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Author Topic: Basic questions and discussions about melting physics  (Read 32018 times)

blumenkraft

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Re: Basic questions about melting physics
« Reply #100 on: September 02, 2019, 07:27:09 PM »
Hence, they is little agreement.

Totally agree to disagree. :)

petm

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Re: Basic questions about melting physics
« Reply #101 on: September 02, 2019, 07:35:11 PM »
Are you talking about rolling 1 six after having rolled five sixes already, or rolling 6 sixes after having rolled none? Completely different questions.

Also, wtf?  ??? ;D

crandles

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Re: Basic questions about melting physics
« Reply #102 on: September 02, 2019, 07:39:28 PM »


Quote
To know whether or not those indicated changes are real or just the impression given by random noise, a good way is to apply changepoint analysis. That confirms that these changes are indeed real, and gives the following “continuous piecewise linear” model of the anomalies.

https://tamino.wordpress.com/2016/07/17/arctic-heat/

Seems like Tamino confirmed it was real/statistically significant back in 2016?

We are rather off topic lets move this to https://forum.arctic-sea-ice.net/index.php/topic,2348
(When will the Arctic Go Ice Free?)
« Last Edit: September 02, 2019, 09:41:44 PM by crandles »

petm

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Re: Basic questions about melting physics
« Reply #103 on: September 03, 2019, 12:55:29 AM »
There is a brief discussion of latent heat fluxes (which are clearly well understood by climate scientists) mentioned up-thread in part 2 of this video (@~7 min). The whole video is well worth watching, as are all of the channel's videos. Solid, current climate science explained well in laymen's terms.



PS. Should I move this to a different thread, and where?

Klondike Kat

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Re: Basic questions about melting physics
« Reply #104 on: September 03, 2019, 03:56:31 AM »
Thanks crandles.  Will do.

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"It is preoccupation with possessions, more than anything else, that prevents us from living freely and nobly" - Bertrand Russell
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binntho

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Re: Basic questions about melting physics
« Reply #106 on: September 03, 2019, 07:58:49 AM »
binntho, you have me mixed up with crandles. It was his/her find and posts:
https://forum.arctic-sea-ice.net/index.php/topic,2709.msg226369.html#msg226369
https://forum.arctic-sea-ice.net/index.php/topic,2348.msg226396.html#msg226396

You are quite right nanning - my mistake, I've corrected it in the post above. Thanks!

EDIT: I've moved the post to the "When will the Arctic go Ice Free" thread as suggested by Crandles. Seems like I'm just now waking up, after 2 hours of hectic activity!

https://forum.arctic-sea-ice.net/index.php/topic,2348.msg226465.html#msg226465
« Last Edit: September 03, 2019, 08:06:12 AM by binntho »
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Feeltheburn

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Re: Basic questions about melting physics
« Reply #107 on: September 23, 2019, 09:33:23 AM »
Felltheburn,

To some extent, you are thinking along the right lines. Alternatively, you are messing up some of the basic physics.

As long as we are in the Tropics, everything is honkydory. Most physical processes take place well above 0C (or 273K). It is when you start using calories and F, your problems begin.

When in the Sub-tropics, some of the precipitation comes down as hail. In recent years, we have seen an increasing hail diameter and more devastating hailstorms spreading north. This may be a sign of the kind of physics you focus on. More evaporative cooling in combination with more humid air should eventually lead to bigger hail, if condensation nuclei are present.

Moving to Mid-latitudes, we begin to have the change-over from freezing rain to warm rain. Apparently, you have forgot to include the temperature of the falling rain. All heat evaporated is not simply sent out to outer space. Some of it will return to Earth as a mild, warm rain, where you would even enjoy getting your underpants wet from time to time.

Now, finally to the physical processes in the Arctic. Your idea that all evaporative cooling will eventually leave the Arctic cold and the World in thermal balance (had it not been for the wicked ocean heat storage), is simply wrong. The moisture advected from southerly latitudes (as well as the minor part of the moisture evaporated from open Arctic waters), will eventually have to come down again as either rain (above 0C), or as snow. In the latter case, we may see the opposite of "sublimation" - that is water vapour going directly into the frozen phase - which of course releases a lot more energy, than just converting water vapour directly into rain.

In the presence of cloud condensation nuclei, we will see hailstorms spreading north, and we will see rain and showers entering the Arctic during winter. What we have not seen yet, is the physical reaction from/to a clean - basically CCN-free - Arctic atmosphere. My guess would be that initially, we will see ice pellets, but eventually we will see drizzle as the Arctic temperatures during winter goes above 0C.

Sitting in Svalbard through the dark "drizzle season" is no great joy I would presume. Maybe it will be a great time to reflect on the basic physics of our time.

Cheers P

Thank you for your thoughtful response. Just trying to understand processes that no doubt affect weather, which is dynamic, but which may not be well accounted for in the climate models.
Feel The Burn!

Glen Koehler

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Re: Basic questions about melting physics
« Reply #108 on: April 21, 2020, 12:18:34 AM »
     The Lebedev formula translates cumulative FDD into ice thickness, and yes, through the 0.58 exponential term represents the declining increase in ice thickness as FDD accumulate.  As shown in the Chris Reyolds posts, that formula can be used to estimate the difference in ice thickness additions between two cumulative FDD values.
   
