Downloaded 10/05/21 06:16 AM UTC of Downstream Turbines in Wind Farm Arrays

WIND ENERGY METEOROLOGY Insight into Wind Properties in the Turbine-Rotor Layer of the Atmosphere from High-Resolution Doppler Lidar BY ROBERT M. BANTA, YELENA L. PICHUGINA, NEIL D. KELLEY, R. MICHAEL HARDESTY, AND W. ALAN BREWER High-resolution Doppler lidar data analyses show that advances in measuring, understanding, and modeling of the atmospheric boundary layer will be required to provide improved meteorological support for wind energy. canning high-resolution Doppler lidar is a remote Analyses of lidar data reveal many aspects of these sensing instrument having the capability to flows important to wind energy. For example, the LLJ S provide high-precision wind speed data at vertical shape evident in the individual profiles is shown to resolutions of less than 10 m through the lowest be poorly represented by standard profiles, such as several hundred meters of the atmosphere. Analyzed the power-law profile often used to extrapolate near- lidar data are presented from two field projects in the surface measurements to hub height. Dealing with U.S. Great Plains. This region has high wind energy the strong spatial and temporal variability of winds resource potential and nighttime meteorological in the rotor layer at multiple scales is a challenge conditions that are difficult to understand and model. that will need to be addressed by multiscale arrays The major nocturnal wind resource in this region is of appropriate profiling instrumentation. Four areas the low-level jet (LLJ). where advancements are most needed are described. Meteorological variability of the wind resource is an important source of uncertainty in the wind AFFILIATIONS: BANTA, HARDESTY, AND BREWER—National Oceanic and Atmospheric Administration/Earth System Research energy (WE) industry. Considering the electric Laboratory, Boulder, Colorado; PICHUGINA—Cooperative power industry as a whole, its imperative is embodied Institute for Research in the Environmental Sciences, University in the often quoted “priority number one,” which of Colorado, and National Oceanic and Atmospheric can be stated—when a customer flips a switch, the Administration/Earth System Research Laboratory, Boulder, lights must come on. In other words, by the time Colorado; KELLEY*—National Wind Technology Center, National the power is dispatched, all sources of uncertainty, Renewable Energy Laboratory, Golden, Colorado including meteorological, must have been resolved. *Retired CORRESPONDING AUTHOR: Robert Banta, NOAA/ESRL, For the WE industry the sources of uncertainty can 325 Broadway, Boulder, CO 80305 be purely meteorological, such as predictions of wind E-mail: [email protected] and turbulence at various lead times; they can be nonmeteorological, such as the percentage of turbines The abstract for this article can be found in this issue, following the table of contents. offline (not operating) at a given time or how much DOI:10.1175 / BAMS - D -11- 0 0 057.1 power is required by the electrical grid; or they can be a combination, such as when strong turbulence In final form 10 October 2012 ©2013 American Meteorological Society bursts damage turbines and take them offline or when turbine wake effects reduce the power output AMERICAN METEOROLOGICAL SOCIETY JUNE 2013 | 883 Unauthenticated | Downloaded 10/05/21 06:16 AM UTC of downstream turbines in wind farm arrays. Here, needed. Such climatologies would have to be based we address meteorology-related uncertainties due to on profile measurements at high enough precision the variability of the winds. and vertical resolution to discern the wind speed Two aspects of the WE industry make it differ- maximum or jet “nose.” Similarly, climatologies of ent from most meteorological applications. First, it ramps, wind gusts, shear, or extreme events in the requires a high level of precision. Errors of less than rotor layer must be compiled from measurements 1 m s−1 in estimating the annual wind resource for a capable of detecting those events. Thus, fine resolu- wind farm can translate into many millions of dollars tion and precision are required in the measurements in annual revenues; other needs for atmospheric even though climatology is a long-term application. information are similarly sensitive. Second, wind Additionally, obtaining needed information for WE information is required in a layer aloft occupied by industry applications, such as forecasting, modeling, the turbine-rotor blades, rather than at the surface and research, often requires meteorological measure- where the preponderance of measurements is taken. ments through a deeper layer than the rotor layer or Unfortunately, a scarcity of measurement data of high over a broader area than one measurement site. enough quality to effectively meet the needs of