by S. Knoop (KNMI), I.L. Wijnant (KNMI),
The creation of the Dutch Offshore Wind Atlas (DOWA) is part of a joint project with ECN part of TNO, Whiffle, and KNMI. The DOWA is a wind atlas based on a 10-year reanalysis, which is an hourly description of the state of the atmosphere using measurements and atmospheric (weather) models. The DOWA aims to provide the developers of offshore wind power in the Netherlands with knowledge on wind conditions additional to the information in the KNMI North Sea Wind (KNW) atlas.
In this report, the DOWA is validated against wind lidar measurements from a Dutch coastal location (Tweede Maasvlakte) of a period of 13 months, and compared to the KNW-atlas. We find for DOWA a mean bias in the wind speed of -0.2 m/s (i. e. DOWA overestimates the wind speed), with a standard deviation of 1.2 m/s, which for KNW are -0.1 m/s and 1.5 m/s, respectively. DOWA captures the diurnal cycle much better than KNW (which suffers from huge jumps in the hourly mean wind every six hours due to the cold starts) and seasonal patterns in the diurnal cycle are also nicely reproduced. A linear regression analysis of DOWA compared to the measurements gives values for the slope of around 0.99 and for R2 of about 0.92, which are better than KNW (0.97 and 0.88, respectively). Thus, DOWA provides an improvement in the hourly correlation. The mean bias in the wind direction is -7 and -6 with a standard deviation of 25 and 28 for DOWA and KNW, respectively, which show both strong dependence on the wind direction, while only the standard deviation is sensitive to the considered range of wind speeds. The validation analysis is also done separately for land wind and sea wind, which reveal quite some differences in the validation results, in particular in the hourly correlation of the wind speed and the wind direction bias.