Wind Farms: Coming Soon to a WSR-88D Near You

Those paying attention to the news lately know that wind power is one of the primary renewable energy sources being aggressively pursued by government and industry, as one solution to our fossil fuel dependence. In July 2008, the Department of Energy (DOE) released a feasibility study on wind energy called 20% Wind Power by 2030, Increasing Wind Energy's Contribution to U.S. Electricity Supply, which can be found at http://www1.eere.energy.gov/windandhydro/pdfs/41869.pdf - a roadmap for reaching the report title's goal. Additionally, 25 states have now established Renewable Portfolio Standards (RPS) that mandate utilities provide a certain percentage of electric power from renewable energy sources. (RPS information is located on the web at http://www.pewclimate.org/ what_s_being_done/in_the_states/ rps.cfm). Soon after the release of the DOE report, oil man T. Boone Pickens announced his Pickens Plan (http://www.pickensplan.com/index.php), which aims to replace natural gas-fueled electric power plants with wind generated power and use the freed-up natural gas to fuel transportation vehicles.

The above efforts, along with increasing climate concerns and the maturing of wind power technology, are fueling the rapid growth of the wind energy industry. During Summer 2008, the U.S. wind energy industry surpassed Germany, becoming the world's leader in wind energy generation capacity with 20GW installed. However, this is still only 2% of the Nation's total electric supply. The DOE report estimates reaching the goal of 20% of the electric supply will require 300GW of wind power capacity. Today's typical wind turbine generates a max power of 1.5MW, so reaching the goal would currently require the installation of about 200,000 wind turbines across the country. As reflected in the graphic in Figure 1, installed wind power capacity is indeed accelerating.

For several reasons (e.g., adequate low-level wind resources, power transmission infrastructure) the distribution of wind farms is not, and will not be, uniform across the country. As shown in Figure 2, the Southeast has the lowest wind resources, while the Great Plains states, which Pickens calls "the Saudi Arabia of wind," have the world's most abundant wind resources. (Information regarding "the Saudi Arabia of wind" can be found at http://www.heraldtribune.com/article/20080726/ COLUMNIST/487392094/2312/OPINION& title=The_Saudi_Arabia_of_wind_power_.) Currently, Texas has the most installed capacity of any U.S. state and is expected to increase its lead in the next few years (Figure 3), with the Texas Public Utility Commission's approved $5 Billion plan to install 18GW of electric transmission capacity (http://www.puc.state.tx.us/nrelease/2008/071708.pdf). The growth in the number of wind farms, and the fact that optimum wind farm locations are similar to WSR-88D siting preferences - relatively high, unobstructed terrain - suggests the number of wind farms developed near WSR-88Ds is likely to increase.




So, why should anyone care about all these new wind farms? As it turns out, rotating wind turbines in the line of sight of the radar can show up very strongly on all three base products (Reflectivity (R) Velocity (V), Spectrum Width (SW)) and some derived products (e.g. precipitation estimates), even with clutter filtering applied.

Wind Turbine Interaction With the WSR-88D
Rotating wind turbines will cause clutter problems when they are in the line of sight of any coherent-type radar designed to detect moving targets,including air surveillance radars (long range and terminal) and Doppler weather radars. The numerous rotating blades of a wind farm appear similar to precipitation, which is also made up of numerous distributed moving targets. The radar clutter filter is ineffective since the filter assumes clutter is stationary. Wind turbine blades now commonly extend 350 ft to over 500 ft above ground level and reflect radar energy very well (greater than 70 dBz in some cases). At distances near the boundary of the radar line of sight and beyond, the wind farms may appear and disappear depending on atmospheric conditions. In the "right" atmospheric conditions, wind farms can be seen on WSR-88Ds at 50+ nm.

Studies by the University of Oklahoma have revealed that wind turbine clutter (WTC) can have negative impacts on the radar's base products (R, V, SW) and derived products such as mesocyclone detection, precipitation estimation, and Velocity Azimuth Dis-play (VAD) Wind Profile over and near the wind energy facility. When wind farms are within ~10 statue miles, they can cause anomalous false echoes in all three radar moments. Figure 4 shows the impact of a wind farm close to the Ft. Drum, NY WSR-88D. Note strong echoes along the leading edge, as well as, weak return behind the turbines due to "multi-path scattering" of the radar beam. In Figure 5, the impact of two wind farms not quite as close to the Dodge City, KS WSR-88D is shown. This reduction of useable radar data can impact weather forecast operations and Federal Aviation Administration (FAA) air traffic routing operations.

