HOW ROTATING WIND TURBINE BLADES IMPACT THE NEXRAD DOPPLER WEATHER RADAR
Rotating wind turbine blades can impact the radar in several ways. Wind turbines can impact the NEXRAD
radar base data, algorithms, and derived products when the turbine blades are moving
and in the radar’s line of sight (RLOS); and, if turbines are sited very near
to the radar their large nacelles and blades can also physically block the radar
beam or reflect enough energy back to the radar to damage the radar’s receiver hardware.
- Radar Receiver: The NEXRAD radar has a very sensitive receiver. The
radar’s Receiver Protector prevents damage from strong reflected signals; however
its upper limit is 53 dBm. Large objects sited very near the radar (< 3 km), such
as turbine nacelles, have the potential to return signals that exceed the limit
of receiver protector and render the radar inoperable.
- Beam Blockage: If sited within a few kilometers of the radar, wind turbines
can partially or fully block the radar beam. This beam blockage attenuates the
strength of the beam and impacts data beyond the wind farm, causing shadows or
spikes in the data through the entire range of the radar (460 km for reflectivity
data, and up to 300 km for velocity and spectrum width data).
- Radar Base Data: Turbines in RLOS can reflect energy back to the radar and
visually contaminate the reflectivity, velocity, and spectrum width data. Forecasters
look for certain “signatures” in the data that indicate the severity of the
storms. The wind farm clutter can sometimes look just like showers and
thunderstorms, or can alter the appearance of a storm (e.g. hook echoes). This
visually corrupted data adds uncertainty to the analysis and could cause
forecasters to delay/miss a severe weather warning or to warn unnecessarily.
- Algorithms and Derived Products: The base reflectivity, velocity,
and spectrum-width data are also used by many algorithms in the radar
processor to detect certain storm characteristics, such as mesocyclones,
relative storm motion, hail, turbulence, etc. Corrupted base data
can cause the radar algorithms to generate false alerts or to miss
alerts. The radar also generates many additional products using
this base data, such as wind profiles and rainfall estimates. Wind
turbine clutter can impact the accuracy of these derived products.
The graph below depicts the relative impact of wind turbines
(or wind farms) on NEXRAD radars and forecasters as a function of distance (on level terrain)
if wind turbines are in the RLOS.
Impacts increase greatly as wind turbines are sited closer to the
radar, especially within 18 km (assuming level terrain), as radar operator workarounds become
more difficult. Turbines sited at least 18 km from the radar
generally only impact the lowest radar scan at 0.5 degrees
elevation, and clutter is confined to the wind farm area. Within
18 km wind turbines cause additional impacts including: clutter
on multiple elevation scans above 0.5 degrees, multipath clutter
down range of the wind turbines, and greater impacts to radar
algorithms. Multipath scattering from wind turbines can
extend the contaminated data up to 40 km beyond the wind
farm. Turbines sited within 3 km of the radar may also
cause significant (>10%) attenuation/blockage of the radar
beam impacting data throughout the entire range
(460 km-reflectivity, 300 km-velocity) of the radar. When turbines are sited within 200 m, construction
or maintenance personnel may be exposed to microwave energy
exceeding OSHA (Occupational Safety and Health Administration)
thresholds. The above distances assume a level terrain and a
Standard Atmosphere Index of Refraction profile. Therefore, actual
impacts may occur closer or further away from the radar than this
chart indicates depending on the terrain and current atmospheric
refraction. Accurate determination of the RLOS and impact
distances requires a detailed site-by-site analysis.
You may wonder why we can’t filter out this clutter since we know where the wind
farms are located. The NEXRAD has a sophisticated clutter removal scheme. Since
weather is usually in motion, the scheme was designed to filter returns that have
essentially no or very low motion. This is effective for removing the returned
signals from terrain, buildings, and other non-moving structures. However, the
radar sees rotating wind turbine blades as targets having motion, hence processes
these returns as weather. At this time there is no filtering scheme available
to identify and remove wind turbine clutter while preserving real weather
returns. Studies are underway to determine if weather radar software can
be developed to automatically identify and then remove the rotating
turbine signature/clutter from the weather signal. Recent results
indicate that automatic identification of turbine clutter will
be easier to achieve than filtering the clutter. Even if a filtering
scheme is discovered, developing the software to run on the
NEXRAD radar would take several years.
Wind turbine clutter has not had a major negative impact on forecast
or warning operations, yet. However, with more and larger wind turbines
coming on line, radars in some parts of the country will have multiple
wind farms in their line of sight. Cumulative negative impacts should
be anticipated – which, at some point, may become sufficient to
compromise the ability of radar data users to perform their missions.
Examples of Wind Turbine Clutter
Zoomed-in Display of WTC-contaminated data from Fort Drum NEXRAD
The image above is a zoomed 0.5 degree elevation Reflectivity product from the
Ft Drum, NY NEXRAD. There is a large wind farm nearby with turbines oriented from
due north through southeast of the radar. The turbines are close enough (within 18 km)
to cause spurious multipath scattering that extends well beyond the wind farm and contaminates
data at multiple scanning elevation angles.
Display of WTC-contaminated data from the Dyess AFB, TX NEXRAD
Sequence (left to right) of 0.5 deg reflectivity images showing thunderstorms developing
over a wind farm (purple rectangle) 18–30 km (10-16 nm) west of Dyess AFB, TX WSR-88D.
Left: thunderstorms have not yet developed, high reflectivity values due to wind turbines alone. Middle
and Right: storm has developed to where in right image a distinct notch structure, indicative of
severe weather, formed – note: turbine and weather echoes indistinguishable.
This radar-estimated Storm Total Precipitation accumulation product from the Dodge City, Kansas
NEXRAD on April 22, 2010 at 1403 GMT depicts how wind farms can impact
radar-derived products. Erroneous 4+ inch radar-estimated Storm Total Precipitation
accumulations (indicated by the arrows) in the image on the left are due to wind farms northeast and
southwest of the NEXRAD. The
anomalous accumulations make estimates of rainfall over an area/river basin more difficult
to determine. However, radar operators can apply exclusion zones to mitigate these anomalous
accumulations, as seen on the right. (Radar precipitation algorithms do not use the returns from the exclusion
zone to accumulate precipitation.)
Dodge City, KS NEXRAD (KDDC) reflectivity (upper right) and mean radial
velocity (lower right) imagery for 0150 UTC on 23 Feb 2007 showing two wind farms
within the radar’s line of sight. The yellow area in the upper left image depicts areas
where the radar line of sight is within 130 m of the ground. The reflectivity and velocity
values are anomalous and can confuse users. The lower left panel shows the effects of the
wind farm to the southwest whose influence has resulted in a false tornado alert generated
by the NEXRAD algorithms. The Weather Forecast Office did not issue a warning because, in
this case, other data indicated that there was no severe weather in the wind farm area.