ROXBURY – Giant wind turbines cropping up across the nation are inadvertently creating problems for Doppler weather radar, because the turning blades are interpreted as precipitation.

Tim Crum of the National Oceanic Atmospheric Association’s National Weather Service Radar Operations Center in Norman, Okla., said Thursday that the phenomenon known as wind turbine clutter can create false predictions of tornadoes or severe thunderstorms. It has even caused the Federal Aviation Administration to reroute aircraft due to false returns.

Crum said they first learned three years ago about problems associated with wind turbine clutter from people seeing significantly different radar images.

“We knew about it, but we didn’t realize how big of a problem it could be, because most of the wind farms that had been out there in the past were smaller and not these wind farms that we’re seeing now with turbines of 400 to 500 feet tall,” Crum said.

“They’re much larger now, and so, instead of seeing a speckle here and there from the smaller ones, we now have some that are very bold,” he said.

The problem increases significantly depending on the line of radar sight distances of turbines from the radar. Some false returns have been observed for turbines within 10 miles to 25 miles of the radar, but it varies with terrain and atmospheric conditions.

And even though the proposed Record Hill Wind LLC project to site 22 wind turbines along two ridges in Roxbury is in the line of sight of the National Weather Service’s Doppler radar in Portland, Crum believes there won’t be any problems because the Roxbury site is about 50 miles distant.

“At that distance, yes, the turbines will show up, but as far as an operational impact to really hinder the forecast office and most people who use it for critical life safety data, probably it won’t be an issue,” Crum said.

“It’s when you get really close – and we’re still trying to define what ‘really close’ is – that it becomes more of an issue,” he added.

Record Hill Wind principal Robert Gardiner said Wednesday in Wilton that he is aware of NOAA’s concerns, but believes they are “completely baseless.”

“It doesn’t make any sense to me that NOAA would be confused if they always see thunderstorms in the same spot of the map 24/7, 365,” said Gardiner, who is a developer in the Roxbury turbine project. “I mean, I think NOAA’s smart enough to figure out that that’s a wind turbine effect and not a permanent thunderstorm.”

“So, it strikes me as this being another case of a little bit of bureaucratic misunderstanding and I don’t want to see people making a big deal out of it, because I think this is absolutely not something that’s going to be a real issue,” Gardiner said.

While Crum agreed with some of Gardiner’s analogy, he said the problem is definitely more than “a little bit of bureaucratic misunderstanding.” That’s why he’s willing to meet or talk with anyone about it.

“There’s a lot of things that he said are right, but I just don’t think that it’s a bureaucratic misunderstanding,” Crum said. “When we first started doing this, developers told us, ‘You can actually see my turbine blades? I can’t see you.'”

“Well, the line of sight visual is different than radar’s, because radars turn at a four-third’s radius. It doesn’t hug the ground…so we can see quite a ways,” Crum said. “If you have a 400-foot-tall turbine, we can see you from 30 to 40 miles away.”

According to the NOAA Web site, in some cases, the disturbed areas are large enough to cause additional forecaster confusion and distraction.

Crum said the radar has a method of reducing ground clutter that is stationary, like buildings, trees and terrain.

“But the radar interprets the rotating blades as something that’s moving, like how weather moves,” Crum said.

Radar currently has no way to determine the number of targets it is sampling within a particular area. Additionally, the reflected energy is constantly changing as the blades change their pitch and orientation relative to radar, according to information on NOAA’s Web site.

Crum said studies are under way at the University of Oklahoma and other institutions to find a solution, but currently, there is no known way to filter out turbine blade clutter.

However, they are encouraging people who operate the radar to be aware of wind power facilities and to not be surprised by the false returns from wind turbine clutter.

Taking a proactive approach, NOAA is also reaching out to the wind power industry to increase awareness and understanding of the issue and how to work together to resolve it.

“We all want renewable energy, but we’ve also got to safeguard the investments that taxpayers have made in the nation’s network of radars,” Crum said.

There are 159 Doppler radar systems around the world, Crum said.

One problem, however, is that there are so many users of the weather radar data who may not be as accustomed to seeing radar images produced by turbine clutter as are weather forecasters.

“We have been trying, especially at the University of Oklahoma, to see if there are ways to change the software in weather radar so that people can understand, ‘Oh, that’s a rotating blade, so let’s don’t consider that effect,'” Crum said.

“But, we’re not even near a solution,” he said.

In the meantime, NOAA is teaching forecast offices about the problem, reaching out to the wind power industry and offering to perform a no-cost case-by-case analysis to determine potential impacts for the nearby NEXRAD.

To learn more about how wind turbines impact the NEXRAD Doppler weather radar system, visit the National Oceanic Atmospheric Association’s National Weather Service Radar Operations Center Web site at:

http://www.roc.noaa.gov/windfarm/windfarm_developers_index.asp.