WIND FARM INTERACTION WITH NEXRAD RADAR

The goals of this web site are to increase awareness and understanding of potential wind turbine interaction with Doppler Weather Radars. We provide a point of contact, summarize our potential impact analysis process, and provide possible mitigation solutions to both wind farm developers and radar users.

The NEXRAD Radar (officially designated Weather Surveillance Radar-1988 Doppler (WSR-88D)), is the key tool National Oceanic and Atmospheric Administration (NOAA) forecasters use to track weather and make life and property protecting weather warning decisions. NEXRAD data also support operations of the FAA National Airspace System, Department of Defense, other government agencies, private industry, and the public. The NEXRAD uses an 8.8 m diameter parabolic antenna that produces a 0.95 degree beam width and is programmed to automatically scan the atmosphere (called Volume Coverage Patterns) by rotating 360 degrees through up to 14 elevation angles from 0.5 to 19.5 degrees. The NEXRAD transmits at a wavelength of about 10 cm (3 GHz) and at a peak power of about 750 kW. The radar has a 460 km range for Reflectivity detection and up to a 300 km range for Doppler velocity and spectrum width detection.

For wind farm developers, early consultation with the NEXRAD Program, via the Radar Operations Center (ROC) is the key to minimizing operational conflicts with NEXRAD network and weather warning program. Research tools and evaluation processes are in place to facilitate early consultation. For instance, you can perform your own preliminary, anonymous NEXRAD impact analysis on the FAA's Obstruction Evaluation / Airport Airspace Analysis (OE/AAA) Database ( https://oeaaa.faa.gov/oeaaa/external/gisTools/gisAction.jsp?action=showLongRangeRadarToolForm ) under "DOD Preliminary Screening Tool" button and then follow-up with a formal analysis request through the National Telecommunications and Information Administration (NTIA) - contact information below. The NTIA acts as a one-stop clearinghouse for developers to reach many interested federal agencies, including the ROC, so please submit your proposal directly to the NTIA. They will forward your proposal to us and we will perform a case-by-case analysis at no cost to determine the potential for impacts to the nearby NEXRAD. If it appears that there could be significant impacts, we would like to work with you on mitigation options to reduce the impacts. While the NEXRAD Program has learned about many proposed wind farms via the NTIA, this represents a subset of the wind farms being planned. It also appears the timing of these notifications is after the wind energy developers have already invested considerable time and money in planning wind farm projects. Advance information on new planned projects, or expansions, would enable impact analysis and siting consultation earlier in the project lifecycle, potentially avoiding costly project changes. A preliminary analysis can be performed on wind farm area coordinates (polygons) and then reanalyzed when individual turbines locations are known.

Please note that the Federal Government cannot approve, disapprove, or recommend any action on part of the developer on private land. We can only provide an estimate of impacts and suggest mitigation options for developers to consider. We urge wind farm developers to take advantage of this consultation early in the planning process to arrive at mutually beneficial siting decisions.

Finally, we are sensitive to the proprietary and competitive nature of development plans and all information is treated as "For Official Use Only, Pre-Decisional, and Not Releasable to the Public".

Please consider reviewing the materials at the links provided below. They are intended to help you and increase our interaction with the wind energy industry.

You can contact us at the Radar Operations Center: Wind.Energy.Matters@noaa.gov

Department of Commerce's National Information Telecommunications Administration (NTIA) Contact:

Mrs. Joyce Countee Henry
U.S Department of Commerce/NTIA
Room 4099A, HCHB
1401 Constitution Ave., NW
Washington, DC 20230
Work Phone: (202) 482-1850 ext. 5526
Fax: (202) 482-4396
Email: jhenry@ntia.doc.gov

You can do your own anonymous analysis of your proposed project's impact on the NEXRAD radar network by using the new NEXRAD Tool on the FAA's Obstruction Evaluation (OE/AAA) web site (DoD Preliminary Screening Tool)

Interpretation of the New FAA OE/AAA DoD Preliminary Screen Tool Results

EXAMPLE OF NEXRAD TOOL ON FAA's OE/AAA WEB SITE

Radar Line of Sight Graphs

The WSR-88D radar performs 360° scans of the atmosphere with elevation angles between 0.5° and 19.5°. The WSR-88D samples up to 14 different elevations angles for each complete scan of the atmosphere. The radar beam increases in height and in diameter as it moves away from the radar, with most of the beams energy at the beam center height. The bottom and top heights of the beam are defined as the points where there is a 50% reduction in the transmitted energy. Below the beam bottom height and above the beam top height the energy drastically decreases. The area between the beam bottom and beam top is referred to as the "beam". There is reduced impact on the radar if the rotating blades are below the beam bottom, especially at "close" distances.

