Can you do a bottom Ekman layer demonstration without a rotating table? That’s the kind of challenge I like :-)
The way I’ve previously showed bottom Ekman layers is by spinning up a cylindrical tank on the rotating table until it reaches solid body rotation, adding dye crystals to visualise the circulation later, and then stopping the tank to create friction at the bottom (and the sides, but we are mainly interested in the bottom since we want a bottom Ekman layer) as the water continues moving but comes under the influence of friction. But what if we just invert the whole thing?
Move the “bottom”, not the water
My initial idea was to use a Lazy Susan (you know, the kind of tray on a swivel base that you can use for your jam and honey etc on your breakfast table, but which you shouldn’t turn too rapidly (ask me how I know)) and to have a cylindrical vase sit on it, which will then be put in rotation and will rotate around and under the (initially still stagnant) water. The friction with the moving vase will then lead to a bottom-intensified circulation.
Problem here: While I have a Lazy Susan at home as well as a vase that would work as “tank”, I am currently in Bergen where I don’t have access to my own equipment. Instead, though, I have access to a rotating table in GFI’s basement which I used to simulate my Lazy Susan idea (Cool, eh? Simulating a non-rotating-table situation on a rotating table ;-)).
The physics themselves obviously work in this setup. However, they will be really difficult to observe for several reasons:
Scales. A small dish (like the one I used; for comparison see the usual tank in the background in the picture above) makes it a lot more difficult to see what’s going on, and in my case the circulation is quickly influenced by the sides of the dish (which is obviously not what we wanted).
Rotation. It’s not difficult to set a Lazy Susan into rotation, but I imagine it will be quite difficult to keep it at a constant rotation for any length of time. But you will only see the nice spiral for as long as you keep the rotation constant. As soon as it changes, so will your currents and that will be clearly visible in the dye (which is why you put it in in the first place).
Documentation. If you want to document your experiment, if want to have your cameras co-rotating with the Lazy Susan, it’s going to be quite difficult to install them (but maybe you would just want one that sits stationary above the center of rotation? That would obviously be easy to do with a tripod)
So all in all: it was a nice idea, but either I haven’t thought it through well enough, or it is a whole lot easier to do with a rotating table. I would imagine that it’s quite hard to observe when you don’t know very well what you are looking for, so it is unfortunately not useful as a demonstration to introduce people to the topic. What do you think? Any suggestions on how to improve this and make it work at home?
How will lee waves look differently if we are using the asymmetrical mountain instead of the symmetric one? And is symmetry actually important at all or are we just looking at different slopes downstream while the upstream slope doesn’t have an influence on the wave field?
There are a couple of reasons why I had not done that before:
It’s longer (1.5 m instead of the 1 m of the other mountain), therefore the tank is, relatively speaking, shorter. And since being close to the ends of the tank leads to weird interferences, this limits the distance over which observations can be made
Since it’s asymmetrical, pulling one way or the other would likely show different wave fields, so you couldn’t just run it back and forth and have students observe the same thing several times in a row
But then it would be really interesting to see what the difference would be, right?
I tried two different stratifications.
Weak stratification, shallow water
Since I just wanted a quick idea of what this mountain would do, I used leftover water I had prepared for the moving mountain experiment. Since there wasn’t a lot left, I ended up with 11.5 cm fresh water, but only 4 cm salt water at approximately 20 psu (since I stretched the 35 psu a little).
What I noticed: A LOT more mixing than with the other mountain! Stratification is pretty much destroyed after the first run, usually we run back and forth a lot. This can be for several reasons:
The water is very shallow, meaning mixing is happening over the whole water column. It might not actually be more mixing than in the other case, but since it’s affecting the whole water column, it might just seem like more because no clearly visible stratification is left above and below the layer which is mixed by the mountain?
The left side of the mountain was bent up a little (as in 2 or 3 cm), meaning that especially on the way back it was flapping up and down on the upstream side, doing a lot of mixing that wasn’t due to the shape of the mountain, just of bits of it being loose.
