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Sicheng Wu and Cristina L. Archer

Abstract

Wind turbines generate wakes, which can potentially influence the local microclimate near the ground. To verify and quantify such effects, the VERTical Enhanced miXing (VERTEX) field campaign was conducted in late summer 2016 to measure near-surface turbulent fluxes, wind speed, temperature and moisture under and outside of the wake of an operational wind turbine in Lewes, Delaware. We found that, in the presence of turbine wakes from a single wind turbine, friction velocity, turbulent kinetic energy, and wind speed were reduced near the ground under the wake, while turbulent heat flux were not significantly affected by the wake. The observed near-ground temperature changes were <0.18 °C in magnitude. Near-ground temperature changes due to the wake correlated well with the temperature lapse rate between hub height and the ground, with warming observed during stable and neutral conditions and cooling during unstable conditions. Of the two properties that define a wake, i.e. wind speed deficit and turbulence, the wind speed deficit dominates the surface response, while the wake turbulence remains aloft and hardly ever reaches the ground. We propose that the mechanism that drives changes in near-ground temperature in the presence of turbine wakes is the vertical convergence of turbulent heat flux below hub height. Above hub height, turbulence and turbulent heat flux are enhanced; near the ground, turbulence is reduced and turbulent heat flux is unchanged. These conditions cause an increase (during stable/neutral stability) or decrease (during unstable stability) in heat flux convergence, ultimately resulting in warming or cooling near the ground, respectively.

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Niranjan S. Ghaisas and Cristina L. Archer

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Layout studies are critical in designing large wind farms, since wake effects can lead to significant reductions in power generation. Optimizing wind farm layout using computational fluid dynamics is practically unfeasible today because of their enormous computational requirements. Simple statistical models, based on geometric quantities associated with the wind farm layout, are therefore attractive because they are less demanding computationally. Results of large-eddy simulations of the Lillgrund (Sweden) offshore wind farm are used here to calibrate such geometry-based models. Several geometric quantities (e.g., blockage ratio, defined as the fraction of the swept area of a wind turbine that is blocked by upstream turbines) and their linear combinations are found to correlate very well (correlation coefficient of ~0.95) with the power generated by the turbines. Linear models based on these geometric quantities are accurate at predicting the farm-averaged power and are therefore used here to study layout effects in large wind farms. The layout parameters that are considered include angle between rows and columns, angle between incoming wind and columns (orientation), turbine spacings, and staggering of alternate rows. Each can impact wind power production positively or negatively, and their interplay is complex. The orientation angle is the most critical parameter influencing wake losses, as small changes in it can cause sharp variations in power. In general, for a prevailing wind direction, the orientation angle should be small (7.5°–20°) but not zero; staggering and spacing are beneficial; and nonorthogonal layouts may outperform orthogonal ones. This study demonstrates the utility of simple, inexpensive, and reasonably accurate geometry-based models to identify general principles governing optimal wind farm layout.

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Cristina L. Archer and Mark Z. Jacobson

Abstract

The formation mechanism of the Santa Cruz eddy (SCE) is investigated using the fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model (MM5). Simulations of 25–26 August 2000 showed that two eddy instances formed on that night, a finding supported by observations. The two eddies had similar behavior: they both formed in the sheltered Santa Cruz, California, area and then moved southeastward, to finally dissipate after 7–11 h. However, the first eddy had greater vorticity, wind speed, horizontal and vertical extents, and lifetime than the second eddy. Numerical simulations showed that the SCEs are formed by the interaction of the main northwesterly flow with the topographic barrier represented by the Santa Cruz Mountains to the north of Monterey Bay. Additional numerical experiments were undertaken with no diurnal heating cycle, no (molecular or eddy) viscosity, and no horizontal thermal gradients at ground level. In all cases, vertical vorticity was still created by the tilting of horizontal vorticity generated by the solenoidal term in the vorticity equation. This baroclinic process appeared to be the fundamental formation mechanism for both SCEs, but more favorable conditions in the late afternoon (including a south-to-north pressure gradient, flow turning due to the sea breeze, and an expansion fan) coincided to intensify the first eddy.

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Cristina L. Archer and Mark Z. Jacobson

Abstract

Wind is the world’s fastest growing electric energy source. Because it is intermittent, though, wind is not used to supply baseload electric power today. Interconnecting wind farms through the transmission grid is a simple and effective way of reducing deliverable wind power swings caused by wind intermittency. As more farms are interconnected in an array, wind speed correlation among sites decreases and so does the probability that all sites experience the same wind regime at the same time. The array consequently behaves more and more similarly to a single farm with steady wind speed and thus steady deliverable wind power. In this study, benefits of interconnecting wind farms were evaluated for 19 sites, located in the midwestern United States, with annual average wind speeds at 80 m above ground, the hub height of modern wind turbines, greater than 6.9 m s−1 (class 3 or greater). It was found that an average of 33% and a maximum of 47% of yearly averaged wind power from interconnected farms can be used as reliable, baseload electric power. Equally significant, interconnecting multiple wind farms to a common point and then connecting that point to a far-away city can allow the long-distance portion of transmission capacity to be reduced, for example, by 20% with only a 1.6% loss of energy. Although most parameters, such as intermittency, improved less than linearly as the number of interconnected sites increased, no saturation of the benefits was found. Thus, the benefits of interconnection continue to increase with more and more interconnected sites.

