Category Archives: observation

Wave train

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When you look at waves, do you sometimes notice the train of smaller waves being pushed forward by the “main” wave? That has always fascinated me. Kind of like in the center of this picture:

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wave train

When we were sailing in Ratzeburg earlier this year, one day there were hardly any wind waves on the lake, so putting a foot in the water from a sailing boat resulted in exactly the phenomenon that had puzzled me for so long.

In the movie below you see it “occurring naturally” and then afterwards “created” like in the picture above. I’m pretty sure it’s the “group velocity is only half the phase velocity” thing, with small waves passing forwards through the group and vanishing, only to be replaced by waves coming from the back of the group. Is this what is happening here? Anyone?

Waves on a slope

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Earlier this year at Forscherfreizeit Ratzeburg – the summer camp at which Conny, Siska, Martin, a bunch of teenagers and myself spent a week sailing, exploring and playing with water – I spent a good amount of time staring at waves hitting the wooden boards that form the slip in the port. They create a nice slope with a very interesting structure, especially at the joints where the angle of the slope isn’t exactly the same.

Watch what happens when the wave approaches the shore (and focus on the left part of the picture, where it is clearer):

At first, it arrives pretty much as an ordinary wave.

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As it is running up the slip, you start seeing the structure of the boards below.

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As the wave becomes steeper and steeper, the front one is being slowed down more than the second one, because it is in shallower water (and we all know that the phase velocity of shallow water waves depends on the water depth, right?).Screen shot 2015-09-26 at 4.41.34 PM

Eventually, they form one steep wave and break.
Screen shot 2015-09-26 at 4.41.54 PM

Watch the movie to see it happen:

For more waves on a slope, check out these posts (Norway, Hawaii).

Fetch and wind waves

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Summer holidays in beautiful Cornwall – what more can you ask for? If only these pictures weren’t from last year… Anyway, one can dream.

And one can find awesome oceanography everywhere. Take for example the picture below. Do you see how the bow lines of the ships have enough slack to hang in the water?

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Mevagissey harbour

If you look closely, this is what you see: Wind waves being dampened out by the swimming ropes, and then gradually building up again on the other side of the rope.

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Wind waves

Oceanography is everywhere! :-)

Continuity

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Last week I went to beautiful Lüneburg with a group of climate scientists to continue working on a very exciting project I’ve been involved in over the last year or so (see “scales in the climate system” funded by CliSAP here). I so enjoyed being with a group of people who talk about converging solutions of discretized differential equations over dinner! I have really spent way too little time with people like that ever since I left oceanographic research and went for instructional design. So it was great to discover that I haven’t lost that side of my life but that I can still happily talk about climate models and eddy covariance measurements!

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Waterfalls in Lüneburg

But the “continuity” in the title of this post is actually referring to something else which I saw during a break we took. In the picture above you see the river going through Lüneburg, which is clearly going downhill, just like every good river should. You also see a couple of fronts, so clearly something is going on there. Watch:

[vimeo 138396707]

I find it super fascinating. Where does the water that comes down the wide waterfall go to if the sea grass (hey, I’m not a biologist! You know, the green stuff in the river!) is going towards the waterfall, too? Is there a vertical circulation involved? But then where does the water actually sink? Yet it doesn’t really look like it could all go in the current along the front. What is going on there?

Isn’t it weird how I always look for continuity? :-)

Curvature of the Earth

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In Ratzeburg, we very much enjoyed our daily early morning swims. One thing that is really nice to observe when you are swimming in a calm lake is how things vanish behind the horizon. Of course, you see the same effect when sitting on the beach or on a boat, but somehow it impresses me most when I’m either in the water or lying flat on my belly on a surf board.

This is what a buoy and boats look like that are fairly close. Even though your eyes (or the camera) are very close to the water level, you still see the water line on the buoy and the boats.IMG_2441

For buoys or boats a little further away it is very difficult to see the water line.IMG_2428

And in this picture you still see boats in the background, but clearly the lowest part above the water line is missing.IMG_2426

The boats in the picture above are maybe about half a kilometer away. Ignoring all effects of refraction of light and just looking at the geometry, the bottom 20 cm of the boats should be missing – which fits well with what we observe. Nice!