Tag Archives: research cruise

Why are they so much slower than I thought? Observing the group velocity vs phase velocity of waves

Have you ever seen a speedboat drive past, looked at its wake moving torwards you, then gotten distracted, and when you look back a little while later been surprised that the wake hasn’t moved as far towards you as you thought it would have during the time you looked away?

Well, I definitely have had that happen many times, and the other day I was sitting on the beach with a friend and we talked about why you initially perceive the waves moving a lot faster than they turn out to be moving in the end. While I didn’t film it then, I’ve been putting my time on the GEOF105 student cruise to good use to check out waves in addition to the cool research going on on the cruise, so now I have a movie showing a similar situation!

But let’s talk a little theory first.

Phase velocity

The phase velocity of a wave is the speed with which you see a wave crest moving.

Waves can be classified into long vs short waves, or deep- vs shallow water waves. But neither deep and shallow, nor long and short are absolute values: They refer to how long a wave is relative to the depth of the water in which it is moving. For short or deep water waves, the wavelength is short relative to the water depth (but can still be tens or even hundreds of meters long if the water is sufficiently deep!). For long or shallow water waves, the wave length is long compared to the water depth (for example Tsunamis are shallow water waves, even though the ocean is on average about 4 km deep).

For those long waves, or shallow water waves, the phase velocity is a function of the water depth, meaning that all shallow water waves all move at the same velocity.

However, what you typically see are deep water waves, and here things are a little more complicated. Since phase velocity depends on wave length, it is different for different waves. That means that there is interference between waves, even when they are travelling in the same direction. So what you end up seeing is the result of many different waves all mixed together.

If you watch the gif below (and if it isn’t moving just give it a little moment to fully load, it should then start), do you see how waves seem to be moving quite fast past the RV Harald Brattstrøm, but once you focus on individual wave crests, they seem to get lost, and the whole field moves more slowly than you initially thought?

That’s the effect caused by the interference of all those waves with slightly different wave lengths, and it’s called the group velocity.

Group velocity

The group velocity is the slower velocity with which you see a wave field propagate. It’s 1/2 of the phase velocity, and it is the velocity with which the signal of a wave field actually propagates. So even though you initially observed wave crests moving across the gif above fairly quickly, the signal of “wave field coming through!” only propagates with half the phase velocity.

Usually you learn about phase and group velocities in a theoretical way and are maybe shown some animations, but I thought it was pretty cool to be able to observe it “in situ!” :-)

At the end of the rainbow you’ll find … home-made surface drifters

For Lars Henrik and Harald‘s GEOF105 class we are deploying home-made surface drifters on the student cruise. Today I had the opportunity to join the cruise again, and since the weather today made for beautiful pictures, I just have to share them here.

First, at the end of every rainbow, as we all know, you’ll find … home-made surface drifters!

Inga and Algot getting the drifters ready for deployment

Inga and Algot getting the drifters ready for deployment

The research ship we are on is the Hans Brattstrøm — cosy ship with a super nice and helpful crew!

We are sailing on RV Hans Brattstrøm

We are sailing on RV Hans Brattstrøm

The drifters themselves are equipped with a sea anchor made from a plastic bucket and four paint roller trays underneath a buoy, and then on top all kinds of equipment to make sure nobody runs over it: A safety flag, a lamp, a radar reflector. And, of course, the GPS sender!

Isn't it cool how those wave rings radiate from our drifter?

Isn’t it cool how those wave rings radiate from our drifter?

What we are using those surface drifters for? To determine the circulation in the fjord right outside Bergen. There are several things that might have an influence: Tides, wind, freshwater runoff from the land… And a tilted sea surface (although it is probably not as tilted as in the picture below…)

Drifter in front of RV Hans Brattstrøm in front of mountains covered in clouds

Drifter in front of RV Hans Brattstrøm in front of mountains covered in clouds

Another amazing day “at sea”, thanks for having me along, Lars Henrik!

Drifter in front of RV Hans Brattstrøm

Drifter in front of RV Hans Brattstrøm

Taking water samples

A big part of any oceanographic research cruise: Taking water samples.

