NEXRAD Radar Operations Center - Wind Farm Frequently Asked Questions

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Radar Operations Center

Wind Farms - Frequently Asked Questions (FAQs)

1.	Question:	How far away from the WSR-88D should we site a wind farm? Do you have a benchmark distance? If so, what is it?
	Answer:	Our benchmark distance is terrain and total turbine height dependent, which varies from site to site. Moderate to Severe impacts begin at the distance where turbines penetrate the second scanning elevation angle (0.9 degrees for most sites). For example, with a 200 m turbine, the second elevation angle begins being impacted at 42 km away for standard scanning strategies. We have also established a benchmark no-build zone of 4 km. For a more detailed description of how the ROC analyzes wind farm proposals visit the page here.
2.	Question:	What is the "Radar-Line-of-Sight" (RLOS) and why is it important?
	Answer:	The radar line of sight/radar beam width can be considered analogous to the beam of light coming from a flashlight. Most of the energy is towards the center, and the energy drops off towards the edge of the beam. As the beam propagates away from the radar, its width increases. For WSR-88Ds, at 111 km (60 nmi) from the radar the beam is approximately 2 km (1 nmi) wide. Anything within the beam out to the maximum range of 430 km is considered visible by the radar, and makes up the RLOS. The beam normally gets higher above the ground as it goes out in range, so targets at or near the ground far away from the radar would not be in the RLOS.
3.	Question:	Does the RLOS ever change?
	Answer:	Yes. The actual RLOS (not the RLOS based on the Standard Atmosphere, but the RLOS based on actual day to day weather) changes during the day as a result of temperature and humidity fluctuations. It also changes as fronts pass or with nearby thunderstorm outflows. As the surface temperature cools, the radar beam bends more towards the earth's surface. This is called super-refraction or "ducting". The net result is that wind farms that are normally out of the RLOS may be in the RLOS at certain times of the day and during certain weather conditions. So, even if wind farm developers site their projects outside the benchmark RLOS, the weather forecasters will occasionally "see" the wind farms on the radar imagery.
4.	Question:	How can WSR-88D systems see wind towers/turbines when I can't visually see the radar from the wind farm?
	Answer:	The path that emitted radar energy (i.e., the radar line of sight) takes depends upon atmospheric density. Density differences are caused by variations in pressure, temperature, and moisture. In a "standard atmosphere", the radar beam takes a path that is approximately 6/5 of the earth's radius. This bending is called "refraction." Thus, radars can "see" targets well beyond the optical line of sight. The figure below is a depiction of the beam's path in a standard atmosphere.
5.	Question:	How powerful is the WSR-88D's transmitted microwave energy?
	Answer:	The WSR-88D radar transmits a pulsed signal at 750 kilowatts (peak power). The maximum time-averaged power (transmitting and listening periods) is about 1500 watts.
6.	Question:	Why can't the WSR-88Ds be reprogrammed to filter out returns from wind turbines?
	Answer:	The WSR-88D's clutter filter scheme only removes clutter that is stationary, such as buildings, trees, and terrain. Unfortunately, both precipitation and wind turbine blades are moving and the filter is not applied to them. The radar has difficulty in separating wind turbine returns from precipitation because the reflected energy is constantly changing as their blades change their pitch and orientation. This is analogous to the movement of precipitation, and trying to filter out moving blades will inevitably alter how the radar sees real precipitation.
7.	Question:	Can't you just move the WSR-88D to a new location, or build a new one?
	Answer:	Moving a WSR-88D radar is very expensive — $1.5Million(M) to $4M — and a new weather radar with similar WSR-88D capabilities could be $10M depending on site acquisition costs and other site-specific costs like radar tower height. In general, moving a radar is not a good solution since these radars were strategically sited to work as a national network with proper coverage while minimizing operating costs. Moving one radar can affect coverage relative to surrounding radars in the network. Given the ever increasing number of wind farms being installed, this can quickly become a costly and futile exercise as new wind farms encroach on the moved radar.
8.	Question:	Can the WSR-88D impact a wind turbine or its maintenance personnel?
	Answer:	Yes, if a wind turbine is sited very close to the radar. When wind turbines are sited very close to WSR-88D radars, the turbines can be adversely affected by the high power (750 kW) radar transmission. Within 200 m (600 ft) of a WSR-88D and in the transmitted beam, this energy can exceed the OSHA 29 CFR Part 1910--Subpart G-Occupational Health and Environmental Control Ch.1910.97 threshold for occupational exposure to microwave energy for construction, operation, and maintenance personnel. Within 18 km (10 nmi) of a WSR-88D, the microwave radio frequency field strength can cause bulk cable interference (inductive coupling) with the turbines' electronic controls if they are not properly shielded (MIL-STD-461D).
9.	Question:	Has the NWS ever missed a weather warning to the public, or given a false weather warning to the public as a result of the wind turbine clutter (WTC) problem?
	Answer:	A warning has not been missed yet, but issuance of warnings has been delayed due to uncertainty within WTC. There have also been false warnings issued due to WTC. Operational forecasters can often distinguish WTC from weather signals using their experience. However, WTC is a distraction that takes forecasters’ time away from evaluating developing weather. Another major concern is the effect of these echoes on automated detection algorithms and users (e.g. media and public) not as experienced or used to the appearance of WTC. And, while the WTC problem is causing relatively minor operational impacts at this time, the expected exponential increase in the number of wind farms near WSR-88D radars is cause for concern. Some WSR-88Ds are already becoming surrounded by many wind farms, forcing forecasters and other users to work around significantly large areas of contaminated radar data.
10.	Question:	Has the FAA diverted aircraft as a result of wind turbine clutter?
	Answer:	Yes. The FAA has re-routed air traffic due to false returns from wind turbine clutter. WSR-88D data streams are fed directly into the FAA's Weather and Radar Processor System at Air Route Traffic Control Centers (ARTCC), and FAA controllers use the data to route aircraft safely around weather. ARTCCs have contacted the Radar Operations Center asking about discrepancies between WSR-88D data showing what appeared to be significant weather that required rerouting and pilot reports not seeing weather in the area. This confusion causes unnecessary and expensive aircraft re-routing and excess fuel consumption.