Support the Arctic Sea Ice Forum and Blog

Author Topic: Overview of Arctic processes (including entrainment)  (Read 662 times)

oren

  • Moderator
  • First-year ice
  • Posts: 7387
    • View Profile
  • Liked: 2837
  • Likes Given: 2667
Overview of Arctic processes (including entrainment)
« on: April 28, 2021, 10:28:23 PM »
Cross-post of a great overview of Arctic processes, thanks to uniquorn.

Click to enlarge.
Edit: Right-click, open in new tab, and then click to enlarge, for some reason clicking directly doesn't work.

« Last Edit: May 03, 2021, 04:31:57 PM by oren »

binntho

  • Nilas ice
  • Posts: 1637
    • View Profile
  • Liked: 603
  • Likes Given: 159
Re: Overview of Arctic processes (including entrainment)
« Reply #1 on: April 29, 2021, 06:47:08 AM »
Very interesting overview indeed. But my old aversio to the word "entrainment" resurfaced and this time I am pretty sure that it is being used incorrectly.

Entrainment basically means when one thing pulls another along with it, but there is no indication of this happening in the picture. Perhaps it is explained somewhere else in more detail, I for one would be interested in knowing which processes cause entrainment (of what? Water? Heat?) between the two main layers.
because a thing is eloquently expressed it should not be taken to be as necessarily true
St. Augustine, Confessions V, 6

J Cartmill

  • New ice
  • Posts: 65
    • View Profile
  • Liked: 17
  • Likes Given: 8
Re: Overview of Arctic processes (including entrainment)
« Reply #2 on: April 29, 2021, 03:42:57 PM »
The mixed layer entrainment zone is the bottom of the mixed layer.

"Deepening of the mixed layer is accomplished by entrainment of the more dense underlying water into the turbulent region above. This process entails a potential energy increase and cannot take place without an energy source--the turbulent kinetic energy of the mixed layer above."

From A General Model of the Ocean Mixed Layer https://www.pmel.noaa.gov/pubs/PDF/garw226/garw226.pdf

binntho

  • Nilas ice
  • Posts: 1637
    • View Profile
  • Liked: 603
  • Likes Given: 159
Re: Overview of Arctic processes (including entrainment)
« Reply #3 on: April 30, 2021, 07:49:50 AM »
The mixed layer entrainment zone is the bottom of the mixed layer.

"Deepening of the mixed layer is accomplished by entrainment of the more dense underlying water into the turbulent region above. This process entails a potential energy increase and cannot take place without an energy source--the turbulent kinetic energy of the mixed layer above."

From A General Model of the Ocean Mixed Layer https://www.pmel.noaa.gov/pubs/PDF/garw226/garw226.pdf
That paper sure is heavy going - but as for "entrainment", it doesn't answer my critique. The word "entrainment" implies that one thing pulls another along with it. So what is pulling what in the above picture, in the quote a above and in the following sentence (from the linked paper, p. 36-37)
Quote
After taking into account any surface buoyancy flux, the balance of the potential energy change went to entrainment--mixing the requisite amount of underlying denser water uniformly throughout the homogeneous mixed region.
If the latter half of this qoute is to be taken as an explanation of the term "entrainment" then it neither fits with the image above (which shows "entrainment" going up and down across the boundary as is also implied in the first qoutation) nor does it explain what it is that is doing the entraining, nor what is being entrained.

The image above seems to misuse the term "entrainment" as do the two quotes from the linked paper.
because a thing is eloquently expressed it should not be taken to be as necessarily true
St. Augustine, Confessions V, 6

crandles

  • Young ice
  • Posts: 2996
    • View Profile
  • Liked: 200
  • Likes Given: 65
Re: Overview of Arctic processes (including entrainment)
« Reply #4 on: April 30, 2021, 01:04:50 PM »
maybe there are different shades of meanings:

https://www.merriam-webster.com/dictionary/entrain

1: to draw along with or after oneself
2: to draw in and transport (something, such as solid particles or gas) by the flow of a fluid
3: to incorporate (air bubbles) into concrete
4: to determine or modify the phase or period of

I am looking at meaning 2 above and thinking extra energy 'draws in' more water into the mixing layer and mixes it as the transport part. Could also consider meaning 3 above.

Arrows both ways - is that odd? Not sure. Could be saying more energy moves the boundary line downwards whereas less energy moves the boundary line upward. Or the arrows could be indicating some water moves from deeper into mixed layer while other water from the mixed layer moves into deeper water. (This will happen to a small extent by diffusion but it will happen a lot faster if there are changes in the energy levels, so it cannot be modelled as a diffusion process you have to consider changes in energy by storms and the climate community calls this an entrainment process.)

