Harvesting wind has an unexpected consequence: Weather radar has trouble telling the difference between turbulence and turbines.

The wind turbine is a symbol of an alternative to an oil-based economy. But on radar, it looks like a storm. That has raised concerns that a proliferation of wind farms could result in the appearance of severe weather where there is none, and even false weather alerts. That collision of weather and wind technology adds some new dimensions to this year's tornado season, which officially kicks off this week with Severe Weather Awareness Week.
A new wind farm in Wisconsin has been appearing as a patch of severe weather north of Milwaukee since February, prompting the National Weather Service to post a website bulletin saying it isn't so.

Weather Service radar covering Minnesota, one of the nation's leading wind energy states, has picked up "echoes" from wind farms but not enough to cause confusion, said Tom Hultquist, science and operations officer for the Twin Cities office of the National Weather Service.

Most of the time, radar beams emitted from the Weather Service office in Sioux Falls, S.D., are able to clear Minnesota's Buffalo Ridge, which bristles with wind turbines, said Phil Schumacher, science officer at that office. But that could change in Minnesota and elsewhere, as more wind turbines spring up within the line-of-sight signals of weather radar.

Buffalo Ridge is about 50 miles from the Sioux Falls radar, but a wind farm in Great Falls, Mont., is about 3 miles from a Weather Service radar. The wind farm in Wisconsin is about 30 miles north of the Milwaukee radar, but on a higher elevation, from which its blades can reflect radar signals.

"Opportunities for potential disruption will increase," said Tim Crum, a lead analyst for the National Weather Service's radar operations center in Norman, Okla. "We recognize and appreciate the need for renewable energy. We're trying to get smarter about what the impacts are."

In recent years, wind farm radar images have suggested a tornado in Dodge City, Kan., confused an emergency manager in Des Moines, looked like flash-flood level precipitation in Lincoln, Ill., and raised concerns about the possibly unnecessary rerouting of air traffic.

Radar sends out electromagnetic energy, which bounces back off solid objects. Weather radar screens out stationary objects, such as buildings. But because elements of weather are always in motion -- rain, clouds, wind -- weather radar is designed to detect motion as weather. And wind turbines, ideally, are in motion.

"We've trained the radar, saying, 'If there's movement, don't erase it,'" Crum said. Wind farms themselves don't move, of course, so a person who knows where a wind farm is located will be able to discount it when looking at radar -- except when there's severe weather in the area.

The Weather Service will be testing a radar upgrade, perhaps next year, in locations near wind farms, to see whether it can filter out the turbines' effects. In the meantime, it is working with wind farm developers on strategies to reduce those effects, Crum said.