Difference between DOWA and KNW

Valuable addition to the KNW-atlas

The KNW-atlas contains climatological and hourly undisturbed wind data (without wind turbine wake effects) based on more than 40 years (1979-2019) of re-analyses data. The wind speeds in the atlas have a climatological accuracy comparable to that of wind measurements. So if you are interested in long-term statistical values such as averages and extremes, the KNW-atlas is hard to beat. However, due to the applied method to create the KNW-atlas, the hourly correlation with measurements (e.g. daily cycle) is rather poor.

Also, the KNW-atlas lacks information above 200 meter height and information that enables further downscaling with Large Eddy Simulation (LES) methods. The Dutch Offshore Wind Atlas (DOWA) presented on this website is a 10 year climatology (2008-2017) with a better hourly correlation than the KNW-atlas, information up to 600 m heights and the option to downscale the information in DOWA from hourly to less than 60 seconds and from 2.5 km to less than 100 m horizontally. Also the domain is larger than that of the KNW-atlas, including the areas where the German wind farms are (being) built and planned (see figure 1).

Enlarge image DOWA and KNW domains
Image: KNMI
Figure1. ERA5-HARMONIE domain (yellow) of 789x789 points and DOWA-subdomain of 217x234 points (red). ERA-Interim-HARMONIE domain of 500x500 points (green) and KNW-subdomain of 170x188 points (blue).
In the table below the main differences between KNW and DOWA are summarized
KNMI North Sea Wind (KNW) atlas Dutch Offshore Wind Atlas (DOWA)
Timespan 1-1-1979 until 31-8-2019 18 UTC (40+ years) 1-1-2008 until 31-12-2018 (11 years)
Variability of the North Sea wind climate Captures the variability of the North Sea wind climate (40 years long enough) Does not capture the variability of the North Sea wind climate (10 years not long enough)
Used models Based on re-analysis ERA-Interim and mesoscale weather model HARMONIE Version 37h1.1 (1979-2013) and Version 37h1.2.bugfix (2013-2019), the latter tested and adapted to guarantee a homogeneous dataset. Based on re-analysis ERA5 (follow-up of ERA-Interim with higher spatial and temporal resolution) and mesoscale weather model HARMONIE-AROME Version 40h1.2.tg2 (improved wind information because turbulence is better modelled)
Method HARMONIE used as downscaling-tool only (data-assimilation of measurements in ERA-Interim only) Additional measurements assimilated in HARMONIE (ASCAT-satellite surface wind measurements and MODE-S-EHS aircraft wind profile measurements)
Climatological information height Climatological information up to and including a height of 200 m Climatological information up to and including a height of 600 m
LES-downscaling Lacks the information required for further LES-downscaling Information required for further LES-downscaling included
Cold starts Cold starts: limited quality of hourly correlation with measurements (e.g. diurnal cycle) No cold starts: better hourly correlation with measurements and representation of the diurnal cycle
Wind shear correction Uniform wind shear correction applied

No wind shear correction required


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