Any object within the beam reflects energy back to the radar. The WSR-88D uses a complex algorithm, called a clutter filter, to perform an analysis on the data to determine if the returned energy is from a desirable target (weather) or not (non-weather clutter). One factor used in the clutter filter process is the motion of the return. Clutter mitigation filters within the WSR-88D can not filter rotating wind turbines due to the motion of the blades. When the turbines are close to the radar, they penetrate more of the beam increasing the amount of energy returned to the radar, resulting in higher reflectivity values, potentially at multiple elevation angles.

Click here for a table with the 9 lowest scanning elevation angles and their associated height in meters above the radar feed horn height, or click here for a visual representation of the tables.

Note: The heights are above the radar feed horn for a Standard Atmosphere. The range of the radar is greater than that listed in the tables and graphs.

In addition to the obstruction evaluation required by the FAA, other federal agencies also analyze wind energy projects for impacts to their assets. Although developers may contact us directly, we recommend developers submit their projects through the National Telecommunications Information Administration (NTIA) as early as possible in their planning process. The NTIA submission preferably should be done well before submitting the project to the FAA for obstruction evaluation. Although we would prefer to have firm turbine locations and heights, we can evaluate impacts with development area coordinates (e.g. polygon coordinates) and estimated maximum turbine heights. The project can then be reevaluated when turbine locations and heights are known. The NTIA contact is:

Mrs. Joyce Countee Henry
U.S Department of Commerce/NTIA
Room 4099A, HCHB
1401 Constitution Ave., NW
Washington, DC 20230
Work Phone: (202) 482-1850 ext. 2215
Fax: (202) 482-4396
Email: jhenry@ntia.doc.gov

See Sections 4.1.6 and 5.9.1 of the American Wind Energy Association (AWEA) Wind Energy Siting Handbook https://www.awea.org/sitinghandbook/ for more information.

The link to the FAA Obstruction Evaluation web site is: https://oeaaa.faa.gov/oeaaa/external/portal.jsp.

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.

The NEXRAD radar transmits a pulsed signal at 750 kilowatts (peak power). The time-averaged power (transmitting and listening periods) is about 1500 watts maximum. If wind turbines are in the main radar beam and within 200 m (600 ft) of the NEXRAD antenna, construction or maintenance personnel may be exposed to microwave energy that exceeds Occupational Safety and Health Administration (OSHA) thresholds for occupational exposure. Within 18 km (10 nm) of a NEXRAD, the microwave radio frequency field strength can cause bulk cable interference (inductive coupling) within the turbines electronic controls if they are not properly shielded.

The ROC learns of wind farm developments through both formal and informal methods. Formally, the Department of Commerce's National Telecommunications and Information Administration (NTIA) acts as clearinghouse for developers to voluntarily submit wind farm proposals for review by several federal agencies, including NOAA. This formal process is recognized by the wind industry in the American Wind Energy Association's (AWEA) Wind Siting Handbook (AWEA 2008). Informally, the ROC occasionally receives notifications directly from developers, or learns of wind farm projects from local forecast offices who email news articles or web links to stories about planned wind farms. The ROC typically receives 10 to 15 notifications per month through the NTIA and 1 to 3 per month directly from developers or third parties. The ROC tries to proactively contact the developers if a third party notifies the ROC of a wind project that has the potential to significantly impact a nearby WSR-88D.

Based on the wind farm proposal the ROC receives, the ROC provides a case-by-case analysis of potential wind farm impacts on WSR-88D data and forecast/warning operations. The ROC uses a geographic information system (GIS) database that utilizes data from the Space Shuttle Radar Topography Mission to create a RLOS map with delineated areas corresponding to a turbine height of 160 m AGL. Multiple radar elevation angles are considered for projects close to the radar.

