Improving potential wind for extreme wind statistics

To correct measured wind for local roughness at the station location, exposure correction factors can be calculated, which convert measured wind into potential wind. Recent KNMI investigations found indications that the exposure correction factors calculated from mean wind speed and wind gusts (gustiness analysis), were too large in the west to north sector in winter. This effect contributes to the curvature problem. Since 2003, for most KNMI wind stations 10 min samples of mean wind speed and standard deviation (sigma) are stored. Therefore, we now can calculate roughness lengths based on sigma-analysis. It is argued that in winter time with north-westerly winds, possible stability changes in a polar air mass within the timeframe of an hour influences the exposure correction factors based on 1-hour gustiness analysis. By comparing both methods, it is shown that a strong improvement in omnidirectional analysis and spatial gradient of exposure correction factors is achieved when sigma is used. The latter exposure corrections are successfully fitted to the gustiness-derived exposure corrections. In this way, the more consistent characteristics of the exposure correction factors calculated from sigma-analysis can be applied onto historical time series.

The improved exposure correction factors were used for extreme wind statistics (return periods of 1000 years). Results are compared to Rijkoort and Wieringa (1983).