Abstract
A common closure for the planetary boundary layer in numerical weather models assumes a direct relation between turbulent fluxes and the mean wind vertical gradient, i.e., the flux-gradient relation or K-theory. This assumption implies that the angle β between the momentum stress vector and the mean gradient of the velocity vector are aligned, i.e., β = 0°. This is not what we observe from measurements. We quantify the misalignment of β in offshore conditions using measurements from a long-range Doppler profiling lidar and numerical simulations from the New European Wind Atlas mesoscale model output. We compare vertical profiles of wind speed, wind direction, momentum fluxes, and β up to 500 m, hence covering the rotor areas of modern offshore wind turbines and beyond. The results show that β ≈ −18° on average, with a lower, but still non-zero, value under stable stability conditions, ≈ −7°. We illustrate that the simulations describe well the mean wind speed and momentum fluxes within the observed levels, but the characterization of wind turning effects could be improved.
Highlights
Current wind turbine rotors operate under vertical wind shear and wind veer conditions over a portion of the planetary boundary layer (PBL), which can surpass the atmospheric surface layer (ASL) or “constant-flux” layer by hundreds of meters, depending on the atmospheric stability and turbulence conditions
Lidar and NEWA-Weather Research and Forecast (WRF) wind climatology Figure 2 shows the concurrent wind roses measured by the wind lidar at 124.5 m and modeled by WRF at 100 m
The size of the selected sector does not have a major impact on the vertical profiles presented below
Summary
Current wind turbine rotors operate under vertical wind shear and wind veer conditions over a portion of the planetary boundary layer (PBL), which can surpass the atmospheric surface layer (ASL) or “constant-flux” layer by hundreds of meters, depending on the atmospheric stability and turbulence conditions. Our comprehension of wind climatology and turbulence above the ASL has to be improved in order to refine the capabilities of numerical models such as the Weather Research and Forecast (WRF) model to simulate the atmosphere. Recent studies took advantage of long-range Doppler profiling wind lidars by probing the atmosphere up to ≈ 1 km to gain insight of the wind climatology beyond the ASL, e.g., by characterizing the Weibull parameters [2], and by comparison with mean wind profiles from WRF PBL schemes [3]. There is an opportunity to better evaluate numerical models and parametrizations of the PBL with wind lidars
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