Lidar measurements for vertical profile

[igarcia]

Dear Wasp team,

For wind vertical profile, I normally let WASP extrapolate to hub height from measurement height. However, i have this case where extrapolation must be from 80m to 120m. Since I have lidar measurements for 3 months up to 240 m, I have decided to extrapolate manually. Could you give me some advice on which lidar measurements to use to calculate the vertical profile? For example should i exclude the highest (240m) or lowest (40 m) measurements? 

Thank you very much,

Kind regards,

Inés

[Morten]

Hi Ines,

There are lots of internal processing in a lidar, which results in time series of horizontal speed and direction for different heights. Presumably, the data are also screened for measurement errors, which are reported by status codes. If lots of data are missing for some heights, e.g. at the top, you might want to disregard those heights. 

Basically, you can only measure the wind component along the laser beam, so a single lidar deduces the horizontal wind by several measurements along slanted beams with different azimuth angles. This deduction usually rely on an assumption of zero vertical wind velocity, so results for the lowest heights might be compromised in complex terrain. It is possible to corrects for effects of non-zero wind velocity by a flow model, as I think we mention this in the WAsP Engineering course. I have heard that some manufactures offers this correction as an optional service inside instrument. Basically it is a correction table, which can be calculated based on the orientation of the instrument and the terrain around the measurement position. 

Hope that this helps a little. We have a large group of lidar specialist here at DTU Wind Energy working on advanced techniques, like multiple coordinated lidars, dedicated scanning patterns and new data processing for turbulence measurements. I must confess that compared to them, I know very little on the topic. 

With best regards,
Morten

Edited July 8, 2021 by Morten

[igarcia]

Thank you Morten! It is really interesting that DTU is conducting so much research on this topic!

Regarding my question, Windpro gives the option to calculate wind shear from time series at various measurement heights. Without lidar, there are not many options because there are 3 heights aprox, but with lidar, I have many more options. I would like to know how many measurements heights (and maybe also which ones) should I input to get the most accurate wind shear exponent. 

Thank you!

Best regards,

Inés

 

[Morten]

Hi Ines, 

The profile between lower and upper tip height is what matters for site assessment, so I would focus on heights in that range. It is also important to use a selection of data with simultaneous measurements from all the heights used for the profile. You may want to skip a height if it has lots of missing observations.   

As I remember, WindPro can plot the fitted profile and the reference points together. This is a good quality check and might help you deciding which heights to include. 

Best regards,
Morten

[Mark Kelly]

Hello Inés,

for vertical extrapolation in resource assessment, to use only 3 months of lidar to "help" can be quite tricky due to the seasonality (depending on the site).

A first step is to get an idea of how much the shear exponent between 80m and 120m varies throughout a typical year, and by sector.  It appears you may only have measurements at 40m and 80m heights, so you can start by checking how much that varies.  If the prediction location is over non-flat/inhomogeneous terrain, then of course the alpha may change quite a bit from 80 to 120m compared to 40-80m, and vary by sector. 

You can at least check how the mast's alpha looks during those 3 months, and then evaluate how much variation there is over 12 months compared to those 3, considering also the sector-wise (directional) aspect. 

I made a M.Sci. project a few years back, where the student (Niels Waars) looked at using MCP on alpha, for just this situation.  Within the context of that work, the trick is to "guess" alpha for the other 9 months in your case, if you know the long-term distribution of alpha. We looked at uncertainty in doing this per length of lidar campaign  (Link to M.Sci. project at DTU, thesis link is there.)  I can upload the thesis if you're not able to download it from the link.

with kind regards / saludos / med venlig hilsen,
--Mark

Edited July 16, 2021 by Mark Kelly

[igarcia]

Thanks Mark and Morten for taking the time to reply. 

I have been able to download the document, and I think I will consider MCP on alpha for future projects, it looks very interesting. For this project, I ended up using an alpha from the mast measurements since seasonality was affecting lidar's alpha. Then, for verification, I compared the wind speed obtained at 120 m with the speed measured by lidar at 120 m for the same reference period. 

Thanks again,

Kind regards,

Inés