     But what about the reverse process?  Is there a melting DD equivalent?  It seems likely that someone has measured that relationship.  A direct physical approach would be to measure ice thinning effects on blocks of sea ice of different thicknesses kept in a controlled setting and exposed to different temperature regimes. 

     A large scale field-based empirical approach might be possible by correlating Arctic seasonal temperature observations with subsequent reductions in Extent, Thickness, and Volume.  In addition to air and/or sea temperature, I suspect that such a study would need to also account for other factors such as cloud cover/solar radiation and wind, and also for the timing of those melting factor inputs on melt pond formation and surface changes to albedo. 

     This question seems so fundamental to understanding Arctic sea ice behavior that it must have been tackled by multiple research projects.  Most of my exposure to Arctic research is from what shows up in climate blogs, lay press articles, and on ASIF.  Minimal effort literature searching has not found anything to address this question, and that's all the time I have available at present. 

     What I am looking for is research relevant to melting DD vs. ice loss, and in particular to the effect of sea ice thickness on subsequent rate of thickness decline in response to melting DD.  The assumption that thinner ice melts faster than thick ice is often stated, and it makes sense due to salinity, structural weakness, surface/volume ratio, etc.  But how much faster does thin ice lose thickness to the same melting energy than thicker ice exposed to the same melting conditions? 

      The Thorndike 1975 ice thickness vs. thickness growth rate curve addresses that question for the winter ice thickening season.  I've been told that simply reversing that curve to get the response for ice thinning in the melt season is not valid.  So what is the melting season equivalent for the Thorndike 1975 ice thickness - thickness accumulation curve?

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Re: Basic questions about melting physics
« Reply #109 on: April 21, 2020, 05:12:18 PM »
The reason thickness is important for freezing is heat has to travel through ice to dissipate. The water below the ice freezes. Melting can occur at the surface so thickness isn't important. Younger ice which is thinner melts easier because it has more salt in it. Multiyear ice survives more freeze thaw cycles that allow salty brine pockets to be released from the ice. Albedo, rain and warm water can also play large roles.

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Re: Basic questions about melting physics
« Reply #110 on: April 21, 2020, 05:24:40 PM »
If you really want to learn more about sea ice formation Tor in post 32 linked to an excellent but long and technical read about ice formation and different types of ice. Physical and dynamic properties of sea ice.
https://apps.dtic.mil/dtic/tr/fulltext/u2/a256303.pdf

kassy

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Re: Basic questions about melting physics
« Reply #111 on: April 21, 2020, 05:45:46 PM »
       But what about the reverse process?  Is there a melting DD equivalent?  It seems likely that someone has measured that relationship.  A direct physical approach would be to measure ice thinning effects on blocks of sea ice of different thicknesses kept in a controlled setting and exposed to different temperature regimes.

We know the average Vol lost over a season so you could get an idea from that?

The blocks won´t really cut it. The type of meltponds make a difference, where the thick ice is, where the winds blow from etc.

I would love to see the underside of the sea ice to see how the keels are distributed.
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.

Glen Koehler

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Re: Basic questions about melting physics
« Reply #112 on: April 22, 2020, 03:25:54 AM »
If you really want to learn more about sea ice formation Tor in post 32 linked to an excellent but long and technical read about ice formation and different types of ice. Physical and dynamic properties of sea ice.
https://apps.dtic.mil/dtic/tr/fulltext/u2/a256303.pdf
      Thanks interstitial!  Gow and Tucker 1991 is a classic document, the kind of comprehensive review I love.  I have only skimmed it so far, but already see numerous nuggets that address a variety of questions that have recently been discussed on various ASIF threads, including BOE estimates, land-fast ice ridging, FYI vs MYI, Fram export, and my question about the effect of thinning on melt sensitivity.

      Published in 1991 and based on many observations in the 1980s, reading what these scientists saw in their field studies is haunting.  Peter Wadhams' work is cited 14 times for 10 different papers in the review.  On 7 of those papers he was lead author.  Wadhams has been criticized lately for overstating the imminence of Arctic degradation.  Looking through this wonderful review I felt an emotional attachment to him, and understand his perspective better. 

      The Arctic those folks studied is becoming a distant memory.  They describe the characteristics of thick MYI that could hardly be found anymore.  One especially evocative statement is about what kind of change in the system would be required to melt out the Arctic, and that such a thing could only happen if atmospheric CO2 levels went to unprecedented levels.  And 29 years later, their extreme scenario is our current world.  No wonder Wadhams is freaked out.  If we knew as much about it as he does and had his historical perspective, we'd be more freaked out too. 

      That is one of the unique qualities of climate change.  With most of the scientific issues with which I become familiar professionally or personally, the more you learn, the more complexity, gray areas and nuance you discover.   So your opinion becomes less definitive.  That is why scientists so often speak in qualified terms, sometimes to the frustration of people who just want a simple yes/no black/white answer.  With climate change, for me at least, (but I think most other people who dig into it), the more you learn the more alarmed you get.  Sometimes to the point that you look around and wonder how life can go on looking so "normal" (at least until COVID-19 hit) when the planet is on fire. 

     God bless Peter Wadhams  Even if his BOE predictions were a bit early, when you see the depth of knowledge that he, Gow, Tucker and that generation of scientists developed back when the tools were much less powerful than today, you can better understand why seeing what has happened to the Arctic over the past 30 years would cause them to be alarmed.

