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JimboOmega

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Stupid Questions about DMI Data
« on: July 26, 2016, 01:08:50 AM »
So I like to watch the DMI data, showing what areas are melting and how much today.

But I have a few stupid questions.

One, the graph at the bottom that shows accumulated ice over the whole season - I notice the "average" ends well above 0.  I assume the extra snow gets lost in calving events. Is that right? Is there a way to view that same graph "corrected" for calving, so as to see the actual mass balance over a whole season?

Second, I see that the southeast is very dark blue, where snow has accumulated, while other areas are more red. Is this anomalous? Or does the southeast usually end the year positive? If it is anomalous... does it matter in the broader picture of things?

Tealight

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Re: Stupid Questions about DMI Data
« Reply #1 on: July 26, 2016, 05:59:43 PM »
I assume you are talking about this site: http://beta.dmi.dk/en/groenland/maalinger/greenland-ice-sheet-surface-mass-budget/

Your assumption about extra ice lost in calving events is correct. The site states several times that it is only surface mass balance. DMI created another website called polar portal which has Accumulated anomaly and Albedo anomaly: http://polarportal.dk/en/groenlands-indlandsis/nbsp/isens-overflade/

Another category on polar portal shows total mass change: http://polarportal.dk/en/groenlands-indlandsis/nbsp/total-masseaendring/

I don't know why they have spread out their model results so much, but everything seems to be still in beta version. At least they have a better land mask than NSIDC and a more useful scientific value than just melt days.

etienne

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Re: Stupid Questions about DMI Data
« Reply #2 on: September 06, 2016, 10:09:32 PM »

Another category on polar portal shows total mass change: http://polarportal.dk/en/groenlands-indlandsis/nbsp/total-masseaendring/

If I follow the link above, I can estimate the loss of greenlandic ice  somewhere around 300 Gt per year, I guess this means around -330 Gm^3 per year.
I calculated a trend on yearly average of PIOMAS data and got a trend around -240 km^3 per year. On Wipneus graphs on https://sites.google.com/site/arctischepinguin/home/piomas/, the trend is steeper (around 330 km^3 per year), but calculated for example on minimal values.

If I am correct, Gm^3 and km^3 mean the same, so the average loss of ice from Greenland would be similar or higher than the loss of ice from the artic ocean. Is this correct ? Can anybody confirm ?

Best regards,

Etienne

oren

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Re: Stupid Questions about DMI Data
« Reply #3 on: September 07, 2016, 12:13:53 AM »
If I follow the link above, I can estimate the loss of greenlandic ice  somewhere around 300 Gt per year, I guess this means around -330 Gm^3 per year.
I calculated a trend on yearly average of PIOMAS data and got a trend around -240 km^3 per year. On Wipneus graphs on https://sites.google.com/site/arctischepinguin/home/piomas/, the trend is steeper (around 330 km^3 per year), but calculated for example on minimal values.

If I am correct, Gm^3 and km^3 mean the same, so the average loss of ice from Greenland would be similar or higher than the loss of ice from the arctic ocean. Is this correct ? Can anybody confirm ?

Best regards,

Etienne

I haven't checked your sources, but can confirm km^3 and GT are the same. But bear in mind comparing Greenland ice loss to Arctic sea ice loss is not necessarily useful as they mean different things. Greenland loss raises sea levels while Arctic loss does not. On the other hand, Arctic loss affects absorption of solar radiation (due to albedo) while Greenland loss almost does not.

Tealight

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Re: Stupid Questions about DMI Data
« Reply #4 on: September 07, 2016, 12:47:29 AM »
If I follow the link above, I can estimate the loss of greenlandic ice  somewhere around 300 Gt per year, I guess this means around -330 Gm^3 per year.

The DMI already calculates Greenland ice loss in water equivalent, but I think PIOMAS calculates real sea ice volume(density 0.9-0.94). You need to factor this into your calculations. Then as oren pointed out those two are very different enviroments and just stating the ice volume loss is similar doesn't mean they melt at similar rates. If you compare the area effected then Greenland loses far more mass per km2 than the Arctic ocean. Especially since most losses in Greenland are near the coast.

