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wdmn

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The science of ice ridging and rafting
« on: July 23, 2020, 10:43:50 PM »
Since there has been continued interest by several posters in the 2020 melt season thread on the question of whether the 2020 HP anti-cyclone caused ice ridging or rafting over open water (i.e. not against shore) due to centripetal forces, I am creating this thread.

There seem to be three pertinent questions:

1) Do ridging and rafting occur during the summer melt season?

2) Do ridging and rafting occur over open water? If so what kind of force is required?

3) Is there a way to answer these questions by comparing pre-anticyclone, and post-anticyclone satellite imagery?

First some definitions:

Ridging: is created by "the flexural failure of opposing ice sheets and subsequent piling of the ice blocks created by the flexural failure on top of and beneath the two sheets." It occurs most frequently between ice sheets of different thickness, and particularly when one sheet is fast (against shore).

Rafting: when one ice sheet overlaps another, and progresses until "the frictional force between the sheets, which increases linearly with the amount of overlap, arrests motion or causes buckling." It tends to occur when ice sheets are almost the same thickness, but can also happen when rubble is created between ice sheets of varying thickness, and the rubble then forces the thinner sheet into a rafting situation.

Taken from this article (hat tip to the Artful Dodger): "Rafting and ridging of thin ice sheets" https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/1999JC900031

I looked through some literature as best I could to answer the three questions above, and provide the initial findings below.

1) Some ridging does occur during the summer, though it is a fraction of what occurs during the winter months:

"Total winter production from November to April (mean value of 4629 km3 over 56 years) is 3.5 times more than the summer production from May to October (mean value of 1324 km3) and occupies about 77% of the annual amount."

"ridge volume has a maximum of 12 150 km3 in June and minimum of 9490 km3 in October. This annual cycle is explained by the fact that before June, ice stays adequately compact in the Arctic basin and ridging continues, but before October the amount of leads exists sufficiently, which reduces ridging."

"the summer seasonal production (May to October) reaches, on average, about 30% of the winter production (November to April) and gives rise to only 23% of the annual production."

Source: "Dynamic–Thermodynamic Sea Ice Model: Ridging and Its Application to Climate Study and Navigation" https://journals.ametsoc.org/jcli/article/18/18/3840/30529/Dynamic-Thermodynamic-Sea-Ice-Model-Ridging-and

2) The same article suggests that compacting forces from anticyclones can cause ridging. However, I had a difficult time deciphering a couple of passages in the paper, which seemed to me contradictory.

"The yearly averaged ice volume systematically grows in the Arctic Ocean (Figs. 1 and 2), especially in the Canadian region, during the periods with anticyclonic circulation (Makshtas et al. 2003), but the prevailing cyclonic regime leads to a shrink of ice cover, as noted by Walsh et al. (1996)... The model also shows that periods with the anticyclonic circulation in the atmosphere lead to a decrease in ridging intensity in the Canada Basin, adjacent parts of the central Arctic, and marginal seas. This decrease causes, on average, the thinning of sea ice in the early 1990s, when cyclonic circulation in the polar atmosphere was well developed."

And the other confusing passage I found, which seems highly relevant:

"The distribution of ridge production is rather nonuniform over the Arctic Ocean. It should be noted that the high ridge production around the islands, comparable to the ridge production in the Beaufort Gyre, is the reflection of ridge accumulation under onshore winds. This natural effect is an essential feature of the coastal area, where the ridged ice zones and rubble fields actually exist. Meanwhile, this ridging around the islands is not reflected in the average ice thickness here because the model does not reproduce grounded ice, fast ice, and ice freezing to the beach. Therefore, winds with changeable direction lead to ridged ice floating away from shore and spreading over the Arctic."

3) There is a method applied to the Baltic Sea to determine winter ridging to assist navigation, and this method will be (or is being?) applied to the Arctic sea ice. It is far beyond my current level of understanding or familiarity with the imagery available to us to know if we could assess the CAB ice before and after the GAAC of this year.

"Estimation of degree of sea ice ridging based on dual-polarized C-band SAR data" https://tc.copernicus.org/articles/12/343/2018/tc-12-343-2018.pdf

"Satellite Observations for Detecting and Forecasting Sea-Ice Conditions: A Summary of Advances Made in the SPICES Project by the EU's Horizon 2020 Programme" https://www.researchgate.net/publication/340548390_Satellite_Observations_for_Detecting_and_Forecasting_Sea-Ice_Conditions_A_Summary_of_Advances_Made_in_the_SPICES_Project_by_the_EU's_Horizon_2020_Programme

Finally, as Gerontocrat asked whether wind would move ridged ice more easily than flat ice. Ridges are called "sails" above the ice, and "keels" bellow, so perhaps ridging does make ice more mobile in the wind (due to having a sail).

ajouis

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Re: The science of ice ridging and rafting
« Reply #1 on: July 23, 2020, 11:25:57 PM »
the formation of the sea ice ridges, they require freezing (-1.8 ) air temperatures to form.
https://www.eolss.net/Sample-Chapters/C05/E6-178-66.pdf