the The lack of measurement data of sufficiently high WE industry exists at the required heights. quality through and above the turbine-rotor layer Wind energy is an important meteorological means that key meteorological phenomena and pro- application, but it consists of many “subapplications,” cesses affecting the winds there are not well character- which we will simply refer to as WE applications ized or understood. The fidelity of numerical weather for convenience. These WE applications range prediction (NWP) forecast models in those layers from hardware design, to resource assessment and is also not well known, although available evidence “prospecting” for favorable wind farm locations, indicates errors and uncertainties too large for the WE to siting and construction of wind farms as well industry’s needs (Schreck et al. 2008). An important as individual turbines, to operations including contribution for meteorology is to minimize the forecasting and performance of maintenance, uncertainties in wind resource characteristics to the through refurbishing and “repowering” of the site extent possible. To do this will require advances in the for continued operation into the future. Other issues state of the art in understanding the lower atmosphere. include environmental impacts of wind farms, Recent progress in measurement capabilities makes turbine wake effects on productivity within the wind this a possibility. Here, we describe insights from one farm, and rotor-layer wind climatology to determine advanced sensor, the High-Resolution Doppler Lidar typical conditions, extreme conditions, or whether (HRDL), developed and operated by Earth System the resource is changing over decades (Schreck et al. Research Laboratory of the National Oceanic and 2008; Shaw et al. 2009). Although each subapplica- Atmospheric Administration (NOAA/ESRL). HRDL tion may have somewhat different requirements for provides detailed WE industry–relevant information precision, frequency, or resolution of wind data, they on the structure and behavior of flow phenomena have some basic needs in common, such as a mean through and above the turbine-rotor layer. inflow wind in the rotor layer, often taken as wind speed at the height of the turbine-rotor hub averaged NEED FOR APPROPRIATE MEASUREMENT. over 10 min, the magnitude of abrupt changes in wind Available episodic studies have shown that complex speed, and peak values of turbulent fluctuations and atmospheric phenomena and strong gradients occur shear encountered within the rotor layer. in the turbine-rotor layer at length scales as small as To provide the best wind information, a a few meters. These phenomena comprise, or control, challenge for meteorology is to characterize the key the inflows to turbines and to wind farms. Mean and meteorological phenomena that affect the flows at fluctuating components of these inflows need to be appropriate time and spatial scales for the applica- accurately characterized for most WE industry appli- tion of interest. Understanding such phenomena is cations. An important first step, therefore, is to char- obviously important for forecasting—forecasters acterize and better understand wind flow features want to understand what they are predicting. But in the rotor layer and their driving mechanisms. such detailed characterization is often important This requires appropriate measurements. A recent for other applications. For example, even though National Research Council report (NRC 2009) climatology and resource assessment are for longer highlights what those measurements might look time periods, accurate climatologies of phenomena like. NRC (2009) describes a phenomenological such as the LLJ, its properties, and its effects may be approach to observational requirements, focusing on 884 | JUNE 2013 Unauthenticated | Downloaded 10/05/21 06:16 AM UTC high-impact weather phenomena and summarizing the work of T. W. Schlatter et al. (2005, personal com- munication). According to this approach, each type of weather phenomenon, such as snowstorms, hurricanes, thunderstorms, tornados, etc., has a characteristic longevity and size (time and space scale). Useful sampling of each type of weather system requires measurements at appropriate spatial densities and FIG. 1. HRDL in the NOAA seatainer lidar laboratory at the National Wind time intervals, “not only Technology Center of the National Renewable Energy Laboratory during the to detect and monitor the Turbine Wake and Inflow Characterization Study (TWICS) in Mar and Apr phenomenon, but also to 2011. (Photo by Scott Sandberg.) describe its internal work- ings and predict its onset and future behavior” (NRC the phenomena of interest. Other considerations are 2009, p. 26). For example, the twice-daily rawinsonde also critical

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