Weibull parameters a and k per wind farm zone and Cabauw
The figures presented here are the 10 year (2008-2017) average vertical profiles of Weibull parameters a (scale parameter) and k (shape parameter) derived from the DOWA. The 10-year average annual profile is given in black, the 10-year average winter profile (months December, January and February) in red, the 10-year average spring profile (months March, April, May) in blue, the 10-year average summer profile (months June, July, August) in green and the 10-year average autumn profile (months September, October and November) in yellow.
The Weibull scale parameter a is a “measure” for the average wind speed. As to be expected, the average wind speed is higher in winter and autumn than in summer and spring.
The Weibull shape parameter k is a “measure” for the shape of the wind speed distribution: the higher k, the smaller the spread in wind speeds. There is a fundamental difference between a Weibull shape profile for land and sea locations.
- On land, there is a strong diurnal cycle (large spread in wind speeds and small value of k) near the surface. The impact of the diurnal cycle becomes less with increasing height: k increases with height (up to 100m). Then k decreases again, just like at sea. The values the DOWA produces for k for Cabauw are conform the ones found in literature.
- At sea, there is no diurnal cycle near the sea surface and the value of k is much larger than on land. The value of k mostly decreases with height (for autumn and winter at all levels, for spring and summer at levels above 40 m) and at 100 m height the value of k is smaller at sea than on land. The DOWA shows that for all offshore wind farms k increases with height below 40 m for spring and summer when the sea is cold and the atmosphere more stable. Or, in other words, in spring and summer the spread in wind speeds near the sea surface is larger (and k smaller) than above (up to 40 m). With a stable atmosphere, there is less “interaction” with higher levels and the sea surface roughness can cause the wind at the lowest levels to decrease more.