Wind energy developers tend to be unaware of the impacts wind turbines can have on the WSR-88D or that the WSR-88D transmits ~750 KW of energy. At close ranges (600 ft), energy levels can exceed safety standards for personnel working at the level of the antenna center point, e.g., turbine construction or maintenance crews. At up to ~10 miles, there is a possibility the WSR-88D could impact turbine electronics without proper shielding. When wind turbines are within 1 km they can cause WSR-88D beam forming problems and partial beam blockage, and return enough radar energy to dam-age the WSR-88D receiver.




The ROC and Proposed Wind Farm Developments
Wind farm siting information is proprietary and closely guarded, due to a very competitive market. Sometimes the Radar Operations Center (ROC) first learns about a planned wind farm when a Weather Forecast Office (WFO) sends ROC management a newspaper article from their local area that discusses a developer's plans. Many developers, however, will follow guidance presented in the American Wind Energy Association (AWEA) Wind Energy Siting Hand-book (http://www.awea.org/sitinghandbook/), which recommends that they submit their proposals for evaluation through the National Telecommunications and Information Administration (NTIA), an agency of the Department of Commerce (DOC). The NTIA forwards each proposal to several government agencies including the National Oceanic and Atmospheric Administration (NOAA). The notifications contain wind turbine dimensions, including blade length, and either location information for each wind turbine or geo-graphic coordinates defining the polygonal area of the development.

As the ROC receives wind farm proposals from the NTIA, a case-by-case analysis is performed of the potential wind farm impacts on the WSR-88D. To date, the ROC has analyzed over 300 wind farm proposals (only a subset of the wind farms). The ROC's current benchmark for concern over a proposed wind farm and proactively contacting developers occurs when the wind turbine blades penetrate into the radar line of sight (RLOS), assuming beam propagation through the Standard Atmosphere.

The ROC has software that creates WSR-88D RLOS maps of the proposed project area and a database of the amount of penetration into the beam, if any. These maps and databases are based upon a data model built from the Space Shuttle Radar Topography Mission 1 arc-second digital terrain data imparting high confidence in their accuracy. The ROC understands radars will be able to see turbines below the RLOS due to side lobes and non-standard atmospheric conditions; however, the Standard Atmosphere propagation model is the benchmark used by the ROC to inform a developer of possible interference concerns, and to offer assistance in learning about mitigation techniques, e.g., terrain masking, orientation of turbines, movement of turbines to a location that will reduce the number of turbines and/or their penetration into the RLOS.

Approximately 15% of ROC analyses show turbine blades in the RLOS, which has led some developers to contact the ROC for more information on the WSR-88D and the potential impacts of their wind farms. Some of these developers have made siting changes to reduce the impacts on the WSR-88D. During follow-up contacts with developers, the ROC invites the local WFO (or military base weather station) and regional headquarters to participate in the call.

ROC Efforts to Mitigate Wind Farm Impacts
The ROC has been working the wind turbine siting and interference issues for over two years on behalf of the WSR-88D tri-agencies. Since the federal government does not have statutory or regulatory authority over private land use, the ROC has been reaching out to the wind energy industry to raise their awareness of WSR-88D locations, and encouraging them to consider potential impacts on the WSR-88D before finalizing their plans. To raise developer's awareness of the WSR-88Ds earlier in the planning process, the ROC worked with the FAA to add a "NEXRAD Toolkit" to the FAA's Obstruction Evaluation/Airport Air-space Analysis (OE/AAA) website (https://www.oeaaa.faa.gov/oeaaa/external/portal.jsp). The NEXRAD Toolkit, which was activated July 1, 2008, was patterned after the Air Force/Depart-ment of Homeland Security Long-Range Radar toolkit, which has been available to wind energy developers for over a year. The addition of the NEXRAD toolkit to the web site provides wind energy developers a single, convenient source from which to gather information to quickly and anonymously evaluate potential wind turbine sites for impacts to both WSR-88D and Long-Range radars.