The ROC then performs a meteorological and engineering analysis using: distance from radar to turbines; maximum height of turbine blade tips; the number of wind turbines; radar azimuths impacted; elevation of the nearby WSR-88D antenna; an average 1.0 degree beam width spread; and terrain (GIS database). From this data the ROC determines if the main radar beam will intersect any tower or turbine blade based on the Standard Atmosphere's Refractive Index profile.

Finally, the ROC estimates operational impacts based on amount of turbine blade intrusion into RLOS, number of radar elevation tilts impacted by turbines, location and size of the wind farm, number of turbines, orientation of the wind farm with respect to the radar (radial vs azimuthal alignment), severe weather climatology, and operational experience. The ROC also compares the wind farm to other operational wind farms to estimate impacts.

The ROC has developed a four zone scheme that takes terrain, distance, and the number of elevation angles impacted into account while. The four zones use terminology that communicates to wind farm developers the desired action. These zones, defined below, are: no build, mitigation, consultation and notification.

1. The No Build Zone is a 3-km radius red circle around the WSR-88D. The ROC is requesting that developers do not build turbines in the RLOS within 3 km of the radar due to the potential for serious impacts, including turbine nacelles blocking the radar beam and potential receiver damage if sited in the radar's near field.
2. The Mitigation Zone, orange areas on the map, is the area between 3 km and 36 km where a 160-meter turbine would penetrate more than one elevation angle. Wind farms sited within the mitigation zone have the potential for moderate to high impacts. Therefore, the ROC will work with the developer to get detailed project information, do a thorough impact analysis, and discuss potential mitigation solutions.
3. The Consultation Zone, yellow areas on the map, is the area between 3 km and 36 km where a 160-meter turbine only penetrates the 1st elevation angle or when a 160-meter tall turbine will penetrate more than one elevation angle between 36 km and 60 km. Due to the increased potential for impact to operations the ROC is requesting consultation with the developer to track the project and acquire additional information for a thorough impact analysis.
4. The Notification Zone, green areas on the map, is the area between 36 km and 60 km where a 160-meter tall turbine will only penetrate one elevation angle, or any area beyond 60 km that a 160-meter tall turbine is in the RLOS. Since impacts are typically minimal beyond 60 km and workarounds are available for penetration of only one elevation angle, the ROC is making consultation optional; however, NOAA would still like to know about the project.

The figure depicts an example of the primary categories of wind farm analysis requests/replies.

An example radar line of sight (RLOS) map generated by the NEXRAD ROC for a wind farm analysis. Four hypothetical proposals: W, X, Y, and Z as described in the text are shown.

Wind Farm A: clearly out of the RLOS, would have no impact on the radar data, except in some anomalous propagation conditions, in which case impacts would be low.

Wind Farm B: Notification zone - low impact on the radar data if turbines were built in the western portion of the proposal area. The ROC would suggest that the developer locate most/all wind turbines in the western portion of the proposed area.

Wind Farm C: Consultation Zone - low impact on the radar data if turbines were built in the western portion of the proposal area. The ROC would suggest that the developer locate most/all wind turbines in the western portion of the proposed area.

Wind Farm D: Mitigation Zone - low to moderate impacts on the radar. The ROC would seek to consult with the developer to determine if there is flexibility to consider impact mitigation techniques and to ensure the developers are aware of potential impact on forecast/warning operations.

Wind Farm E: Encroaches into No-Build Zone. Potentially high impacts on the NEXRAD for the portion of the proposal in the red area. The ROC would seek to consult with the developer to ensure they are aware of the likely impact on forecast/warning operations, the NEXRAD system, and the wind turbines/personnel.

Introduction:

Since 2006 the Radar Operations Center (ROC) has had an outreach program to constructively engage the wind energy development community (NEXRAD Now January 2010, Wind Farms and the WSR-88D: An Update and ROC web: https://www.roc.noaa.gov/WSR88D/WindFarm/WindFarm_Index_GreatFalls.aspx?wid=*). One important component of the program is the determination and quantification of the potential wind farm and wind turbine interaction with neighboring WSR-88D installations. Given the variability and complexity of the potential interactions there is no simple closed form solution wherein you have a configuration and location as input and a quantified degradation as a result. The ROC has made increasing use of graphical information system (GIS) tools to reduce some of the complexity.