And the shape of the “reservoir” that is being built up upstream of the mountain is different to what I have observed before: Running in either direction, the reservoir didn’t built up smoothly, but as a hump that was pushed in front of the mountain. Maybe because the internal wave speed in this case was very close to the speed of the mountain, something like 7cm/s, so the disturbance created by the mountain couldn’t propagate upstream. Is that an upstream hydraulic jump we are seeing there?!
Since I was not satisfied with this at all, I ran a second experiment:
Strong stratification, deep water
First, I tried to set up the same stratification as for this lee wave experiment with the symmetrical mountain because I thought that would be easiest to compare. But I aborted that after having moved the mountain just a little because it was mixing so much that there stratification was destroyed completely and nothing could be seen. I ended up putting more dense water in and ended up with 12 cm pink (35 psu) and 4 cm clear freshwater. And this is what this looked like:
You now see a wave train with wave lengths longer than in the symmetrical case. Probably due to the longer length of the obstacle (even thought the waves are still shorter than the obstacle)? Or what sets the wavelength?
This time, with a faster internal wave speed of around 10cm/s while the mountain is still pulled with 7cm/s, we don’t see the “hump” in the upstream reservoir — the signal can propagate faster than the mountain and thus smoothes out.
So that is what I think is going on here. While the first experiment mainly showed effects of the stratification compared to previous experiments, the second one might provide some insight on the different slopes of the mountain, although I am not sure in what way. Do you see something I didn’t observe? How would you expect the different slopes to influence the lee waves?
I am so glad I tried this and I’m looking forward to thinking about this more! :-) Any insights you’d care to share with me?
This blog post is meant as guidelines if someone other than me might have to set up this demonstration at some point… Have fun! :-)
Setting up the stratification
If I am working fast and nothing goes wrong, this takes almost 2.5 hours. Make sure you have enough time to set this up! Filling the tank takes time and there is not much you can do to speed up the process once you’ve started…
Fill in what will end up being the top layer: 5 cm at 0 psu. For this, connect the tap to the bottom inlet in the left corner of the mountain with one of the hoses. When you are done, make sure to close the lock at the tank!
Move “mountain” over inflow to contain mixing to the volume underneath the mountain (better for your nerves, trust me)
Prepare the future bottom layers one by one (35 cm at 35 psu). We will need four full fillings of the 80l barrel (which doesn’t empty all the way because the tap is slightly elevated from the bottom, in case you were calculating ;-)), each with 2.8kg salt dissolved in it. To prepare that, connect the hose from the tap to the outlet of the barrel, put in the salt, put in the dye, use a paddle while you fill the barrel with water to stir. This way the salt will be pretty much dissolved by the time the barrel is full.
Note: Make sure the barrel is located high enough so that gravity will pull the water down in the tank from the barrel!
Note: When the barrel is filled, close the lock at the barrel before disconnecting the hose to reconnect it to the tank!
Fill in the bottom layers into the tank one by one. While one layer is slowly running into the tank, you have time to measure the salt for the next one.
Pulling the boat
Here is a sketch of the contraption that pulls the boat:
Put 4 or 5 gram in the little zip lock bag (called “weight” in the sketch above). This only works when the ship
Set up bumper to stop the ship before weights reach the floor (too much slack on the line, line might come off pulleys)
Stern rope on one of the tank’s braces is set up so the line is stretched as far as it can safely go
Check that there are marks on the tank which help measuring the speed of the boat (6 marks over 3 meters work well)
If there is suddenly too much friction in the system, check: Did the pulley on the left edge of the tank fall down? Did the rope come off the pulleys (sometimes happens if there was too much slack in the system, e.g. if the bag has been lifted or the bumper is too far left)
If the boat is moving a lot faster in the beginning than in the end, even though waves haven’t caught up with it, and it bothers you, move the two fixtures that hold the line at the ceiling closer together. Ideally, they should be in the same place, but this didn’t work for us because of tangling lines. Compromise between “constant” force and being able to run the experiment at all…
Ask students to observe:
Speed of the boat (actually take the time for a set distance)
Development of the boat’s speed over time, especially when waves are catching up with it
Generation of internal waves. Is there one, are there many? What are their wavelengths and speeds?
