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Wind Energy Science The interactive open-access journal of the European Academy of Wind Energy
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© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Research articles 08 Jan 2019

Research articles | 08 Jan 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Wind Energy Science (WES).

Low-level jets over the North Sea based on ERA5 and observations: together they do better

Peter C. Kalverla1, James B. Duncan Jr.2, Gert-Jan Steeneveld1, and Albert A. M. Holtslag1 Peter C. Kalverla et al.
  • 1Wageningen University, Meteorology and Air Quality Section, PO Box 47, 6700AA Wageningen, The Netherlands
  • 2ECN part of TNO, Wind Energy, Westerduinweg 3, 1755LE Petten, The Netherlands

Abstract. Ten years of ERA5 reanalysis data are combined with met-mast and LiDAR observations from ten offshore platforms to investigate low-level jet characteristics over the Dutch North Sea. The objective of this study is to combine the best of two worlds: (1) ERA5 data with large spatiotemporal extent but inherent accuracy limitations due to a relatively coarse grid and an incomplete representation of physical processes, and (2) observations that provide more reliable estimates of the measured quantity, but are limited in both space and time. We demonstrate the effect of time and range limitations on the reconstructed wind climate, with special attention paid to the impact on low-level jets.

For both measurement and model data, the representation of wind speed is biased. The limited temporal extent of observations leads to a wind speed bias on the order of 1ms−1. In part due to data-assimilation strategies that cause abrupt discontinuities in the diurnal cycle, ERA5 also exhibits a wind speed bias of approximately 0.5ms−1. Representation of low-level jets in ERA5 is poor in terms of a one-to-one correspondence, and the jets appear vertically displaced (`smeared out'). However, climatological characteristics such as the shape of the seasonal cycle and the affinity with certain circulation patterns are represented quite well, albeit with different magnitudes. We therefore experiment with various methods to adjust modelled low-level jet rate to the observations or, vice versa, to correct for the erratic nature of the short observation periods using long-term ERA5 information. While quantitative uncertainty is still quite large, the presented results provide valuable insight into North Sea low-level jet characteristics. These jets occur predominantly for circulation types with an easterly component, with a clear peak in spring, and concentrate along the coasts at heights between 50–200m. Further, it is demonstrated that these characteristics can be used as predictors to infer the observed low-level jet rate from ERA5 data with reasonable accuracy.

Peter C. Kalverla et al.
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Peter C. Kalverla et al.
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Publications Copernicus
Short summary
A common assumption in the design of wind turbines and wind farms is that the wind field is quite uniform. This assumption is violated during so-called low-level jet events, when there is a distinct peak in the wind speed. Low-level jets modify loads on the turbines, and also affect power production. To understand their impact and facilitate better planning and design, we present a detailed climatology of these events over the North Sea, based on offshore measurements and meteorological models.
A common assumption in the design of wind turbines and wind farms is that the wind field is...