Author Archives: mglessmer

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

Rainbows in regnbyen Bergen

Yesterday when approaching Bergen airport, I saw something super cool: The lower half of a rainbow!

Even though I grabbed my phone and snapped a picture in record time, I didn’t manage to capture it. Bummer! But that doesn’t keep me from writing about it while showing you a “normal” rainbow I took a picture of a couple of minutes later.

Rainbow seen from a plane approaching Bergen airport

Rainbow seen from a plane approaching Bergen airport

Have you ever seen the lower half of a rainbow?

But can you imagine it? A u-shaped rainbow?

Have you ever seen anything like that before? It’s not something that we are used to seeing, at least not if we are looking a) at rainbows that are occurring on natural rain “curtains” and b) while we are on the ground. Let me explain…

Under perfect conditions, a rainbow is a full circle

Imagine you are a floating in space, looking at a curtain of rain drops. The sun is shining from behind you onto that curtain. What you then see on that rain curtain is a full rainbow circle, purple towards the middle and red towards the outside.

The size of the rainbow depends on how far away from the rain curtain you are. Imagine looking at the shadow that your head is making on the rain curtain. The line from your eyes to the shadow of your head will be our reference. Now imagine looking at any point on the rainbow. The line from your eyes to any point on the rainbow will be at a 40 to 42 degree angle to the reference line (40 degrees if you are looking at a purple point, 42 if you are looking at a red point, anything in between for the other colors).

Tweaking the size of the rainbow

Now imagine moving the rain curtain farther away. The angle between the reference line and the line to the rainbow stays the same, but the further away the rain curtain, the larger the rainbow. And vice versa: The closer the rain curtain, the smaller the rainbow!

So now imagine a nice curtain of droplets that you can walk towards and away from (sprinklers! garden hoses!) — the further you walk away, the larger the rainbow gets. And the closer you come, the more it shrinks again.

Standing on the ground, you only see the upper half

If you walk close enough to the rain curtain, you can actually see a full rainbow. But typically when we think of rainbows, we think of those occurring naturally, and then the rain curtains aren’t as neat and tidy as those from a sprinkler, and rainbows that we see are usually far far away, and thus really big. And that is why we aren’t used to seeing the lower half of a rainbow: Where the lower half would be there isn’t any rain curtain for it to appear on, because there is ground there! And the only way not to have the rainbow hit the ground is either have it close enough in front of us so it’s too small to even reach the ground, or to look at it from a plane that is high enough above the ground that even a large rainbow has enough space above the ground to fully appear on the rain curtain.

Next steps

So where do we go from here? I need to a) play with sprinklers and take pictures of rainbows! b) draw illustrations of the stuff I tried to describe above, and c) hope that I’ll be faster next time to finally get my u-shaped rainbow picture from a plane!

Thinking about the Doppler effect as of a boat sailing against the waves!

I can’t believe I haven’t written about this on my blog before, thanks Markus Pössel for reminding me of this great way to understand the Doppler effect!

Doppler effect, or why ambulances change their sound as they race past you

Doppler shift is everywhere, but it’s maybe not obvious how to imagine what’s going on if you think of sound waves.

But look at the picture below. Can you imagine the sound of those waves lapping against the shore?

Now imagine taking a speed boat riding out on the water. Can you feel how you are bouncing over the wave crests, and notice how you are meeting them a lot more often than when you were standing on the beach, looking out on the water?

Or imagine being a surfer, riding that perfect wave. You are staying with the same wave crest for a really long time, while in front of you creat after crest breaks on the beach.

Yes, the Doppler effect is as easy as this! As you are moving with or against the waves, their frequency changes. Totally obvious when you think about waves on water, right? But the same happens with sound waves, and in their case, a changed frequency means that the sound appears to change pitch. If the ambulance is coming towards you, the sound gets higher and higher, and then as it races away, it gets lower again. So now you don’t even have to look when you hear an ambulance, you know whether it’s coming or going! (Just kidding! Please definitely look out, anyway, and don’t get run over!)

Balances of dyed and un-dyed waters

Oh look, a plume of (almost) un-dyed water hitting the green lake!