Winter sea ice extent: 15,000,000km2
Summer sea ice extent: 5,000,00 km2
Greenland melt area: 850,000 km2 (assuming 50% of total area affected from mass loss)

Edit: Do you have a particular goal with the comparison or is it just "playing with numbers"?

etienne

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Re: Stupid Questions about DMI Data
« Reply #5 on: September 07, 2016, 10:09:54 AM »
Hello,

I was wondering if the heat absorbed by the ice melt of Greenland could cool the artic ocean, just like when you put ice cubes in orange juce, if this could explain why the trend regarding the sea ice extend seems so linear. Since values are similar, and if like this year melting seems to be related to SST, I guess we can say that Greenland "protected" the artic sea ice.

Melting 1 t of ice needs about 9 kWh of energy, so with 300 Gt, it's about 2700 TWh absorbed by Greenland and by Artic Sea Ice.

Best regards,

Etienne


Tealight

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Re: Stupid Questions about DMI Data
« Reply #6 on: September 08, 2016, 02:07:52 PM »
Ok now I know why this is in the Stupid Question thread. It can also be answered with any level of detail.

Here area some points
1. Meltwater from Greenland is 0°C warm and sea ice melts at -1.8°C so any meltwater does melt sea ice.
2. Probably over 95% of all Greenland mass loss is on the west and east coat, which is the Atlantic and not the Arctic Ocean. Current SST around Greenland is far above average: http://polar.ncep.noaa.gov/sst/ophi/color_newdisp_anomaly_north_pole_stereo_ophi0.png
3. 2012 was the biggest melting year in Greenland and the Arctic Ocean.
4. Yearly mass loss is not the same as mass that melted during the melting season. Snow accumulation is about 500Gt per year and if Greenland loses 300Gt a year 800Gt of ice/snow is melting.
5. 2700TWh is nothing for Arctic dimensions. In my Energy Budget Calculation the Arctic loses around 5500PWh of heat to space during the melting season. The calculation is not perfect reality but gives a good sense of dimension

Iceismylife

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Re: Stupid Questions about DMI Data
« Reply #7 on: September 08, 2016, 06:06:50 PM »
...
1. Meltwater from Greenland is 0°C warm and sea ice melts at -1.8°C so any meltwater does melt sea ice.
...
A question about this.

Fresh melt water on the surface of the ice sheet is at 0°C.  But ice under pressure has a lower melting point.  So when the melt water flows to the bottom of the ice sheet then over ice at the bottom it should be at the local melt point below 0°C.  Flowing towards the surface it should slush and mix with sea water.  So shouldn't it be below fresh water freezing at the surface?

etienne

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Re: Stupid Questions about DMI Data
« Reply #8 on: September 08, 2016, 06:52:31 PM »
5. 2700TWh is nothing for Arctic dimensions. In my Energy Budget Calculation the Arctic loses around 5500PWh of heat to space during the melting season. The calculation is not perfect reality but gives a good sense of dimension

Hello,

What do you mean with "loses heat to space" ? I am talking about heat absorbed by ice that transforms ice into water.

Best regards,

Etienne

RoxTheGeologist

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Re: Stupid Questions about DMI Data
« Reply #9 on: September 08, 2016, 08:12:55 PM »

1. Meltwater from Greenland is 0°C warm and sea ice melts at -1.8°C so any melt water does melt sea ice.

Conversely, the sea ice melting will freeze the freshwater in pretty much an identical amount. It really depends on the degree of mixing how much freeze/melt happens.

Tealight

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Re: Stupid Questions about DMI Data
« Reply #10 on: September 09, 2016, 11:19:52 PM »
Hello,

What do you mean with "loses heat to space" ? I am talking about heat absorbed by ice that transforms ice into water.

Best regards,

Etienne

It is just a value from a model I developed. What it represents is less important than the order of  relative magnitute.
http://forum.arctic-sea-ice.net/index.php/topic,1543.msg77790.html#msg77790

Another way of comparing 300Gt to the Arctic dimensions is by dividing 300Gt by 15,000,000 km2 - roughly the winter sea ice extent. So (300*10^9)/(15*10^6*1*10^6) = 0.02 t/m2 or 20 kg/m2. This is only a 2cm thick layer of ice.

Fresh melt water on the surface of the ice sheet is at 0°C.  But ice under pressure has a lower melting point.  So when the melt water flows to the bottom of the ice sheet then over ice at the bottom it should be at the local melt point below 0°C.  Flowing towards the surface it should slush and mix with sea water.  So shouldn't it be below fresh water freezing at the surface?

The melting point of water doesn't change until around 100bar of pressure (1000m depth) and even if the melting point is lower it doesn't mean it immediately cools to that point. It can still emerge below the surface at 0°C.


« Last Edit: September 09, 2016, 11:25:16 PM by Tealight »