« Last Edit: April 22, 2020, 09:23:15 PM by Glen Koehler »

uniquorn

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Re: Basic questions about melting physics
« Reply #113 on: April 22, 2020, 12:01:26 PM »
Quote
The Arctic those folks studied is becoming a distant memory.

There are 10 active thermistor buoys in the arctic today in ice that is just about to start melting. What an opportunity to look at near real time data yourself.

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Re: Basic questions about melting physics
« Reply #114 on: April 22, 2020, 06:57:51 PM »
      Thanks interstitial!  Gow and Tucker 1991 is a classic document, the kind of comprehensive review I love.  <snip>
Your welcome. I found it useful as well that is why I remembered it.

oren

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Re: Basic questions about melting physics
« Reply #115 on: June 04, 2020, 02:59:17 PM »
This would be a good thread to discuss bottom melt mechanics.

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Re: Basic questions and discussions about melting physics
« Reply #116 on: June 04, 2020, 03:03:01 PM »
One thing I was thinking while reading was that we tend to forget bottom melt, and focus on top melt from all sorts of different causes, insolation and WAA, stormy weather, rain and condensation.

But I seem to remember reading a paper saying that top- and bottom melt follow each other in magnitude, so that e.g. 2012 saw both rapid top melt and unusually strong bottom melt and that the same pattern is repeated in other years - sluggish top melt coinciding with slow bottom melt and vice versa.

I can't remember where i saw this, perhaps it rings a bell with somebody else, but I don't really see where the variation in bottom melt should come from. Is there a causal link between the two, i.e. does increased top melt cause increased bottom melt? Or is there a shared external cause?

I'm sure there are lots of people here who knows a lot more than I do about this, it surely would tickle my knowledgebone to hear from some of them!

oren

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Re: Basic questions and discussions about melting physics
« Reply #117 on: June 04, 2020, 03:07:31 PM »
In short bottom melt is affected by:
* Core ice temperatures, which go up in partial correlation with top melt. AFAIK significant bottom melt can happen before significant top melt.
* Insolation transmitted through the ice, which goes up in correlation with top melt
* Movement of the ice, due to various processes at the ice-water interface.

Off the top of my layman's head, ice that moves through water, especially when there is a lower ice concentration and open water between the floes, melts faster - because of the increased energy transfer, because the cold fresh water below the ice is replaced, and because of some Ekman pumping.

binntho

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Re: Basic questions and discussions about melting physics
« Reply #118 on: June 04, 2020, 03:15:10 PM »
Some of the papers that I looked at through the links in the (somewhat dubious?) paper that Phoenix linked to seems to indicate that insolation through the ice is the major source of bottom melt, although those papers were written in the late noughties when ice movement was much less pronounced than it is now.

Ekman pumping from movin ice may well be a growing factor, a chilling thought when you consider that Polarstern drifted at double the speed expected. I wonder how big the effect is of this increased movement, perhaps the halocline is under severe attack? Or perhaps not?
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oren

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Re: Basic questions and discussions about melting physics
« Reply #119 on: June 09, 2020, 11:22:59 PM »
A comment on the main thread got me wondering. Sea ice melting point is -1.8C while fresh snow melts at 0C. What happens when air temp is -1C and the ice is covered by a snow layer?

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Re: Basic questions and discussions about melting physics
« Reply #120 on: June 09, 2020, 11:26:51 PM »
No expert here, but isn't this what is meant by snow having an insulating value, at least within that narrow range?
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Re: Basic questions and discussions about melting physics
« Reply #121 on: June 09, 2020, 11:45:52 PM »
Colder ice probably should prevent melting. Otherwise sea ice will melt under snow. After it, snow will melt in salt water.

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Re: Basic questions and discussions about melting physics
« Reply #122 on: June 10, 2020, 12:38:02 AM »
we have some data.. at least 10 Tbuoys every day for a while
air temps left, ocean temps right, snow/ice in between
« Last Edit: June 10, 2020, 12:43:25 AM by uniquorn »

Phoenix

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Re: Basic questions and discussions about melting physics
« Reply #123 on: June 10, 2020, 04:57:29 AM »
A comment on the main thread got me wondering. Sea ice melting point is -1.8C while fresh snow melts at 0C. What happens when air temp is -1C and the ice is covered by a snow layer?

I think the sea ice melting point is actually 0C. The freezing point which is -1.8C at an average salinity. At higher salinity, the freezing point goes down.

My understanding is that sea water has a lower freezing point because extra energy is required to remove the salt ions from the solution. The result is fresh water ice + salty brine.

Going in the other direction, the ice is fresh water and the melting point is the usual 0C.

binntho

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Re: Basic questions and discussions about melting physics
« Reply #124 on: June 10, 2020, 07:05:27 AM »
A comment on the main thread got me wondering. Sea ice melting point is -1.8C while fresh snow melts at 0C. What happens when air temp is -1C and the ice is covered by a snow layer?

I think the sea ice melting point is actually 0C. The freezing point which is -1.8C at an average salinity. At higher salinity, the freezing point goes down.

My understanding is that sea water has a lower freezing point because extra energy is required to remove the salt ions from the solution. The result is fresh water ice + salty brine.

