A scicomm comic on Rossby waves and hands-on teaching

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Last year in pre-social distancing times, Torge and I brought hands-on rotating tank experiments into his “atmosphere and ocean dynamics” class. The “dry theory to juicy reality” project was a lot of fun — the affordable DIYnamics rotating tables are great to give students hands-on experiences in small groups and to see — by running the same experiment on four rotating tables in parallel — how the same experimental setup can lead to very different realizations because of tiny differences in boundary conditions.

Instead of a classical lab report, we asked students to write a pupular science text about an experiment of their choosing. We got lots of great results (see all of them on our blog “Teaching Ocean Science“), but there is one that particularly stood out to me, and the author, Johanna Knauf, kindly agreed to me publishing it here. Enjoy!


I am super impressed with this comic, and also increadibly flattered and touched. This comic is the most meaningful feedback on my teaching and science communication I ever got and that I can possibly imagine. Thank you, Johanna!

P.S.: Curious about how we modified the project to work with social distancing? Check it out here!

One of the most difficult #friendlywaves I’ve ever gotten! Did I get it right?

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Florian sent me a #friendlywave — a wave picture he took, with hopes that I might be able to explain what is going on there. And this one had me puzzled for some time!

This is what the picture looks like:

What I knew about it: Florian was on the ferry from Wisschafen to Glückstadt, crossing the Elbe river.

In the picture itself, there are several features that jumped at me. First, drawn in with the lightblue line below: A sand bank parallel(-ish) to the island’s coast line.

Then, the ship’s wake (shown in red) breaking right near the ship (orange) and turning (green) and breaking (yellow) where it runs on the sand bank.

Florian wrote he was watching the ferry’s wake and noticed something curious: There seemed to be a shallow part, where the waves suddenly became a lot faster! And could I explain what was going on?

Looking at the picture, there were two possibilities for what he might have meant (and, spoiler alert — I completely jumped on the wrong one first!).

Below, I’ve drawn in the part of the wake that is running on the shallow sand bank (green) and how those wave crests continue on the other side of the sand bank (red). I’ve also drawn in some mystery wave crests in blue. Those were the ones I chose to focus on first, since Florian had written that he noticed waves behaving weirdly and suddenly becoming much faster. So if we are talking fast, we are talking really fast, right?

So how do we explain those blue wave crests?

There is a limit for the maximum speed a wave can have. That limit depends on the wave’s wave length: The longer a wave, the faster it travels. In deep water, i.e. water deeper than 1/2 the wavelength, the wave travels at this maximum speed (see green lines in the plot below).

But as it comes into shallower water, it gets slowed down (see black lines in the plot below — those are just a quick sketch, there are complicated equations to calculate it exactly).

In shallow water, i.e. water that is smaller than 1/20th of the wave length, the phase speed only depends on water depth: The shallower the water, the more the wave is being slowed down (see the red lines in the plot below).

Sorry about the quality of the sketch — I don’t have Matlab or anything else useful on the computer I have available right now, so I drew this in ppt! Take it with a pinch of salt, but qualitatively it’s correct!

So looking back at Florian’s picture, for the blue waves to have been caused by Florian’s ferry, there are two options:

A) they would have to have wave speeds faster than the ferry’s bow wave and wake

B) the ferry would have had to come from the direction of the island, so that the waves propagated in that deeper channel behind the sand bank before the ferry made its way around the sandbank.

Option A is impossible, because wakes travel at maximum wave speed (similar to a sonic boom in the atmosphere, where sound is travelling at maximum speed, forming a cone with the air plane at its tip, only here it’s a 2D version, a V-shaped wake with the ship at its tip). So if the wake is traveling at maximum speed already, then the blue waves can’t go faster than that.

For option B, looking Florian’s ferry up on a map, I saw that that ferry goes around a small island, which is the land you see in his picture. But a quick glance at the map shows that even though the sand bank seems to end where the ship would have had to have gone in order to create those waves, the island is still very much in the way. So this can’t be the solution, either.

This map is published at http://map.openseamap.org under a CC BY-SA 2.0 license

So let’s take another good look at the original picture.