The term is certainly used by weather and climate modellers eg
https://glossary.ametsoc.org/wiki/Entrainment_coefficient

If a term gets widely used by a community of people, like weather and climate modellers, surely it reaches a point where the word acquires an extra meaning rather than saying the whole community is misusing the term.
« Last Edit: April 30, 2021, 01:18:31 PM by crandles »

uniquorn

  • Young ice
  • Posts: 3436
    • View Profile
  • Liked: 1596
  • Likes Given: 326
Re: Overview of Arctic processes (including entrainment)
« Reply #5 on: April 30, 2021, 01:39:07 PM »
Maybe oren should have linked to the original paper, but that may have been off topic. These are all images at least...

oren

  • Moderator
  • First-year ice
  • Posts: 7387
    • View Profile
  • Liked: 2837
  • Likes Given: 2667
Re: Overview of Arctic processes (including entrainment)
« Reply #6 on: April 30, 2021, 03:44:41 PM »
Thanks for the response, uniquorn.
Binntho, if you wish to discuss that image further please use the original thread in which it was posted (Salinity and waves, if I am not mistaken).

J Cartmill

  • New ice
  • Posts: 65
    • View Profile
  • Liked: 17
  • Likes Given: 8
Re: Overview of Arctic processes (including entrainment)
« Reply #7 on: April 30, 2021, 03:52:59 PM »
My simplistic explanation:

At the surface air gets entrained (pulled) into the water by breaking waves as air bubbles. If there is no breaking then gravity waves don't entrain air.

At the bottom of the mixed denser water can be entrained (pulled) up into the mixed layer through turbulence.

The mixed layer in the Arctic is extremely complicated, that figure is one of the best oceanography diagrams I have ever seen.

SteveMDFP

  • Nilas ice
  • Posts: 1956
    • View Profile
  • Liked: 429
  • Likes Given: 27
Re: Overview of Arctic processes (including entrainment)
« Reply #8 on: April 30, 2021, 04:17:19 PM »
I would think entrainment would be the right word, if by "turbulence" we mean vortices.  A surface water vortex will entrain water below into the vortex.  Perhaps also with other modes of turbulence.  The net would be vertical mixing.

kassy

  • Young ice
  • Posts: 3555
    • View Profile
  • Liked: 1400
  • Likes Given: 1382
Re: Overview of Arctic processes (including entrainment)
« Reply #9 on: April 30, 2021, 10:55:07 PM »
There are different meanings like crandles points out in #1898. Here it is the technical terms as used in ocean modelling. So what are they actually discussing?

The entrainment process in the ocean mixed layer is the product of the entremaint velocity (We) and the difference of temperature between the mean temperature in the mixed layer (T) with the temperature just below the mixed layer base (T(-h)).
The entrainment is composed of three terms:
1. the entrainment due to the vertical velocity (w*|T-T(-h)[)
2. the entrainment due to the tendency of the mixed layer depth (dh/dt*[T-T(-h)])
3. the entrainent due to "advection of the mixed layer depth"  ((U(-h).Grad(h))

https://www.researchgate.net/post/What-is-the-entrainment-process-in-the-ocean-mixed-layer

See link for a bit longer explanation in relation to ekman pumping.

And an old 1995 paper. Not that relevant but probably easier to digest then the 2016 article because methods and questions were more limited then.

https://journals.ametsoc.org/view/journals/phoc/25/12/1520-0485_1995_025_3089_dsmlw_2_0_co_2.xml?tab_body=pdf

Everything flows and that is even more true for the oceans.

In the picture it is going up and down because it is a graphic simplification for a whole season and it also ignores specific local factors like river inputs for example so it can and will vary.
Þetta minnismerki er til vitnis um að við vitum hvað er að gerast og hvað þarf að gera. Aðeins þú veist hvort við gerðum eitthvað.

binntho

  • Nilas ice
  • Posts: 1637
    • View Profile
  • Liked: 603
  • Likes Given: 159
Re: Overview of Arctic processes (including entrainment)
« Reply #10 on: May 02, 2021, 02:42:54 AM »
Thanks for the comments on "entrainment". Every definition of the word involves two "things" moving, where the movement of one pulls the other along.

The word seems to be used as a technical term in oceanography, but none of the above examples actually explains what is going on. What is being entrained? What is "thing A" that is moving and puling "thing B" along with it?

So my question regarding the image still stands - what is being entrained by what? Or should another word have been used, such as turbulence or flow? From the definitions above it seems clear that kinetic energy is involved, so is the term being used to describe the transfer of heat by the flow of water across the boundary? If that is the case, why do the arrows point in both directions?