Here is a group of students practicing how to arm Niskin bottles that will go into the ocean open on both ends, and that will then, when the whole rosette is on its way up again, be closed one after another at depths that promise to be interesting in terms of water properties.

Arming those Niskin bottles is actually not as easy as it looks, there is a strong spring going through the bottle, connecting the lids. And it is actually pretty painful if you accidentally close the bottles while some part of your body is between the bottle and the lid. Ask me how I know…

When the bottles are all open, the rosette can be lifted off the deck and into the sea.

Usually, rosettes are equipped with instrumentation in addition to the Niskin bottles, usually a CTD, measuring conductivity (to calculate the salinity from), temperature, and depth (actually measuring pressure, which converts easily into depth). I contributed to a very nice movie about how CTDs work a couple of years ago, check it out!

And now the rosette is finally in the water.

Water samples in physical oceanography are mainly used to calibrate the sensors on the CTD, which give (pretty much) continuous measurements throughout the whole depth of the water column. And that’s also what we want to use our water samples for — we have a hand-held conductivity probe that is right now producing values that cannot be correct. How we are going to deal with that? We (and you!) will find out tomorrow! :-)

Home-made surface drifters

A bicycle safety flag, a plastic bucket, four paint roller trays — what are those people doing there?! Until now this might almost count as kitchen oceanography!

Home-made surface drifters

But it’s only almost kitchen oceanography; at least my kitchen isn’t usually stocked with GPS trackers, which is what is mounted on this contraption. Let alone the research ship we used to deploy it. So this must surely count as real oceanography then!

Lars Henrik and students deploying a surface drifter to measure the surface current in a fjord

Lars Henrik and students deploying a surface drifter to measure the surface current in a fjord

Above, you see  Lars Henrik and his students deploying a surface drifter. The red buoy keeps it floating at the surface, the chain hanging below is heavy enough to make sure it stays upright. The bucket and four paint roller trays act as sea anchor so the whole thing moves with the water rather than being blown about by the wind. A safety flag, radar reflector and light make sure nobody accidentally sails over it, and the GPS sender lets the position be tracked.

For example like this:

Screen shot of the map and the drifter positions from my mobile phone

Screen shot of the map and the drifter positions from my mobile phone

Above, you see what it looked like when we had already deployed three of our four surface drifters (the red ones that are moving so slowly that the software tells us they aren’t moving at all), while the fourth one is still onboard the ship, moving to the position where it will be deployed (the green one moving at 3km/h).

Follow their positions on your mobile device!

Following surface drifters’ paths in real time is pretty awesome in itself, but what makes it even better is that the GPS positions can be accessed online from any device. Below, for example, you see the positions on my phone with the drifters behind it in the water (if you look really closely, that is. But they were there!).

My mobile phone with the drifters' positions and the drifters in the background

My mobile phone with the drifters’ positions and the drifters in the background

What you also see is that three of the drifters have huddled together after a couple of hours out in the fjord. Nobody really knows why yet, but that’s what we are here to find out!

Just from observing the wind and the movement of the drifters throughout the day, it seemed that the surface circulation in this fjord is dominated by the wind over the tides. But there will be a Master’s thesis written on the data we collected today (plus a lot more data and a regional ocean model!) so we’ll soon know how good my assumptions are and what really drives the surface currents here.

Three of the drifters huddling together due to currents that have yet to be understood

Three of the drifters huddling together due to currents that have yet to be understood

Come time to recover the drifters, the weather wasn’t quite as nice as earlier throughout the day. Just to give you an impression of the conditions under which the drifters were recovered:

Algot and Inga recovering a drifter

Algot and Inga recovering a drifter

Yep, if you look at the sea state, there is nothing to complain about, really, just a little water coming from the sky! But it was cold water… ;-)

And everything got recovered safely and made it back to port — ready to be deployed again tomorrow to gather more data and understand the fjord a little better. Exciting times! Thanks for letting me be part of this GEOF105 adventure, Lars Henrik!

The drifters coming home to the port of Bergen

The drifters coming home to the port of Bergen

“Wimmelbild” of a research ship

Wanna know why I am drawing a research ship “Wimmelbild”*? Check out the blog post over at our #SciCommChall blog to find out why!

