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Wind Energy Science The interactive open-access journal of the European Academy of Wind Energy
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https://doi.org/10.5194/wes-2020-113
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/wes-2020-113
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  14 Nov 2020

14 Nov 2020

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This preprint is currently under review for the journal WES.

WRF-Simulated Low-Level Jets over Iowa: Characterization and Sensitivity Studies

Jeanie A. Aird1, Rebecca J. Barthelmie1, Tristan J. Shepherd2, and Sara C. Pryor2 Jeanie A. Aird et al.
  • 1Sibley School of Mechanical and Aerospace Engineering, Cornell University, Ithaca, New York, USA
  • 2Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, New York, USA

Abstract. Output from high resolution simulations with the Weather Research and Forecasting (WRF) model are analyzed to characterize local low level jets (LLJ) over Iowa. Analyses using a detection algorithm wherein the wind speed above and below the jet maximum must be below 80 % of the jet wind speed within a vertical window of approximately 20 m–530 m a.g.l. indicate the presence of a LLJ in at least one of the 14700 4 km by 4 km grid cells over Iowa on 98 % of nights. Nocturnal LLJ are most frequently associated with stable stratification and low TKE and hence are more frequent during the winter months. The spatiotemporal mean LLJ maximum (jet core) wind speed is 9.55 ms−1 and the mean height is 182 m. Locations of high LLJ frequency and duration across the state are seasonally varying with a mean duration of 3.5 hours. LLJ are most frequent in the topographically complex northwest of the state in winter, and in the flatter northeast of the state in spring. Sensitivity of LLJ characteristics to the: i) LLJ definition and ii) vertical resolution at which the WRF output is sampled are examined. LLJ definitions commonly used in LLJ literature are considered in the first sensitivity analysis. These sensitivity analyses indicate that LLJ characteristics are highly variable with LLJ definition. Further, when the model output is down-sampled to lower vertical resolution, the maximum LLJ wind speed and mean height decrease, but spatial distributions of regions of high frequency and duration are conserved.

Jeanie A. Aird et al.

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Jeanie A. Aird et al.

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Short summary
Low-level jets (LLJ) are pronounced maxima in wind speed profiles affecting wind turbine performance and longevity. We present a climatology of LLJ over Iowa using output from the Weather Research and Forecasting Model (WRF) and determine the rotor plane conditions when they occur. LLJ characteristics are highly sensitive to the identification criteria applied. LLJ characteristics also vary with different model output resolution but spatial distributions of areas of occurrence are preserved.
Low-level jets (LLJ) are pronounced maxima in wind speed profiles affecting wind turbine...
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