Going in the other direction, the ice is fresh water and the melting point is the usual 0C.
Common misconception, or rather simplification. Newly formed sea ice is not fresh but contains quite a lot of small brine pockets which will quickly start to melt the surrounding ice as the temperature approaches the melting point at that level of salinity (which is actually quite high within the brine pockets in fresh ice)

The older the ice, the purer it gets, and multiyear ice is pure enough that it will not taste salty when melted, but new ice is very salty when melted. Snow deposited on fresh ice will probably retard the salt expulsion due to insulation.

Given the size of the arctic and the ongoing freezing process whenever open water appears during winter, as well as the dearth of multiyear ice, it is probably truer to say that the average melting point is somewhere below 0 degrees, but by how much presumably depends on location and circumstances.

The two plateaus in the DMI 80N graph are interesting in thic context. It is tempting to interpret the -3C plateau as being when the temps hit the melting point of the freshest and briniest ice. And the -0.5C second plateaus when it hit the "normal" melting point. I don't remember seeing such plateaus before, an indication perhaps of much earlier melt than usual.
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binntho

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Re: Basic questions and discussions about melting physics
« Reply #125 on: June 10, 2020, 07:16:07 AM »
Speaking of temperature plateaus, since rental cars started having outside temperature displayed in the dashboard, I've noticed that temperatures with snow on the ground will tend to stick at 0C for days on end.
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Re: Basic questions and discussions about melting physics
« Reply #126 on: June 10, 2020, 07:17:30 AM »
A comment on the main thread got me wondering. Sea ice melting point is -1.8C while fresh snow melts at 0C. What happens when air temp is -1C and the ice is covered by a snow layer?

I think the sea ice melting point is actually 0C. The freezing point which is -1.8C at an average salinity. At higher salinity, the freezing point goes down.

My understanding is that sea water has a lower freezing point because extra energy is required to remove the salt ions from the solution. The result is fresh water ice + salty brine.

Going in the other direction, the ice is fresh water and the melting point is the usual 0C.
Common misconception, or rather simplification. Newly formed sea ice is not fresh but contains quite a lot of small brine pockets which will quickly start to melt the surrounding ice as the temperature approaches the melting point at that level of salinity (which is actually quite high within the brine pockets in fresh ice)

The older the ice, the purer it gets, and multiyear ice is pure enough that it will not taste salty when melted, but new ice is very salty when melted. Snow deposited on fresh ice will probably retard the salt expulsion due to insulation.

Given the size of the arctic and the ongoing freezing process whenever open water appears during winter, as well as the dearth of multiyear ice, it is probably truer to say that the average melting point is somewhere below 0 degrees, but by how much presumably depends on location and circumstances.

The two plateaus in the DMI 80N graph are interesting in thic context. It is tempting to interpret the -3C plateau as being when the temps hit the melting point of the freshest and briniest ice. And the -0.5C second plateaus when it hit the "normal" melting point. I don't remember seeing such plateaus before, an indication perhaps of much earlier melt than usual.

The salinity of surrounding ocean water can also have an effect. It may be fresher during melt season but it will still lower the liquid solid phase change temperature compared to pure water.

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Re: Basic questions and discussions about melting physics
« Reply #127 on: June 10, 2020, 08:45:09 AM »
A comment on the main thread got me wondering. Sea ice melting point is -1.8C while fresh snow melts at 0C. What happens when air temp is -1C and the ice is covered by a snow layer?

I think the sea ice melting point is actually 0C. The freezing point which is -1.8C at an average salinity. At higher salinity, the freezing point goes down.

My understanding is that sea water has a lower freezing point because extra energy is required to remove the salt ions from the solution. The result is fresh water ice + salty brine.

Going in the other direction, the ice is fresh water and the melting point is the usual 0C.
Common misconception, or rather simplification. Newly formed sea ice is not fresh but contains quite a lot of small brine pockets which will quickly start to melt the surrounding ice as the temperature approaches the melting point at that level of salinity (which is actually quite high within the brine pockets in fresh ice)


If one were to zoom in with a powerful enough microscope, they would see that the ice and the brine are discrete entities.

In liquid state, sea water is H2O molecules bound by hydrogen bonds (intermolecular bonds) with salt ions (Na+ and Cl-) dissolved in solution.

During the freezing process, the salt ions and some of the H2O molecules are separated from the rest of the H2O molecules. The crystal lattice of ice is composed of only H2O molecules bound together by intermolecular bonds. Consider this lattice to be similar to a house which is held together with wood and screws.

The brine exists in the spaces within the lattice, but is not part of the lattice itself. The brine is like a sofa inside a house. It fits inside the house, but it is not a component of the ice house and does not impact the strength (heat) required to dismantle the ice house.

As you indicate, the brine does exit the ice lattice over time... by escaping through the spaces in the lattice. Most of the brine exits within a year of ice formation.

The lattice portion of the ice house is the same in the Arctic as the ice in your freezer and melts at the same temperature.




binntho

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Re: Basic questions and discussions about melting physics
« Reply #128 on: June 10, 2020, 10:39:23 AM »
A comment on the main thread got me wondering. Sea ice melting point is -1.8C while fresh snow melts at 0C. What happens when air temp is -1C and the ice is covered by a snow layer?

I think the sea ice melting point is actually 0C. The freezing point which is -1.8C at an average salinity. At higher salinity, the freezing point goes down.

My understanding is that sea water has a lower freezing point because extra energy is required to remove the salt ions from the solution. The result is fresh water ice + salty brine.

