Advances in current weather radar technology have paved the way for better rainstorm, hailstorm and flash flood prediction, according to University researchers.

The dual polarization differential reflectivity technique, developed by Thomas A. Seliga, professor of electrical engineering and associate dean for graduate studies and research, not only significantly improves conventional radar measurements of rainfall rate, it provides additional information on concentration, size, water content and phase (ice/water) of precipitation particles.

"I was challenged by the long-standing problem of a radar's ability to measure rainfall rate. I arrived at this polarimetric method to solve the problem," Seliga said.

Kultegin Aydin, assistant professor of electrical engineering and Seliga's research collaborator, explained polarization is a property of an electromagnetic wave related to the behavior of the electric field in space. If the electric field lies along a straight line, it is linearly polarized. Linearly polarized waves can be in a horizontal, vertical or any intermediate plane. Conventional radars usually operate using either horizontally or vertically polarized radar waves.

Polarization radars are essentially conventional radars which have been modified to alternately send and receive both horizontally and vertically polarized radar signals, Aydin explained.

"Differences in returned signals characterize various forms of precipitation particles which have produced the scattering," Seliga added.

Aydin said, "[Polarization radars] increase the information available from conventional radars, so you can tell more about what's out there.".

The polarization radar technique is a simple extension of previous technology that is easy to understand, readily interpretable both qualitatively and quantitatively, and applicable to practical problems like flash flood forecasting and severe storm warnings, Seliga said.

An estimation of the size and distribution of raindrops in space is possible with the polarization radar. Aydin explained raindrops lose their spheroidal shape as they fall through the atmosphere. Air pressure against the bottom of a falling raindrop flattens it, giving it an oblate spheroidal shape, which has a longer horizontal than vertical dimension. The ratio of the polarized signals is proportional to the ratio of the horizontal and vertical dimensions of the raindrop, which in turn is related to its size. The power of the returned signal yields further information about the number of raindrops in the storm, Aydin added.

"The principle for differentiating hail from rain is essentially the difference between the shape of raindrops and hail," explained Aydin.

Raindrops have an oblate (flattened at the ends) spheroidal shape, whereas hailstones have irregular shapes. This distinction in shape, which is characteristic of the precipitation particle, is detected by the pattern of the returned radar signal, he said.

"In flash flood forecasting the rainfall rate and accumulation are very important factors," Aydin said. The polarization radar provides hydrologists with the same information as rain gauges, but with more frequent and comprehensive spatial coverage, he added.

Currently, Seliga and Aydin are collaborating with colleagues in Italy to use polarization radars for predicting flash floods in the cities of Florence and Rome.

Although flash floods are not a major problem in State College, parts of Pennsylvania are susceptible to flash flooding, especially during hurricane season, Seliga said.

"The great Johnstown flood of 1889 killed about 4000 people in Pennsylvania," said Erik Pytlick (sophomore-meteorology), tower worker at the University weather station.

Seliga and Aydin hope to receive appropriate funding support in order to collaborate their research efforts with the University meteorology department in the future.