The initial phase starts during ridge formation and is characterized by the formation of
freeze-bonds. Three different heat fluxes are important: a) the surface flux ( qsur ), into the cold surrounding air, b) the oceanic flux ( qocean ), from the ocean beneath and c) the
internal fluxes ( qre ), in between the cold pieces of ice and the warm water pockets
inside the keel (Figure 3). The surface flux freezes the water pockets from the top and downwards and creates a cold front that defines the consolidated layer. The initial cold content of the ice is partly spent in making freeze bonds and partly consumed by the oceanic flux. The fraction that goes into making freeze bonds depends on the initial ice temperatures, the block thicknesses, the ridge size and the oceanic conditions. When all the ice and water below the cold front is isothermal that is at the freezing point of the surrounding water the initial phase ends.
The rubble beneath the consolidated layer is thermally insulated by the freezing front on top of it, and feels only the water below. Since the conditions are isothermal there is no longer any cold reserve available and the rubble decays continuously. The rubble transforms from individual ice blocks with freeze bonds to an ice skeleton with a hierarchy of pores, from a few centimeters and up to meter(s).
In the decay phase the ridge is heated both from the top and from the bottom. The ridge now either melts completely, or it transforms into a second-year ridge during the summer. Several processes take place. On the surface the warm air and the sun radiation melts the snow and the surface ice and creates relatively fresh melt-water. Its freezing point is above the temperature in the rubble so it will freeze as it drizzles down in the keel. This freezing process release heat and increases the temperatures in the rubble. In this way the decay phase includes both melting and freezing. Freezing can take place as long as there is cold capacity (ice temperature less than the freezing point of the melt water) in the keel. However, another mechanism can contribute to further consolidation. If the pore water salinity is changed cyclically, either by periodic surface melting or by tidally driven river runoff the ridge could actually expel heat into the surrounding water
and contribute to further freezing (consolidation). This mechanism is only shown in laboratory investigations and in simulations. Finally the ridge keel could collapse and in this way decrease the porosity and increase the degree of consolidation. By the end of the melt season the ridge has become a second-year ridge.
After a thousand steps on the ice, it cracked.
The Man looked down at the infinite blue of the sea.
On the horizon, standing still, the polar bear had just scented his next meal.

 Less than 3000 cubic kilometers this Piomas minimum.

oren

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Re: The science of ice ridging and rafting
« Reply #2 on: July 23, 2020, 11:54:45 PM »
Very good thread. Add the assumption of deep summer, ice core temp is at the melting point of -1.8C, and air temps are above that, so the ice is structurally weaker than in winter and there can be no freezing together of floes or rubble. And that we are interested in events in the middle of the ocean. It is quite obvious that some ridging can occur even in summer with enough pressure against a static shore.

People on the melting season thread have mentioned seeing ridging on the Pole Cam, may it rest in peace. Does anybody have access to those images? Or can anyone post accurate recollections of such events?

Michael Hauber

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Re: The science of ice ridging and rafting
« Reply #3 on: July 24, 2020, 02:17:05 AM »
the formation of the sea ice ridges, they require freezing (-1.8 ) air temperatures to form.
https://www.eolss.net/Sample-Chapters/C05/E6-178-66.pdf

The initial phase starts during ridge formation and is characterized by the formation of
freeze-bonds. Three different heat fluxes are important: a) the surface flux ( qsur ), into the cold surrounding air, b) the oceanic flux ( qocean ), from the ocean beneath and c) the
internal fluxes ( qre ), in between the cold pieces of ice and the warm water pockets
inside the keel (Figure 3). The surface flux freezes the water pockets from the top and downwards and creates a cold front that defines the consolidated layer. The initial cold content of the ice is partly spent in making freeze bonds and partly consumed by the oceanic flux. The fraction that goes into making freeze bonds depends on the initial ice temperatures, the block thicknesses, the ridge size and the oceanic conditions. When all the ice and water below the cold front is isothermal that is at the freezing point of the surrounding water the initial phase ends.
The rubble beneath the consolidated layer is thermally insulated by the freezing front on top of it, and feels only the water below. Since the conditions are isothermal there is no longer any cold reserve available and the rubble decays continuously. The rubble transforms from individual ice blocks with freeze bonds to an ice skeleton with a hierarchy of pores, from a few centimeters and up to meter(s).
In the decay phase the ridge is heated both from the top and from the bottom. The ridge now either melts completely, or it transforms into a second-year ridge during the summer. Several processes take place. On the surface the warm air and the sun radiation melts the snow and the surface ice and creates relatively fresh melt-water. Its freezing point is above the temperature in the rubble so it will freeze as it drizzles down in the keel. This freezing process release heat and increases the temperatures in the rubble. In this way the decay phase includes both melting and freezing. Freezing can take place as long as there is cold capacity (ice temperature less than the freezing point of the melt water) in the keel. However, another mechanism can contribute to further consolidation. If the pore water salinity is changed cyclically, either by periodic surface melting or by tidally driven river runoff the ridge could actually expel heat into the surrounding water
and contribute to further freezing (consolidation). This mechanism is only shown in laboratory investigations and in simulations. Finally the ridge keel could collapse and in this way decrease the porosity and increase the degree of consolidation. By the end of the melt season the ridge has become a second-year ridge.

The paper does not say that ridges cannot form without freezing air.  It only describes the typical life cycle process where a ridge forms with freezing air which allows the ice to consolidate through further freezing, then melt during summer and possibly survive as a multi-year ridge.
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Michael Hauber

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Re: The science of ice ridging and rafting
« Reply #4 on: July 24, 2020, 02:27:39 AM »
Very good thread. Add the assumption of deep summer, ice core temp is at the melting point of -1.8C, and air temps are above that, so the ice is structurally weaker than in winter and there can be no freezing together of floes or rubble. And that we are interested in events in the middle of the ocean. It is quite obvious that some ridging can occur even in summer with enough pressure against a static shore.

People on the melting season thread have mentioned seeing ridging on the Pole Cam, may it rest in peace. Does anybody have access to those images? Or can anyone post accurate recollections of such events?

So the ridge cannot consolidate, but there is nothing to stop it forming.

