CONTACT: GARY GALLUZZO
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Iowa City IA 52242
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UI engineers improve tropical rainfall measurements
IOWA CITY, Iowa -- A set of 20 rain gauges recently installed at the
Iowa City Airport by University of Iowa researchers may help meteorologists
learn more about the effects of tropical rainfall on global weather and
Witold Krajewski, professor of civil and environmental engineering in
the University of Iowa College of Engineering and researcher in the Iowa
Institute of Hydraulic Research (IIHR), says that the Iowa City Airport
experimental site will provide information useful for studies ranging from
rainfall forecasting methods to climate change. The site, developed with
funding from the National Science Foundation and the Army Research Office
of the Department of Defense, will benefit projects funded by NASA and
the National Weather Service.
For example, the UI researchers participate in a NASA-funded project
to improve the accuracy of satellite-based rainfall measurements. The
project, Tropical Rainfall Measuring Mission (TRMM), involves a satellite
that carries numerous sensors used to estimate tropical rainfall; however,
the type of information provided is limited and needs to be validated with
ground-based observations. The UI researchers also operate a cluster of
rain gages on Guam, similar to that at the Iowa City Airport, and provide
NASA with data to evaluate the performance of the TRMM satellite.
"We know that remote sensing estimates of rainfall are good for
showing spatial coverage, but they are correct only within a factor of
two or so," Krajewski says. "For scientific studies, such as
those related to climate, we need very good quantitative information.
With good information on rainfall differences over short distances, we
will be able to make more precise estimates of rainfall using both space-borne
and ground-based radar."
Taken together, the 20 rain gauges serve as a high-density network to
collect information on how rainfall amounts vary over short distances.
The gauges, mounted in pairs on wooden platforms, each stand about two
feet high and are eight inches in diameter. When rain fills their internal
buckets with a pre-determined amount of water, they tip over and empty
their contents, while automatically relaying the information to an IIHR
base station. The distance between the buckets ranges from one meter on
up to 1,000 meters, so as to give readings over a variety of short distances.
Krajewski, who previously received a four-year, $438,000 NASA grant
for TRMM research, says that long-term predictions of climate variability
depend upon an accurate inventory of tropical rainfall patterns. Scientists
estimate that more than two-thirds of the world's rainfall occurs over
the oceans and jungles of tropical regions, with satellites being the only
reliable means of providing that information. Additionally, the tropics
are the birthplace of many weather anomalies, including "El Nino"
events in which warm ocean currents trigger worldwide droughts and floods.
In addition to the rain gauges, the UI project consists of a vertically
pointing Doppler radar and a 2-D video distrometer an instrument
that measures the size and shape distribution of raindrops. The UI instrument,
one of only two in the United States (the other one is owned by NASA),
was recently used in conjunction with a UI workshop for training researchers
who will participate in related NASA climate change experiments. Future
TRMM experiments include one scheduled to begin in December 1998 in Brazil,
and another to be held next summer in the Kwajalein Atoll in the Pacific.
The IIHR team, composed of members of the UI hydrometeorology group
led by Witold Krajewski and IIHR researcher Dr. Anton Kruger, was invited
to participate in TRMM experiments based on its past contributions in the
areas of analytical, numerical, and experimental research on rainfall observation
and estimation. During the past summer, the IIHR team was located at the
Triple-N Ranch, a wildlife preserve area near Melbourne, Fla., collecting
data from tropical rainfall systems including some hurricanes.
Asked why the team set up the rain gauges with winter approaching, Krajewski
says that UI researchers are ahead of schedule: "We know it will
snow more than it will rain in the next few months, but at least we will
be ready when the spring storms come."