Methods to Estimate Surface Roughness Length for Offshore Wind Energy

Table 4

Statistical properties of surface roughness length during the CW Historical (2003–2007) and IMPOWR (2013-2014) campaigns at the Cape Wind tower obtained with the analytical, Charnock, and statistical methods. Note that the statistical method is not intended to give realistic values of surface roughness length, which are listed here in the last column just for completeness.

	IMPOWR		CW Historical
	Analytical	Charnock	Analytical	Charnock	Statistical
ALL
Mean (m)
Median (m)
Count	18,764	18,819	99,309	100,393	207,676
DJF
Mean (m)
Median (m)
Count	3,257	3,263	23,066	23,309	49,213
MAM
Mean (m)
Median (m)
Count	9,559	9,594	26,479	26,931	49,876
JJA
Mean (m)
Median (m)
Count	2,266	2,276	15,650	15,871	52,860
SON
Mean (m)
Median (m)
Count	3,682	3,686	34,114	34,283	55,727
STABLE
Mean (m)
Median (m)
Count	7,339	7,006	24,687	25,536	25,093
NEUTRAL
Mean (m)				2.29
Median (m)
Count	1,776	2,882	7,567	7,625	7,346
UNSTABLE
Mean
Median
Count	9,649	8,931	67,055	67,228	62,292