Ridging is driven by two things - A high enough force pushing ice together.  And week enough ice that instead of resisting the force the ice buckles allowing parts to be forced up and over each other in some form or another.  During summer the ice will be weaker, so riding will be easier.  There will be less opportunities for ridging as most of the time the ice pack is disperse, and is typically only forced up against the coast along the Greenland/CAA north shore, in contrast to winter when ice can be forced against Alaska, Siberian and Laptev shores, as well as the islands on Russian side.  So 30 percent production of ridge ice in summer compared to winter is a fair bit in my opinion.
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Michael Hauber

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Re: The science of ice ridging and rafting
« Reply #5 on: July 24, 2020, 03:03:27 AM »
Image of recent Arctic below

Much of the central pack has faint white lines which are similar in pattern to that expected from ridging.  Note that these lines are absent from areas to left and top right marked with ovals.  These areas have large areas beside them allowing relief of pressure, considering that the main pressure is built up by an anti-cyclonic circulation centered somewhere to the bottom right of the image.  In the top right is an anomalous white streak in an area where the pressure would be relieved by adjacent open water.
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wdmn

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Re: The science of ice ridging and rafting
« Reply #6 on: July 24, 2020, 03:13:24 AM »
Michael, please note that the study was looking at the ice up until 2003.

That is relevant because the 30% summer (23% figure in terms of total annual ridging - which, if it were evenly distributed over the summer months would be 4.6% per month) figure has likely declined. My justification for believing that is the time period covered by the study, and the fact that volume of ridged ice peaked in June:

"This annual cycle is explained by the fact that before June, ice stays adequately compact in the Arctic basin and ridging continues..."

In the study, summer was counted as May-October. So almost all of that summer ridging happened before peak melt. The paper states that the lowest monthly production is in August, when the most energy would be in the system:

"Only a small percentage of the level ice volume is involved in the ridging processes during each month. This amount varies between 0.2% and 7.4% and has an average of 2.2% for August when monthly ridge production is minimal..."

Obviously conditions have changed a lot in the arctic, and especially this year where so little of the ice has been close to shore. This has to be acknowledged, since the ice is already reached an extent that would have been close to annual minimum in almost all of the years the study looked at, and lower than August. Moreover, the paper seems to be saying that almost all of this ridging occurs along the coast.

Let's also recall the reason why this discussion came up in the first place: it was claimed that the GAAC created enough ridging/thickening of the ice this year that it would impact the September minimum by preventing/slowing melt. Is there anything in the papers put forward so far that suggests that is likely?

ajouis

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Re: The science of ice ridging and rafting
« Reply #7 on: July 24, 2020, 08:15:12 AM »
michael yes it says it needs below sea ice freezing air, it considers ridge formation only when there is a combination of block that get glued together by new ice, while consolidation happens in summer when the ice gets less salty and smoother from melt. It technically depends on what you consider ridging but to only consider whether there is rubble makes it a shallow definition, ridging being a source of ice thickening, which can only be true with the mechanism described above. Also you have provided no counter info on ridge formation, or had anything that would have indicated the processes described were only typical, and the source is categorical itself, nit making exceptions or qualifying the processes described as solely typical
« Last Edit: July 24, 2020, 08:25:59 AM by ajouis »
After a thousand steps on the ice, it cracked.
The Man looked down at the infinite blue of the sea.
On the horizon, standing still, the polar bear had just scented his next meal.

 Less than 3000 cubic kilometers this Piomas minimum.

Pmt111500

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Re: The science of ice ridging and rafting
« Reply #8 on: July 24, 2020, 08:25:30 AM »
My basics-cartoon on this, three years back, does not delve in to the mathematical modelling. Republishing, free to use, if the need arises.

gandul

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Re: The science of ice ridging and rafting
« Reply #9 on: July 24, 2020, 09:13:58 AM »
It's very close-minded to shut the door to the possibility of ridging or piling up of some kind in Summer just because studies are focused to Winter, naturally. But we have just passed a 20-day compressing event which is quite abnormal as well.

It's clear the ice has received a major beating across the CAB. But I won't go with the herd dismissing what naturally comes to mind given known physics because it's against their dream to see major record this year.

Ice piling up must have happened in substantial degree. I buy ajouis argument (supported by science) that to consolidate these pressure piles into ridges below freezing temps are needed. But that doesn't mean the CAB ice, although having lost 50-100 cm of thickness due to melting, is not more protected because of cohesion and piling up rather than being torn apart. Which is going to happen anyway as storm is forecasted to rip apart whatever mess is left down there. So probably the herd will be delighted soon and forget this discussion.

And remember, the water under the CAB ice is really cold

wdmn

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Re: The science of ice ridging and rafting
« Reply #10 on: July 24, 2020, 09:37:37 AM »
It's very close-minded to shut the door to the possibility of ridging or piling up of some kind in Summer just because studies are focused to Winter, naturally.

And it's pretty unhelpful to join this discussion by leading off your post with a straw man.

oren

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Re: The science of ice ridging and rafting
« Reply #11 on: July 24, 2020, 09:40:59 AM »
So the ridge cannot consolidate, but there is nothing to stop it forming.

Ridging is driven by two things - A high enough force pushing ice together.  And week enough ice that instead of resisting the force the ice buckles allowing parts to be forced up and over each other in some form or another.  During summer the ice will be weaker, so riding will be easier.  There will be less opportunities for ridging as most of the time the ice pack is disperse, and is typically only forced up against the coast along the Greenland/CAA north shore, in contrast to winter when ice can be forced against Alaska, Siberian and Laptev shores, as well as the islands on Russian side.  So 30 percent production of ridge ice in summer compared to winter is a fair bit in my opinion.
My intuition is that when the ice is weaker riding will be harder. Initially pressure may cause ice floes and especially their weaker edges to crumble, allowing the pack to be compacted without ridging, and accelerating the melt by transferring the mechanical energy into breaking the ice bonds.
Of course, with a strong enough pressure floes can be forced to over-raft one another even in such a situation, but I sincerely doubt this can happen a lot in the open ocean, not against land. And when it does happen, I sincerely doubt this makes the ice more resistant to melting. The floe that was over-rafted has its keel trail deeper in the water, at a time when floes are being transported very fast by the wind. This surely enhances bottom-melt and causes more damage than any potential advantage of said over-rafting.

binntho

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Re: The science of ice ridging and rafting
« Reply #12 on: July 24, 2020, 11:33:42 AM »
Good thread. Although I'd like to point out to Michael that if he thinks anticyclonic ridging during summer can be seen in satellite imagery, he has some maasssssssive explaining to do.

