Turbulence Characteristics of Wind-Speed Fluctuations in the Presence of Open Cells: A Case Study

Open cellular structures are frequently observed accompanying cold fronts over the North Sea. Through a two-day case study, measurements from two sites that are 100 km apart, and both covered by open cells, show that the turbulence is characterized by (1) considerable energy in the spectral gap region; (2) similar large-scale wind variation from surface to 100 m. These observations challenge existing algorithms for calculating parameters relevant to wind energy, including the turbulence intensity. We suggest that, in the presence of open cells, the stability effect is more related to the large-scale process, while the conventional parameter, the surface-layer Obukhov length, is less suitable. This issue is also revealed by the comparison of measurements with an unstable-boundary-layer spectral model. A mesoscale spectral model A a_1 f^-2/3 is proposed to include the stability effect, when combined with a boundary-layer turbulence model for neutral conditions. The stability effect is introduced to this mesoscale model in a simple manner through calibration, with the coefficient A obtained from regression using standard 10-min time series (from measurements or numerical modelling). The combined model successfully reproduces the power spectrum of wind-speed fluctuations for the two-day open-cell event.