And while you are there, why not join us in our #SciCommChall? :-)

*In case you are wondering what the translation English of “Wimmelbild” might be: No idea how to properly translate it! Apparently they are used in the “I spy” books in the US, in “Where is Waldo?” in the UK, sometimes called “busy pictures”, sometimes called “look-and-see” pictures. How would you call something like this?

Swell and wind waves

Sometimes waves are very regular and mostly of the same length. Those are the ones that I usually talk about when I talk about interference of waves. But of course, other times, there are different kinds of waves with different histories and different lengths, and those do interfere, too. For example in the picture below, there are long swell waves caused by a distant storm, and then small wind waves on top of those, caused by a local breeze.

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The really long swell you can’t even see in the picture, because waves with a couple hundred meters wavelength and just a dozen or so centimeters height are just really hard to photograph… But you get the idea!

Wavelets on bow wave

The other day (well, the other day when I was still at sea and wrote that blog post. Been quite a while since…), when sailing in calm waters, I noticed the wavelets of a bow wave.

And I cannot not see them these days! No matter how much the other waves try to disguise any trace the boat might be trying to leave to prove its existence, the bow wave wavelets put up a fight to be noticed.

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Below, you see the direction the ship is sailing in (yellow), the wash from the broken bow waves (green) and the wavelets that form the bow wave (red).

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And they look extremely pretty in the setting sun, too!

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If you like pictures like this, you’ll love my book! Stay tuned!

Watch the dispersion relation in action

Remember how we talked about how waves seem to propagate extremely slowly into that calm patch that occurs when a boat pulls away from a dock? Well, the other day I noticed that there is even more physics you can see when watching a similar situation: You see how long waves propagate much faster than short waves (that is for deep water waves, in shallow water the wave speed only depends on water depth, not on wave length)

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Here you see a formerly smooth patch of water where the Håkon Mosby was until a minute ago, and you see how long waves have already propagated into that smooth patch while shorter waves are everywhere in the choppy water around the smooth patch, but have yet to propagate into it. Now that I think of it I’ve seen this many times before, I just never noticed. It’s even visible in the video I posted with the other blog post.

And here is a video. Note how the long waves invade the smooth spot of water long before the shorter waves do:

Recovering an oceanographic mooring

So in my previous post we deployed a mooring (in fact, those pictures were from the deployment of several different moorings). Now how do we get such a monster back on board again?

Recovering a mooring is always slightly nerve-wracking, because even though we’ve tried very hard to forget about this possibility during the year the mooring was out there in the ocean, it is never 100% certain that we will actually be able to recover it. It might not be there any more, or it might be out of batteries. I have been on cruises where we have had to give up on recovering moorings, or on another one where we had to dredge for a mooring (and found it!). Luckily, on this cruise things went smoothly and the way they were supposed to:

A sound signal is sent and establishes contact with a releaser that connects the anchor with the rest of the mooring. After establishing the position of the mooring, a signal is sent and the releaser lets go of the anchor: The mooring floats up to the surface!

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Then, the task is to spot the orange floatation thingies in the waves.

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And bring the ship close enough to actually connect a hook to it.

 

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Once it is on the hook, it needs to be brought on board.

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Remember, it’s hundreds to thousands meter of rope we are talking about! Luckily the Håkon Mosby is (as all research ships are) equipped with plenty of winches and cranes and a super helpful, knowledgeable and skilled crew.

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Instruments are brought on board individually (or, in this case, a releaser).

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After having been out in the ocean for a year or two, they are sometimes overgrown with stuff. And in this particular case, that bio stuff was stinky!

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But pretty, I have to admit. I think it’s some sort of cold water coral. I think. Any biologists here?

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But there is always a lot of stuff to be recovered.

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And also pretty interesting: This is the first time I got a good look into one of those orange flotation thingies. I knew there was a glass sphere inside, but it was nice to actually see one. I had previously seen one that had imploded – it ended up pretty much pulverized. But so this is what we knock around on deck and throw out into the ocean:

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But yeah. In a nutshell, this is how moorings are recovered.