Low Level Jets (LLJs)

[PierreB]

Airborne wind energy: Optimal locations and variability (Cristina L. Archer, Luca Delle Monache, Daran L. Rife) https://www.sciencedirect.com/science/article/abs/pii/S0960148113005752

In the Introduction:

In this altitude range, 200–3000 m AGL, wind speed generally increases monotonically with height, with higher gains (shear) in the boundary layer and more modest increases above it [3], [4], [5], [10]. However, a variety of weather phenomena can invalidate the general rule-of-thumb that better AWE resources are available at higher altitudes, for example low-levels jets (LLJs). LLJs are narrow, nocturnal wind speed maxima with cores centered below 1000 m that form at a several locations worldwide due to a favorable combination of synoptic conditions, regional topography, and local stability [11], discussed later. Here we include LLJs as a subset of the broader category of wind speed maxima (WSM), defined as jet-like wind profiles centered below 3000 m regardless of the formation mechanism and diurnal variation. Our hypothesis is that locations with WSM are ideal for AWE applications because wind speed and wind power densities near the WSM are as high or higher than those normally found elsewhere at greater elevations, but at much lower altitudes. The height of 3 km is not based on physical properties of the atmosphere, but rather on practical limitations of current and near-future AWE systems. Since tethers longer than 5–6 km can weigh more than a ton, flying altitude will unlikely exceed 3 km as tether angles remain lower than 30°. As such, only jets that are located below 3 km are of practical interest for AWE applications and are therefore the focus of this paper.

Another and recent publication: Global Climatology of Low-Level-Jets: Occurrence, Characteristics, and Meteorological Drivers ( E. W. Luiz, S. Fiedler) First published: 04 May 2024 https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2023JD040262

Some excerpts:

3 Results
3.1 Global LLJ Occurrence
[…] The global mean in the frequency of occurrence of LLJs is 21%. It is important to mention that this is an average over all grid points and the regions closest to the poles have a higher weight. The mean frequencies for LLJs with a simultaneous appearance of an inversion measured by Δθ v > 0.001 Km−1 from surface until the LLJ core (in the lowest 100 m a.g.l.) occur in 15% (12%) of the profiles. The slight difference in the occurrence frequency of the LLJs with inversion layers at different altitudes can be explained by the neutral to unstable stratification close to ocean surfaces that become more stably stratified with increasing height. Specifically, the frequency of occurrence of LLJs averaged over oceans for all LLJ cases, with a stable stratification below the LLJ core and those with a surface temperature inversion in the lowest 100 ṁ a.g.l is 15%, 8%, and 5%, respectively. On land, the differences associated with a temperature inversion are less apparent, with average frequencies of occurrence of LLJs of 28%–32%. On average, the frequency of occurrence of LLJs on land is higher than over the oceans, because many regions over the ocean have very small LLJ frequency of occurrence. This can be seen by the mask made for regions with a frequency of occurrence smaller than 15% from Figure 4.
[…]
3.2 Global LLJ Characteristics

Globally averaged, the mean wind speed of LLJs is about 12 ms−1, with a difference at the order of 10% (<1 ms−1) when we compute the average over ocean and land regions separately. The mean height of the LLJ cores differs more between the ocean with 419 m against land with 325 m. The global average of the LLJ core height is 371 m. Accounting for the co-occurrence of a temperature inversion for LLJs has a marginal influence on the globally averaged wind speed and height of the LLJ cores. The average height of LLJs for regions with frequencies of occurrence higher than 15% and a co-occurring temperature inversion, measured by Δθ v > 0.001 Km−1 underneath the LLJ core, is for instance 387 m.

These documents (above all the last one) could help to better understand the possible directions of AWE.

[tallakt]

We did talk about this during the AWEC conference. I know nothing about LLJs but was intriegued after C Archer’s talk.

Some takeaways

• LLJs are sometimes fixed in coordinate and over a period of time
• LLJs require detailed data analysis. Older data sets are probably not useful
• LLJs can occur at lower altitudes
• LLJs are changing fast due to global warming
• LLJs are not nice winds, probably would be very rough on a wind turbine

All in all, for me LLJs are below the «not worth it line» for a few of these reasons as of today. Maybe with better analysis tools and already established AWE platforms this could change though.

LLJs would be mostly interesting for mobile AWE in my opinion, and ships seem to be the lowest hanging fruit in that respect.

The extent of the LLJs are important as well. Any single kite AWE turbine would need large wings and thus also large flight patterns. The large scale would be a consequence of having to reach high altitudes. Those would have to fit neatly into the LLJs.

Having an abrupt increase in wind speed though could mean that higher elevation angles could make sense, eg 45 deg rather than 25…

Multi kite AWE is stuff of dreams at this point.

[PierreB]

Cristina Archer summarized the airborne wind energy resource at the 10th International Airborne Wind Energy Conference (AWEC 2024) in Madrid.

I put again the video:

On the Phenomenal Airborne Wind Energy Resource

At 10:15:

On average, little “gain” above 500 m

At 11:30:
Cristina Archer indicated something like that 500 m is a good altitude.

At 11:35:

Exception: Low-Level Jets (LLJs)

• Nocturnal;

• Form during dry, cloud-free nights;

• Strongest in early morning and decay after dawn;

• Anti-cyclonic turning of wind (in northern hemisphere);

• Peaks often lower than 500 m;

• Jet winds often super-geostrophic;

• Need high resolution to resolve them.

I would remark that, in spite of the exception of LLJs, these often occur within 500 m (“Peaks often lower than 500 m”).

This would mean that 500 m altitude is a good base for AWES, whether or not they exploit LLJs, these being a considerable plus to the extent that they are exploitable.

And that is approximately the altitude that several AWE projects, including SkySails, are working on. Perhaps harnessing LLJs require more resistant wings, but this would not fundamentally change the AWES perspective regarding target altitude.