The Problem:

Early on, ROC engineering and operations personnel involved in resolving blockage and clutter issues determined that the extent of an object's (including wind turbines) penetration of the radar beam was directly related to the level of impact on the radar's ability to process the weather signal in that area. Further, based upon earlier engineering studies, inserting large objects within particular ranges from the radar had more significant impact than merely creating clutter localized to the object.

With this knowledge in hand, when the ROC began to evaluate wind power development proposals received from developers via the National Telecommunications and Information Administration (NTIA) one of the first calculations was the radar line of sight (RLOS) and other beam parameters at the locations of proposed wind turbines. However, the reality of the wind energy developers' proposal cycle was that the specific locations of the turbines were not known until very late in the development cycle. Usually the locations were identified only immediately before the projects were shovel ready. At this point it becomes very costly to make changes. It was inopportune for the developer as there were major commitments in time and money made before this point.

A typical proposal consisted of the geographic coordinates of an area and the anticipated wind turbine hardware configuration, i.e. a mast height and turbine blade diameter. That was all. Fewer than five percent of the submissions contained a number of turbines contemplated for the area. Sampling RLOS penetration at a number of points in the front, rear, and middle of the area was not really predictive of the potential for problems to arise and consumed significant time and resources.

The Solution:

Building upon prior software and GIS (geographical information system) development products, the ROC developed several GIS-enabled databases to model a wind farm's potential areal interactions with the WSR-88D network. In addition to these they developed historical databases of the proposed wind farm areas and wind turbine locations for analyzing potential impacts, doing future ad hoc analysis, and for WSR 88D Hotline responses to field site inquiries. As a first step, the ROC developed a model of the radar coverage coinciding with common wind turbine heights above the ground level (AGL). This model evolved into three GIS databases that portray 130m, 160m, and 200m AGL coverage. Figures 1, 2, and 3 are graphic representations of the coverage areas for the ROC's test-bed system in Norman, OK.

For each radar site, the ROC created a polar terrain grid based on 3600 one tenth degree rays, each with 230 1 km bins or 828000 bins. Using 1 arc second Space Shuttle Radar Topography Mission (SRTM, see https://www2.jpl.nasa.gov/srtm/) digital terrain elevation data (DTED), the maximum terrain height for the particular bin was calculated and normalized to the 0.5 km radial center of the bin.

Associated with each bin are several parameters needed to evaluate potential RLOS penetration and degree of impact. The parameters are the radar range, azimuth from the radar, terrain height in meters MSL (mean sea level), bottom of the radar beam height in meters AGL, and the first radar elevation angle providing terrain clearance for this bin. As a note, the elevation angles correspond to a composite of the angles of the most commonly used volume coverage patterns (VCP). A separate radar specific database table contains range dependent parameters that are used in coordination with the above bin specific parameters. The parameters are slant range, beam width in meters, bottom of beam height in meters MSL, bottom of beam height in meters MSL for the next elevation angle, the row specific elevation angle, and the x, y coordinate pair for the radar location.

The databases described above and SQL (Structured Query Language) manipulations are used to produce geographic presentations and detailed tabular data that become the basis for the analytical impact reports provided to the NTIA, WSR 88D agencies, and developers.

In the case of wind turbine submissions while we use the same databases and similar procedures, we have enhanced our analytical capability by using the USGS National Elevation Data set (NED, see https://seamless.usgs.gov/about_elevation.php) for the terrain height of the wind turbine baseplate height.

Below are two of the primary SQL retrievals for isolating wind turbines penetrating the RLOS. The first identifies all wind turbines within the specific (XXXX) radar's area of coverage:

Select * from XXXXRangeBins, WindTurbines

Where XXXXRangeBins.Obj intersects WindTurbines.Obj and range > 0

Into FirstJoin

The next retrieval identifies those wind turbines penetrating the RLOS and places additional information from the range specific database table along with each wind turbine identified:

Select * from FirstJoin, XXXXAngleHeights

Where range = SlantRange and BoBHtMetersMSL ≤ AMSL_m and BoBHtMetersMSLNextAngle > AMSL_m and FirstAngleTerrainClear ≤ ElevationAngle

Into SecondJoin

At this point all of the data to be used in the analysis along with its corresponding geographic information have been extracted. Now some additional calculations and unit reformatting are performed to produce a file ready to be placed into a spread sheet. Below is the SQL query that performs this function.

