24 Days of #KitchenOceanography — Wind-driven mixing
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Tag Archives: wind-generated waves
Strong gusts of wind -> lots of energy transferred to the water -> wind waves with large amplitudes
We are still in the “interesting weather” period here in Kiel. Feels more like April than like September, but I am not complaining. I love the rapid change between dark clouds and blue skies and sunshine! Also I like how much more interesting wave pattern get if the wind comes in gusts rather than blowing just consistently the same.
Below, you see strong gusts of wind in the dark areas with the high surface roughness, but you also see that the small waves in the foreground have higher amplitudes and more pointy peaks than we usually see. Additionally, there are longer wave length waves coming in with crests more or less parallel to the images lower edge. And on top of all of this, there is the seagull’s wake. Can you still spot it even though it’s superposed on all the other waves?
Below, you clearly see the different wind strength in different areas. The shiny, flat surface with lower wind speeds, the rougher areas, and the comparatively short waves with large amplitudes in the foreground that show that there really is a lot of energy input over a relatively short fetch.
Below, in some regions we can also see hints of a checkerboard interference pattern of longer waves that were reflected at the sea wall, with the small, short wavelength waves superimposed.
Here is another look at these waves. I find them so fascinating!
And below is another strong gust of wind visible. And do you see the wave crests parallel to the edge of the floating part of the pier, created by that part of the pier moving in the waves?
And, just in case you didn’t know: At the end of the rainbow, you will find a … research ship!
From wind-driven capillary waves to gravity waves on a calm lake
The picture above I thought was too pretty to not put on my blog (because my blog’s main function to me is still my personal brain dump), but the picture below is actually interesting from a physics point of view.
In the middle of the lake, the surface looks a lot rougher and crumpled than the water surrounding it. That’s because there the light breeze is generating small capillary waves, whose restoring force is surface tension. But if we look closely towards the lower edge of the “crumpled” area, we see that the water isn’t as calm and the surface isn’t as flat as they appeared to be on first glance — there are longer waves propagating out of the crumpled area. Those waves are gravity waves, and they can propagate for longer distances without having constant new energy input by the wind.
But why doesn’t the crumpled area that is directly influenced by the wind extend all the way to the shore? Of course, on one side of the lake it would be sheltered from the wind by the trees and other things growing there. But on the other side, it’s in a way sheltered by the trees, too, even though the mechanism there is different. There, we don’t have wind until the very edge of the lake, because the current of air is deflected upwards by the trees, so an area of low velocity is formed, kind of like the area surrounding a stagnation point in an idealized model.
Wave watching in Kleinwaabs — and my first real Insta story!
So today (and tomorrow and the day after) is the big event that I have been working towards all year in my not-so-new-anymore job: The GEO-Tag der Natur! If you are curious about what’s going on there, check out our Instagram account @geo.tag.der.natur that Kati is doing an amazing job with!
As you can imagine, the weeks running up to this weekend were quite busy and a little stressful, too. So last Sunday I went to the beach to hang out with friends and do some wave watching! Because nothing has a more calming effect on me than watching water…
For example below we see nicely the effect of the wave (and wind) breakers on the wave field. In the lee of the wave breaker, the water is completely calm, whereas towards the right of the bay waves form and grow larger and larger.
And below we see a pretty cool “diffraction at slit” example: Straight wave fronts reach the slit between two wave breakers, and as they propagate through the slit, they become half circles.
But to relax and get my thoughts away from my job, I tried something new: I created and posted my first ever Instagram story! I’m not quite sure it’s my format, but I definitely had fun! What do you think? Would you like to see more of those? (I only just realized the story is in german and my blog in English. Posting anyway… Would anyone like to see this kind of stuff in English? Then please let me know and I’ll see what I can do…)
https://vimeo.com/341862851
(P.S.: Since I made this for Instagram, the format of the video was optimized for viewing on a mobile phone. Therefore it looks crap embedded in a blog. But some you win, some you loose…)
What do you do to relax and get your mind off of work? Wave watching and posting about it on social media? Have you ever tried that? Or what else would you recommend?
Observing a breeze making waves on Parsteiner See
Yesterday, I happened to be at Parsteiner See for work.
At first, the sea was completely calm and the only waves were the ones we made doing our photo shoots, like so:
Other than that, the lake was completely calm.
But then suddenly, I spotted a breeze going over the lake. It becomes visible in the dark areas with higher surface roughness, where capillary waves have formed.
As the wind keeps blowing over the capillary wave area, those waves grow and then at some point become “regular” gravity waves, that travel out of the region where they are directly forced by the wind. See below: In the background you still see the area with higher surface roughness, whereas in the foreground gravity waves are coming towards the shore.
As the wind keeps blowing over the surface, forming capillary waves over larger and larger areas, those areas all show up as darker and rougher.
And the cute little waves keep coming to the shore :-)
Atmospheric & water wave watching
What I find really fascinating about watching waves in the atmosphere rather than on water is that all the waves that become visible are not surface waves like on water, but internal waves. Which we have to go to great lengths to make visible in water (for example by adding dyes in tank experiments) but which we can’t just visually observe in the sea in the same way as we can in a transparent atmosphere.
In the atmosphere, however, we also don’t see every internal wave going on, either, we need very specific conditions for them to become visible. So whenever I see one, I start pondering why we see exactly what we see, why there are clouds in some places and not in others. Below, for example, we see the troughs of an internal waves in cloud stripes, but the crests don’t form clouds. Fascinating how just displacing air by a little bit can cause clouds to form and to disappear!
And things become super cool when you combine atmospheric wave watching with “normal” wave watching like in the picture above. There you see the rough surface with tiny little wind waves in the background, waves coming around the break water, the calm water in the lee of the break water, sheltered from the wind, and then the reflection of the atmospheric waves on the water.
