Methods to Estimate Surface Roughness Length for Offshore Wind Energy

Figure 2

Frequency distribution of surface roughness length from the 2013–2014 IMPOWR campaign with two sonic anemometers located at 12 m AMSL and (a) the analytical method and (b) the Charnock method and the 2003–2007 CW Historical campaign with (c) the analytical method using a sonic anemometer located at 20 m AMSL, (d) the Charnock method using a sonic anemometer located at 20 m AMSL, and (e) the statistical method using pairs of sonic and cup anemometers at 20, 41, and 60 m AMSL. The mean and median z₀ are displayed on each figure. The x-axis is logarithmic and the y-axis has different maxima in each figure. Note that the statistical method is not intended to give realistic values of surface roughness length; thus, its distribution is shown here in (e) just for completeness.

(a)

(b)

(c)

(d)

(e)