Going in the other direction, the ice is fresh water and the melting point is the usual 0C.
Common misconception, or rather simplification. Newly formed sea ice is not fresh but contains quite a lot of small brine pockets which will quickly start to melt the surrounding ice as the temperature approaches the melting point at that level of salinity (which is actually quite high within the brine pockets in fresh ice)


If one were to zoom in with a powerful enough microscope, they would see that the ice and the brine are discrete entities.

Absolutely. But the brine pockets start melting the ice as soon as the melting point of the brine/ice interface is reached, well before the melting point of the ice inside the ice crystals is reached.

EDIT: The internal temperature of the ice and the brine pockets is the same. As soon as any melting starts on the interface of the two, the temperature stops rising. So ice with brine pockets will never reach 0 degrees, yet still melt out totally.
« Last Edit: June 10, 2020, 10:48:49 AM by binntho »
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oren

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Re: Basic questions and discussions about melting physics
« Reply #129 on: June 10, 2020, 11:00:43 AM »
 I know for certain that the temperature at the ocean-ice interface is -1.8C, not 0C. So for bottom melt I'm quite clear on the answer.
It would appear that top melt when there's some snow or even meltpond cover might begin at 0C. Thus  my hypothetical -1C might melt nothing, at least for a while.

oren

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Re: Basic questions and discussions about melting physics
« Reply #130 on: June 10, 2020, 11:08:43 AM »
And for those who like to look at air temps in the arctic as a sign of warming, here's another thought  (which I hope is true). Temps above wet ice/melted snow are pegged at near 0C. Any lower and the top layer starts freezing and releasing heat. OTOH above open water temps can be cooler, down to -1.8C in extreme cases, unless the water aurface has been heated by the sun.
Thus under certain conditions (cloudy?) summer air temps could be cooler over open water than over the ice.

binntho

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Re: Basic questions and discussions about melting physics
« Reply #131 on: June 10, 2020, 11:17:54 AM »
I know for certain that the temperature at the ocean-ice interface is -1.8C, not 0C. So for bottom melt I'm quite clear on the answer.
I agree with you on this and should have mentioned it. But the discussion has been, as I understand it, mostly about air temperatures, starting with the hypothetical clean snow on briny ice.
because a thing is eloquently expressed it should not be taken to be as necessarily true
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Re: Basic questions and discussions about melting physics
« Reply #132 on: June 10, 2020, 11:52:13 AM »
I know for certain that the temperature at the ocean-ice interface is -1.8C, not 0C. So for bottom melt I'm quite clear on the answer.
It would appear that top melt when there's some snow or even meltpond cover might begin at 0C. Thus  my hypothetical -1C might melt nothing, at least for a while.

The ocean / ice interface is a dynamic place with respect to freezing temperatures. As the freezing season goes on and the wet side of the interface becomes saltier with brine leaking, the freezing point goes down below -1.8C. The ice side of the interface is irrelevant in terms of freezing temperature (it's already frozen).

The flip side of this occurs during the melting season when only the ice side of the interface is relevant as the water/brine below is already in liquid state. Here, the melting point is 0.0C at normal atmospheric pressure or 760 torr. Small deviations in pressure as the water gets deeper will result in slightly different melting points.

You can see that the NSIDC takes care to mention only the freezing point of sea water is -1.8C. You might try to find a credible reference anywhere on the internet which supports a melting point of the same temperature.

https://nsidc.org/cryosphere/seaice/index.html

To illustrate the impact of salinity on freezing point, here is a study which demonstrates the freezing point of Dead Sea water to be -32C.

https://scialert.net/fulltext/?doi=jas.2005.1334.1339

binntho

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Re: Basic questions and discussions about melting physics
« Reply #133 on: June 10, 2020, 12:48:00 PM »
The flip side of this occurs during the melting season when only the ice side of the interface is relevant as the water/brine below is already in liquid state. Here, the melting point is 0.0C at normal atmospheric pressure or 760 torr. Small deviations in pressure as the water gets deeper will result in slightly different melting points.

The brine below the ice controls at what temperatures bottom melt starts.

The brine within the ice controls at what temperatures melting starts within the ice and at the surface of the ice.

As long as bottom melt is active, the temperature is pegged at the melting point of the ice/water interface - which as oren pointed out is at -1.8, since any expelled brine from the freezing process quickly dissipates.

Thermal radiation enters the ice from above and in new and first-year ice, brine pockets within the ice warm up at the same rate as the ice itself. As soon as the ice starts melting at the interfaces of the brine pockets and the surrounding ice, the temperature STOPS RISING. All the extra heat (energy) is used to melt the ice.

Since the brine pockets can quite easily be saltier than the underlying ocean, the resulting starting melting point could quite well be as low as -3 degrees or even lower. As melting progresses, the brine pockets increase in size and their salinity goes down, raising the melting point.

BUT the temperature within the ice never goes above the melting point of the active melting interface. So if the ice contains brine pockets, the temperature of the ice never reaches 0C. Which makes it very obviously wrong to say that the melting point of that ice is 0C.

What about the surface, I hear you ask. We've covered bottom melt and "internal" melt, which is what happens as a result of thermal radiation (i.e. sunlight). Melting caused by conduction from air takes place at the surface, but again, the brine pockets reach all the way to the surface. So the melting ice will always be interacting with brine, and thus melting of the surface will start when air temperatures are at the melting point of the brine-pocket/ice interface.

