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HOW WIND TURBINES IMPACT THE NEXRAD DOPPLER WEATHER RADAR
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Wind farms can impact coherent (Doppler) radars in three ways if the
turbine blades are moving and they are within the radar’s line of sight. If
close enough (within a few kilometers) they can partially block a
significant percentage of the beam and attenuate data down range of the
wind farm. They can also reflect energy back to radar and appear as clutter
on the radar image and contaminate the base reflectivity data. The base
reflectivity data is used by radar algorithms to estimate rainfall and to
detect certain storm characteristics. Finally, they can impact the velocity
and spectrum width data, which are also used by radar operators and by a
variety of algorithms in the radar’s data processors to detect certain
storm characteristics, such as mesocyclones, relative storm motion,
turbulence, etc. The WSR-88D has a sophisticated clutter removal scheme.
Since weather is always 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. Unfortunately, the radar sees rotating wind turbine blades as
targets having reflectivity and motion, hence processes these returns as
weather. |
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Wind farms at “extremely close” ranges to radars have all the impacts listed
above and additional ones. Inter-turbine scatter and multi-trip/multi-path
returns create false signals down-radial from the real wind farm echo regions.
These down-radial returns have been observed for turbines located within 10
miles (16 km) of the radar, and can extend down radial for 25 miles (40 km) or
further. In some cases the disturbed areas are large enough to cause additional
forecaster confusion and distraction, and to affect forecasts and radar data
(particularly Velocity Azimuth Display Wind Profile) assimilations into
numerical models. |
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If wind turbines are in the main radar beam and within 600ft (183m) of radars
(in the near field), damage to both the radar’s and turbines’ electrical
components might occur, Turbines within 1 mile of a NEXRAD can prevent the
radar’s beam from properly forming, thus causing significant radar estimation
errors down range from the turbines. |
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In summary, WTC can have negative impacts on radar
capabilities and products (radar imagery) over and near the wind energy
facility. This reduction of useable radar data can also impact weather forecast
and warning operations, DoD military operations, and FAA air traffic routing
operations. Examples of these impacts are:
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Thunderstorm or winter storm characteristics
could be masked or misinterpreted, reducing warning effectiveness in the
vicinity of and downrange of the wind energy facility.
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False signatures contaminating Doppler velocity
data in the vicinity and downrange of the wind energy facility could reduce
forecasters’ situational awareness, particularly during hazardous/severe
weather events.
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Data masking or contamination if thunderstorms
develop over the wind energy facility may negatively impact warning
effectiveness.
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False precipitation estimates could negatively
impact flash-flood warning effectiveness.
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Needless and expensive FAA aircraft re-routing from
false returns due to WTC.
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Examples of WTC Impact
Zoomed-in Display of WTC-contaminated data from Fort Drum NEXRAD
Some Wind Farms look like thunderstorms and can confuse users
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This Reflectivity image (0.5 degree scan) from
the Des Moines NEXRAD (KDMX) on July 19, 2007 at 0236
GMT shows how it can be difficult to distinguish high radar reflectivity
returns (yellow and red pixels) due to wind turbines from strong storm returns
(see annotations). The wind farms vary in distance from approximately 115 km to
160 km from the NEXRAD. In this case an emergency manager, monitoring the
severe weather situation using the Des Moines NEXRAD, mentioned confusion as
the storm moved into their area of responsibility. This can happen at close
range also.
KTFX (Great Falls, MT) NEXRAD Velocity (0.5 degree) (Zoomed)
This KTFX Velocity image shows multipath echoes downrange with erroneous velocity values
This radar-estimated Storm Total Precipitation
accumulation product from the Des Moines IA NEXRAD on
July 19, 2007 at 0512 GMT includes the period of time and storms shown in the
figure before this one. Erroneous 5+ inch radar-estimated Storm Total Precipitation accumulations (indicated by the
arrows) are due to wind farms northwest 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.
(Radar precipitation algorithms do not use the returns from the exclusion zone
to accumulate precipitation.)
Reflected energy from wind farms within the NEXRAD RLOS negatively impacts the radar data and products. This is illustrated by the situation of two
wind farms within line of sight of the Dodge City, KS NEXRAD (KDDC) in the figure below.
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.
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