Classification and properties of non-idealized coastal wind profiles – an observational study

Abstract

Abstract. Non-idealized wind profiles frequently occur 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 observational study, we classified non-idealized profiles as the following wind profiles having negative shear in at least one part of the lidar wind 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 these non-idealized 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) mostly occurred during unstable conditions, in contrast to low-level jets that primarily occurred in stable stratification. There were indications that the strong shear zone of 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 favorable 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 in the wind speed from below.

Characteristics of the low‐level jets observed over Dunkerque (North Sea French coast) using 4 years of wind lidar data

“…The contrast between the sea‐surface temperature and the air temperature is therefore stronger in spring than in summer, generating stronger and more frequent breezes; this would also explain why the fraction of strong jets is, by far, the highest in April. The annual cycle was not shown for the WiValdi site (Wildmann et al ., 2022) or the FINO1 platform (Wagner et al ., 2019a), but the annual cycle observed in IJmuiden is similar to what was retrieved in the central Baltic Sea (Hallgren et al ., 2022) and on the US East Coast (Aird et al ., 2022).…”

The winds are twisting: analysis of strong directional shear across the rotor plane using coastal lidar measurements and ERA5

“…In general, winds from the sector 45°-220°represents pure open sea conditions. For winds from the 220°--295°sector the properties of the advected air is influenced by Gotland and from the 295°-355°sector, Östergarnsholm affects the flow, further details in Rutgersson et al 2020 andHallgren et al 2022).…”

“…The large area covered by the rotor also implies that wind turbines are exposed to a great variation of meteorological conditions both horizontally and vertically across the rotor, such as changes in wind speed (e.g., Kettle, 2014;Møller et al, 2020;Debnath et al, 2021;Aird et al, 2022;Hallgren et al, 2022;Foody et al, 2023), wind direction (Kalverla et al, 2017;Englberger and Lundquist, 2020), temperature (e.g., Janzon et al, 2020;Gadde and Stevens, 2021) and turbulence intensity (Türk and Emeis, 2010;Svensson et al, 2019;Bodini et al, 2020). These factors impact not only the performance and power production of a single wind turbine, but also the behaviour of the wake behind the turbine and thus the power production form the wind farm as a whole.…”

“…On the synoptic scale, frontal systems can make the winds turn with height (e.g., Browning and Monk, 1982), associated with rapid changes in the properties between air masses. Among mesoscale phenomenon affecting the directional shear are the sea and land breeze circulations (Simpson, 1994;Miller et al, 2003;Hallgren et al, 2023), convective cells with a high degree of turbulent motions (Clarke, 1970;Svensson et al, 2017;Sanchez Gomez and Lundquist, 2020), non-ideal wind speed profiles (Kalverla et al, 2017;Hallgren et al, 2022), and internal boundary-layers in, e.g., the coastal zone or over forested areas (Hsu, 1979;Arnqvist et al, 2015). Also, topography induced directional shear can occur, such as e.g., katabatic winds and channeling in terrain with a high degree of complexity (Rotach et al, 2008;Heinemann and Zentek, 2021;Liu and Stevens, 2021).…”

Brief communication: On the definition of the low-level jet

“…Compared to a falloff definition of LLJ, a shear definition considers the wind speed rate of change with height, rather than only the change of wind speed independent of the height. Strong negative shear is associated with a low degree of turbulence, enhanced entrainment fluxes (Doosttalab et al, 2020) and possibly a separation of atmospheric layers with different turbulent properties (Banta et al, 2006;Hallgren et al, 2022), which in turn results in different effects on the loads on the wind turbine and wake recovery rates (Doosttalab et al, 2020;Gadde and Stevens, 2021). For an extreme example, assume that the vertical wind speed profile is discretized with an interval of 50 m. If the wind speed decreases by 0.99 m s −1 between two levels (and then slowly increases to the next level), the profile will not be classified as an LLJ according to a 1 m s −1 falloff threshold, although having a very strong shear of −0.0198 s −1 in this layer.…”