Select Proj_name, id, ICAO, Azimuth, range, SE_m, BoBHtMetersMSL, Blade_Tip_Height, AMSL_m BoBHtMetersMSL "Meters into the Beam", BeamWidthMeters, (AMSL_m BoBHtMetersMSL) / BeamWidthMeters * 100"Percent Penetration", ElevationAngle, FirstAngleTerrainClear, distance(OBJX, OBJY, SiteX, SiteY, "km")

From SecondJoin

Order by ElevationAngle desc, FirstAngleTerrainClear, COL11 desc

Into ReportReady

After the SQL processing we are ready to produce both graphic and statistical data. Figures 4 and 5 depict the Fictitious Turbine Project, its relationship to the ROC's test-bed system, KCRI, and the KCRI range cells with their azimuth and range for the Fictitious Turbine Project turbines penetrating the KCRI RLOS.

Now we have the graphic identifying the azimuth and range of the KCRI range cells containing those turbines penetrating the RLOS.

Figure 6 is a screen capture of the spreadsheet containing the details of each wind turbine penetrating the RLOS.

Below is a similar set of SQL retrievals for producing an areal analysis for a wind farm.

Select * from XXXXRangeBins, WindFarms

Where XXXXRangeBins.Obj intersects WindFarms.Obj and range > 0

Into FirstJoin

Now for the second retrieval:

Select * from FirstJoin, XXXXAngleHts

Where ( range = SlantRange ) and ( BoBHtMetersMSL - TerrainHtMetersMSL ≤ Blade_Tip_Height ) and ( BoBHtMetersMSLNextAngle - TerrainHtMetersMSL > Blade_Tip_Height ) and FirstAngleTerrainClear ≤ ElevationAngle

Order by Proj_Name

Into SecondJoin

The report reformatting query:

Select Proj_Name, ICAO, Azimuth, range, TerrainHtMetersMSL, BoBhtMetersAGL, Blade_Tip_Height, (Blade_Tip_Height-( BoBHtMetersMSL - TerrainHtMetersMSL ))"Meters into the Beam", BeamWidthMeters, ( Blade_Tip_Height - ( BoBHtMetersMSL - TerrainHtMetersMSL ))/ BeamWidthMeters * 100"Percent Penetration", ElevationAngle, FirstAngleTerrainClear

From SecondJoin

Order by Proj_name , ElevationAngle desc , FirstAngleTerrainClear , Col10 desc

Into ReportReady

As a part of the areal analysis we calculate the area in the RLOS and the total area within the proposed boundaries. The following retrieval calculates the area and number of bins in the RLOS.

Select Proj_Name, count(*), sum(SphericalArea(obj, "sq km"))

From SecondJoin

Group by Proj_Name

Order by Proj_name

Into RLOSnums

This retrieval calculates the total area and number of bins.

Select Proj_Name, count(*), sum(SphericalArea(obj, "sq km"))

From FirstJoin

Group by Proj_Name

Order by Proj_name

Into TOTnums

After the SQL processing graphical and statistical data can be produced. Figures 7 and 8 depict the Hypothetical Farm Wind Energy Project, its relationship to the ROC's test-bed system, KCRI, and the Hypothetical Farm Wind Energy Project's area in the KCRI RLOS.

The areal relationship between the KCRI radar and the Hypothetical Farm southeast of the radar is depicted above.

The area in black within the northwest portion of the Hypothetical Farm's boundaries signifies the potential for wind turbines placed in that area to penetrate into the first elevation angle of the KCRI RLOS as identified in the SQL retrievals above.

Figure 9 is a screen capture of the portion of the spread sheet that is developed from the report data generated above.

In this case there would be over 7400 rows of data in the spreadsheet, each with information about the location and potential for RLOS penetration presented within that 1 km by 0.1 degree bin.

