Category Archives: observation

Commissioned wave watching: Eckernförde edition on a beautiful calm and sunny Sunday!

Recently, more and more of my friends send me pictures of waves they spotted when walking along a lake side or taking a ferry ride. I love how contagious wave watching is, and I love sharing my fascination with you! :-)

Here are some pictures that Fred sent me of his lovely Sunday walk today. There are at least five interesting things that I notice in the picture below. How about you?

  1. Look at the beautiful interference pattern where two wave fields are almost perpendicular to each other, creating the checkerboard pattern! As you see in the picture below, there is one wave field coming in at a 45ish° angle to the sea wall, so its reflection is at 90ish° to the original wave field.
  2. In the background you see the surface roughness changing and the water seeming darker where there is a breeze going over the water, creating small ripples that reflect the sky in a different way than the smooth surface closer to us.
  3. See the waves the seagull made where it landed on the water?
  4. Looking at the foreground, do you see the tiny ripples that show up not so much on the surface of the water, but rather at the sandy ground, because they focus the light?
  5. And notice how you can look into the water in the foreground but not in the background? That’s the awesome phenomenon of total internal reflection where, if you look at water at an angle that is smaller than a critical angle, you cannot look into the water any more but just see light reflected at the surface! One of the things I never understood we had to learn about in school, but that I find super cool now.

And in the picture below, what do you see?

What I find most interesting in the picture above is how the reflection of that storehouse tower looks different in areas with different surface roughnesses. Where there is a breeze on the water in the background and in the foreground, it’s a lot more spotty than in the calm and smooth surface in between. And the checkerboard waves pattern (now you see the seawall that created the reflection, btw) carries through to the reflections, too, with the blue crisscross going into the white area where a cloud is reflected.

And then the phenomenon of total internal reflection is really clearly visible here with a lot of reflections on the water (or just more interesting things to reflect than just a blue sky in the previous picture) and a view down to the ground only in the very foreground of the picture.

Thanks for sharing these beautiful pics, Fred!


How to use home-made surface drifters to teach oceanography

Very early knowledge about oceanography stems from beach finds that had to have been transported to that beach from far away because the finds themselves (pieces of trees, or coconuts, or whatever) were not native to their finding places so the ocean must have provided a connection between their place of origin and the beach they ended up on. And in early oceanographic research, messages in bottles or even wood pieces marked with identifying numbers were deployed at known times and regions and then recovered wherever they made landfall to get a better idea of ocean currents. And as oceanography got more and more sophisticated as a discipline, such lagrangian (i.e. current-following) data has become an important part of oceanographic research, especially over the last two decades with profiling ARGO floats.

Position of 3930 ARGO floats that were active in the 30 days before January 18th, 2019. Source:

ARGO data is available to anyone and, via its Google Earth interface, easily accessible in teaching. But of course this is only a passive resource, you cannot deploy drifters wherever you would like for teaching purposes. Now imagine if you had cheap drifters* available for use in teaching, how cool would that be?

Last year I was involved in discussing the design of home-made surface drifters and later got the chance to join the student cruise (as part of Lars Henrik and Harald‘s GEOF105 class at the University of Bergen, Norway) where the drifters were tested, both in their functions as drifters and as a teaching tool. They are an amazing addition to the student cruise and a great learning opportunity! But there are also a lot of challenges that arise when with working with drifters — or opportunities to think about interesting problems! What more could an instructor (or a student!) want? :-)

Building home-made surface drifters

While in our case the drifters were developed and built before the class started, discussing design criteria with students would be a really interesting task in an applied oceanography course. The design we ended up working with with is described here.

Building those relatively cheap drifters provided us with the opportunity to have students handle them to learn to use oceanographic instrumentation without them, or us, being too concerned about the welfare of the instrumentation. It also provided us with a fleet of four drifters that we could deploy and recover on four day-long student cruises and have them right in the vicinity of where we were taking Eulerian measurements at the same time, so we would end up with a complementing data set and could discuss the benefits of each of the two kinds of measurements and how, when they come together, they tell a much more interesting story than any of them could on their own.

