Case Study of Uncertainties Connected to Long-term Correction of Wind Observations

Abstract: Good knowledge about on-site wind climate is necessary in wind power assessments. Due to yearly and natural variability, short-term on-site measurements do not themselves give information about the wind climate. Long-term reference data can be used to correct the short-term observations to better correspond to a normal year. The method doing this is commonly referred to as MCP, which stands for measure, correlate, predict. By performing long-term correction, uncertainties are however introduced. In this report, uncertainties connected to long-term correction processes have been studied. Uncertainties connected to the length and season of the on-site measurements are analysed. A long-term on-site series of 29 years have been used along with different long-term correction methods and reanalysis datasets. The results show that uncertainties decrease with increased length of on-site measurements. There are however seasonal influences; summer periods tend to overestimate the wind climate while winter periods underestimate the climatic mean wind for all period lengths. A relatively high coefficient of correlation between target site data and reference data is not always seen to imply small errors. In the most extreme case, one-year long-term corrected on-site measurements are in this report seen to deviate from the true climatic mean wind with more than 0.75 m/s. An additional study evaluating how variations in wind climate within a wind farm can be predicted by short-term measurements has been performed. The short-term measurements may be observed by a sodar and be correlated to on-site met mast measurements. High-resolution model data from an imaginary wind farm has been used. Similar results as in the other studies are given in this study. It is found that systematic errors have a seasonal dependence. Six months long measurement period centred in summer or winter and by long-term correction extended to one year, are seen to deviate as much as 0.6 m/s from the true yearly mean wind. Equal length measurements, centred in spring and autumn, are seen to deviate less than 0.2 m/s.