During late 2008 and early 2009, an energy wind farm consisting of 36 turbines began operation in east central Dodge county on Butler Ridge in Herman township. This is about 2 miles east of Iron Ridge, just north of Rubicon and just south of Mayville and Theresa.
Butler Ridge has an elevation of 1170 feet above mean sea level, which is about 300 feet above the elevation of the town of Hustisford, located just to the west. The Butler Ridge wind farm is about 30 miles directly north of the Weather Surveillance Doppler Radar located at the National Weather Service office near Sullivan (KMKX) in far eastern Jefferson county.
Unfortunately, the Butler Ridge wind farm and its turbines are within the radar line of sight (RLOS) of the NWS doppler radar in eastern Jefferson county. The height of the wind turbine towers are about 260 feet above the ground, and the turbine blades are about 300 feet in diameter. Hence the top of the wind turbine rotors are about 400 feet above the ground on Butler Ridge.
At this height, the rotating turbine blades of the wind farm impact the KMKX Doppler Radar beam. As you can see in the above image depicting most of southeast Wisconsin, the rotating wind turbines are having an affect on the radar beam.
A small part of the electromagnetic energy radar beam sent from the radar is reflected back by the rotating turbines. The radar processes this “returned energy” as an area of precipitation and plots it accordingly on the map. This contamination of the base reflectivity image as illustrated in the below image, has an effect on the radar algorithms used to estimate rainfall and to detect certain storm characteristics.
The rotating turbines also impact the velocity base data as you can see from the below image. This velocity data is used by radar operators and by a variety of algorithms in the radar’s data processors to detect certain storm characteristics such as mesocyclones, tornado vortex signatures, and relative storm motion.
The KMKX doppler radar has a sophisticated clutter/interference removal scheme, however the scheme was designed to filter out spurious returned (reflected) energy that has little or no motion. This is effective for removing the returned signals from terrain, buildings, and other non-moving structures. Unfortunately, the radar sees the rotating wind turbine blades as targets having reflectivity and motion, hence processes these returns as weather.
The below two hour animation from the evening of April 1, between 915 pm and 11 pm CDT shows the persistent interference from the Butler Ridge wind turbine farm on the KMKX base reflectivity radar image.
Wind turbine clutter or interference that shows up on the base reflectivity and velocity images produced by the doppler radar can have several impacts including:
- Thunderstorm or winter storm characteristics could be masked or misinterpreted, reducing warning effectiveness in the vicinity of, and downrange of the wind farm.
- False signatures contaminating Doppler velocity data in the vicinity and downrange of the wind energy facility could reduce forecaster’s situational awarness, particularly during hazardous/severe weather events.
- Data masking or contamination if thunderstorms develop over the wind farm may negatively impact warning effectiveness.
- False precipitation estimates could negatively impact flash-flood warning effectiveness.
The best mitigation technique is to avoid locating wind turbines in the radar line of sight (RLOS) of the doppler radar. The National Weather Service is conducting an outreach program to ensure the wind energy industry and developers are aware of NWS Doppler Radar locations and the potential impacts on radar data. The NWS Radar Operations Center (ROC) works with these developers and suggests mitigation options to consider.
The NWS continues to learn about wind farm impacts on radars, weather forecast office operations, and other users where radars and wind farms are already in close proximity. Based on this information, the NWS will develop training materials for radar operators and weather forecasters on how to identify, mitigate, and partially work around wind turbine impacts during forecast and warning operations.
For much more information on how wind turbines impact doppler weather radar, check out the ROC website.
National Weather Service Weather Forecast Office
This article is the work of the author(s) indicated. Any opinions expressed in it are not necessarily those of National Wind Watch.
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