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Questions & Comments
 

FREQUENTLY ASKED QUESTIONS (FAQs)


Q1. How far away from the NEXRAD should we site a wind farm? Do you have a benchmark distance? If so, what is it?
Q2. What is the "Radar-Line-of-Sight" and why is that important?
Q3. Does the RLOS ever change?
Q4. How can NEXRAD systems "see" wind towers/turbines when I can't visually see the radar from the wind farm?
Q5. How powerful is the NEXRAD's transmitted microwave energy?
Q6. Why can't the NEXRAD be reprogrammed to filter out returns from wind turbines?
Q7. Can't you just move the NEXRAD to a new location, or build a new one?
Q8. Can the NEXRAD impact a wind turbine or its maintenance personnel?
Q9. Has the National Weather Service ever missed a weather warning to public, or given a false weather warning to the public as a result of the wind turbine clutter problem?
Q10. Has the FAA diverted aircraft as a result of wind turbine clutter?

Q1. How far away from the NEXRAD should we site a wind farm? Do you have a benchmark distance? If so, what is it?

REPLY: Our benchmark distance is terrain dependent and varies from site to site, but on flat terrain the distance would be approximately 18 km (10 nm). This benchmark is based on the distance at which turbines begin to penetrate the radar line of site (RLOS) of the second scanning elevation angle (0.9 deg.). We have also established a benchmark no-build zone of 4 km. For a more detailed description of how the ROC analyses wind farm proposals visit: HOW THE ROC ANALYZES WIND TURBINE SITING PROPOSALS.

Q2. What is the "Radar-Line-of-Sight" and why is that important?

REPLY: The radar line of sight/radar beam width can be considered analogous to the beam of light coming from a flash light. Most of the energy of the flashlight, just as with the radar, is in the beam of light/radar beam. In radars this is the distance between the "half power" points or where the energy in the beam is down 3 dB from that at the center of the beam. For the NEXRAD the beam width is approximately 1 degree. As the beam propagates away from the radar, its width increases. For NEXRADs, at 111km (60 nm) from the radar the beam is approximately 2km (1 nm) wide. Obstacles in the radar line of sight can block the radar signal and reduce the ability of the radar to see targets further downrange. The figure below is a depiction of the radar line of sight.

Main beam/radar line of sight is defined by half-power points

Q3. Does the RLOS ever change?

REPLY: 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 changes. It also changes as fronts pass or with nearby thunderstorm outflows. Typically after sunset, the surface temperature cools causing the radar beam to bend 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.

Q4. How can NEXRAD systems "see" wind towers/turbines when I can't visually see the radar from the wind farm?

REPLY: 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" representative of the atmosphere on a day with enough wind to mix the lower atmosphere well, the radar beam takes a path that is approximately 4/3 of the Earth's radius. This bending is called "refraction." So, the NEXRAD, like other 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.

Q5. How powerful is the NEXRAD's transmitted microwave energy?

REPLY: The NEXRAD radar transmits a pulsed signal at 750 kilowatts (peak power). The maximum time-averaged power (transmitting and listening periods) is about 1500 watts.

Q6. Why can't the NEXRAD be reprogrammed to filter out returns from wind turbines?

REPLY: The NEXRAD'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. Trying to filter out moving blades will inevitably alter how the radar sees real precipitation. Here's why. A single radar volume sample (gate) at 48 km (26nm) from the radar is approximately a square kilometer. Thus, for a typical wind farm, the radar may receive reflected energy from many turbines within that gate, each with multiple rotating blades. These numerous rotating blades appear similar to precipitation, which is also made up of numerous distributed moving targets. Yes, there are fewer blades than raindrops within a sample volume, but the blades make up for their smaller numbers by reflecting significantly more energy back to the radar. However, the radar has no way to determine the number of targets it is sampling within a particular gate. Also, the reflected energy is constantly changing as the blades change their pitch and orientation relative to radar, with some blades moving towards the radar, some moving away, and some not appearing to move at all (perpendicular). This is analogous to the movement of precipitation within a volume sample.

Q7. Can't you just move the NEXRAD to a new location, or build a new one?

REPLY: Moving a NEXRAD radar is very expensive and is not a good solution since they are strategically sited to work as a national network with proper coverage while minimizing operating costs. Relocating 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.

Q8. Can the NEXRAD impact a wind turbine or its maintenance personnel?

REPLY: Yes, if a wind turbine is sited very close to the radar. When wind turbines are sited very close to NEXRAD radars, the turbines can be adversely affected by the high power, 750 kW, radar transmission. Within 200 m (600 ft) of a NEXRAD 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 nm) of a NEXRAD, 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).

Q9. Has the National Weather Service ever missed a weather warning to public, or given a false weather warning to the public as a result of the wind turbine clutter (WTC) problem?

REPLY: A warning has not been missed yet, but there have been some false warnings issued due to WTC. Operational forecasters can often distinguish WTC from weather signals using their experience. However, WTC can be a distraction and can take 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 NEXRAD radars is cause for concern. It is easy to envision some NEXRADs becoming surrounded by many wind farms and forecasters and other users having to work around significantly large areas of contaminated radar data.

Q10. Has the FAA diverted aircraft as a result of wind turbine clutter?

REPLY: Yes. The FAA has re-routed air traffic due to false returns from wind turbine clutter. NEXRAD 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 NEXRAD Radar Operations Center asking about NEXRAD radar data showing what appeared to be significant weather that required rerouting, but pilots reported not seeing weather in the area. This confusion causes unnecessary and expensive aircraft re-routing and excess fuel consumption.