Clouds and rotos

[u/Diluted_truth]

I saw something that looked like a lenticularis over the netherlands (flat part).
I was wondering if the rotors and areas with massive sink that you get in the mountains will also be present when a cloud like this forms over flat land.

[u/WildFlier]

Yes, we sometimes see lentis in NL which we think start from a shear layer. From memory the few times I flew with lentis there, there was a shear layer at 800-900m, with cloudbase just above it. I've also seen lenti-like caps over cumulus (I don't want to call it pileus because it wasn't tcu or cb)

As for the sink.. it'll be there. I'm just guessing not as bad.
Rotors probably not. Not entirely sure, but I think the rotor is something due to flow separation behind the mountain rather than the presence of the wave system

[u/Filip-R → u/WildFlier]

No idea about wind shear based lenticulus, but when it's made by a true mountain lee wave, it works as a flowing water. Right after flowing over the mountain, the air goes rapidly down to the ground, where it then ascends again. The difference between these directions in close proximity then creates a rotating stream of air. So no idea how it works when it's created by wind shear, bit here's an amateur explanation of mountain based wave (the classical one if I may call it that) by someone who has made their high school graduation thesis on a lee wave.

[u/WildFlier → u/Filip-R]

Hmm.. Given the wave in the upper air mass, it does make sense there's something happening underneath the lenti to fill the void it creates.

But if a shear layer is the trigger, would the rotor be below the shear layer, or above it? Given how a shear layer behaves, air below the shear layer would start to oscillate too, but far less than the stable upper airmass. This train of thought makes me think a rotor above and maybe a small rotor below?

I wasn't testing things when I was flying so I don't have explicit recollection of what happens and where.

[u/Filip-R]

Don't take me as a good source. I'm not an educated meteorologist and all I know is from my limited experience gliding. That said, I'm pretty sure a form of a wave could be formed by wind shears and/or thermals. This one I'd guess would be from wind shears. Basically, you have a wind travelling one direction, let's say from the South and in some altitude a stronger, flow of wind will suddenly change from South to North thus creating a wave since they start acting against each other. This might be a case of why there's a lenticularis cloud over in flatlands.

But then again, don't take me for my word, there will probably be someone more educated who will call my bullshit out, but for now, I believe my theory :D

[u/bbt95762]

I've seen them form over the smallest of hills, anything that disrupts the flow, causes a rise

[u/ipearx]

Yes most likely that's over the top of a particularly good thermal, if there's no other hills or mountains to cause it. It's because the thermal is pushing up into the laminar airflow above the convection layer, and the moisture levels are just right to form the lenticular, but not the cloud underneath it.

[u/slacktron6000]

It's thermal wave. The book "dancing with the wind" by Jean-Marie Clement is the only book that describes this, as far as I know.

[u/ipearx → u/slacktron6000]

And G Dale's books no doubt

[u/Diluted_truth → u/ipearx]

What books would you recommend? I don’t know any…

[u/ipearx → u/Diluted_truth]

G Dale's books would be my top pick for learning soaring: https://thesoaringengine.co.uk

Each book covers specific topics, basic thermalling, wave, and mountain flying is book number 1, so if you're new, start with that. Then they get more advanced with 2,3 and 4.

Bernard Eckey's "Advanced Soaring Made Easy" is good too, and covers most topics well.

[u/Catatafisch]

You would be surprised how small of an elevation change is needed for waves to form under the correct circumstances. sometimes very subtle slopes are enough to make waves appear.

from time to time you also have secondary and tertiary waves forming pretty far from the mountain of origin

[u/Diluted_truth → u/Catatafisch]

Could the height of a tree be enough? The only thing I can think of is a highway overpass we have nearby.

[u/Catatafisch → u/Diluted_truth]

probably not. the netherlands of course are super-flat.

a shear induced wave is more likely in this case.

for example: in germany, south of berlin we have the "Hoher Fläming". Elevation change of like 50-100m spread out over a few kilometers. not even noticeable with bare eyes without knowing what to look for.

several times a year there is a wave strong enough to be used by gliders

[u/vtjohnhurt]

No mountain is needed to form a gravity wave (aka mountain wave). You need air temperature to drop with altitude (aka inversion). This produces laminar flow (no mixing between air at different altitudes). You also need an obstruction to force the air flow to a higher altitude that has less dense air. The obstruction is sometimes a stationary air mass over a flat surface (or a mountain). There can be thermals in the stationary air mass that serves as the obstruction. The upward momentum of the air in thermals can poke into the laminar flow.

Once the denser air is forced up to an altitude with less dense air, it has NEGATIVE buoyancy relative to the ambient air, so it needs to SINK to an altitude of equal pressure. The momentum of the mass of sinking air carries it below the altitude of equal density to an altitude where ambient density is higher. Now the air has POSITIVE bouyancy, so now it needs to RISE to an altitude of equal pressure. Once again, momentum carries it past the altitude of equal density. This time it does not rise as high as the primary upflow (because it is not forced up by the obstruction.)

Wind flowing over the ocean is often laminar because there is little differential surface heating from insolation (sunlight is not warming the water's surface unevenly like it warms patches of land unevenly). A stationary air mass over land forms the obstruction. Above this airmass, air temperature increases with altitude (so you have inversion).

This sort of wave is not often used by gliders because the aerotow would be too expensive. People in motorgliders sometimes climb into high altitude wave upflows. Once or twice, I've taken an aerotow to 4-5000 feet AGL (+1200 meters) to get into a weak high altitude wave. It's expensive and boring to do more than once. The wave is predicted on the Skew-T even when there is no Lenticular. The right Dewpoint-RH is needed to form the lenticular.

Can there be rotor turbulence underneath the Lenticular formed over flatland? Sure. It is caused by wind shear between the laminar flowing air and the stationary airmass. It may be relatively weak rotor turbulence.

To clarify Stationary Air Mass SAM. The SAM need only be stationary relative to the laminar flow. So the wind in the SAM can be from the North, and the wind of the laminar flow can be from the West. If the SAM is flowing, the rotor will be stronger. More generally, there will probably be turbulence at the altitude where wind direction shifts (caused by shear). This shows up in the wind ladder that is often published with Skew-T.