Generation of surface waves and their size relative to the internal waves. Why?
Below are movies of a couple of experiments which you could use in teaching instead of running the experiment for real (if for some reason running the experiment is not possible. But I would totally 100% recommend doing the experiment!). For a fun video, watch the one above (the ones below are cut to only show the tank so might be a little boring less exciting ;-))
Ship pulled with 5g in the bag
Ship pulled with 4g in the bag (for a repeat, see experiment 4!)
Ship pulled with 3g in the bag
Ship pulled with 4g in the bag (again, because we like repeat experiments ;-))
This blog post is meant as guideline if someone other than me might have to set up this demonstration at some point… Have fun! :-)
Lee waves are the kind of waves that can be observed downwind of a mountain in the clouds, or downstream of an obstacle in a current as a series of undulations with crests parallel to the disturbance.
Why move the mountain?
Students sometimes find it hard to imagine that a moving mountain should be equivalent to flow across a ridge. It helps to discuss how it would be really difficult to set up a flow in a tank: A huge amount of water would need to be moved without too much turbulence. Instead, it’s a lot easier to imagine the water is moving by moving a mountain through the tank, so the water is moving relative to it if not relative to the lab.
The size of the tank is 60×1.5×5 dm, so it can hold a total of 450l of water.
The mountain we use is 10.5 cm high and 1 m long and it’s symmetric, so pulling it either way shows similar lee waves (which is why I’ve always used it). There is a second, asymmetrical mountain on the shelf that I have never used*.
Setting up the stratification
The stratification that we’ve found works well is 10 cm at 35 psu (here dyed pink) and 9 cm at 0 psu. This leads to an internal wave speed of approximately ~11cm/s.
Prepare the dense water in a barrel that sits high enough so gravity will bring the water down into the tank (see picture below). For the 80l barrel, you need 2.8kg of salt and 1/3 tea spoon of dye MAX.
Elin’s GEOF213 class observing lee waves
You achieve the stratification by filling in the fresh water first through the bottom left inlet, moving the mountain over it, and filling in the dense water. That way the mixing is contained to the volume underneath the mountain which will be a lot better for your nerves (believe me!).
Moving the mountain
The system that pulls the mountain can go at two speeds: “fast” and “slow”, “slow” meaning 5m in 1:11min (7cm/s) and “fast” meaning 5m in 0:36min (14cm/s).
Here is where you run the mountain from:
Troubleshooting if the mountain doesn’t move:
you might be trying to pull the mountain in the wrong direction (into the wall)
the mountain might not be located on the sledge well. There is a tongue on the sledge that needs to sit in the groove in the mountain
the mountain might not be sitting well in the tank so an edge digs into the side
the belt that pulls the tank might not be tight enough (always make sure the two weights at both ends of the tank are actually hanging down to put tension on the belt!)
the belt might have come off the axle that drives it (the white plastic above the left end of the tank)
Elin’s GEOF213 class observing lee waves
As you see in the pictures above (or the movie below), there is a lot to observe!
Lee waves (not one, but a whole train!)
Different flow regimes: supercritical shooting down the lee side of the mountain, then a hydraulic jump, and then a normal flow
The reservoir upstream of the mountain that builds up as the mountain is moving
Even after the mountain has stopped, you see waves travelling through the tank and being reflected at the ends
Here is a movie of the lee wave experiment. Feel free to use it in teaching if you like! And let me know if you need the movie in a higher resolution, I am happy to share!
*Yes, this was true at the time of writing. But I am setting up that experiment as we speak. Write. Read. Whatever. Will post movies tomorrow!
A ship that is continuously pulled with a constant force suddenly slows down, stops, and then continues sailing as if nothing ever happened? What’s going on there? We will investigate this in a tank! And in order to see what is going on, we have dyed some of the water pink. Can you spot what is going on?