So all in all, Phoenix, new sea ice has a melting point well below 0, first year ice also has a melting point below 0 but not by much, and multiyear ice has a melting point at 0.

The reason that you will not find any direct information online about the melting point of sea ice is that it is never simple nor obvious (as opposed to the freezing point of sea water).
because a thing is eloquently expressed it should not be taken to be as necessarily true
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Re: Basic questions and discussions about melting physics
« Reply #134 on: June 10, 2020, 01:15:17 PM »
Might explain why we've been having a really tough time pinning down our "ice:ocean" interface recently.

We've been observing very strange fluctuations in temperature at the bottom of the ice in the raw data, which is bugging the code we used to make;

www.mosaic-ice.com:8501



The raw data can be viewed here;

https://data.meereisportal.de/gallery/index_new.php?lang=en_US&active-tab1=method&active-tab2=buoy&singlemap&buoyname=2019T56
Im working on a satellite-miner to detect changes in small ice-caps/ snow-fields. Send me recommendations to optimise the program with.

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Re: Basic questions and discussions about melting physics
« Reply #135 on: June 10, 2020, 01:38:30 PM »

So all in all, Phoenix, new sea ice has a melting point well below 0, first year ice also has a melting point below 0 but not by much, and multiyear ice has a melting point at 0.

The reason that you will not find any direct information online about the melting point of sea ice is that it is never simple nor obvious (as opposed to the freezing point of sea water).

I agree that multiyear ice has a melting point of 0 at 1 atm pressure. Let's just agree to disagree on the variability of melting point with ice age. If you have links to respected sources which support that version, I'll be happy to look at them.

binntho

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Re: Basic questions and discussions about melting physics
« Reply #136 on: June 10, 2020, 01:48:47 PM »

So all in all, Phoenix, new sea ice has a melting point well below 0, first year ice also has a melting point below 0 but not by much, and multiyear ice has a melting point at 0.

The reason that you will not find any direct information online about the melting point of sea ice is that it is never simple nor obvious (as opposed to the freezing point of sea water).
I agree that multiyear ice has a melting point of 0 at 1 atm pressure. Let's just agree to disagree on the variability of melting point with ice age. If you have links to respected sources which support that version, I'll be happy to look at them.

This is what we call common sense + a longstanding interest in the physics of ice.

I did a very quick web search at the beginning of this conversation, "melting point of ice",  I just did it again now, here is a quote from the very first page suggested by Google:

Quote from: NSIDC link=https://nsidc.org/cryosphere/seaice/index.html
New ice is usually very salty because it contains concentrated droplets called brine that are trapped in pockets between the ice crystals, and so it would not make good drinking water. As ice ages, the brine eventually drains through the ice, and by the time it becomes multiyear ice, nearly all of the brine is gone. Most multiyear ice is fresh enough that someone could drink its melted water. In fact, multiyear ice often supplies the fresh water needed for polar expeditions. See Salinity and Brine in the Characteristics section for more information.

Pure common sense tells you that ice that tastes salty when melted does not have a 0C degree melting point. There is nothing to disagree about here, it's just the way things are. Ice with salt interlaced can't even reach 0C - it's physically impossible.
« Last Edit: June 10, 2020, 01:55:29 PM by oren »
because a thing is eloquently expressed it should not be taken to be as necessarily true
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Re: Basic questions and discussions about melting physics
« Reply #137 on: June 10, 2020, 01:59:54 PM »
Thank you for the answers. Obviously FYI melting point is not 0C, I'll take binntho's description.
In any case, no need for further back and forth, unless someone else wants to step in with additional info.

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Re: Basic questions and discussions about melting physics
« Reply #138 on: June 10, 2020, 02:44:01 PM »
Thank you for the answers. Obviously FYI melting point is not 0C, I'll take binntho's description.
In any case, no need for further back and forth, unless someone else wants to step in with additional info.

It would be nice if you didn't use the word "obviously" here. It is a way of saying that you are obviously right and i am obviously wrong which is a form of one upsmanship at the same time you are basically telling me to shut up which is an example of authority silencing dissenting perspectives.

We can agree to disagree politely.

binntho

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Re: Basic questions and discussions about melting physics
« Reply #139 on: June 10, 2020, 02:56:37 PM »
Thank you for the answers. Obviously FYI melting point is not 0C, I'll take binntho's description.
In any case, no need for further back and forth, unless someone else wants to step in with additional info.

It would be nice if you didn't use the word "obviously" here. It is a way of saying that you are obviously right and i am obviously wrong which is a form of one upsmanship at the same time you are basically telling me to shut up which is an example of authority silencing dissenting perspectives.

We can agree to disagree politely.
Is this an example of what they call microagression? I don't know. But claiming that people can agree to disagree in factual discussions is just rubbish. There are no dissenting perspectives when it comes to physics. And obviously oren is free to use his authority to tell us to shut up. I can even hear him shouting while reading this, so I'm shutting up now with immediate effect!  8)
because a thing is eloquently expressed it should not be taken to be as necessarily true
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Re: Basic questions and discussions about melting physics
« Reply #140 on: June 10, 2020, 03:08:45 PM »
After reading the responses, it is obvious - to me. I am fine with agreeing to disagree as long as the back and forth is over.

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Re: Basic questions and discussions about melting physics
« Reply #141 on: June 10, 2020, 03:29:50 PM »
After reading the responses, it is obvious - to me. I am fine with agreeing to disagree as long as the back and forth is over.

