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Vertical Wind Speed development

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I would like to have a deeper understanding of the vertical development of wind speed in WAsP.

More in detail my company bought some data (MAST product - .tab file) from VORTEX which I extracted at different heights (from 50 m to 130 m with a step of 10m)  and I used them in WAsP to generate the GWC. What I experience is a decreasing behavior of the AEP the higher is the elevation at which that the predicted wind speed at 130 m from the data extracted at 50 m is 7,94 m/s and the predicted wind speed at 130 m from the data at 60 m is 7,81. The decreasing behavior goes all the way to 130 m where the wind speed is 7.29 . I experience the same decreasing development also for the AEP and for the percentage difference between a predicted wind speed from a given height to 130 m and the actual wind speed at 130 m.

. I created then a reference site that matches the met mast but with a height of 130 m

I know that the vertical development of the wind speed is expressed with the log-law, but I was wondering if it could be possible to have some deeper insight into this.


Andreas Wolf Ciavarra

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Hi Andreas, WAsP will try to clean your input .tab file from local effects and then apply local effects where you do your prediction. If your input height is different your are not guaranteed to get the same generalized wind climate out, simply because the vertical profile models are imperfect. Only when you use a fixed height and let WAsP (with stability set to neutral) and calculate at some heights it will be a log profile.

You would also need different roughness and elevation maps and stability info from the WRF simulation for your generalization step. So in other words, you can't easily use WAsP this way to downscale WRF results..



Edited by Rogier
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Hi Rogier,

Thanks for your reply. Could you elaborate more regarding the sentence "the vertical profile models are imperfect"?

More in detail, how good or how bad does WAsP work in the vertical direction? Is the vertical wind profile only dependent on the log law, or are other components contributing as well?

My issue is that I am trying to calculate the power of a wind turbine at 135 m by using .tab files from VORTEX extracted at different heights (from 50m to 130 m with a step of 10m), and I got a power production that is inversely proportional to the height at which I get the data from. In other words, the AEP at 135 m calculated with a .tab file extracted at 50 m is by far higher than the AEP at 135 m calculated with the data extracted at 130 m. So I was wondering how the wind speed is calculated in height.


Andreas Wolf Ciavarra

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What I actually meant to say was "the vertical profile models are different": Vortex and their models will give you one vertical profile model, WAsP will given you another one. For the last profile model I can give you some numbers: for typical vertical extrapolations between 20 and 200 m you can see some results of the WAsP model here:


So vertical extrapolation errors are typically around 1-2% on wind speed using the latest WAsP (the report above also contains horizontal extrapolations which have larger errors). This is only under best model set-up conditions where you use measurements as input.

Whether you trust the vertical profile model of Vortex more than WAsP depends on your site. There is large uncertainty on the data from vortex because they are from numerical weather prediction models for a coarse resolution (1 km? can't really judge what model it is from your message). They typically also switch off parametrizations to make it run faster I think.

You may be able to get a generalized wind climate that is independent of height by obtaining the roughness, elevation and atmospheric stability information from Vortex, but even when everything is setup perfectly it might still not match, because you are mixing two models.

In other words, if you trust the vortex vertical profile model I would use the 135 m hub height, if you trust the WAsP vertical profile model you will have to pick your 50 m height as input :) I would say definitely use the 135 m output, simply because you don't have the materials to setup a model chain here to make WAsP perform well (i.e. roughness map, elevation map, stability info from mesoscale model + the same for microscale modelling). If you had measurements and good model inputs, I would trust the WAsP method more than Vortex.


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