The phenomenon of “dead water” is probably well known to anyone sailing on strong stratifications, i.e. in regions where there is a shallow fresh or brackish layer on top of a much saltier layer, e.g. the Baltic Sea, the Arctic or some fjords. It has been described as early as 1893 by Fridtjof Nansen, who wrote, sailing in the Arctic: “When caught in dead water Fram appeared to be held back, as if by some mysterious force, and she did not always answer the helm. In calm weather, with a light cargo, Fram was capable of 6 to 7 knots. When in dead water she was unable to make 1.5 knots. We made loops in our course, turned sometimes right around, tried all sorts of antics to get clear of it, but to very little purpose.” (cited in Walker, J.M.; “Farthest North, Dead Water and the Ekman Spiral,” Weather, 46:158, 1991)
When observing the experiment, whether in the movie above or in the lab, the obvious focus is on the ship and the interface between the clear fresh water layer (the upper 5cm in the tank) and the pink salt water layer below. And yes, that’s where a large-amplitude internal wave develops and eats up all the energy that was going into propulsion before! Only when looking at the time lapse of the experiments later did I notice how much more was going on throughout the tank! Check it out here:
The setup for this experiment is discussed here and is based on the super helpful website by Mercier, Vasseur and Dauxois (2009). In the end, we ended up without the belt to reduce friction, and with slightly different layer depths than we had planned, but all in all it works really well!
When setting up the stratification for the Nansen “dead water” demo (that we’ll show later today, and until then I am not allowed to share any videos, sorry!), I went into a meeting after filling in layer 4 (the then lowest). When I came back, I wanted to fill in layer 5 as the new bottom layer. For this experiment we want the bottom four layers to have the same density (so we would actually only have one shallow top layer and then a deep layer below [but we can’t make enough salt water at a time for that layer, so I had to split it into four portions]), and I had mixed it as well as I could. But two things happened: a) my salinity was clearly a little fresher than the previous layer, and b) the water in the tank had warmed up and the new water I was adding with layer 5 was cold tap water. So I accidentally set up the stratification for salt fingering: warm and salty over cold and fresh! Can you spot the darker pink fingers reaching down into the slightly lighter pink water? How cool is this??? I am completely flashed. Salt fingering in a 6 meter long tank! :-D
In a presentation about science communication I gave on Monday, I recommended a couple of resources for scientists interested in science communication. For example the amazing climatevisuals.org for advice on which images to use to communicate about climate change (plus lots of images that even come with explanations for what purpose they work well, and why!). And of course my #scicommchall to get people inspired to try out a new micro scicomm format every month.
But here is an (open access!) book I wish I had known about then already but only came across two days after my presentation: “Communicating Climate Change” by A. K. Armstrong, M. E. Krasny, J. P. Schuldt (2018).
This is a book aimed at educators who want to communicate climate change in a literature-based and effective manner. It consists of four parts: A background, the psychology of climate change, communication, and stories from the field, which I will briefly review below (and you should definitely check out the real thing!). It’s nice and easy to read, and there are “bottom line for educators” at the end of each chapter as well as recaps at the end of each part, making it easy to get a quick overview even if you might not have the time to read the whole thing in detail.
This part of the book begins with an introduction to climate change science, reporting state-of-the-art science on climate, greenhouse gases, evidence for climate change, and climate impacts. It then moves to how climate change can be addressed: by mitigating or adapting to its effects, how it is important to reduce greenhouse gas emissions, and how that can be achieved both on an individual level and by collective action. It ends with a “bottom line for educators” summary that stresses that climate change is real, that misinformation campaigns are an unfortunate reality, and that educators can contribute to solving the problem.