Remember those wave crests that I marked in blue? Well, upon closer inspection it turns out that they are tidal gullys and not wave crests! (Which is what Florian confirmed when I asked whether he remembered the situation) Guess I have been barking up the wrong tree all this time!

So back to the wave crests that I marked in red:

What we see here is exactly the depth dependence of the phase speed that I plotted above. Right at the sand bank, the water is shallowest and waves are slowed down (we see that both in the green wave crests that seem to be falling back and start breaking as they get closer to the sand bank [both indicating that the water is getting shallower], and in the red wave crests right at the sand bank). But as the water gets deeper again on the far side of the sand bank (which depth measurements in the map above seem to confirm), the phase speed picks up again (as it has to — see my plot above) and the wave crests accelerate again. Hence we have the weird phenomenon of waves suddenly speeding up!

Very long explanation, I know, but still pretty cool now that we solved it, right? I love #friendlywaves — if you have any mystery wave pictures, please do send them my way! :-)

Marine energy: Tides. A guest post by Manel Camacho

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Today’s guest post is brought to you by Manel Camacho. Manel and I met on Twitter and bonded over our joint love of wave watching — me as a dedicated amateur, Manel as PhD student in marine energy. Today, Manel is giving us a glimpse into what marine energy is all about. Enjoy! :-)

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As a person that grew up in the coast of a tropical country will be weird for me to say that I was afraid of the sea during most of my childhood. I remember going outside of our parked car at the sand to meet the sea, was my 1st time and the day was warm; no clouds anywhere and no wind. Same moment an enormous wave came into my direction, I ran inside and never went out for all day.

Ironic as it is despite this fear, 25 years later I was doing a Ph.D. on marine energy. Marine energy engineering is the subject that involves the use of our oceans to produce electricity, the sea by itself it is involved in one of the biggest energy exchanges on the planet earth. The energy exchange involves the gravitational pull of the sun/moon and also the sun thermal radiation.

The first force that I will talk about causes a phenomena called “tide”, as we know the moon and the sun can exert a gravity force over the earth. The gravity force as we know tries to pull out any mass into its direction, as the continents and the sea bottom are made of dense rock they resist this pull; however the seas can deform more easily, the force will pull out the seas above its normal level. This rising and decay of the sea level is called “tide”. The difference on elevation from the low level to the higher level can be over half a meter in open ocean, when its pulled is called “high tide” ad when its not “low tide”. 

We might think that half a meter of difference is not too much, but when the ocean reaches a depth of more than 3000m is considerable. Comparing lets take a circle of 1 meter radius in the sea and lift it 1 meter, we’ll the same force to do this is to pull up almost 95 liberty statues just one meter above the ground.

Now that we know how much force is need it to pull that single cylinder of water, imagine again the force necessary to make an entire ocean to vary half a meter on level. The forces of the sun/moon will cause the ocean to bulge, then earth rotation will cause these bulges to move across the globe. When we are at the coast and we see the water rising at night and day, what we see is the water reacting to these forces creating this massive but imperceptible lumps in the ocean.

The water height increases more at the coast than in open sea, this will cause that certain parts to rise several meters at the “high tide”. These sudden increases on water will cause a flood on the coast, if it moves to the coast is called “flood” or away from the coast “ebb”. These flows will cause strong currents in certain locations, these currents work similar to the wind blowing; the currents produced then can be used to harvest energy, the easiest way to do it is using a device similar to a wind turbine.

Devices used to extract energy from the tidal stream currents are called, tidal stream turbines. These devices use knowledge learn from disciplines as wind energy, marine, civil and offshore engineering as naval, mechanical and aerospace areas.

As any new device to produce energy many problems need to be solved, problems related to: how it affects to the animals, to the people and its way of living, how they will survive in extreme weather and many many more. But thats something to discuss further.

#WaveWatchingWednesday

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A week’s worth of #WaveWatching pictures for you. Enjoy!

Starting off strong: I love living in Kiel!

Totally different vibe the next morning, looking very winter-y somehow.

And then another early morning on my way to the beach. Below you see the locks on Kiel canal from the bridge that crosses the channel. I really love this view but I have to admit — if the ferry ran this early in the morning already, I’d totally take it to cross the channel rather than cycling that bridge!

But arriving at the beach always makes it worth it!

