1:45 PM - 2:10 PM
[MAG32-01] Multi-lidar investigations of turbulence structure over the ocean
★Invited Papers
Keywords:Doppler lidar, Wind Energy, Atmospheric turbulence
In recent years, there has been considerable interest in measuring the wind conditions offshore with dual-Doppler wind lidars. The results are generally very good for the mean wind speed as well as the turbulence, albeit to a lesser degree. There is still some uncertainty about the turbulence characteristics at heights of 100 to 400 m above the ocean when compared to models present in the wind turbine design standard IEC 61400-1. In particular, there is hardly any information on correlations or coherences offshore between points separated horizontally with distances relevant for modern wind turbines rotor. We have conducted an experiment on the west coast of Denmark measuring over the North Sea. We show that we can measure two-point spectral coherence of offshore atmospheric turbulence, similar to the work done by E. Cheynet et al, Atmospheric Measurement Techniques (2021) 14, 9. This is done by five coordinated, pulsed Doppler lidars standing on the coast of the North Sea with beams intersecting almost perpendicularly. The six crossing points are 150 to 250 m above the ocean and have lateral separations of up to 200 m, reflecting the scale of modern offshore wind turbines. Pointing accuracy of the Doppler lidars is of particular concern. We deploy drones equipped with gps antenna, fly them over the ocean to intercept the lidar beams, such that we know their position precisely. We compare the measurements with spectral and cross-spectral models. The model of Syed and Mann (Boundary-Layer Meteorology, 2024, vol 190), in general fits the spectra well and predicts the lateral coherences well. However, there are cases where the measured lateral coherence of the v-component is much larger than predicted. This seems not to be due to malfunction of the instruments, but rather due to non-turbulence processes in the atmosphere, e.g . interval gravity waves. We will also touch upon the potential consequences for loads on wind turbines.