And to gandul and others - I dislike phrasing like "Ice piling up must have happened in substantial degree". Totally groundless without a shred of evidenced, not only making a "must have happened" claim but adding "substantial" to it as well!

Why "must" it? And where is the evidence for "substantial"? Please give reasoning and evidence for any claims, and don't just repeat them at nauseam.
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Tor Bejnar

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Re: The science of ice ridging and rafting
« Reply #13 on: July 24, 2020, 05:27:33 PM »
I think the "above freezing air temperature" requirement is a red herring. 

Why?  Take two 2-cm thick slabs of ice from your freezer; place one on top of the other in a basin ('cause it will melt some); wait a half hour.  How many slabs of ice do you have?  One!  The air was warm but the ice 'redistributed' the heat and froze the two together.

Now, in the Arctic during the mid- to late summer, there is not a great deal of rafting because the ice is generally mobile and there is not much putting one floe on top of another.

I think that much mechanical ice thickening happens when the ice acts like a bunch of turtles on a barely submerged log.  Each turtle's tail is under water and each turtle's belly is on top of its neighbor's back.  Along coastlines the first-year ice can get tens of meters thick, grounded on the shallow seabed and pushed way up into the air on top.  This definitely happens every winter near Utqiagvik, and never (any more, at least) in late summer.  On the northern Greenland coast, on the other hand, I expect it happens year round (except when there is a coastal polynya there).

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ajouis

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Re: The science of ice ridging and rafting
« Reply #14 on: July 24, 2020, 05:52:05 PM »
Tor, respectfully you didn't understand it, it is a below freezing air requirement, because otherwise no ice to serve as glue. Also I think you are making an hyperbole in how thick how quickly the ice can get because the biggest floe ever recorded was 45 meters thick and 100 years.
After a thousand steps on the ice, it cracked.
The Man looked down at the infinite blue of the sea.
On the horizon, standing still, the polar bear had just scented his next meal.

 Less than 3000 cubic kilometers this Piomas minimum.

Tor Bejnar

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Re: The science of ice ridging and rafting
« Reply #15 on: July 24, 2020, 07:06:57 PM »
I'll agree that I don't understand some things!  :)
However, you wrote
Quote
the formation of the sea ice ridges, they require freezing (-1.8 ) air temperatures to form.
The ice needs to be freezing cold, yes, but the air doesn't, per my experiment (usually done with lots of ice cubes in iced tea in a glass).

Sea Ice Thickness Surveying with Airborne Electromagnetics - Grounded Ridges and Ice Shear Zones near Barrow Alaska
Stefan Hendricks and Thomas Krumpen, Alfred Wegener Institute · February 2014
DOI: 10.4043/24552-MS

Their graphic shows a pressure ridge pretty darn close to 20 m thick (~18 meters down and ~2 meters up).  Does this qualify as "10's of meters thick"?

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ajouis

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Re: The science of ice ridging and rafting
« Reply #16 on: July 24, 2020, 07:20:48 PM »
(sure Tor and how old was it?) (edit: next time don’t bury the lead) plus keep in mind the general loss of thick ice in the last few years.

Your experiment is flowed because it doesn’t have the same variables, plus it isn’t even what you originally said. What I quoted is from an engineering perspective so I do think they would have accounted for other forces if there were, anyways show just one source that says it isn’t true or become one (preferably get published) then you can refute the point several sources made.
If you want to pursue the experiment keep in mind that in reality the ice is not much cooler than its environment as opposed to your experiment, plus mechanical movement means the temperature needs to be actually lower than freezing point, of course salt the water.
After a thousand steps on the ice, it cracked.
The Man looked down at the infinite blue of the sea.
On the horizon, standing still, the polar bear had just scented his next meal.

 Less than 3000 cubic kilometers this Piomas minimum.

gandul

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Re: The science of ice ridging and rafting
« Reply #17 on: July 24, 2020, 07:38:50 PM »
Good thread. Although I'd like to point out to Michael that if he thinks anticyclonic ridging during summer can be seen in satellite imagery, he has some maasssssssive explaining to do.

And to gandul and others - I dislike phrasing like "Ice piling up must have happened in substantial degree". Totally groundless without a shred of evidenced, not only making a "must have happened" claim but adding "substantial" to it as well!

Why "must" it? And where is the evidence for "substantial"? Please give reasoning and evidence for any claims, and don't just repeat them at nauseam.
No, you give evidence that it can’t happen. I state obvious Science. You let me know why it is not obvious that, after 20 days of constant convergence of ice streamlines toward a center slightly offset toward the Pacific wrt to the Pole, with 1030+ hPa practically the whole 20 days, there must be MANY places where ice, melting but yet of >1m thickness, being fragile solid material that easily buckles under compression, has been piling up. Explain to me otherwise how the ice becomes super plastic and deforms so as to keep as a 2D sheet, or a regular bunch of floes very tight but no lifting whatsoever. Find the proof or shut it. It is you who have to produce proofs, or conjectures that are scientifically well poised, please

You explain to me the process  by which you melt at exactly the rate needed and its three-dimensionality (it’s not enough with top/bottom melting to make space to the compressing ice) so that melt alone can explain the huge, unprecedented reduction in extent (with a long area hiatus, by the way). There’s been a lot of melt but you underestimate the mechanical effects of convergence, just as many people still don’t understand the destructive effects of divergence as well.