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Cristina L. Archer, Mark Z. Jacobson, and Francis L. Ludwig

Abstract

A shallow cyclonic circulation that occurs in the summertime over the Monterey Bay (California) is investigated. Since it is often centered offshore from the city of Santa Cruz and has never been studied in detail before, it is named the Santa Cruz eddy (SCE) in this study. Its horizontal size is 10–40 km, and its vertical extent is 100–500 m. The SCE is important for local weather because it causes surface winds along the Santa Cruz coast to blow from the east instead of from the northwest, the latter being the climatological summer pattern for this area. As a consequence of the eddy, cool and moist air is advected from the south and southeast into the Santa Cruz area, bringing both relief from the heat and fog to the city.

The SCE is unique in its high frequency of occurrence. Most vortices along the western American coast form only during unusual weather events, whereas the SCE forms 78%–79% of the days during the summer. The SCE frequency was determined after analyzing two years of data with empirical orthogonal functions (EOFs) from a limited observational network and satellite imagery. An explanation of the formation mechanism of the SCE will be provided in Part II of this study.

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Cristina L. Archer, Joseph F. Brodie, and Sara A. Rauscher

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The goal of this study is to evaluate the effects of anthropogenic climate change on air quality, in particular on ozone, during the summer in the U.S. mid-Atlantic region. First, we establish a connection between high-ozone (HO) days, defined as those with observed 8-h average ozone concentration greater than 70 parts per billion (ppb), and certain weather patterns, called synoptic types. We identify four summer synoptic types that most often are associated with HO days based on a 30-yr historical period (1986–2015) using NCEP–NCAR reanalysis. Second, we define thresholds for mean near-surface temperature and precipitation that characterize HO days during the four HO synoptic types. Next, we look at climate projections from five models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) for the early and late midcentury (2025–34 and 2045–54) and analyze the frequency of HO days. We find a general increasing trend, weaker in the early midcentury and stronger in the late midcentury, with 2 and 5 extra HO days per year, respectively, from 16 in 2015. These 5 extra days are the result of two processes. On one hand, the four HO synoptic types will increase in frequency, which explains about 1.5–2 extra HO days. The remaining 3–3.5 extra days are explained by the increase in near-surface temperatures during the HO synoptic types. Future air quality regulations, which have been successful in the historical period at reducing ozone concentrations in the mid-Atlantic, may need to become stricter to compensate for the underlying increasing trends from global warming.

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Cristina L. Archer, Sicheng Wu, Yulong Ma, and Pedro A. Jiménez

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As wind farms grow in number and size worldwide, it is important that their potential impacts on the environment are studied and understood. The Fitch parameterization implemented in the Weather Research and Forecasting (WRF) Model since version 3.3 is a widely used tool today to study such impacts. We identified two important issues related to the way the added turbulent kinetic energy (TKE) generated by a wind farm is treated in the WRF Model with the Fitch parameterization. The first issue is a simple “bug” in the WRF code, and the second issue is the excessive value of a coefficient, called C TKE, that relates TKE to the turbine electromechanical losses. These two issues directly affect the way that a wind farm wake evolves, and they impact properties like near-surface temperature and wind speed at the wind farm as well as behind it in the wake. We provide a bug fix and a revised value of C TKE that is one-quarter of the original value. This 0.25 correction factor is empirical; future studies should examine its dependence on parameters such as atmospheric stability, grid resolution, and wind farm layout. We present the results obtained with the Fitch parameterization in the WRF Model for a single turbine with and without the bug fix and the corrected C TKE and compare them with high-fidelity large-eddy simulations. These two issues have not been discovered before because they interact with one another in such a way that their combined effect is a somewhat realistic vertical TKE profile at the wind farm and a realistic wind speed deficit in the wake. All WRF simulations that used the Fitch wind farm parameterization are affected, and their conclusions may need to be revisited.

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Anna C. Fitch, Joseph B. Olson, Julie K. Lundquist, Jimy Dudhia, Alok K. Gupta, John Michalakes, Idar Barstad, and Cristina L. Archer
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Cristina L. Archer, Brian A. Colle, Luca Delle Monache, Michael J. Dvorak, Julie Lundquist, Bruce H. Bailey, Philippe Beaucage, Matthew J. Churchfield, Anna C. Fitch, Branko Kosovic, Sang Lee, Patrick J. Moriarty, Hugo Simao, Richard J. A. M. Stevens, Dana Veron, and John Zack
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