Here are plots of average windspeeds in Bayern in January 2018 at 100m,
COSMO_REA6 on its ~ 6 × 6 km grid
ERA5 on its 0.25° × 0.25° grid.
We see big differences over the Alps in the south and over hilly areas in north Bayern.
Data sources:
COSMO_REA6 .../U_100m.2D.201801.nc4
ERA5
Of course, comparing two datasets doesn't tell us much about why they differ. Experts in wind reanalysis might say, "of course, ERA5 is lower when ..."; I'd welcome reasons with runnable code.
Well, if you're comparing anything at all, BIG differences are more interesting than little differences -- you don't see much if everything's grey. In January 2018 there were 100 kmh storms across Europe, Storm David aka Orkantief Friederike.
It's entirely possible that I've made mistakes along the way — mostly home-made code sticking to numpy. (COSMO_REA6 apparently has no CRS, so I regrid with KDTree, don't like smoothing / interpolating timeseries.)
Spot checks are always good. Can anyone post max windspeeds at some int lat/lons, please ?
MacKay, Sustainable Energy -- without the Hot Air 2009, 368p, pdf free
This rough answer is not exact, but it's accurate enough to inform interesting conversations.
See also: Windspeeds-and-windfarms-in-Bayern, Windfarms-in-Germany, Windpower-Weibullmodel, under my gists.
cheers
— denis-bz-py t-online.de 24 June 2023