This short post deals with offshore wind farm wakes, and one of their peculiar features: large Turbulence Kinetic Energy (TKE) streaks at the border between the mean flow and the wake. Quoting the WIPAFF summary paper:
The reduction of the wind speed by wind turbines leads to an area of low wind speed which can generate large horizontal shear at the boundary between the undisturbed wind field and the wake. For flights perpendicular to the wake at hub height, the measurements show strongly enhanced turbulence parameters at the edges of the wakes. For pronounced far-reaching wakes, the turbulent kinetic energy (TKE) at the edges od the wake is still at the same level several 10 km behind the wind park (Figure 10b). During the airborne measurements, the enhanced aircraft vibrations during entering and leaving the wake were noticeable to the crew. Turbulence was found to be particularly enhanced for high wind speed gradients between the wake and the undisturbed flow (Figure 10a, b), and for denser wind park geometries. In contrast, within the wake, turbulent kinetic energy is reduced even compared to the undisturbed flow (Figure 10b).
It turns out that, in stable conditions, these streaks are visible too on SAR images (because the mean flow friction is small, and the radar picks up small scale turbulence created by the wake), see an illustration below (using oceandatalab‘s SAR sea surface roughness product):
I am collecting’em all, you will find them HERE. Note: you do no need to use the oceandatalab product (which uses a GMF), you can use the Sentinel Hub EO-Browser interface instead, together with the sea roughness script from Annamaria Lungo, see here and image below.
Interestingly, they are also visible on Sentinel 2 and Landsat 8 data (red band, mostly), see examples below:
I see them mostly to the left. Maybe does this have to do with gradient in the mean flow (from coast to offhsore), but then: what are they also located to the left when the wind blows from the South along the coast of England? See examples below.
Really, these TKE streaks remain a mystery to me. Maybe these are not TKE streaks, maybe these are regions with larger wind speeds (still, I doubt it). Other WIPAFF papers like this one by Siedersleben et al shows indeed large TKE at the edge of the wind farm (or cause by upstream wind farms), but is it what we are seeing on these SAR images?
As you can see I am very interesting in these SAR artefacts showing us what I believe are TKE streaks to the left (mostly) of offshore wind farms. I would really like to look into this further, and in particular into the optical measurements (Sentinel 2 and Landsat): can these be used to understand what the SAR actually sees?
Comments/suggestions are welcome,
Rémi
