+/- range in Oewc and Oewc-POT results

[Sinisa Knezevic]

Dear,

1) How do you calculate the +/- range around the extrapolated wind speed?
I assume it's a sort of measure of uncertainty of the result.

2) Is that range in any way connected to coefficient of variation of the annual maximum wind speed defined in IEC 61400-1 Ed.4 section 11.3.2 Wind condition parameters (Footnote 31)

Thank you and kind regards
Sinisa Knezevic

[Morten]

Dear Sinisa Knezevic,


1) The extreme wind in WEng may be calculated by three alternative methods called the
annual maximum (AM), peak-over-threshold (POT) and spectral correction (SC) methods.
The theory differ for the three methods, but to generalize we could say that they
first fit a statistical model for extreme events and then used that model
to estimate the level of the extreme event with a fifty year return period.
The uncertainty of the 50-year extreme wind estimate depends on the uncertainty of the fitted model. Read more in the WEng help file section 'WAsP Engineering modelling| Extreme winds', specifically in the subsections called 'Extreme wind estimators' for the AM method, 'Extreme wind POT method' for the POT method and 'Spectral correction' for the SC method.


2) The footnote in IEC 61400-1 Ed.4 expresses the coefficient of variation (COV) by the alpha and beta parameters of the Gumbel distributions. Thus, COV depends on the fitted model and not directly on the uncertainty of the fit. However, the uncertainty is closely related to the Gumbel alpha parameter, so you could say that there is an indirect relation. Unfortunately, WEng no longer reports the fitted alpha and beta parameters, but if you look in the 'WAsP Engineering modelling| Extreme winds| Extreme wind estimators' section of the WEng help file you will find an easy way to calculate them by yearly maxima exactly as in WEng when using the AM method. Alternatively, you can read the 100-year event from the graphs in WEng showing extreme winds as function of return period and then use the formula in the footnote of the IEC standard.


With best regards,
Morten