The next chapter then deals with what is known on attitudes and knowledge about climate change in different audiences internationally and at different ages, explaining that attitudes are actually a pretty bad predictor for behaviour, but nevertheless important to know about if you are an educator! For example, if you want teens to be concerned about climate change, a useful approach might be to involve their parents along with them, since what family and friends believe about climate change is very important to what an individual teenager believes, as is how often they discuss climate topics with their friends and family. Again, the “bottom line for educators” breaks this down into advice, for example to focus on different topics depending on how concerned about climate change a given audience already is, or to focus on areas in which a common ground between them and their audiences exists in order to generate a constructive and positive dialogue even though there might still be areas in which they do not agree with their audiences (which they should think about beforehand, hence the importance to know about the audience’s attitudes).
The next chapter suggests possible outcomes for climate change education — how do we know if a climate change communication activity was successful? — and stresses the importance of defining these goals in the first place. Outcomes can be defined on the level of individuals, of communities, of the environment, or of resilience of all of the above. For individuals, outcomes could for example be literacy (understanding essential principles, knowledge of how to assess scientifically credible information, ability to communicate, ability to make informed and responsible decisions) of climate change, or attitudes and emotions, the feeling of confidence that you can reach your goals, or environmentally friendly behaviour. For communities, outcomes could be positive development of youth, building of social capital (e.g. trust or positive action), the belief that the community can reach a goal together, or action taken together by the whole community. Focussing on the environment, an outcome could be adaptation to, or mitigation of, climate change.
The next chapter then presents three climate change vignettes — three examples of how different educators address different audiences in different settings — and a discussion of why they chose to design their activity a certain way and react to questions or comments the way they did.
The psychology of climate change
This part of the book presents psychology research on why knowledge about climate change is not sufficient to actually change behaviours.
Identity research especially is very helpful, as it explains how in order to feel like you are part of a group (something that we as humans are hard-wired to crave) we tend to conform with our group’s norms and values. We might be part of different groups at different times as well as simultaneously (for example our family as one and our colleagues as another, or inhabitant of a city, or student of oceanography), and contexts trigger specific identities that might even not be completely congruent with each other. When new information is presented, we interpret it in a way that does not threaten our identity in the context the information is presented in. Therefore, in order to not threaten anybody’s identity and making it impossible for them to take on our message, it is important to make sure that climate change is not communicated as something polarising or political, but rather choose to trigger identities that are inclusive, like for example “inhabitant of place x”, and focus on outcomes that benefit that community independent of what other identities might exist, by for example protecting a local beach.
Psychological distance is another lens through which climate change communication can be viewed. The more distant a problem seems, the less important it is perceived. Therefore focussing on local relevance rather than global, on places that are important to people, on communities they care about, might in some cases be helpful — although not always; the results of the research on this are not conclusive yet.
Then a few other relevant psychological research areas are discussed, like for example “terror management theory”. This leads to the recommendation to avoid “doom and gloom” presentations of climate change that might kick people into a defence mechanism of ignoring the topic to protect their emotional well-being in the moment, and to focus on hope and positive action instead. Then there is the “cognitive dissonance theory”, according to which we try to ignore information that conflicts with what we think we already know or threatens other goals we might have. The recommendation here is to give people ideas of easy things they can do to combat climate change to combat cognitive dissonance.
This part of the book presents three aspects of communicating climate change: How we frame it, which analogies and metaphors we use, and how we, as a messenger, can build trust.
“Framing” is about how a message is featured in a story line to help the audience interpret it in a certain way, by making certain aspects of it especially visible, for example economic aspects or tipping points. When thinking about framing a climate change message, it is important to think about audiences and their identities and to avoid wording that will trigger identities which make it difficult to accept the message. Depending on the desired outcomes, climate change communications could, for example, be framed for solutions, hope, or values. There are ways to build entire climate change communication programs around those frames, and there are several examples given for how this might be done.
The next chapter focusses on analogies and metaphors. For example, “osteoporosis of the sea” (which I had never heard in use before) has been found to be a successful metaphor for ocean acidification. However, as all metaphors, it only highlights similarities between issues and neglects to mention the dissimilarities which makes them tricky to use because it’s hard to make sure people don’t take a metaphor so far that it breaks down. In fact, to address this problem, the authors recommend to explicitly talk about where the analogy or metaphor will break down.