Taking the ferry back home… Love this picture! A turbulent wake, a feathery wake, those clouds… What more could anyone want?

….and we are back on the next day! Waves breaking pretty much right on the beach because the beach profile has a step shape right at the water line. Looks surreal to have those long smooth waves, than a tiny bit of breaking, then nothing but sand…

All those bubbles in the white water of the breaking waves!

I actually took the picture below because of the birds’ wakes in the center. Weird how it turned out!

And here is one just because it’s pretty!

Always surprising how many fossils there are on the beach, even though this is the 5th day in 8 days of me collecting on the really short stretch from there to the lighthouse! How many more are hiding underneath our feet and we’ll never know?

Took a very similar picture as on the day before, but I love all the different parts of the wake, the clouds, the reflections. So beautiful and calming.

Running, seal watching, swimming in Kiel fjord, now my well-deserved coffee. Have a nice Sunday everybody!

On Monday, a colleague from GEO visited me to do an interview on wave watching. Amazing day!

Taking the ferry & admiring the wakes. Isn’t it fascinating for how long turbulence persists & wipes out any waves / prevents formation of new waves? Love the different surface textures!

Also fascinating how differently wakes look depending on weather conditions!

Love how dramatic this looks!

…and how turbulent patches are so smooth and reflect the building so well. And then a sharp boundary and we are back to the normal surface roughness of wind waves!

I can’t get over how fascinating this is!

Also love the pictures for their beauty. Below: After we had arrived at a stop, the ferry has just started sailing again (see where the wake changes between smooth and turbulent and then those large eddies of the propeller rotating for propulsion?)

Very nice pattern in the waves this morning, showing constructive interference (where the crests are high and the troughs are low) and destructive interference in between where the surface is completely flat. So fun to watch!

And that’s it! Hope you enjoyed and hope it inspires you to do some wave watching yourself! :-)

Wave watching in preparation of a guided tour! (Day 2)

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Woke up at 4:11 this morning and was AWAKE. So the obvious thing to do is catch the sunrise from the bridge across Kiel canal!

And I was lucky that there was quite a lot going on in the locks already, too!

Look at this beautiful wake! Early morning wave watching is really the best because the light from a shallow angle makes all the wave features stand out beautifully.

Ha — caught the wake reflecting on the side of the canal!

I love early mornings! Another example of wave features that come out really clearly in morning light but that would be really difficult to capture if the sun was higher up already.

Anyway, on to the beach! Aren’t you happy I got up this early?

Maybe I should make people get up for sunrise on my wave watching tour! Would definitely weed out all the people who aren’t 100% dedicated to wave watching. I might end up alone on the beach, but I would probably enjoy that just as much as I enjoyed it this morning!

Definitely a very dramatic sky again!

Yesterday I found tons of fossils on a really short stretch of the beach, so I wanted to see if that was just a fluke. Today, the beach looked very different! Remember how yesterday there was a band with small pebbles all along the water line? Not today! Think of all the fossils that the sea claimed back over night!

Still a lot of stuff to be found today, though. I especially love the non-fossilised fishy at the bottom of the pic below. Isn’t he cute?

But on to wave watching. With only these long waves present, it’s really nice to observe how the wave crests behave as they reach the beach. Below: Breaking, but simultaneously…

…already pulling back. That wave front looks so … unreal?

For your morning meditation, watch the movie below (sound on!)

The peninsula that connects the lighthouse with the beach makes for super interesting wave watching, because the waves on either side of this small strip of sand usually look very different. Today: the far, upwind side is again a lot rougher than the lee side in the foreground.
But also very cool to observe here is how the sand ripples underneath the waves are formed by the wave field.

The picture below was raken right at the edge where, in the picture above, the sand dam meets the lighthouse’s island (you see the reinforced edge of the island at the bottom of the picture) and the ripples clearly show where the beach falls dry in between waves washing over it (smooth, no ripples), where the wave field is mostly regular (regular ripples), and where there is chaos of waves being reflected back and forth (small messy ripples).

Moving on towards the right (away from the dam), ripples get more and more regular. You still see the reinforced edge of the island at the bottom of the picture.

A little further to the right still, and the regular ripples reach all the way to the edge of the island.