I am really fascinated by the balance between green water leaking out of the pipeline and into the rain drainage, the rain falling on the lake, and the rain water coming into the lake through the rain drainage system. Right now, the water coming out of the drainage is a lot less green than the water in the lake, which is itself being diluted by rain. So much so that you can see a clear plume entering before it is mixed so much, entraining so much lake water, that you loose track of it in the green.

This makes me think about all kinds of stuff: how long between it raining on the catchment area that drains into the lake and the water actually reaching it? And how large might the catchment area be relative to the area of the lake (i.e. how large are the respective influences on the color)? So much entertainment just stemming from a little green dye :)

The first autumn storm and its impact on dye tracer and water level

Last night it rained a lot. So the first thing to do this morning was to check what that had done to my green lake!

The dye is now a lot more diluted, but overall it still looks surprisingly green seeing that there is a lot of rain water draining into the lake. To give you an idea of how much more water is going through now than when I last showed pictures of the green stream: Look at how clearly you see the inflow into the lake in the picture above! And remember the little waterfall in the picture below? There is a lot more flow now.

Another thing that has gotten a lot easier to see now is where the dye goes into the Kiel fjord. Because the flow rate is a lot higher, so the flow itself is clearly visible, independent of the tracer, but also because … well, there isn’t a lot of water left in Kiel fjord!

This is what it looks like this morning: That little stream is water from the lake going into the fjord. Usually there is about a meter more water here!

It looks actually pretty cool to see exactly what the sea floor looks like.

Even though there are no tides in the Baltic (well, hardly any), we do have some large changes in water levels sometimes. They are due to changes in wind or pressure; in this case there was a lot of wind last night that pushed a lot of water out of the Kiel fjord into the Baltic.

What typically happens now is that this water doesn’t stay away indefinitely, but once the winds stop, forms a “seiche”, a standing wave, with a period of a little more than a day.

Of course I am going to check if there is water back by tonight, and then gone again tomorrow morning! Assuming, of course, that the winds stay calm. Otherwise that would influence where the water goes, too.

What I found really interesting, too, is that I saw a lot of herons now that I’ve hardly ever seen in this part of Kiel fjord before. It makes sense — usually there is too much water so they have nowhere to stand — but it was still weird to see five at once, and more as I walked along the fjord.

And — at last! — it was possible to see from land what those two sticks in the water are warning about: The stone in the middle! I had never actually seen that before. Now I know! And now the water can come back; wave watching is more fun when the waves have slightly shorter periods than the seiche’s 27 hours… ;-)

…Update in the afternoon…

After more rain throughout the day, we now actually see a clear plume of the rain water going through the green lake, with a little mixing on the sides as the green water is entrained!

And some water is back in Kiel fjord. Phew. So there is wave watching to be done right away:

Below, we see a really nice example of waves changing their direction as they run into shallow water, since their phase velocity depends on water depth (more about that here).

Wave reflections

This still is one of my favourite kinds of wave field to look at. So calming. (Who am I kidding. All waves are my favourite waves, of course! I am addicted to wave watching…)

Here is a short movie for you where you see the crests coming in from the bottom right and then being reflected at the wall. Isn’t this nice? :-)

 

It’s all about the right equipment: That’s why I now own a UV lamp! I see a lot of fluorescent tracer spotting in my future!

Before I start gushing about my awesome new UV lamp (thanks for encouraging that purchase, Uta! :-)), some other updates on the state of green in the park across the road from my house (don’t know what I am talking about? Check out previous posts on the fluorescent dye tracer).

The lake is still bright green and very well mixed, similar to what it looked like in this post. But what is a lot easier to see now is the green water coming out into the Kiel fjord. It was very hard to see on the pictures I took the other day on our fluorescent night walk, and I didn’t see any by eye the first couple of days, but for the last days it has been clearly visible:

It’s still a lot clearer by eye than on the pictures, but even in these pictures you see the plume going out of the storm drain, don’t you?

In other news: my UV lamp arrived today and I am so excited!

