Union meeting during May 1998. Researchers will
publicly share all data in September on a to-be-announced NASA web site.
This past December, Jackson learned that the Mir space station had resumed
collecting data over Oklahoma. He won't need to go to Oklahoma this June to
verify the data. He's glad that with last summer's work, scientists around the
world will be able to process satellite data for years to comewithout
crawling on their hands and knees with buckets and soil spatulas.By
Agricultural Research Service Information Staff, 6303 Ivy Lane, Greenbelt,
Maryland 20770, phone (301) 344-2748.
Thomas J. Jackson and
William P. Kustas are at the
USDA-ARS Hydrology Laboratory, Bldg. 007, 10300 Baltimore Ave., Beltsville, MD
20705-2350; phone (301) 504-8511, fax (301) 504-8931.
"World Weather: The Soil Has a Role Too" was published in
the April 1998 issue of Agricultural Research magazine. Click here
to see this issue's table of contents.