You explain, please, but don’t come correcting from your extremely high place if you don’t come with convincing arguments yourself. Please.


oren

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Re: The science of ice ridging and rafting
« Reply #18 on: July 24, 2020, 07:42:18 PM »
Tor, I think the experiment is problematic because in our case this being July and sunny ice core temp is at -1.8C, as many Mosaic buoys have shown. So not much "freezing energy" in the system. The only thing that can glue floes together is freshwater, than can freeze in contact with the salty but colder ice. I doubt this can provide serious structural stability.

Ajouis, I recommend reading the very short Wikipedia entry about Stamukhas (grounded pressure ridges).
https://en.wikipedia.org/wiki/Stamukha

Quote
Stamukhi tend to occur in belts that are parallel to the shoreline, along coastal shoals, at water depths of about 20 m, but that can reach 50 m

Of course, this is not directly relevant to our discussion as the big question was about pressure ridges and over-rafting in the open ocean.

Tor Bejnar

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Re: The science of ice ridging and rafting
« Reply #19 on: July 24, 2020, 08:52:37 PM »
Some good points, Oren.

I'm not a physicist, but I'd guess physically putting -1.8C ice on top of 0.0C ice will yield some bonding across the threshold, enough to keep the top floe from casually sliding off the bottom one, if held in place for a while. The salty water will certainly slow or prevent bonding, but the water created by ice-to-ice compression (heat) where the ice actually connects would be much fresher and might freeze at -1C.  Is there a physicist in the house?  Neither surface will be actually flat, so a certain amount of 'one floe's bumps' matching 'the other's depressions' will hold the surfaces together, at least under calm conditions.

In the open sea under atmospheric high pressure, I do not expect there to be much piling of ice on ice, some, but not much.  Under a cyclone, there would be much more, but the turbulence will cause salty water to be 'everywhere', and whatever time it takes for two slabs of ice to stick together would likely be countered by waves causing the top floe to slip off.  (When some of the ice is 40 below, not much time is required!)

As soon as two floes are stacked, of course, the air temperature becomes irrelevant except for how it affected the ice surface and near-surface temperatures.  During a hot day in May, the air may be warm, but the ice is still very cold (at least in its core).
« Last Edit: July 24, 2020, 09:26:42 PM by Tor Bejnar »
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Michael Hauber

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Re: The science of ice ridging and rafting
« Reply #20 on: July 25, 2020, 12:12:41 AM »
michael yes it says it needs below sea ice freezing air, it considers ridge formation only when there is a combination of block that get glued together by new ice, while consolidation happens in summer when the ice gets less salty and smoother from melt. It technically depends on what you consider ridging but to only consider whether there is rubble makes it a shallow definition, ridging being a source of ice thickening, which can only be true with the mechanism described above. Also you have provided no counter info on ridge formation, or had anything that would have indicated the processes described were only typical, and the source is categorical itself, nit making exceptions or qualifying the processes described as solely typical

So what do you think will happen when a pile of rubble shaped like a ridge happens in summer?  It won't freeze together (or maybe it will if Tor is correct).  But it will still make the ice thicker.  And then when the winds change and the ice separates it will all fall down and spread out again.
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Re: The science of ice ridging and rafting
« Reply #21 on: July 25, 2020, 12:18:46 AM »

My intuition is that when the ice is weaker riding will be harder. Initially pressure may cause ice floes and especially their weaker edges to crumble, allowing the pack to be compacted without ridging, and accelerating the melt by transferring the mechanical energy into breaking the ice bonds.
Of course, with a strong enough pressure floes can be forced to over-raft one another even in such a situation, but I sincerely doubt this can happen a lot in the open ocean, not against land. And when it does happen, I sincerely doubt this makes the ice more resistant to melting. The floe that was over-rafted has its keel trail deeper in the water, at a time when floes are being transported very fast by the wind. This surely enhances bottom-melt and causes more damage than any potential advantage of said over-rafting.

So is your argument that the ice will still crumble together, but it won't get thicker because the ice will be melted by the release of mechanical energy?

I don't think that ice floes being pushed up against each other mid ocean is something that happens a lot.  But the recent circumstances are exceptional and I have not seen anything like the same degree of compactness in MODIS imagery since 2011.

I would not argue that the rafting/ridging would make the ice more resistant to melt, and would not be suprised if it does melt a bit faster because it is sticking up or down and exposed to more energy transfer.  But it will take a while to melt back to its original thickness.
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Re: The science of ice ridging and rafting
« Reply #22 on: July 25, 2020, 12:40:01 AM »
Good thread. Although I'd like to point out to Michael that if he thinks anticyclonic ridging during summer can be seen in satellite imagery, he has some maasssssssive explaining to do.


Whats to explain?  Are the white streaks that look like ridging not visible to you?  I cannot prove that they are for sure ridging, but look at the facts:

-ridging is proven to happen mid ocean.  So you can't say that they can't be ridges because they are mid ocean
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Re: The science of ice ridging and rafting
« Reply #23 on: July 25, 2020, 12:43:07 AM »

Let's also recall the reason why this discussion came up in the first place: it was claimed that the GAAC created enough ridging/thickening of the ice this year that it would impact the September minimum by preventing/slowing melt. Is there anything in the papers put forward so far that suggests that is likely?

