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Author Topic: Greenland 2019-20 SMB (Snowfall & Melting) Year  (Read 1378 times)

gerontocrat

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Greenland 2019-20 SMB (Snowfall & Melting) Year
« on: August 30, 2019, 02:33:13 PM »
I am starting this thread so these posts conform to the DMI Greenland Year September to August as in their website http://polarportal.dk/en/greenland/surface-conditions/.

Also, if I ever get to see new GRACE-FO data on overall Greenland Ice Sheet Mass this looks like a good place to put it.

The Greenland 2019 melting Season thread is staying open, as there will be many 2019 reports coming out to the end of the year and beyond.
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blumenkraft

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #1 on: August 30, 2019, 10:18:32 PM »
That naming will surely prevent weekly questions about the meaning of SMB. ;)
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gerontocrat

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #2 on: September 08, 2019, 01:57:40 PM »
http://polarportal.dk/en/greenland/surface-conditions/ as at 7 September


MELT continues to fall in line with the average,

PRECIPITATION has been higher than average,

so as a result, in this first week of the new DMI Greenland year, SMB gain is above average
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Quote from DMI
Quote
The Greenland Ice Sheet evolves throughout the year as weather conditions change. Precipitation increases the mass of the ice sheet, whilst greater warmth leads to melting, which causes it to lose mass. The term surface mass balance is used to describe the isolated gain and loss of mass of the surface of the ice sheet – excluding the mass that is lost when glaciers calve off icebergs and melt as they come into contact with warm seawater.
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P-maker

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #3 on: September 11, 2019, 07:35:59 AM »
Gerontocrat,

Thank you for keeping track of this. Please check the units and the numbers on your second graph. It sure seems like a lot ice per square km.

Thanks

Alexander555

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #4 on: September 11, 2019, 12:17:42 PM »
It's not that much. A ton is something close to 1 by 1 by 1 meter. A km2 is 1 million m2 . So a layer of 1 km2 and 1 meter thick is a million tons. That makes 7000 tons close to nothing.

nukefix

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #5 on: September 11, 2019, 12:21:30 PM »
1 gigaton of ice ~= 1km^3

gerontocrat

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #6 on: September 11, 2019, 01:50:32 PM »
1 gigaton of ice ~= 1km^3

Late at night, I mixed up my gt, tons & '000s of tonnes when trying to do the analysis in a hurry.
That'll teach me..

So I am deleting the post and doing it again.

Damn & Blast, sorry all. Mea Culpa

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blumenkraft

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #7 on: September 11, 2019, 01:57:55 PM »
No worries, thanks for your great work! :)
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Alexander555

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #8 on: September 11, 2019, 02:20:16 PM »
Are you sure it was wrong ? Normaly it can not be that much more than 7 mm a year. If i'm right the loss is from glaciers calving.

SteveMDFP

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #9 on: September 11, 2019, 02:35:38 PM »
1 gigaton of ice ~= 1km^3

Late at night, I mixed up my gt, tons & '000s of tonnes when trying to do the analysis in a hurry.
That'll teach me..

So I am deleting the post and doing it again.

Damn & Blast, sorry all. Mea Culpa

Well, what's a few orders of magnitude among friends?

gerontocrat

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #10 on: September 11, 2019, 02:41:56 PM »
Try again...... Only got it wrong by a magnitude of 10^3
The per km2 calculations are now in million tons per km2, i.e. tons per m2. 1 ton per m2 equates to a water equivalent height change of 1 metre, or about 1.09 metres of solid freshwater ice.

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Apart from the DMI daily daily data, we have occasional papers such as this one....
https://www.pnas.org/content/116/19/9239
Forty-six years of Greenland Ice Sheet mass balance from 1972 to 2018

and hopefully GRACE-FO data on a monthly basis (still only up to May 2019).

There is of course, always a problem. The PNAS paper, which is incredibly good, and has a super spreadsheet to download, summarise the individual glacial sub-regions into 7 basins, as does  the GRACE-FO data. BUT, the boundaries are not quite the same. (see 1st image).