Where to deploy the drifters

If you have a limited number of drifters available (four in our case), you have to think long and hard about where to deploy them. Of course you can just dump them into the water anywhere and see where they end up. But in order to figure out the best spot, it is really helpful to have a clear idea of what influences the currents in the regions you are interested in, and what path the drifters might take, depending on the location of their deployment.

On the three first days of the student cruise, we saw the drifters move against the predicted tidal current (“predicted” tidal currents, because we didn’t look at direct observations of the tidal current, so we don’t actually know if it is behaving the way the prediction predicted) and, at times, also against the main wind field. Nevertheless, we expect the wind to have a large influence on the flow in the surface layer, hence the day at sea starts with a briefing on the weather forecast.

Students presenting the weather forecast for the cruise day in the ship's messe

Students presenting the weather forecast for the cruise day in the ship’s messe

In addition to thinking about a deployment strategy for specific weather conditions, it is helpful to think about how trajectories from different days will be compared to each other. Therefore we chose to deploy on two sections over four days, thus repeating each section twice.

How to track your drifters

There are many ways to track drifters. In the early days, acoustic signals were used to know where drifters moved within an array of sound sources. These days, most tracking is done using GPS. In our case, we used readily available GPS tracking units that were then mounted on the drifters (see below).

GPS units being fixed to the drifters onboard RV Hans Brattstrøm

GPS units being fixed to the drifters onboard RV Hans Brattstrøm

Looking at the features of the GPS units we used, they were apparently mainly designed to tracking cars when you’ve lend them to your kids. In any case you can set alarms if velocities are too high, if they leave a pre-defined area, etc.. Interesting to see what kind of products are on the market!

Looking at how to track the drifter, i.e. the specifications of the GPS sender, might also be a very interesting exercises to do with students. How often should it “call home”, what battery lives are needed, how will the data be transferred, where and how can it be accessed, stored, processed?

How to deploy your drifters

Even when you know where to deploy the drifters, that doesn’t tell you how to deploy them. And even from a small research ship like the Hans Brattstrøm it is not immediately obvious how to do it.

Deploying a drifter

Very good reality check on how difficult it is to get instrumentation in place to measure oceanographic data!

How to interpret your data

Speaking of oceanographic data — how do you actually interpret it? Below you see a snapshot of our four drifters in action. This is actually on of the more interesting times when it comes to velocities: We do have two drifters moving with 4km/h and then one with less than 3km/h (which shows up as not moving because of some algorithm in the website). But what does this actually tell us?

Position and approximate velocities of our four drifters at the end of day 4

Interpreting drifter data becomes very difficult very quickly when you are in a flow field that changes over time. We did have the tidal forecast and the wind forecast, but both only in a coarse resolution in space and time and so it gets really difficult to imagine how they might have influenced the currents and thus the trajectories of the drifters!

How to protect your drifters from damage

Even in a fjord that is sheltered from the wind and big waves of the open ocean, the sea is still a harsh environment and large forces will act on the drifters. If we want to be able to recover the drifters in one piece, we have to make sure that they are actually sturdy enough to stay in one piece.

One of our drifters capsized for unknown reasons. Luckily Algot was still able to recover it!

Another point to consider is how much buoyancy a drifter will need to stay afloat, yet to be submerged enough into the water to actually follow the surface current rather than being pushed through the water by winds, or pushed over by the winds as the one above.

How to find your drifters again

As we think about how to protect the drifter from damage, we also need to think about how we can make sure the drifter stays upright so the GPS antenna stays above the water level. Even with fairly good visibility and low waves, and despite the brightly colored flags and radar reflectors on the drifters, they were pretty difficult to spot!

Even though we can see the drifter’s position through an app on my phone, it is really difficult to spot it out on the water!

How to recover your drifters

Even on a small vessel like the one we used for the student cruise, the water is actually pretty far away from where you can stand on the deck, so recovering a bulky and heavy item out of the sea is not as straight forward as one might think!