I never actually claimed that.  My reasons for thinking a minimum are unlikely are primarily based around the implications of no open water within the ice pack.  When I first stated this theory I did think it would make things worse(better) but not sure whether that would be 0.1% or 10% etc.  Now I know it is included in PIOMAS.  So it might be the reason why PIOMAS is not lower, but its not a reason to think that there is more ice than PIOMAS says.  Perhaps PIOMAS is underestimating the amount of ridging and there is more ice than PIOMAS says.  Or perhaps PIOMAS has too much ridging and there is less ice.
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Re: The science of ice ridging and rafting
« Reply #24 on: July 25, 2020, 01:45:19 AM »
Good thread. Although I'd like to point out to Michael that if he thinks anticyclonic ridging during summer can be seen in satellite imagery, he has some maasssssssive explaining to do.


Whats to explain?  Are the white streaks that look like ridging not visible to you?  I cannot prove that they are for sure ridging, but look at the facts:

-ridging is proven to happen mid ocean.  So you can't say that they can't be ridges because they are mid ocean

Michael,

If you glanced at the articles I shared in the first post of this thread, you would see that assessing ridging from satellites is usually quite difficult. There's also the fact that some of the white streaks you are referring to are 100s of kms long, but there are no floes that large. Finally, you only provided an "after photo," so that even if those white streaks are ridges, you haven't provided any evidence that they were formed during the GAAC.

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Re: The science of ice ridging and rafting
« Reply #25 on: July 25, 2020, 08:30:19 AM »

Michael,

If you glanced at the articles I shared in the first post of this thread, you would see that assessing ridging from satellites is usually quite difficult. There's also the fact that some of the white streaks you are referring to are 100s of kms long, but there are no floes that large. Finally, you only provided an "after photo," so that even if those white streaks are ridges, you haven't provided any evidence that they were formed during the GAAC.

Well difficult enough to require an entire paper to be written about techniques for assessing ridges from satellite data.  However this is to assess ridging well enough for navigation purposes which ideally needs to know the height of the ridge and the presence of all ridges big enough to impact a ship, and not just that may be big enough to be seen on MODIS imagery.

And yes the streaks are 100s of km long, and they are not formed with one individual floe.  Ridges can only form when ice is pressed together in one continuous sheet, which is why they are much less common in summer.  The pressure builds up, and perhaps at first only one floe breaks and you get a ridge a few meters long.  But then this weakens the ice, and so ice at the end of the ridge is under even more force, and so the next floe breaks.  So the ridge can then extend for as far as the pressure continues and the ice sheet is continuous.  The sea ice has been compacted into a single ice sheet well enough that now that dispersion is occurring cracks are forming that are visible at the same scale and are many hundreds of kilometers long.  The process of lead formation is basically the same as for ridges where the ice splits locally due to stress becoming stronger than ice strength, which causes more stress at the end of the crack which then extends.

location
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Re: The science of ice ridging and rafting
« Reply #26 on: July 25, 2020, 09:45:23 AM »
Michael,

You are quite tenacious. Through this back and forth, I finally think we are getting to a clearly formulated hypothesis.

The papers I included in the first post above, and most reading on the internet confirms that ridging occurs primarily between ice sheets of different thicknesses. For a ridge to form 100s of kms long can only occur if there is a crack in the pack and a lead forms (i.e. new, thin ice). Subsequently, "the two sheets of ice on either side of a lead can collide with a force large enough to form ridges," out of the thinner lead ice. (NASA article on MOSAiC https://earthobservatory.nasa.gov/images/146508/drifting-with-broken-sea-ice)

So then, your hypothesis is that the GAAC formed pressure ridges in areas where there were new leads (formed maybe as late as April?) with thin ice that could be crushed.

For reasons already stated in this thread, it seems unlikely that such thin ice on newly formed leads would have maintained its structural integrity through May and June (and the insolation during the GAAC).

Nevertheless, as evidence for your hypothesis you present the MODIS images showing what you say are pressure ridges.

If we grant that these are pressure ridges, you still have yet to prove that these ridges formed during the GAAC.

I was on the NISDC website and found this: "ridges are initially thin and transparent with very sharp edges from blocks of ice piling up; also see keels" (https://nsidc.org/cryosphere/glossary/term/ridging).

This makes sense given that they are formed over open water in areas with thin, lead ice, but it doesn't bode well for your MODIS evidence, since if these were ridges formed during the GAAC, they should be "thin and transparent," and likely not show up from space.

In summary, your hypothesis has been clarified as: "pressure ridges were formed during the GAAC in areas where new leads were crushed by the movement of the pack on either side of the lead."

But,

The only evidence you have provided, even if it does depict pressure ridges, does not establish that these ridges were formed during the GAAC, and we now have additional evidence that suggests such ridging -- if it did occur -- is highly unlikely to be visible on MODIS.
« Last Edit: July 25, 2020, 10:08:28 AM by wdmn »

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Re: The science of ice ridging and rafting
« Reply #27 on: July 25, 2020, 12:17:09 PM »
I don't think ridges are only formed on leads.

Also I can't see why ridges cannot be formed on ice sheets of similar thickness.  One paper does say that rafting occurs more often on similar thickness, but others don't mention this.  Whether rafting or ridging the end result is the ice becomes more compact and gains thickness to compensate.

The claim that ridges start out thin and transparent seems quite bizarre even if it is from NSIDC.

From your link, this image looks like a reasonable fresh lead - the blocks are distinct with sharp edges that haven't been worn away, or filled in with snow, picture taken March 11.  I cannot see how this could be described as transparent or thin.




from rubble ice

This picture is intriguing and seems to have similar characteristics to the satellite images I showed earlier.  Similar patterns can often form at smaller and larger scales.



Also revisiting the MODIS images I would estimate the width of the white streaks is somewhere near 2km.  So not a standard ridge.  But looking at above image it is possible for many ridges to form near each other to form a larger area with a brighter white colour.