I am truly trying to understand ocean processes, and was quite enthused by the image until my eyes stumbled on the two red arrows with the word "entrainment". This has no explanatory value on it's own, and nobody here seems to know what is meant.
because a thing is eloquently expressed it should not be taken to be as necessarily true
St. Augustine, Confessions V, 6

kassy

  • Young ice
  • Posts: 3555
    • View Profile
  • Liked: 1400
  • Likes Given: 1382
Re: Overview of Arctic processes (including entrainment)
« Reply #11 on: May 03, 2021, 12:58:21 AM »
Well a greek dude once stood by a river long enough to utter panta rhei. This works even better for oceans but you cannot see them from ancient Greece.

The arrows point in both ways because it they are 2 graphics displaying processes that happen over a season.

The oceanography definition is about layers mixing and there are so many inputs going into that  question. It really depends on the location to see which drags which. Are you near a big river? Are you near the Atlantification or Pacification? (or any big ocean trend). And on top of that you get weather so yes the interface goes up and down along the season. 
Þetta minnismerki er til vitnis um að við vitum hvað er að gerast og hvað þarf að gera. Aðeins þú veist hvort við gerðum eitthvað.

binntho

  • Nilas ice
  • Posts: 1637
    • View Profile
  • Liked: 603
  • Likes Given: 159
Re: Overview of Arctic processes (including entrainment)
« Reply #12 on: May 03, 2021, 06:54:55 AM »
I must admit to some vigorous headscratching. Is it possible that in this forum of extremely clever Arctic Enthusiasts, nobody is able to explain a central feature of a simple school-book image? The excellent Uniqorn is the original poster (according to Oren although no link or further information is provided), one of our most industrious contributors. Perhaps he could find time off his busy schedule to explain these basic facts to one confused groundling?

Specifically: What is meant by the double read arrows and the term  "entrainment". What is being entrained and what is doing the entrainment?
because a thing is eloquently expressed it should not be taken to be as necessarily true
St. Augustine, Confessions V, 6

oren

  • Moderator
  • First-year ice
  • Posts: 7387
    • View Profile
  • Liked: 2837
  • Likes Given: 2667
Re: Overview of Arctic processes (including entrainment)
« Reply #13 on: May 03, 2021, 11:06:07 AM »
Binntho are you being difficult on purpose?

Here is the original post, where you can also read the original article:
Stratified Ocean Dynamics of the Arctic: Science and Experiment Plan
https://apl.uw.edu/research/downloads/publications/tr_1601.pdf

I have already pointed you to that thread ("Salinity and waves" not too hard to find) in addition to requesting that you continue your already-answered questioning there.

https://forum.arctic-sea-ice.net/index.php/topic,2417.msg307112.html#msg307112

binntho

  • Nilas ice
  • Posts: 1637
    • View Profile
  • Liked: 603
  • Likes Given: 159
Re: Overview of Arctic processes (including entrainment)
« Reply #14 on: May 03, 2021, 11:41:26 AM »
No Oren I am not being difficult on purpose. But I would like to ask why you automatically answer my posts like this, assuming malfeasance and evil intent. I am simply amazed that nobody here is able to answer this question. To me, this lack of knowledge and lack of willingness to know is astonishing.

You cross-post a diagram because it is supposed to be so very good - and then neither you nor anybody else is able to answer a simple question about a central feature of the diagram.

Why use the word "entrainment" and not eg. flow or diffusion or induction or turbulence or radiative transfer or conduction. What are the two phenomena involved. Why show it with a double red arrow.

But thanks for the link. In your original crosspost you neither mention where the crosspost is from nor give a link, which I would think should be the minimum expected.

But unfortunately, the article linked does not go very far in answering the questions I have put forward - why use the word eintraiment, why the double red arrow, what are the two phenomena involved. The closest the arcticle comes to an explanation is the following:

Quote
How do atmospheric forcing, diverse ice conditions, and existing vertical and
lateral stratification interact to govern mixed layer deepening (entrainment) and
shoaling (restratification).(p19)

Quote
... changes in internal wave properties, turbulent fluxes, and
entrainment of subsurface heat into the mixed layer ... (p. 32)

So "entrainment" is the same as "mixed layer deepening" according to the first quote, and the second states that it is heat which is being entrained. But neither quote explains the use of the word "entrainment", and what it is that entrains the heat, nor does it make the image in question any more logical, with the double red arrow.
because a thing is eloquently expressed it should not be taken to be as necessarily true
St. Augustine, Confessions V, 6

uniquorn

  • Young ice
  • Posts: 3436
    • View Profile
  • Liked: 1596
  • Likes Given: 326
Re: Overview of Arctic processes (including entrainment)
« Reply #15 on: May 03, 2021, 12:39:32 PM »
apologies for off topic. There is an interesting review

Understanding Arctic Ocean Circulation: A Review of Ocean Dynamics in a Changing Climate
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2018JC014378

which proposes one method of 'mixing and entrainment' from a paper in 1985

Quote
a horizontally uniform vertical entrainment velocity
is an interesting concept. I think it's possible that flows in different directions might cause the arrows to point downwards rather than upwards but there is no example provided. As ever, there is some discussion...