The calculations necessary to produce the polar terrain grid and GIS database files are far too lengthy and beyond the scope of this short presentation.

Conclusion:

A distinct advantage of this GIS database approach is having much of the time consuming calculation already completed with results available as intermediates for the final site specific analyses. This approach provides the ROC with the ability to provide timely analysis to the stakeholders in the wind energy - WSR 88D community.

The best mitigation technique is to avoid locating wind turbines in the radar line of sight (RLOS) of a NEXRAD. This strategy may be achieved by distance or terrain masking. Mitigation of impacts, if turbines are in the RLOS, can be achieved by reducing the number of turbines in the RLOS, the amount of blade penetration into the RLOS, greater separation from the radar, or through selective turbine siting; e.g., to reduce the azimuthal extent of the turbines with respect to the radar. Each situation requires a case-by-case analysis.

The NEXRAD Program's Radar Operations Center (ROC), working with some wind energy developers the past two years, has developed another promising mitigation option for developers to consider: Operational Curtailment. This involves the developer working with the local NWS Weather Forecast Office (WFO) or military Base Weather Station (BWS) to define severe weather scenarios and the maximum number of hours per year under which the wind farm operators will stop the wind turbines, upon request from the local WFO or BWS, to allow forecasters to receive uncluttered radar data over the wind farm area. Stopping the turbines allows the radar's clutter filter algorithm to eliminate any wind turbine clutter (WTC) and is the next best mitigation option after not siting in the RLOS. The clutter filter algorithm can only remove non-moving clutter and there is no know filtering technique to remove WTC while preserving the weather signal. The Wind Farm Owner/Operator and WFO could sign a voluntary Letter of Intent (LOI) agreement detailing the weather scenarios, maximum number of curtailment events and hours per year, points of contact on each side, and the procedures for implementing curtailment. NOAA currently has three signed LOIs in place with two wind energy companies. A generic LOI template is available for download here: LOI Template

In summary, here is a prioritized list of mitigation options for developers to consider.

1. Site Wind Turbines (WT) out of the Radar Line of Site (RLOS) (see Fig 1 below).
2. If within the RLOS, sign an LOI agreement with the local forecast office to briefly (30 to 60 min) stop the turbines during significant severe weather events (this typically eliminates WTC in most circumstances. However, if WTs are very close (< 3km) to the radar, the WTs can still physically block the radar beam, which is an even more severe issue than clutter because it affects the entire range of the radar.
3. Within the RLOS, site WTs as far as possible from the radar (assuming level terrain) (see Fig 2 below).
4. In hilly areas, use terrain masking by placing WTs on the far side of the hill below the peak (see Fig 3 below).
5. Try to minimize azimuthal spread relative to radar, especially when siting very close to the radar (see Fig 4 below).
6. If azimuthal spread is large, provide open radials from radar by bunching turbines into groups with space between these groups. When the wind turbines are very close (within 18 km) to the radar, this provides clear radials of data when multipath effects (ghosting, beam blockage) are occurring down radial from the wind farm (see Fig 5 below).
7. When choosing the height of wind turbines, if shorter wind turbines would not penetrate into the RLOS, we would prefer the shorter turbines. However if shorter turbines would still be in the RLOS, then fewer, taller turbines over a smaller area may be preferable (see Fig 6 below).

 

Future Considerations:
It may be advantageous for the wind industry to investigate the potential benefits of using "stealth" technology in wind turbine design to reduce the radar cross section of the tower and blades.

The NEXRAD Program is executing a multi-pronged short-term strategy, and developing a long-term strategy, to mitigate the impacts of wind farms on NEXRAD data, products, and operations.