Establishing trust in the climate change messengers: This is tricky as people tend to trust other people that hold values similar to their own. Therefore it is helpful to think about the messenger and to use trusted middle persons. [There is are actually some very interesting work on trust out there, for example by Hendriks, Kienhues and Bromme (2015) that isn’t mentioned in the book, but that I’d be happy to summarise for you if anyone is interested!]
Stories from the field
The book ends with a part called “stories from the field” in which examples of different climate change communication activities, focussing on different goals, audiences, messenges and happening in very different settings, are given and the design choices that were made explained in detail. Also for each of the story, an example is given how the message is phrased in actual interaction with the target audience. All of this is super interesting to read because all the theory the book provided in the previous chapters is applied to real world cases, which makes it easy to see how they might be applied to your own climate change communication activities. Also these best practice examples are inspiring to see and give me a sense of hope.
To sum up: I really enjoyed reading this book! So much so that continuing reading it was more important than getting a good Instagram pic of my latte while writing this blogpost. I would really recommend anyone interested in climate change communication to check it out! When I finished my talk on Monday, on my second to last slide I put the African proverb along the lines of “if you think you are too small to make a difference, try going to sleep with a mosquito in the room”. I used this to talk about using messages of hope in climate change communication, and then also applied it to science communication — don’t think you are too small to make a difference there, either! And that’s a message that this book conveys really well, too, providing a good idea of what one could do and how one might go about it, and inspiring one — or at least me — to do so, too.
So you thought filling water into a tank was boring? Not on my watch!
This is how we fill up the tank: Through a hole at the bottom. Which leads to a very nice fountain that slowly submerges as the water level rises:
…and to tons of nice waves, which are great to observe!
Propagation of waves
Below you see waves propagating. Can you spot the water’s orbital movement, i.e. water particles moving in circles, even though the wave phase is propagating from left to right?
After a while, waves are reflected at the end of the tank and propagate back, setting up a different, very cool, pattern:
Now the wave phase does not seem to travel any more! Instead, there are fixed points in space where water levels oscillate between maximum and minimum, and in between there are other points where the water level stays more or less the same. How cool is that?!
…And this is just filling the tank. Just wait how cool it gets when we are actually running our demonstrations! :-)
Top pic: Ice cubes (left) and leftover uranine solution from when I was preparing those ice cubes Bottom left pic: Melting the ice cubes in a water bath to see if they start glowing again when melting. Bottom right pic: Yes, they do! :-)
Why do I find this exciting? Because that means that a phase change of the water switches fluorescence as a tracer of that water on and off. Or the other way round: Seeing water fluoresce (or not) tells you what state the water is in without having to figure out anything else about that water.
…and funny how that the apprenticeship as chemical technician that I started in parallel to finishing high school (and didn’t finish because other stuff — oceanography — became more interesting) does come into play in the most unexpected moments :-)
I had frozen a second tray* with ice cubes dyed with uranine (you know, the green stuff I found in the lake near my house the other day?). Like the other ones, they clearly froze back-of-the-freezer-forward, and the most interesting thing to me: Only the parts that weren’t frozen were fluorescing in the fridge!
For comparison, here a picture of the leftover water (so same dye concentration as the ice cubes) in the beaker, and the tray with frozen ice cubes next to it. You can see the beam of the UV lamp on the table* so you believe me that the ice cubes were lit with UV light, too, they just don’t fluoresce!
*Ailin and Steffi, did you see I got you a gold colored ice cube tray? :D Although, had I thought that I would be looking at the ice cubes themselves this much, I would probably have gotten two whites instead of a white and a gold one… But now you at least have a pretty ice cube tray in the lab :-)
**and a stain that I am only now noticing, but it wasn’t me; I was working super carefully since today I am not dressed for the lab and I don’t want that stuff on my clothes!