BOULDER—The buildup of ice on airplanes in flight is a major wintertime hazard for small and commuter planes. The National Center for Atmospheric Research (NCAR) is testing a new system that may pinpoint water droplets in clouds that cause icing, potentially enabling pilots to avoid dangerous areas.

The system, known as S-Polka, combines two existing radars that use different wavelengths. By studying the differences between the images that are reflected back to each radar, scientists hope to find tiny water droplets that are difficult to distinguish using either radar alone. The project is being funded by the National Science Foundation, which is NCAR's primary sponsor, and the Federal Aviation Administration.

"This will take out a lot of the guess work," explains Marcia Politovich, director of NCAR's icing program. "We think it will show exactly where the water is. That information could ultimately turn into an important warning system for pilots."

Scientists and engineers at NCAR are deploying S-Polka at NCAR's Marshall facility southeast of Boulder. The system consists of a powerful polarized radar, known as S-Pol, that operates at a frequency of 3,000 MHz, and a polarized Ka-band radar, that operates at 35,000 MHz. The S-Pol radar produces detailed images of clouds and precipitation, whereas the Ka-band radar can detect weaker clouds that are not precipitating. By comparing the images from each radar, researchers hope to find areas in clouds that harbor water droplets.

Finding cloud water droplets has long posed a scientific challenge. The droplets are 50 microns or less in diameter, or just one-tenth the size of raindrops. They may remain in liquid form even when the surrounding air temperature drops below freezing. They are most dangerous at this time because they adhere to the wings of aircraft and then freeze, reducing the plane's aerodynamic properties. Unfortunately, existing radar often cannot detect the droplets if they are surrounded by larger raindrops or snow. Even if small cloud particles are detected, a radar signal cannot indicate whether they are water droplets or ice crystals.

"When it comes to cloud particles, we can't interpret the standard radar echo," explains NCAR's Jothiram Vivekanandan, the lead scientist on the project. "This research is very challenging."

The two radars have been mounted on a single pedestal at the Marshall facility. They are precisely aligned to look at the same defined area at the same time. Researchers will compare the radar images with data collected from a University of North Dakota Citation research airplane flying in the test area to determine whether the radar system is pinpointing water droplets. After data are collected, the researchers will focus on creating algorithms (mathematical procedures) that will identify and measure droplets within the radar images accurately. If all goes well, the instrument will undergo final tests in a couple of years and be considered for implementation at airports.

Notable Icing Crashes

• In-flight icing downed the small plane carrying 1950s rock 'n' roll legends Buddy Holly, Ritchie Valens, and The Big Bopper (J.P. Richardson). All three musicians and the pilot died when their plane crashed soon after take-off from Mason City, Iowa, on February 3, 1959.


• An American Eagle ATR-72 went into a high-speed dive and crashed near Roselawn, Indiana, on October 31,1994. As the plane circled for a half hour waiting to land in Chicago, ice forming on the wings caused the crew to lose control. None of the 68 people aboard survived.

An Embraer 120RT en route from Cincinnati crashed on approach to the Detroit airport on January 9, 1997, killing all 29 people on board. At the time, other aircraft in the area were reporting various amounts of icing, from minor to very heavy.