SHORT-TERM STRATEGY

a. Creating awareness of the problem

We are conducting an outreach program to ensure the wind energy industry and developers are aware of NEXRAD locations and potential wind farm impacts on NEXRADs earlier in the development process so that they site developments at more favorable locations with respect to NEXRAD impacts. The Radar Operations Center (ROC) works with developers by providing no-cost site-by-site analysis of proposed wind farms and suggests mitigation options to consider.

b. Increasing knowledge of the problem and solutions

We continue 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, and in collaboration with the Warning Decision Training Branch (WDTB), a forecaster awareness training course was developed that introduces forecasters to the appearance of wind turbine clutter (WTC) in NEXRAD products and provides some information on the "work-arounds" that are available. This training course can be found at: https://www.wdtb.noaa.gov/modules/windfarms/index.html. In the absence of predictive modeling software, we are also learning how observed impacts at one site can apply to similar proposals for evaluation.

c. Generation of identification tools

A joint effort between the National Severe Storms Laboratory (NSSL) and the ROC has resulted in the development of Advanced Weather Interactive Processing System (AWIPS) overlays that indicate wind turbine locations. Creation of the overlays was accomplished using 12-months of accumulated radar estimated precipitation data (see https://www.nssl.noaa.gov/projects/q2/q2.php for more information). Wind farms create "hot spots" in these data. The overlays are added to the AWIPS display to help Weather Forecast Office and River Forecast Center staff identify potential areas of wind turbine clutter.

d. Collaborating with other Federal Agencies

The NEXRAD radar is not the only radar affected by wind turbine clutter. The long-range Air Route Surveillance Radars (e.g. ARSR-4), used jointly by the DOD and the Department of Homeland Security (DHS) for tracking aircraft, have also been impacted by large wind farms. The ROC is working with the DOD/DHS Long-Range Radar Joint Program Office to build off of their experience with this issue. We are collaborating with the FAA, DoD and DHS on the DHS-funded contract for the Wind Turbine/Radar Modeling Tool.

e. Supporting Experimental Signal Processing Technique Investigations

The ROC is providing limited funding for studies of potential signal-processing techniques by the Atmospheric Radar Research Center (ARRC https://arrc.ou.edu at the University of Oklahoma (e.g., Isom et al 2009). One goal of these sophisticated signal processing methods is to automatically identify the turbine-corrupted data through spectral features, temporal continuity, etc., flag it, and potentially recover the underlying weather information. When this detection algorithm is finalized, the NEXRAD has a flexible, open architecture signal processor that would enable relatively low cost and timely implementation of the new signal processing technique.

In addition to detection, signal processing methods based on real-time, telemetry-based algorithms are being explored by the ARRC. These knowledge-based techniques would exploit wind turbine data of blade rotation rate, orientation, etc., and are a good example of the benefits of collaboration with wind farm operations. An initial phase is currently being conducted in a controlled laboratory environment using scaled models and scattering experiments (Fig 1). Further, electromagnetic simulations are being conducted to determine expected radar cross-section from turbines, which will be used to validate laboratory measurements and to enhance adaptive signal processing algorithms.

Finally, more advanced radar designs are being studied as a long-term solution to wind turbine interference. In particular, OU's ARRC is investigating the possibility of using adaptive null steering with phased array radars as a means of mitigating clutter from moving targets (Palmer et al 2009). Promising results have already been obtained using sophisticated numerical simulation techniques previously developed by the ARRC (Cheong et al 2008). The potential exists for a possible solution to this difficult problem, but further research and additional funding sources are needed.

Success of these ARRC studies could depend on partnering with other federal agencies and/or the wind energy industry. However, these techniques, if achievable, will take several years to operationally implement. For further details, see the related paper entitled Spatial Filtering of Wind Turbine Clutter Using Adaptive Phased Array Radars (8B.6).

LONG-TERM STRATEGY

Our current (short-term) strategy is labor-intensive and the workload will increase as more developers seek to place wind farms on land with the best wind resource, which is often near weather radars. A more effective approach will be needed to ensure this critical, national radar asset is not significantly degraded. In cooperation with the wind industry and other federal agencies, the ROC is exploring two possible areas: new national guidelines for the wind industry and new funding, federal and/or industry, to find technical solutions to the interference problem.