OK, if you want to stay with the word obvious, I will leave it on the same terms and say that it is obvious to me that you and binntho are wrong.

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Re: Basic questions and discussions about melting physics
« Reply #142 on: June 10, 2020, 05:59:11 PM »

If one were to zoom in with a powerful enough microscope, they would see that the ice and the brine are discrete entities.

In liquid state, sea water is H2O molecules bound by hydrogen bonds (intermolecular bonds) with salt ions (Na+ and Cl-) dissolved in solution.

During the freezing process, the salt ions and some of the H2O molecules are separated from the rest of the H2O molecules. The crystal lattice of ice is composed of only H2O molecules bound together by intermolecular bonds. Consider this lattice to be similar to a house which is held together with wood and screws.

The brine exists in the spaces within the lattice, but is not part of the lattice itself. The brine is like a sofa inside a house. It fits inside the house, but it is not a component of the ice house and does not impact the strength (heat) required to dismantle the ice house.

As you indicate, the brine does exit the ice lattice over time... by escaping through the spaces in the lattice. Most of the brine exits within a year of ice formation.

The lattice portion of the ice house is the same in the Arctic as the ice in your freezer and melts at the same temperature.

As one who in my chemistry career used salt-ice baths to achieve lower than 0ºC temperatures I wish to correct the physical chemistry referred to in this post.

When water freezes into ice, the hydrogen bonds make a hexagonally shaped network of molecules inherent to the structure of ice.
When a solute is added to water the ordering of the solvent molecules is disrupted. This means that more energy must be removed from the solution in order to freeze it.
When salt is added to water, the resulting ions in the water disrupt the usual network of hydrogen bonds made upon freezing. As a result, the freezing point of the solution is lower than it is for the pure solvent. This is termed freezing point depression.  As the ice warms up the network of hydrogen bonds (referred to above as the lattice) requires less energy to be broken up so the melting point is lower.
Because the solubility of the salt decreases with temperature some of the salt is rejected, forming brine pockets.  These brine pockets get eliminated over time but some salt remains in the ice disrupting the structure. In multiyear ice the salt content will ultimately reach the solubility of the lowest temperature the ice has reached so in thick MYI you'd expect lower salinity at the top vs. the bottom.

Here's an amusing video illustrating the difference in melting between saline and pure water ice.


Glen Koehler

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Re: Basic questions and discussions about melting physics
« Reply #143 on: June 11, 2020, 03:47:45 AM »
     Inside Climate News article on aerosol drop impact on Arctic sea ice
https://insideclimatenews.org/news/08062020/sulfate-emissions-coronavirus-arctic-heatwaves

     "Overall, this winter wasn't particularly warm, but now that's flipped around in the last month and we're really seeing the effects," says Mark Serreze, director of the National Snow and Ice Data Center (NSIDC). "Big holes are opening up along the Siberian coast where it's been the warmest."

     "This Central Arctic heatwave may not be a one-off event only occurring in spring 2020, researchers suggest. Rather, if levels of global industrial air pollutants continue to fall due to the Covid-19 pandemic, the current Arctic warmth could be a bellwether of what's to come later this summer when sea ice melt annually kicks into high gear."

     "Indeed, in a 2017 study, scientists posited that the sulfate aerosols released due to human activity masked the decline in Arctic sea ice in the mid-20th century, before the Clean Air Act went into effect, and actually led to periods of ice growth."

     "Using earth system computer modeling, his simulations showed that sulfate aerosol reductions in Europe since 1980 could potentially explain a significant fraction of Arctic warming over that period. Specifically, the Arctic received approximately 0.3 watts per meter squared of energy, warming by 0.9 degrees Fahrenheit on average as Europe's sulfur emissions declined."

     " "We conclude that air quality regulations in the Northern Hemisphere, the ocean and atmospheric circulation, and the Arctic climate are inherently linked," his 2016 Nature Geoscience study stated. "

Freegrass

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Re: Basic questions and discussions about melting physics
« Reply #144 on: June 11, 2020, 04:59:37 AM »
Great article Glen. It has already been discussed here that aerosol reduction has a warming effect in summer, but a cooling effect in winter. Is that why the antarctic sea ice extent is close to normal this season?


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Re: Basic questions and discussions about melting physics
« Reply #145 on: June 11, 2020, 03:37:31 PM »
Hi FG - My guess (and that is all it is) is that aerosol reduction could also cause increased radiation losses out to space during the winter as the aerosols can also add to the insulating effect of the atmosphere, thus fewer particles = less insulation = more winter heat loss. 

      During the 24 hour nights of polar winter there is no counteracting cooling effect of aerosol particles reflecting incoming shortwave radiation.  So the cooling effect, which dominates during summer is not active during winter.

      But I don't pretend to understand the details of these interactions, just thought I'd take a shot it since you asked.  It would be great to get an answer from somebody who studies this stuff.
 
« Last Edit: June 11, 2020, 04:12:47 PM by Glen Koehler »

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Re: Basic questions and discussions about melting physics
« Reply #146 on: June 11, 2020, 03:51:46 PM »
I think it more of the vast expanse of cold that makes the difference.  What little effect the aerosols have is meaningless, when temperatures are consistently below freezing.  Even the coastal areas average daily below freezing temperatures year round.