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Re: The science of ice ridging and rafting
« Reply #28 on: July 25, 2020, 05:50:49 PM »
I didn't say it "only forms on leads." You presented images you claimed (without any evidence) showed pressure ridges from the GAAC. They were 100s of kms long. Either they formed along leads, or a ridge formed along 100s of kms of pack ice between floes of all varying thicknesses during the summer months in unison (in spite of all of the different thicknesses potentially involved). The latter seems even more unlikely the former, as I'm sure anyone who is reading this thread will agree. That is why I concluded what your hypothesis must be -- I was giving you the benefit of the doubt. It seems clear now that you don't actually have an hypothesis, beyond that there was ridging during the GAAC -- potentially so significant as to effect the PIOMAS numbers -- and you'll throw anything that you think might stick.

The "one" paper that says that is the one looking at the mechanics of ridging versus rafting. Your statement is very disingenuous. However, it is also said in many other places, such as the report from Mosaic I sent, and -- according to wikipedia -- in the text book Weeks, W. F. (2010) On sea ice. University of Alaska Press, Fairbanks, p 664: "The blocks making up pressure ridges are mostly from the thinner ice floe involved in the interaction, but it can also include pieces from the other floe if it is not too thick."

Granted that the white lines could be pressure ridges, if those "ridges" are 2km in width you've got even more explaining to do. Not even rafting has been documented anywhere close to that.

As for the photo, where does it say the ridge was freshly formed, or that it was formed in summer? And even if you find some examples of ridges forming from multiyear ice (covered with accumulated snow), how does that prove that such things were occurring in the GAAC? Because they would better match your MODIS imagery? Your argument is circular.

When hunting for ice ridges, radar satellites are used. Not sure if we access to those, but if so you might look there.

I also note that all discussion of ridges from the MOSAiC stories describe the events as occurring in winter during freezing season: "But the ice dynamic wasn’t just a logistical problem; it was also an outstanding object of study – pressure ridges, and their effects on energy transfer, lead formation and biological activity, which were revealed – we finally had the chance to monitor and investigate all these things that take place on the ocean in winter, yielding a treasure trove of new data"

« Last Edit: July 25, 2020, 06:41:23 PM by wdmn »

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Re: The science of ice ridging and rafting
« Reply #29 on: July 26, 2020, 12:12:08 AM »
I don't think you are understanding my arguments.

I gave evidence.  You may choose to disagree with it but claiming I provided no evidence is a closed mind.
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Re: The science of ice ridging and rafting
« Reply #30 on: August 03, 2020, 03:09:20 AM »
Going over some older material, I found this on the Mosaic thread which I thought was of interest to this thread.

Here is an English Translation of a transcript of the latest Podcast from the MOSAIC Website that is only available in German.

The Podcast was posted last Wednesday (January 22)  at 5:57 PM on the MOSAIC Website (in German) - the recording itself was probably made 8. January

Installment 11 – thick ice, four-legged visitors and slight frostbite

In the meantime, the team around the new leader of the MOSAIC-Expedition, Christian Haas, has accustomed itself to life on the ship and to the camp on the Ice Floe. In this installment the Sea Ice physicist goes into detail about the composition of the ice and how it is continuously changing. Apart from this, Dr. Haas also reports on measurements made by other scientific disciplines and explains how an aircraft landing-strip is made on the ice. The continual decrease in temperatures and the ongoing polar night present further challenges for the members of the expedition. And, this week, the camp got another animal visit.

…..[Ed.: Just so that transcript readers don’t miss out on the atmosphere of the podcast they should know that it is preceded and ended by sound recordings of strong wind and creaking ice….]

Arctic Drift – The audio logbook.

Christian Haas:   At the moment we are at 87 degrees 8 minutes North. During the MOSAIC expedition the ship this the most Northerly that the ship has been. [Ed.: According to the positions reported on MOSAIC webpage this would date the time that this recording was made a being around 8. January]

Commentator: In the meantime, the leg 2  Team has adapted to arctic conditions. The crew around the new  Expedition leader Christian Haas has familiarised itself  with the Icebreaker  Polarstern and the condition on the home floe. Dr. Haas himself is head of the Sea Ice Physics section of the  Alfred Wegener Institute and can precisely explain what an ice floe is and why the ice in the arctic is constantly changing.

Christian Haas: we are always using the term “ice floe”, but everyone probably imagines something different under this term…and at this time of year, in the middle of winter, there aren’t really any, anymore. When the  Polarstern arrived here at the beginning of October, it really was the case that there were individual ice floes drifting in the water. They were separated from each other by water or thin ice. But the ice and the ice floe formed a unit and could be regarded as a swimming platform.   The MOSAIC ice floe had a diameter on the order of two to three kilometres. But since we have been here and the winter has begun, the whole area around us has frozen solid, so that one can’t make out individual ice floes, because the borders between them are not visible, except with the help of Satellite data. Nonetheless it’s the case that the ice floe isn’t a plate, it isn’t a simple uniform plate of ice, but, as before, it regularly fractures and is displaced by shear zones. Till now we have just had a lot of luck that such shear zones and fractures didn’t go directly through our camp but were some distance away. Just yesterday we made an exploratory tour with snowmobiles to the West and East and at a distance of about two to three kilometres in each direction we found tears and shear zones.  With that we could say that the floe is  two to three kilometres in size, but the Northern and Southern boundaries haven’t been found yet. 

Commentator: The ice and the alterations in it are being constantly observed. Using different kinds of measurements it is possible to completely understand the displacement of the ice. Many researchers view these displacements as a danger, because they can lead to interruption in their research. Others welcome the possibility being able to  observe and analyse them directly. 