ROC staff have participated in AWEA meetings to present briefings, papers and posters, and meet developers and other industry players. The ROC is also collaborating with the Department of Homeland Security/Department of Defense's Long-Range Radar Program Office to identify common issues and solutions to the WTC problem. Later this year, the ROC plans to submit proposed updates to the AWEA Wind Energy Siting Hand-book that address siting issues related to the WSR-88D. A wind farm/WSR-88D interaction section has been added to the ROC web site at https://www.roc.noaa.gov/wsr88d/WSR88D/WindFarm/WindFarm_Index_GreatFalls.aspx?wid=*. The ROC is refining and updating the page, splitting it into two sections - one section for WSR-88D data users and the other for wind energy developers to provide more tailored information.

The ROC has also been supporting WTC mitigation investigations at the University of Oklahoma Atmospheric Radar Research Center. The goal is to automate wind farm detections and to invoke signal processing techniques that can filter out the effects of rotating wind turbines on WSR-88D data and products. While progress is being made in this effort, it will be several years before any software will be implemented on the WSR-88D. Additionally, the ROC has discussed the WTC issue with the NOAA General Counsel office to determine what we and the WFOs can do and/or say when interacting with wind power developers.

How WFOs Can Mitigate Wind Farm Impacts
A WFO's first line of defense is developing an understanding of the problem. To learn more about the WTC issue, visit the ROC web site at (https://www.roc.noaa.gov/wsr88d/WSR88D/WindFarm/Research.aspx?wid=dev) where several papers and briefings have been posted.

The ROC is attempting to track and evaluate all wind farm proposals within or near a WSR-88D's line of sight. Since ROC notification by developers is not required, there will be some wind farm developments of which the ROC will not be alerted through the NTIA or otherwise. Therefore, it is very important that upon hearing of a planned wind farm development, the WFO send an email to the ROC points of contact listed at the end of this article.

Also, WFOs should notify the ROC if they are currently dealing with WTC and encounter cases that impact forecasts and/or warning operations. Forecasters may want to document wind turbine clutter impacts for their particular radar, with the goal of developing a "climatology" of the clutter, i.e., how often, under what conditions, what products are affected, etc. To better understand the interaction between wind turbines and the WSR-88D, the ROC is interested in collecting significant impact cases (missed or delayed weather warnings) from around the country, and if warranted, make a case for action by policy makers. A clearer picture of the impacts may also help in the development of a formal policy for working with the wind energy industry, and avoiding over-reaction or under-reaction to this issue.

In the mean time, WSR-88D users can apply some simple mitigation strategies to alleviate the clutter problem. One of the most important is to apply exclusion zones over the wind farm area, to prevent false accumulations in the precipitation products. When applicable, forecasters can look at higher elevation angles or adjacent radar coverage to "see over" wind farms. Finally, speak with fore-casters in other weather offices that have more experience dealing with wind turbine clutter.

The Future
Wind power will rapidly expand in the U.S over the next few decades because of its appeal as a clean, alternative energy source. As a result, the number of wind farms installed in the line of sight of WSR-88Ds will also rapidly increase. Presently there is not much the ROC can do regarding developments close to WSR-88D radars, as the federal government has no regulatory authority over wind farm developments on private land. Some WFOs and military bases, particularly those in West Texas and the Great Plains, will be more affected than others and perhaps feel as if they are being surrounded by wind farms. However, it is imperative that forecasters keep this problem in perspective - it's a clutter issue and largely confined to the lowest radar elevation tilt. Yes, the wind farms may impact the radar data and products, but the key is whether or not they affect WFO's forecasting and warning operations. Forecasters must be ready to document these operational impacts, if a successful case for action is to be made. In the mean time, WSR-88D users will have to include wind farm signatures, and possible impacts on data and products in their forecast and warning process, and work around the issue to the best of their ability.

The ROC, for its part, will continue its outreach to the wind energy industry and continue to proactively contact developers to mitigate impacts. The ROC will also work with AWEA to update its guidance with respect to the WSR-88D in their Siting Handbook, and fund investigation into mitigation techniques. Lastly, the ROC hopes to collect impact cases and/or fund case studies of wind farm impacts, with the goal of developing and fine tuning an overall NOAA policy to address the wind turbine clutter problem.

Radar Operations Center points of contact for wind farm issues are:

Tim Crum (Tim.D.Crum@noaa.gov, 405-573-8888)
Ed Ciardi (Edward.J.Ciardi@noaa.gov, 405-573-3439)
Major John Sandifer, USAF (John.B.Sandifer@noaa.gov, 405-325-2095)



Tim Crum
ROC Director's Office

Ed Ciardi
Wyle Information Systems/ROC Engineering Branch

John Sandifer
USAF/ROC Applications Branch