Validation of KNW atlas with publicly available mast observations (Phase 3 of KNW project)

The KNW (KNMI North Sea Wind) atlas is based on the ERA-Interim reanalyses dataset which captures 35 years (1997-2013) of meteorological measurements and generates 3D wind fields consistent with these measurements and the laws of physics. This dataset is “downscaled” using the state-of-the-art weather forecasting model, HARMONIE with a horizontal grid of 2.5 km. The result is a high resolution dataset of 35 years: the KNW dataset. In this report the KNW dataset is validated against publicly available wind measurements from three tall offshore wind masts: OWEZ, FINO1 and MMIJ (Meteorological Mast Ijmuiden). The difference between the measured wind speeds (averaged over periods when the mast measurements were undisturbed by nearby wind parks) and the KNW values is less than 0.2 m/s for all masts and all measurement heights. The ERA-HARMONIE wind speeds were tuned to match the measurements made at KNMI’s 200 m tall meteorological mast at Cabauw and the same wind shear correction factor was applied uniformly throughout the whole KNW atlas area. The validation results imply that the accuracy of the long term average wind speeds of the KNW atlas is comparable to that of the measurements (cup and sonic anemometer and LIDAR). The same can be said of the estimates of the once in 10 year extreme wind speeds for heights around wind turbine hub height.

This report deals with the validation of the KNW wind atlas in the vertical dimension with measurements from three tall wind masts. In another report we validate the atlas on a horizontal plane 10 m above the North Sea (Wijnant, 2015). The reports are identical from the introduction up to and including section 1.1.3. The KNW dataset that is validatedhorizontally has not been corrected for the model’s underestimation of the vertical shear of the horizontal wind speed (since this has little effect on the wind speed at 10 m) whereas the vertically validated dataset has been corrected. The vertically validated dataset falls just within the area bounded by 50.25-54.75 NB and 1.50-8.25 EL (publicly available from the middle of 2015) whereas the horizontally validated dataset covers the whole of the North Sea. To avoid confusion: the KNW datasetis different from the North Sea wind climatology described in KNMI TR343 (Wijnant et al, 2014). Both the KNW dataset and the climatology of TR343 are based on re-analyses model ERA-Interim, but the “downscaling” procedures used are different. The KNW dataset followed up the wind climatology of TR 343 so quickly that the TR 343 dataset was never made available to a wider public