Christian Haas: The ship’s radar, that every 10 minutes makes an image of the surroundings within a radius of 5 kilometres, helps us a lot. When one looks at a time series of these images it’s like looking at a film of the ice movement. Most of the time the ice is stable, but sometimes one sees shear events, where, because of a difference in the extent of ice-drift in different regions, a part of the floe suddenly slides by between several metres up to as much as 100 metres relative to the other part. These zones produce tears and the formation new pack ice ridges.

Christian Haas: For most of our colleagues here the tears and the formation of pack ice ridges are seen as a hazard, because they interrupt research. But for us as researchers and  for the whole MOSAIC project of course it’s an important process that we want to investigate.  This is because we want to better understand why the ice in the arctic has declined so much during the last few decades and to find out what processes result in the ice becoming thicker or thinner. The growth of pack ice ridges, the deformation of the ice and the sliding of pieces of ice on top of each other  is a very important process and can make ice much thicker than it would become through solely as the result of freezing through contact with the cold atmosphere. For this reason the sea ice researchers and remote sensing experts who are involved in our project are very thrilled to be able to observe such deformation events at first hand and to be able to see how the ice can continuously become thicker through floes fracturing and sliding on top of each other. 

Commentator: In the meantime, the floe ice is circa one metre thick and has doubled in thickness since the beginning of the expedition in October.  In comparison, the so called “pack ice ridges” are considerably thicker. To investigate them more thoroughly various instruments have been installed in the ice.

Christian Haas: We see here that some pack ice ridges are up to three metres high. Pack ice ridges are like icebergs, that means that roughly a tenth of appears above the surface and nine-tenth of them are under water. It follows that where there are pack ice ridges the ice can be 10, 20 or even more metres thick. We have observed this with our remotely controlled ROV, with which we were able to make  sonar measurements of the ice depth and we have already found ice thickness of over 10 metres. In our last big action, we installed a number of instruments in some of these pack ice ridges. We call this the “Pack Ice Ridge Observatory” and we want to use it to observe on the one hand how the underside is eroded by currents and by the warmth that is present in the sea water and on the other hand  how the pack ice ridges are frozen from above. In addition, we want to know how, because of their rough surfaces, they are affected by turbulence in both air and water and whether this is important for their growth or melting.

Most ice measurements are made by drilling holes in the ice and then placing instruments underneath the ice or in it. That’s exactly what we have to do here. We have placed large measuring devices, that require large holes to be drilled, at the periphery of the pack ice ridges and underneath the ice. These are for instance instruments that measure water currents and turbulence.  Then we embedded thermistor chains over the whole ridge as well as in the thickest ice, that was more than 8 metres thick in places. With these chains we can observe how the ridges cool, how they freeze in the centre, and how the processes of erosion and disintegration  take place on their undersurface.

Commentator: Research in other scientific disciplines is also ongoing. A great deal of weather data is being collected both on the Polarstern and  in the Ice Camp. Still lower temperatures than the current low temperature of minus 35 degrees have been measured there.

.....

Commentator: Christian Haas is looking forward to the coming weeks and the upcoming research work, although the conditions for it won’t be made easier by the continuing polar night and the further decrease in temperatures. He is very focused on keeping the goal of the MOSAIC expedition in his sights.

Christian Haas:  We are still moving into February and March, that’s actually the coldest season in the arctic. That means that weather conditions will become even more extreme. Nonetheless, I believe we will continue to work enthusiastically on the ice. Because now, just as we are slowly beginning to get good time-series measurements of atmospheric conditions and of ice and water conditions, the information is becoming increasingly interesting and we are getting nearer to fulfilling the goal of the MOSAIC expedition to investigate the interaction between the atmosphere, ocean and  ice and, and we shouldn’t forget that, the Biology of the arctic. So I hope that we will continue to be able to work unhindered. Naturally one or two ice deformation events should also take place, if possible, at quite a distance from the  ship. We will continue to expand the radius of the area that we move in. Apart from that, I hope that we might indeed get a storm that will bring us some snow. Till now the snow has been very sparse, the snow cover is between eight and twenty centimetres. And what we also hope for, even if it may sound paradoxical, is that at some stage we get an intrusion of warm air that will temporarily give rise to very high temperatures and even to a little rain, as has been seen with increasing frequency in past years and about which there has been much speculation as to the effect it has on ice cover and ice growth. We now have a unique opportunity to observe the phenomena on the spot and that is absolutely necessary to better understand these processes.

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Re: The science of ice ridging and rafting
« Reply #31 on: August 03, 2020, 03:34:36 AM »
Oceans & Sea Ice NPI
@OceanSeaIceNPI
Jul 2
Our team on @MOSAiCArctic is establishing a new ridge site dubbed "Jaridge" for our @RCN_Norway #HAVOC project to study ridges in the last phases of the expedition. Photo shows a ROV multibeam mapping of the ice draft at the site.
 

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Re: The science of ice ridging and rafting
« Reply #32 on: August 03, 2020, 04:02:58 AM »
What looks like the aftermath of a pressure ridge. I lack info on where and when photo was taken, but the Instagram credit is:
Photo: Jean Negrel (@ins_jeann), then @norskpolarinstitutt


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Re: The science of ice ridging and rafting
« Reply #33 on: August 03, 2020, 07:30:04 AM »
What looks like the aftermath of a pressure ridge.
You mean the vertical bits of ice? From what I've seen in photos and read in descriptions, these would be all over the place in winter and could reach severeal meters height and width.

Winter-generated pressure ridges would still be visible during summer melt, i.e. in the image above, the vertical bits are most likely the remains of winter-generated ridges.
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Re: The science of ice ridging and rafting
« Reply #34 on: August 03, 2020, 01:00:53 PM »
What looks like the aftermath of a pressure ridge.
in the image above, the vertical bits are most likely the remains of winter-generated ridges.
This is what I meant.