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Re: Basic questions and discussions about melting physics
« Reply #147 on: June 11, 2020, 08:44:09 PM »
It would be great to get an answer from somebody who studies this stuff.
I agree, but it looks like nobody is home today...  :(
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Re: Basic questions and discussions about melting physics
« Reply #148 on: June 13, 2020, 10:00:15 PM »
Some Mosaic Tbuoy charts showing overlays of temperature profiles through the ice from jun1-13
click to run ~1.5MB

Air where the lines diverge on the left.
Then snow or melt ponds?  T56, T68 and T75 reach +3C close to ice surface.
Ice is the thickening band in the middle but that is the temperature profile changing as it slowly warms. The thickness is calculated from the number of thermistors along the x-axis. Multiply by 2 as they are 2cm apart.
ocean on the right.

images where you can read the dates are here

static image of T56 with close look at the ice/ocean interface
« Last Edit: June 13, 2020, 10:27:20 PM by uniquorn »

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Re: Basic questions and discussions about melting physics
« Reply #149 on: June 14, 2020, 09:57:26 PM »
I'm struggling to interpret the charts above. Thought this may be helpful here

On the thermodynamics of melting sea ice versus melting freshwater ice
Wiese, M., Griewank, P., & Notz, D. (2015). On the thermodynamics of melting sea ice versus melting freshwater ice. Annals of Glaciology, 56(69), 191-199. doi:10.3189/2015AoG69A874
Quote
The timespan needed to warm freshwater ice is much shorter than for sea ice, because we actually completely transform the sea ice into water by heating the ice to its liquidus temperature. This is also the reason why the calculation gives a longer time for the warming of the ice with a lowerbulk salinity: the ice has a higher solid fraction, which needs more energy to be completely changed into liquid.
Becausethe typical timescale for the heating of the sea ice is much longer than the diffusive timescale of ~3 hours, the temperature profile during the heating will no longer be linear(seeFig.3). Instead, we expect a temperature minimum in the interior of the ice, which indeed we find both in the experiments and in the simulations. This implies that we can no longer roughly separate an initial period of warming of the ice from a consecutive period of thinning of the ice, as was the case for freshwater ice. For sea ice, the ice still keeps warming significantly in its interior as the surface is already at its liquidus temperature (cf. blue lines in Fig. 1).
This slow heating of the ice’s interior explains the initial shape of the observed and modelled evolution of the melt rates that is shownin Figure2c and d: as the ice’s interior gets warmer during the heating,the heat flux into the ice’s interior becomes smaller. Therefore, more energy remains available to thin the ice at the bottom and at the surface, which explains the initial increase in melt rate. Compared to freshwater ice, the initial melting is slower because more energy is used to warm the interior of the sea ice and to decrease its solid fraction. At later stages, however, this initial investment of energy pays off, and the by then much less solid sea ice thins faster than freshwater ice. To understand why the thinning of the sea ice eventually slows down again, we need to consider the vertical inhomogeneity of the bulk salinity within the ice. Since we do not have direct measurements of bulk salinity available, we use for our analysis the simulated bulk salinity evolution shown in Figure4.
The simulations show clearly that after the onset of surface warming, flushing sets in, which transports cold and salty brine from higher up in the ice to the region close to the ice–ocean interface. Heat diffusion from the ice–ocean interface into the ice and the negative heat advection of the cold flushing brine cools the lower layers for a short time after the onset of melting. This cooling keeps the highly saline lowest layers with a low solid fraction from being melted away by the oceanic heat flux.In the 12 gkg^-1 simulation the sum of the diffusive heat flux from the ice–ocean interface and the negative heat advection of the cold brine is stronger than the oceanic heat flux, which causes some ice to grow at the ice–ocean interface(Fig.4).
However, once the heat flux from the interior ice is depleted, a relatively rapid bottom ablation occurs, since only ice with a comparably low solid fraction needs to be completely dissolved to thin the bottom ice. This initially slow melt followed by a rapid thinning is visible in all sea-ice experiments and simulations(Fig. 2).
Quote
Once the salty layer at the bottom is fully eroded, bottom ablation slows down significantly, since then ice with a much higher solid fraction is present at the ice-ocean interface. For example, ice grown in water with an initial water salinity of 28 gkg-1 has a very low bulk salinity close to 0gkg-1 about 95 hours after the start of the experiment and the bottom ablation decreases from this moment on (Fig. 4). It is primarily this decrease in bottom ablation that causes the observed and modelled slowdown of the thinning after the initial acceleration. This process is amplified by the fact that flushing continues to very effectively desalinate the ice, which is seen in the almost complete loss of bulk salinity in the simulations as time progresses. Hence, the solid fraction of the remaining ice becomes higher and higher, making it more and more difficult to thin that remaining ice. The melt rates of the sea ice therefore approach those of freshwater ice towards the end of the experiment, both in the laboratory and in the numerical model. In line with expectations, we always find that sea ice is fully melted faster than freshwater ice, because sea ice has a lower overall solid content and hence requires less time to melt for any given heat flux than the fully solid freshwater ice.
Quote
For all experiments, ice was grown to a thickness of 0.1 m at a temperature of -20°C. Once that thickness was reached, the air temperature was either increased directly to +10°C or increased stepwise to simulate a slower transition from freezing to melting conditions.
So perhaps not entirely relevant to 1.5m sea ice but should cover two bottles of ice in a cooler.
« Last Edit: June 14, 2020, 10:36:33 PM by uniquorn »