Correlation and coherence of mesoscale wind speeds over the sea

Abstract

A large offshore observational dataset from stations across the North and Baltic Seas is used to investigate the planetary boundary‐layer wind characteristics and their coherence, correlation and power spectra. The data from thirteen sites, with pairs of sites at horizontal distances of 4 to 848 km, are analyzed for typical wind turbine nacelle heights. Mean wind characteristics, correlation and coherence are also calculated for analogous wind data from simulations with the Weather Research and Forecasting (WRF) model.Results indicate a general good agreement for the coherence calculated based on measurements and the WRF‐derived time series. By normalizing the frequency axes with the distance and mean wind speed, it can be demonstrated that, even for data with a wide range of distances, the coherence is a function of the frequency, mean wind and distance, which is consistent with earlier studies. However the correlation coefficient as a function of distance calculated from WRF is higher than observed in the measurements. For the power spectra, wind speed and wind speed step change distribution, the results for all sites are quite similar. The land masses strongly influence the individual wind direction distribution at each site. The ability of the WRF model to reproduce the coherence of the measurements demonstrates that its output can be used to estimate the coherence of fluctuations for the integration of offshore energy. The power spectra of WRF time series underestimates the high‐frequency fluctuations. Due to the large number of measurement sites, the results can be used for further plausibility validation for mesoscale model runs over the sea.