So here is a water sample I took out of the green stream, photographed in normal daylight and then lit by my UV lamp. Pretty cool, ey? :-)

Who wants to come fluorescent water-spotting with me? :-)

Wave watching: Refraction and diffraction of waves

A little more wave watching, today with a focus on how waves change direction when they run into shallow water. Let’s look at this beautiful wave and see what happens when it reaches the shallow shore.

Above, you see the wake of the pilot ship, consisting of many wavelets that propagate as parallel wave crests towards the shore.

Below, you see that the wave is propagating at an angle to the shore (something around 45 degrees, maybe?). If you focus on the wave crest that is just offshore of that little obstacle in the water (curious enough, a piece of brick wall), you clearly observe that angle. But then looking at the next wave crest in-shore, it is almost parallel to the shore! Assuming that both crests come from the same wave field, so that the second one was in the same position as the other one only moments before (which I know it was because I observed it), something clearly happened between then and now.

Refraction of waves

Why do waves change direction as the water depth changes? As waves run from deep into shallow water, at some point they start to “feel” the bottom, which slows them down.

Or, more scientifically speaking, the dispersion relation for shallow water waves is a function of water depth: The shallower the water, the slower the waves. That means that if a wave crest is running on a slope with one side being in shallower water while the other one is still in deeper water, it will change direction towards the shallow water because the shallow side of the crest is slowed down while the deeper side keeps on moving faster, thus forcing the whole crest around a curve.

But in this picture series there is more to see: See how the wave crest gets deformed after it has passed that obstacle?

Diffraction of waves

This is a process called diffraction: The change of direction after a wave crest has passed either through a slit and then starts radiating from that slit as circle segments, or, in this case, an obstacle. The wave passing an obstacle is, in a way, the same as the wave passing through two wide slits which are very close to each other, only separated by the obstacle: The edges of the wave crest at the edges of the “slits” also start radiating out as circle segments!

One spot, so many things to observe!

And there are, of course, ships. What I wanted to show on this picture is a close-up of the turbulent wake of the ship, but it’s really difficult to see so I’ll let that pass for today.

And the picture below shows so much cool stuff: Waves radiating from that pylon. Ripples on the surface by a gust of wind. Wave crests getting a lot steeper as they run up on the slope. And, my main reason for posting: I really like how the wave is spilling as it breaks! :-)

Fluorescent night walk — following the stream through the lake into Kiel fjord!

Luckily some of my friends are crazy enough to bring the UV lamps and go on a night walk with me, following the green fluorescent stream! (Don’t know what I am talking about? Check out the previous posts (post 1, post 2) on why there is fluorescent dye in a lake across my street and why that is exciting)

Following the water

It looks very spooky when all of a sudden in the middle of a park you come across something looking like the picture below. Well, you would probably not come across it if you didn’t know where to look, but you get my point. And once you found it, you can follow it downhill.

But don’t let yourself get distracted by signs on the trees, someone is trying to lead you in the wrong direction ;-)

Because what we were looking for was, of course, the same lake I have been posting about today and yesterday, except now it looks like the picture below. If you thought it was creepy by day you know nothing of creepy!

Creepy, but also fascinating! Of course I have to inspect it more closely.

Below my hand holding the UV torch while I was looking at all kinds of critters in the water (poor things!)

Science is, of course, team work. Especially when you want pictures, too ;-) Thanks Maria and Tom for such a spontaneous and exciting adventure!

Below, Tom is shining the UV lights down the little water fall so we can take pictures.

And here you see the view from the upper lake down the water fall into the lower reservoir. Next time I will definitely not do such a fluorescent night walk without a tripod and a better camera than my phone!

It might have been a bit of a hassle to find if you didn’t know where to look, but since I know exactly where that lake drains into Kiel fjord, we could follow the fluorescent water out the storm drain into the fjord!

Here we are at the top of the sea wall, looking down, and you see eddies of fluorescent water coming out of the storm drain and into the fjord. Super cool to see that the flow was coming out on the edges of the drain, and that it was eddying. And that, even though there was not a large flow coming out, it could be seen quite far into the fjord, at least as far as our torches could still light the surface. Very very cool tracer oceanography! That was one exciting evening!! :-)