Technician Algot and a student recovering one of the surface drifters

Making sense of your drifters’ trajectories

This is not something I can cover in this post, of course — it’s what Inga will do for her Master’s thesis. Below, you see her plotting trajectories from the four days together with the predicted wind fields of the respective days.

, and think about how we can make sure the drifter stays upright so the GPS antenna stays above the water level.

Inga looking at analyses of the drifters’ trajectories which she will explain in her Master’s thesis

But there are several aspects I find especially interesting for discussions with students:

  • At which depth range did we place the anchor of the drifter, i.e. what “surface current” are we actually tracking, the real surface, or an average over the top 0.5 meters, or the top 1 meter? And what would “average” even mean? Or something else?
  • When we have Eulerian data from, say, tidal gauges, weather stations, etc, how do we bring those together with the Lagrangian data provided by the drifters?
  • Knowing what we know now, what could we learn for future deployment strategies?

There are so many super interesting questions to be discussed using this fairly inexpensive instrumentation that it is a great opportunity that should not be missed!

*of course, ARGO uses profiling floats that actively measure data and send them home, whereas we use surface drifters that only send their position and nothing else. But maybe we can mount data loggers on them next time? :-)

Forget about big waves, today we are focussing on the details!

Today we are focussing on tiny waves right near the shore inside the sheltered harbor. See how below there are two wave fields, one with longer waves with crests that are parallel to the water’s edge, and then shorter ones propagating at a right angle relative to the first field?

Where the rope swims on the water you see how the short wind waves are stopped and only start forming again at a distance downwind of the rope.

The same here: Where there are ropes floating on the water, the water’s surface looks a lot smoother because the wind waves that propagate perpendicularly to the ropes are erased. But there are some wave crests parallel to the rope, formed by the rope hitting the surface and being pulled out again!

Below, the ropes don’t actually touch the water’s surface, but we have cool reflections of waves with crests parallel to the two walls that form the corner. The water level is right at the height where there is a little ledge on the wall that gets flooded with wave crests arriving and then falls dry during wave troughs. This causes this cool pattern of wave crests that seem to be interweaved right at the corner.

Sometimes looking really closely at small scale pattern is even more fun than looking at the sea and all the big and flashy (or splashy?) stuff going on there!

More wave watching, this time in Kiel

Beautiful morning arriving back in Kiel… Looking downwind, the weather might seem pleasant (especially when focussing on the sunrise).

But looking upwind however, the wind rows on the water as well as the white caps on the waves indicate that it’s quite windy!

Very cool: the turbulent wake of a ship interrupts the wave field and therefore, with its different surface roughness, is clearly visible!

And below you see so many things: The sand bank running from the lighthouse towards the next headland becomes visible as waves are breaking  on it. The turbulent wake of that blue ship we saw above already is still clearly visible, as is its V-shaped wake. And you see our own wake as the feathery pattern that runs all the way from the bottom edge of the picture to way behind the blue ship!

And here our own wake becomes even more prominent as we turn. Laboe in the background…

Here is another ship, waiting to enter the locks of the Kiel canal. It’s moving only very slowly (so hardly any wake visible), but you see how it’s sheltering the water from the wind so the downwind water appears completely smooth right at the ship!

And here are some more wakes and sheltered spots of water surfaces. Locks of the Kiel canal in the background!

And another look at the locks. Do you notice how the wind rows still indicate that it’s quite windy, but how it’s a lot less windy than it was further out?

And then we are in the Kiel fjord. This is the upwind shore — see how waves are only slowly forming and building up with longer and longer fetch?

And then in the sheltered port a different kind of waves: Our bow propellers mixing the inner Kiel fjord!

Sunset wave watching in Gothenburg. Wakes under different light conditions!

Wave watching from high up gives you a whole new perspective on wakes, and depending on the lighting, features in the wave field become more prominent or fade away.

See for example below the ferry: You very prominently see the turbulent wake right behind the ship, and you see the waves of the wake opening up in a V-shape.

Above, there is still a lot of ambient light from the sky. Below though, the same ferry, similar spot, 30 minutes later: The turbulence is a lot harder to see since colors fade away, but the V-shaped wake becomes really clear since one slope of the waves reflects the city’s lights while the other reflects the darkness.

