WAsP Extrapolation

[carriv9]

Good Morning,

I recently compared the WAsP extrapolation from 57m to 80m to the extrapolation done by other software (Windographer). The difference between both extrapolations was so small that it didn't really create a relevant difference. However, I do know that Windographer uses the power law exponent to extrapolate values. This being said, I know .tab files for WAsP do not carry any sort of power law exponent or shear exponent. So what is the process that WAsP uses to create extrapolations? It is only based on terrain meaning that the extrapolation is only as good as your .map file?

Additionally, is the "U" value the the mean of the best-fit Weibull distribution, or the mean of the actual wind speed distribution?

Thanks!

[Mark Kelly]

Hello,

WAsP performs vertical extrapolation using the logarithmic law as a basis, which means that the roughness length determines the shear. The vertical extrapolation modeling also accounts for long-term stability, through the parameters Hoffset and Hrms; the model is most sensitive to the parameter Hrms (which is negative by default, because stable conditions dominate on average). Thus the extrapolation also depends on Hoffset, which can be modified to change the (implicit) shear.

One must also ask: how does Windographer obtain the shear exponent? If you have measurements at two heights above the surface-layer (at least 40-50m), then this makes some sense.

The Weibull-distribution is fit to the 1st and 3rd moments, above the peak--thus a compromise for matching power-density as together with mean wind speed, as well as possible.

With kind regards,
--Dr. Mark Kelly, Wind Energy Dept., Danish Technical University
for WAsP support
support@wasptechnical.dk

[carriv9]

Thank you Dr. Kelly this is an excellent explanation of how WAsP performs vertical extrapolations. To answer your question, Windographer does calculate the shear exponent only when there are two or more heights above surface layer. Additionally, does that mean the "U" value is not actually the "mean wind speed"? What would the "A" and "U" value be?

Again, thank you.

[Mark Kelly]

Hi, again A is the Weibull scale factor, and U is the mean wind speed. Recall that for a Weibull distribution, U = A*Gamma(1+1/k), where Gamma is the gamma-function and k is the Weibull-shape factor.

[carriv9]

Thank you!