Abstract
It is well known that when eddies are small, the eddy fluxes can be directly related to the mean vertical gradients, the so-called flux-gradient relation, but such a relation becomes weaker the larger the coherent structures. Here, we show that this relation does not hold at heights relevant for wind energy applications. The flux–gradient relation assumes that the angle (β) between the vector of vertical flux of horizontal momentum and the vector of the mean vertical gradient of horizontal velocity is zero, i.e., these vectors are aligned. Our observations do not support this assumption, either onshore or offshore. Here, we present analyses of a misalignment between these vectors from a Doppler wind lidar observations and large-eddy simulations. We also use a real-time mesoscale model output for inter-comparison with the lidar-observed vertical profiles of wind speed, wind direction, momentum fluxes, and the angle between the horizontal velocity vector and the momentum flux vector up to 500 m, both offshore and onshore. The observations show this within the height range 100–500 m, β=−18∘ offshore and β=−12∘ onshore, on average. However, the large-eddy simulations show β≈0∘ both offshore and onshore. We show that observed and mesoscale-simulated vertical profiles of mean wind speed and momentum fluxes agree well; however, the mesoscale results significantly deviate from the wind-turning observations.
Highlights
Numerical weather prediction models, such as the Weather Research and Forecast (WRF) mesoscale model [1], are widely used to simulate the atmosphere’s dynamics
When L computed by the sonic anemometer is compared to the output from New European Wind Atlas (NEWA)-WRF, we find that 60% of the observed stable profiles are correctly simulated as such, whereas the number increases to 94% for the observed unstable profiles
We extend the work from Santos et al [11] and present novel lidar observations that show a clear misalignment between the vertical gradient of the mean wind vector and the stress vector, β, up to 500 m for both offshore and onshore conditions
Summary
Numerical weather prediction models, such as the Weather Research and Forecast (WRF) mesoscale model [1], are widely used to simulate the atmosphere’s dynamics. A number of atmospheric processes cannot be resolved at the mesoscale resolution, so we have to rely on model parameterizations to characterize turbulence within the planetary boundary layer (PBL). Some limitations in mesoscale models are inherent from parameterizations of the PBL. These models use a turbulence closure of the vertical transfer of momentum, which is responsible for well-known and long-standing biases related to enhanced diffusion and underestimation of the turning of the wind, especially under stably stratified conditions [2], as well as strongly baroclinic boundary layers [3]. Accurate observations of the wind and turbulence measures across the PBL, e.g., using remote sensing technologies, are valuable for the evaluation of PBL parameterizations commonly used in mesoscale models
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