Preprints
https://doi.org/10.5194/wes-2021-161
https://doi.org/10.5194/wes-2021-161

  04 Jan 2022

04 Jan 2022

Review status: this preprint is currently under review for the journal WES.

Classification and properties of coastal wind profiles with negative gradients – an observational study

Christoffer Hallgren1, Johan Arnqvist1, Erik Nilsson1, Stefan Ivanell1, Metodija Shapkalijevski2, August Thomasson1, Heidi Pettersson3, and Erik Sahlée1 Christoffer Hallgren et al.
  • 1Department of Earth Sciences, Uppsala University, Uppsala, Sweden
  • 2Swedish Meteorological and Hydrological Institute, Norrköping, Sweden
  • 3Finnish Meteorological Institute, Helsinki, Finland

Abstract. Wind profiles with a negative gradient are frequently occurring over the Baltic Sea and are important to take into consideration for offshore wind power as they affect not only the power production, but also the loads on the structure and the behavior of the wake behind the turbine. In this study, we classified non-normal profiles as wind profiles having negative shear in at least one part of the profile between 28 and 300 m: low-level jets (with a local wind maximum in the profile), profiles with a local minimum, and negative profiles. Using observations spanning over 3 years, we show that the non-normal wind profiles are common over the Baltic Sea in late spring and summer, with a peak of 40 % relative occurrence in May. Negative profiles (in the 28–300 m layer) were mostly occurring during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the the zone with strong shear during low-level jets could cause a relative suppression of the variance for large turbulent eddies compared to the peak of the velocity spectra, in the layer below the jet core. Swell conditions were found to be favourable for the occurrence of negative profiles and profiles with a local minimum, as the waves fed energy into the surface layer, resulting in an increase of the wind speed from below.

Christoffer Hallgren et al.

Status: open (until 15 Feb 2022)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on wes-2021-161', Anonymous Referee #1, 09 Jan 2022 reply

Christoffer Hallgren et al.

Christoffer Hallgren et al.

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Short summary
Non-normal wind profiles with negative shear in part of the profile (such as e.g. low-level jets) are frequently occurring in coastal environments and are important to take into consideration for offshore wind power. Using observations from a coastal site in the Baltic Sea, we analyse in which meteorological and sea state conditions these profiles occur and study how they alter the turbulence structure of the boundary layer compared to normal profiles.