Quote
Bathymetric influences (aside from those of the straits) and recirculations within the Arctic basin are not represented in estuary models. Nor do they account for recirculations in the vicinity of the connecting straits. Further, it is unclear whether the required mixing between the surface fresh layers and the inflowing Atlantic Water is realistic. In an alternative framework, the wind directly drives the topography‐following Atlantic Water circulation. In the next section, we describe studies which have shown how the prevailing wind field over the Arctic is such that the wind‐stress curl can set the observed ocean transport.

though if this is a semantic issue with the use of the word entrainment it may be best to take that up with the authors, Mary‐Louise Timmermans and John Marshall or perhaps AAgard et al.



The second image is from the SODA Experiment Plan so we may need to wait for results. If anyone finds more recent articles hopefully they will share on an appropriate thread.


crandles

  • Young ice
  • Posts: 2996
    • View Profile
  • Liked: 200
  • Likes Given: 65
Re: Overview of Arctic processes (including entrainment)
« Reply #16 on: May 03, 2021, 12:44:20 PM »
I am simply amazed that nobody here is able to answer this question. To me, this lack of knowledge and lack of willingness to know is astonishing.

...and then neither you nor anybody else is able to answer a simple question about a central feature of the diagram.

I also am amazed you say this. There are answers on this thread. If you don't think they are good enough explanations then have the courtesy to explain why rather than just pretending there are no answers being given to you.

Phil.

  • Frazil ice
  • Posts: 374
    • View Profile
  • Liked: 47
  • Likes Given: 1
Re: Overview of Arctic processes (including entrainment)
« Reply #17 on: May 03, 2021, 03:20:27 PM »
Specifically: What is meant by the double read arrows and the term  "entrainment". What is being entrained and what is doing the entrainment?

'Entrainment' is a term used in fluid mechanics to describe the mixing that occurs when two fluids flow relative to each other.  The red arrows indicate that mixing, if one of the fluids is more saline than the other the shear at the interface results in some of the more saline being transported into the other layer and some of the less saline being transported in the opposite direction.

johnm33

  • Frazil ice
  • Posts: 112
    • View Profile
  • Liked: 35
  • Likes Given: 7
Re: Overview of Arctic processes (including entrainment)
« Reply #18 on: May 06, 2021, 10:24:00 PM »
The first image shows, roughly, the center of mass of the ice.
I suggest that whenever the ice moves, in any way, cracks open then freeze over resulting in a net movement away from the center +/- of this area. Unless the ice is constrained on the Barents side by the island chains, or the Asian coast, the build up of thick ice is limited. Or, in other words, the day to day expansion of the pack will merely tend to push ice from say 85/180 towards Fram,  and thicken ice in the opposite direction. Moving north the ice will have a little excess momentum and tend to accelerate with the Earths rotation, moving south it will conversely lack momentum and lose ground, leading to a base rate of  rotation in a clockwise direction that would naturally form part of the energetic input of the Beaufort Gyre. If constrained on the Barents side then the center of mass would 'roll' it's center towards the 120E meridian and here the pressure would force thick ice to form against the coast of Greenland,[+Laptev] and everything south of lat85 would move south and rotate towards Fram.
The second image is from earlier today, it indicates a general high pressure over the Arctic possibly adding to the momentum through Fram and some indication that although the ice may compress a little south of the pole on the 180 meridian the general drift towards the upwelling at the pole should be coming from that direction too.  [The low was 1011, i think the mean is 1012, but for thinking I always reckon from 1010 up or down]
 

johnm33

  • Frazil ice
  • Posts: 112
    • View Profile
  • Liked: 35
  • Likes Given: 7
Re: Overview of Arctic processes (including entrainment)
« Reply #19 on: May 11, 2021, 11:20:09 AM »
The first image shows, top left, a very persistent ice formation grounded at +/- 50m which suggests that the flow out through Fram is below that depth. I suspect that that flow is coming directly from the Canadian side, their are hints in the surface ice suggesting both large movements and some smaller gaps orthogonal to the direction of movement suggesting a more general surface acceleration towards Fram. We have a new moon with it's exagerated tidal flux upon us and atmospherics almost perfectly suited to enhanced flow through the Faroes gap arctic gateway and easing flows into Barents and onward towards Kara/Laptev.
 As the ice loses integrity the upper layer of water which is more or less constrained by it will begin to acquire momentum from the lower layer, below 50-70m, and will begin to accelerate through Fram taking the ice with it. I think.
Meanwhile the general flow of surface ice towards Fram may be moving the Beaufort freshwater lens north, if thats the case it's own inertia will also carry it east.