Another ferry coming in, another wake… Below the surface roughness becomes clearly visible with the turbulent wake right behind the ferry and the bow waves fanning out.

That was one brilliant mini cruise! Thanks for joining me, Frauke, and for staying out on deck with me — despite the freezing temperatures — until we were far out of the port and the light was gone completely. The sacrifices we bring in order to wave watch… ;-)

Wave watching on Kiel Canal: Bulbous bows and how they shape the wave field

Even when I fully intend to just go for a Saturday afternoon walk to catch up with a friend, this is what happens…

I get distracted by waves. Like the crisscrossing pattern of waves and their reflections that you see below.

Or the amazing bow waves of ships passing by. Isn’t it fascinating what a huge amount of water is displaced by the ship’s bulbous bow, piling up into a mountain in front of it, then the sharp dip where the actual ship begins? (If you want to read about why ships are built with a bulbous bow, check out this old blogpost).

Having a bulbous bow alone does not always lead to the same bow wave. Which is fairly obvious when you think about it, of course the speed of the ship or the shape of the bow influence the wave field that is created, but also how heavily the ship is loaded, i.e. how deep the bow is in the water.

What you can see very nicely on the sequence of pictures of bows and bow waves in this post are bulbous bows going from fairly far out of the water (above) to fully submerged (towards the end).

And I just love the sharp contrast of the smooth water piling up and then the turbulence and breaking waves right there. Interesting example of subcritical and supercritical speeds, btw: The ship travels faster than the bow wave (so the bow wave can’t overtake the ship, but always stays behind it, forming a two-dimensional Mach cone).

The ship in the picture below is the odd one out in this blogpost: It does not have a bulbous bow but just pushes water in front of it. This is an interesting example of a bow shape that is clearly not optimized for energy efficiency when traveling large distances, but then the purpose of that ship is obviously a different one. But isn’t it amazing how such a small ship creates waves larger than all the other much bigger ships do, just because they have better bow shapes?

But beautiful wakes nonetheless. I love those tiny ripples riding on top of the wakes!

And, of course, the checkerboard pattern of a wave field and its reflection.

Here is another example of a ship with a bulbous bow, this time it is almost submerged. Since they are designed to be fully submerged, this ship is loaded in a way that is closer to what it was made for, and you see that the generated waves are smaller than the ones in the pictures up top.

And look at its wake — really not a lot going on here, especially when compared to the much smaller ship a couple of pictures higher up in this post!

Now for a ship that is hardly creating any waves at all, the mountain of water that it’s pushing in front of its bow looks especially weird since the bulbous bow isn’t visible any more.

See? (And isn’t it cool how the chronological order of pictures in this post just coincided with ships laying deeper and deeper in the water? I love it when stuff like that happens :-D)

And then, of course, I had to include some more pictures of beautiful wakes…

Do you see, comparing the picture above and below, how the first one was taken when the wake had just reached the shore, and the second one the wake was reflected on the shoreline already?

Not many things make me as happy as wave watching :-)

P.S.: Ok, one last bonus picture (non-chronological, we saw it some time during the walk. But that’s ok, I wasn’t going to include it until the post was already done and I decided that you just HAD to see this): Someone who is clearly not using their bulbous bow to their advantage. But at least I get to show you what they look like when they are not in the water. And we got to speculate about how annoying it is to be on a ship with such a strong tilt all day :-D

Minimal distractions from #wavewatching on a foggy Sunday morning in Kiel

I’ve been wondering. Are foggy mornings where all you can see are waves (and a couple of seagulls) opportunities on which everybody else sees the world like I always see it (i.e. mainly focussed on waves), or do “normal people” just see the seagulls and, well, fog?

Below, for example, the first thing I notice are the wakes. And then possibly the birds creating them (since, of course, I am interested in what caused the waves) and the algae growing in the water (because I am looking down to see if there are more waves to see). But even looking at the picture now, I focus on the wakes and on how amazing it is that you see the middle one going all the way across the picture. And that the seagull in the front only started swimming at the middle of the image, since that’s where its wake starts.