And walking a lot further to the right, ripples are long and regular (those are the dark structures — all the messy light structure is the sun ding weird things in the water).

And here is a cool movie of a wake arriving and meeting a shallow stretch. For details of what’s going on there, check yesterday’s post!

And there we have it. Those are the waves that can typically be observed at Falckenstein beach (that is — those are the special ones. Of course we’ll also do all the basics! But as a faithful reader of my blog, I’m sure you know all those already :-))

Wave watching in preparation of a guided tour! (Day 1)

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Had a really early start yesterday (today again, but those pics will come in a separate post :-)) because I wanted to cycle to the beach to prepare a guided wave watching tour that I’ll be running for Ocean Summit later this year.

Quick detour on the way — I never realized what a cool spot this is in Christiansprieß! I will be back! Love the combination of linden trees (very continental in my perception) and the industrial maritime vibe of the docks!

But let’s get on to where we want to go: The beach at Falckenstein is ideal for #WaveWatching newbies!

The lighthouse is on a peninsula, meaning that the waves on the upwind side…

…always look a lot more dramatic than the ones on the lee side.

But even though the wind waves on this side are a lot smaller, this side is by no means boring!

There are very cool things to observe here, too. Let me explain…

When there are no ships, this side is pretty much flat except for small wind waves.

But as soon as a ship passes by and creates a wake, the coolest things happen!

The wake travels towards the beach. So far so not especially exciting. Ship still passing…

BUT! Check out what happens. I’m first filming the wake as it is arriving in the distance and then… (Turn your sound on — suuuper relaxing! :-))

Isn’t it fun how the waves appear as out of nowhere when the water becomes shallow enough for them to feel the sea floor and become steeper?

 

Remember, those were straight wave crests traveling towards the beach, only they get wrapped around the shallow bit and now two sides of the same wave crest even cross each other’s path!

Love the cool pattern created in this way.

And there are several shallow spots where cool stuff happens. Another one here (again, the wake is arriving in the distance, and then I focus on where the waves are going to do fun things).

How amazing is this? (And look at the shallowest bit, where the ripples in the sand have been wiped away by the waves, whereas in the deeper bits they happily exist!

Again, straight wave crests being wrapped around a shallow bit.

So fascinating how the waves appear out of nowhere and then behave in such a way!

You don’t want to know how many of these pictures and movies I have on my phone, this is really a curated selection ;-)

Ok, one last one of this specific spot. Don’t you just love the pattern that’s just there because the water is really shallow in this one spot?!

Here is another movie. Love it.

Wave crests arrive almost perpendicular to each other, even they were one straight crest just meters further offshore!

This is seriously the last picture of that sort.

Have a regular beach pic to relax ;-)

There was actually something that distracted me from wave watching (and that does not happen easily!). Due to the fairly low water level, a whole band of pebbles was exposed on the beach. Usually I find this beach booooring — just fine sand with some shells on top.

Not today, though!

I really enjoy collecting fossils, and I have never found so many on this beach! Usually I only find one or two per trip there.

What I also noticed today was that there were some shells overgrown with these thingies.

Even though they looked fairly solid, they came off at the lightest touch, so I didn’t end up collecting one of those. But they looked cool!

Yep, perfect day at the beach!

Oh, you thought I was done wave watching? Here is an interesting spot as seen from the lighthouse (see my shadow at the bottom of the picture). Another shallow spot that bends waves as they arrive, ending up in waves crossing each other.

Like so!

And here is a video of what that looks like in motion:

And from the lighthouse island, you can see more cool wave pattern like this one where one wave field is coming towards us, and another one moving away from us (the wake of a passing ship, actually). Crests from both fields meet at almost a 90 degree angle, creating this zigzag water line.

Also, looking into the water reveals the cutest sand ripple pattern!

…and how the sand ripples get washed out when the water becomes too shallow and waves lap over them constantly. (Poor stranded jellyfish!)

There were SO MANY JELLYFISH!

Here is another view down from the lighthouse, showing how waves lapping on the concrete slaps radiate a new wave field.

And this I just thought looked cool.

Anyway, time to go home! Can you spot the lighthouse and the Oslo ferry in the background? (Obviously, the picture was taken for the nice wake in the foreground ;-))