A. New National Guidelines:

New national guidelines might include one or more of the following:

(1) A Memorandum of Understanding (MOU) between the wind energy industry and federal agencies, similar to an existing British MOU, with agreement on:

1. No-build zones very near radars
2. Notification of federal agencies with Doppler radar assets
3. Consultation to mitigate impacts
4. Halting wind turbine blades during significant weather events

(2) A National "clearing house" for developers to submit wind farm proposals to all federal agencies with radar assets - DHS, DOD, FAA, and NOAA. This clearinghouse would work similar to the FAA's Obstruction Evaluation Office for determining obstructions in navigable airspace; refer to FAA Regulation Part 77

(3) Federal Statutory Authority

1. Mandatory developer notification of projects and government evaluation of potential wind farm impacts, similar to FAA Regulation Part 77
2. Define no-build zones very near radars

B. New Funding:

New funding might be used to help develop radar-based and/or wind-turbine based solutions:

(1) Radar-based mitigation funding to:

1. Develop modeling software that produces estimated radar impacts
2. Develop signal processing technology that eliminates wind turbine clutter - a difficult technical challenge for which there may not be a solution
3. Build additional radars for an alternate view of impacted areas

(2) Wind turbine-based mitigation funding to develop radar-friendly "stealthy" wind turbine blades and towers.

The goals of this web site are to increase awareness and understanding of a relatively new issue-Wind Farm (WF) interaction with Doppler Weather Radars - by providing:

• Example imagery of wind turbine - radar interactions,
• A summary of our process to assess potential impacts,
• Potential mitigation solutions to both wind farm developers and radar users/operators, and
• A point of contact at the WSR-88D Radar Operations Center.

For radar users/operators, we know that the WSR-88D is your primary tool for tracking severe weather, making severe weather warning decisions, and non-severe weather decisions. We also appreciate that WSR-88D data supports operations of the FAA National Airspace System, the Department of Defense, and other government and private entities. We are working hard to educate the wind energy industry about the impacts of wind farms on the WSR-88D, and asking wind farm developers to consult with us as early as possible in their development process to minimize project risk and operational conflicts with the Nation's weather radar network and NOAA's NWS's severe weather warning program. If you have a concern about a wind farm (WF) project, please contact us and we will try to get all the interested parties together to discuss the issues.

Forecaster awareness of wind farm locations, their potential impacts to base data and radar algorithms, and appropriate work-around strategies can go a long way to avoiding problems caused by wind turbine clutter (WTC). This is both a training and operations issue. WFOs should incorporate WTC awareness and mitigation into their operating procedures, such as shift change briefings. The ROC will work with the Warning Decision Training Branch to incorporate WTC awareness and mitigation strategies into their Distance Learning Courses.

Based on our investigation and experience, we have identified the following impacts wind farms can have on WSR-88D data and products:

• False or anomalously large radar-estimated precipitation amounts (especially Storm Total Precip) due to reflection from towers/turbines;
• False low radar-estimated precipitation amounts due to radar beam blockage (very close wind farms (WFs) only);
• False echoes downrange from wind farms due to multipath effects (close WFs only);
• Anomalously large reflectivity values due to reflection from towers/turbines;
• False storm identification due to reflection from towers/turbines;
• Incorrect velocity values due to contamination by turbine blade motion;
• Velocity dealiasing errors;
• False mesocyclone and tornado vortex detection due to anomalous velocity values;
• Incorrect VAD wind profiles; and
• Potential loss of low-level tornado/severe weather signatures because of blockage from towers/turbines.

Forecasters can learn to recognize most wind farm signatures, reduce impacts somewhat through proper radar configuration, and attempt to accommodate or "work around" the wind farm impacts in their decision process (Burgess et al 2008). For example, forecasters can:

(1) Exclusion zones can be set up to prevent known areas of wind turbine clutter from contaminating precipitation products. See Figures 1 and 2 below, which demonstrate the effectiveness of applying exclusion zones. Note that exclusion zones do not affect base data, which may be used years later for climatological or other studies. Currently, up to 20 exclusion zones can be implemented. Each zone has five parameters to enter:

• Begin Azimuth
• End Azimuth
• Begin Range
• End Range
• Max Elevation Angle

(2) Invoke clutter suppression. This approach only excludes stationary targets and is not effective on clutter arising from turbine blades in motion.

(3) Look at higher elevations to "see over" wind farms. This can result in the loss of low-altitude information crucial in some forecast situations, e.g., onset of a tornado.

(4) Look at adjacent radar coverage. However this further reduces the ability to see low-level phenomena.

(5) Talk with WFOs who have more experience dealing with WTC.