I do like to have some birds or structures in my pictures for visual interest, but the reason I took the picture below was to show how foggy it actually was this morning. And how calm the Kiel fjord was with hardly any waves and therefore very nice reflections of even the flag pole. That is very unusual.

And then, in the image below, I finally spotted the Sweden ferry coming into Kiel port. I knew it had to be there because I had heard the engine for a couple of minutes already and it sounded very close, but it was nowhere to be seen. Can you spot the blue underwater hull and possible the blue writing saying Stena Line at the lower end of the upper third of the picture? But the reason I took that picture was mainly so that, when talking about the wake later (and of course I set out to take pictures of that wake specifically. Yep, that’s how I structure my Sunday mornings), I would have a reference for how foggy it was.

Because as much as I like watching ships, what I am even more interested in are their wakes. I really like how the otherwise calm surface clearly shows very detailed reflections that are only distorted by the wake. Look for example at the little life guard stand and its reflections — the poles holding up the roof are reflected on at least four different waves, repeating that very distinct part of the structure in different places depending on the surface slope of the wave. Or, to the left of the bottom reflection of that life guard stand, the reflection of the railing that seems to be mirrored. How awesome is that?

But then I also really like interferences of waves as shown below. Little seagull and its wake riding on the remaining wake of the Sweden ferry…

Or little random wave rings radiating out from a rock that is submerged and then resurfaces with each wave reaching it.

And what I find super fascinating in the picture below is how you see the longer waves in the undulations of the dark reflection of the structure of the jetty (careful though, it would never be a straight line since each individual pillar and the shoulder on top of which the actual gangway sits creates a little edge in the reflection), and then the short wavelength waves creating a noise on top.

And then, last but not least, the thing that I love to look at every single time: the waves and their reflections on the sea wall, creating a crisscross pattern.

Now imagine you had been on this walk with me. Would you have seen all the wave stuff, I saw?What are the kinds of things that you would have noticed that I clearly did not (since I did not mention them here)? 

I see a new hashtag in my future #oceanographyofpuddleswithreflectionsofElbphilharmonie

There is a theme these days: one of Hamburg’s most famous sights, the concert hall called Elbphilharmonie.

And not only is it a pretty impressive buildings, it’s located right at the port of Hamburg, and, more importantly, right across from my new work place!

Yesterday morning there was a lot of ice on the puddle in front of it (above), this morning, by the time I got there, it was crushed (below).

And when I left tonight, there was some drizzle in the air and I did not feel like checking on the puddle.

Are you going to see this every day now? No, fear not — there are some cool blog posts in the pipeline! :-)

Here is to new beginnings!

From dawn til dusk (which wasn’t actually as long a time as it sounds ;-)), first day in my new job as programme manager of the citizen science project on biodiversity “GEO-Tag der Natur“. I am looking forward to great views on the way to and from work! And I am suuuper excited to be starting this job! I will tell you more about it once I had the chance to settle in a little.

Even though wave watching is not part of the job, I could not help but notice those puddles. Not only because of the reflections of Elbphilharmonie on them, but because when I arrived there was a little ice on them (see above)! Which was gone when I left, but there were some tiny wind ripples (see below). Which one do you think is more beautiful? I can’t decide!

Where has all the water gone?

You might have noticed that in today’s first post there was a lot less water in the Kiel fjord than in yesterday’s post (starting this year strong on the blogging front! I like it! And don’t worry, I won’t be keeping up this pace :-D). But look how little water there actually is!

In the picture above you see two navigation signs that are usually necessary there, because the rock in the middle is submerged far below the water surface. But not today!

And also in these locations you would typically see water coming all the way to the sea wall and sometimes even higher than that. So what happened? Strooong winds!

And the even more interesting thing will happen in a day or so, when the winds die down and all the water that got pushed out into the Baltic Sea comes rushing back into Kiel Fjord! Unfortunately I will most likely not be able to document it due to travel. Someone should pay me for documenting important oceanographic events in Kiel Fjord all day every day :-)