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Release: Immediate

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 climate.

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."