And you thought it couldn’t get any better? Well, you were wrong! Now there are also some waves on the water, plus soap bubbles! :-)
Now, for a thought experiment: What would soap do to the waves? Would destroying surface tension actually matter? I think not in this case, or t least not close to land in the picture above, since the waves are mainly gravity waves, not capillary waves. But what do you think?
Wind-driven waves in natural flumes for #flumefriday
On a bike tour with my friend Frauke in Greetsiel two or three weeks ago, she pointed out how well one could see that the waves on the puddles left in the Wadden Sea close to low tide were wind-generated. That was that for the bike tour — now I had to take pictures.
Below you nicely see the ripples that are created where there are longer stretches of puddle aligned with the wind direction, i.e. where there is enough fetch. And you see how the waves get diffracted behind topography, fanning out downwind of slits! The wind is coming from the right here, almost in the direction pointed out by the looks-like-an-arrow-but-is-plastic marine litter.
Here we are looking in the opposite direction, the wind now going left-to-right. Do you see the one slit in the lower half of the picture and how wave crests propagate almost perpendicularly to the wind direction, just because there are waves going through that slit? Pretty cool, me thinks!
The really shallow water with all the stones in it made it really easy to look at waves from different directions. Below, we are looking downwind, at the back of the waves.
And below we are looking upwind. See how different things look now? You still see the wind pushing the waves, the front slope of the wave is a lot steeper than the back slope.
I love the picture above, makes me want to put my hand in the water and play with the waves :-)
The wonders of a Wadden Sea. Or what someone addicted to #wavewatching thinks they are
As someone living on the German Baltic Sea coast, I don’t spend a lot of time on the North Sea coast (except, actually, my week-long vacation after Easter with my godson and his family, and when my friend Frauke and I went to Sylt earlier this year, or when Frauke and I are going back to the North Sea next weekend. So maybe that’s actually not so little time on the North Sea coast compared to most other people?).
Anyway. I really like the North Sea, especially because I like the flat landscape where the highest points are dykes.
What I really dislike, though, is getting my feet muddy. But that’s pretty much the whole point of a North Sea vacation, according to my godson and his family.
On the other hand, though, having the opportunity to actively and directly influence water depth (or, as normal people would probably say, leaving footprints in the mud) makes for some pretty cool wave watching!
It’s a little hard to see, but if you look at the picture above, you see that the sun is coming from kinda behind my left shoulder, and the picture below is taken from a similar perspective (just telling you so you can interpret the footprints and resulting waves). So the left edges of the footprints are actually coming up and partly out of the water.
The wind is coming from the right, and you see the locally generated wind ripples and how they get defracted around the obstacles created by the foot prints!
Pretty cool, eh?
In the picture below, the wind is coming from the left and you see the muddy wakes of the fresh footprints! This I think is pretty awesome, especially because you at the same time see the refraction of waves around the obstacles.
What I also think is pretty cool are the little spaghetti piles of sand that the worms living in the mud leave behind.
And that, for each of the piles, there is a funnel somewhere close by, and a worm connecting the two inside the mud!
But then when the water is gone completely, it’s still pretty here, but also a liittle boring. Don’t you agree?
Ok, but it’s still pretty. But Wadden Sea and tides take the fun out of wave watching for quite substantial amounts of time every day, and I don’t approve of that ;-)
Night swimming… or at least night-time wave watching
Looking at Kiel fjord in the picture below, it is quite obvious from the shape of the waves that those waves are some ship’s wake.
Why is that obvious? Because the waves a) have a very short wavelength for their height, and b) are also all of the same wavelength. What I mean by that is a) on Kiel fjord, if we see waves that high that are driven by the wind, their wavelength is a lot longer since the waves have been building up over a long distance. For short waves to display such an amplitude, the waves would have to run up a fairly steep slope which I know is not the case in this location (and which would also lead to two or three high crests in the shallowest part of the water, not to as many as far out as we see here). B) we don’t see a spectrum of wavelengths as we would expect in a wind-driven wave field. In fact, the water surface doesn’t display any ripples or other evidence of wind at all.
And what do you see when you look at water at night? :)
Wave watching from a train
You know how they say that the journey is the destination? That was certainly the case for my spontaneous mini-vacation yesterday (and how awesome is it that my #BestTravelBuddy is up for a cross country trip on a day’s notice?). We went all the way from the east coast to the west coast — which in Germany admittedly isn’t that terribly far — to visit the island Sylt in the North Sea for a day.
Even the train ride itself is spectacular, though, at least if you are as easily excited as we are. Wave watching from the bridge across the Kiel canal in Rendsburg (below): A super neat wake of the ship, showing the turbulent wake as well as the feathery V-shaped wake. And as you can see from the rows of foam on the water that are a sign of Langmuir circulation (more about that here): It was pretty windy, too!
But it got even better when we reached the west coast. This is my kind of train ride!
Below is a view of the dam that connects the island Sylt with the main land, and here again you see how windy it is, and this is in the lee of the island. In the lee of those shallow dams you see that it really doesn’t take long for the surface roughness to increase again.
So are you excited to see the wind-ward side of the island now? I’ll post some wave watching from that side soon, but I first have to wade through literally thousands of pictures to cut it down to a handful. I’m already down to about the 100 best, but now I can’t decide which ones to post, because I like them all…
But here is a picture of the train ride back. Do you notice how there are regions with really low surface roughness on either side of the dam, suggesting that this dam is sheltering the water surface from the wind in two directions? Of course it isn’t — it’s just ebb tide and the smooth surface areas towards the right of the dam are wet sand that look similar to a smooth water surface.
So that’s my wave watching from the train! Excited to go back soon! :-)