So the SMB, discharge and Net Mass Balance basin data in the PNAS paper cannot be matched simply to the GRACE-FO Net Mass Balance basin data. Scientists, bless 'em!
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The PNAS paper analyses the Greenland ice sheet (GIS) mass balance change down to individual glacial basins and summarises the data into the 7 drainage basins.

I attach 2 graphs. One looks at cumulative ice mass loss by basin and in total, the other looks at mass change per km2.

The cumulative mass loss graph shows that
- the South West basin in recent years has switched from mass gain to mass loss,
- The North West and the South East are losing the greatest mass of ice,
- overall mass loss from 1972 to 2018 equivalent to nearly 14 mm of sea level rise,

The cumulative mass loss graph per KM2 shows that
- The North West (NW) and the South East (SE) are losing the greatest mass of ice per square kilometre by far,.
- average NW basin height loss 1972-2018 circa 5.5 metres (water equivalent),
- average SE basin height loss 1972-2018 circa 6.5 metres (water equivalent),
- overall GIS height loss 1972-2018 nearly 3 metres (water equivalent).

Of note also is that of the 260 sub-basins/glaciers in the PNAS data, in 1972 only about 35 were losing mass. In 2018, that figure is over 200. It is a different icesheet.

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I will be (I hope) working on comparing the PNAS data (which starts in 1972) with the GRACE-FO data from 2002. I also will be digging out the data on individual glaciers for posting on the threads for those glaciers.

Have I got it right this time?
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gerontocrat

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #11 on: September 11, 2019, 06:59:49 PM »
More data derived  from the PNAS paper  https://www.pnas.org/content/116/19/9239
Forty-six years of Greenland Ice Sheet mass balance from 1972 to 2018


I had a look at three individual glaciers mass loss from 1972 to 2018. The attached graph shows that the glaciers losing the most ice are not always the ones losing the most by area

e.g. JAKOBSHAVN_ISBRAE  has lost 100GT (335 GT) more ice than the 2nd highest, STEENSTRUP-DIETRICHSON (225 GT).

But when looked at as Million Tons per km2 (which equates to a water equivalent  loss in height in metres), the JAKOBSHAVN_ISBRAE loss is 4.3 MT per Km2, while the STEENSTRUP-DIETRICHSON loss is 23.4.

So while JAKOBSHAVN_ISBRAE is number 1 in total ice mass loss, on the per Km2 measure it is at No 130 out of 260, compared with STEENSTRUP at No. 2 and No. 32.

The PETERMANN_GLETSCHER has lost 56 GT, but due to its large size this is only 0.75 million tons per km2, making it 32nd highest it total mass loss, and no 200 out of 260 in loss per km2.

I.e. looking at only the cumulative mass loss of a glacier is not a good guide to intensity of its decay. Mass loss per unit of area matters more.
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Pure Speculation:-
The area calculated by the PNAS paper of the ice sheet is 1.81 million km2.
The area of the ice sheet quoted most often is 1.71 million km2

My speculation is that in 1972, that was the area of the ice sheet, suggesting in the nearly 50 years since, the ice sheet has retreated by 100,000 km2.

This is not a small area, and means that for at least part of the year, the energy from insolation is not being used for melting ice, but for heating the land, which quickly releases the heat to the atmosphere. This is a positive feedback? It must be happening to some extent, (e.g. the OK glacier in Iceland is now bare land for a good part of the year)

Of course, this 100,000 km2 might just be the result of problems in getting accurate data for 260 sub-regions of Greenland.

"Para a Causa do Povo a Luta Continua!"
"And that's all I'm going to say about that". Forrest Gump
"Damn, I wanted to see what happened next" (Epitaph)

Alexander555

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Re: Greenland 2019-20 SMB (Snowfall & Melting) Year
« Reply #12 on: September 11, 2019, 07:11:00 PM »
I see it now, it's cumulative mass loss. So the worst part, the green line. Lost 500 000 ton between Jan 2016 and Dec 2018. That's like 25 cm a year.
« Last Edit: September 11, 2019, 07:18:22 PM by Alexander555 »