by S. Knoop (KNMI), P. Ramakrishnan (Delft University of Technology), 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 order to improve upon the KNW-atlas, the DOWA uses an updated version of the global ECMWF reanalysis, as well as an updated version of the HARMONIE numerical weather model that was used to transform the global reanalysis into a regional wind atlas. The method that was used to make the atlas was changed as follows: the ‘cold starts’ within the global reanalysis used in KNW were removed and at three-hour intervals aircraft and satellite measurements were assimilated.
The DOWA is validated against wind speed and direction measurements from the Cabauw meteorological mast, for the 10-year period of DOWA and at heights between 10 m and 200 m. The validation results are compared to the KNW-atlas. It is found that the average difference (bias between DOWA wind speeds and those measured at Cabauw varies between -0.1 m/s to 0.3 m/s for the different heights. Significant differences between DOWA and KNW are only found at 10 and 20 m altitude, where KNW performs better. For heights above 20 m there is no significant difference between DOWA and KNW regarding 10-year averaged wind speed bias. The diurnal cycles is better captured by DOWA compared to KNW, and the correlation is slightly improved in DOWA.
In addition, a comparison with the global ECMWF ERA-Interim and ERA5 reanalyses, used for KNW and DOWA respectively, is made, in particular highlighting the added skill provided by downscaling those global datasets with HARMONIE.