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Nightvid Cole

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Re: 2015 sea ice area and extent data
« Reply #600 on: July 09, 2015, 01:54:29 PM »
I doubt that draining melt ponds would cause the concentration to go up that much in any one place. A melt pond that has been there long enough to lose its fresh water would have also been there long enough to have a good shot at turning into an outright melt hole, especially on first-year ice. If this happens, the area not covered by ice is still there, only now in the form of a melt hole instead of a melt pond. On the other hand, if the ice disintegrates into much smaller floes and spreads to cover the surface again, then this could cause concentration to go up.

I would argue that a more plausible explanation for these upticks is clouds. Though thin clouds don't affect the lower-frequency SSMI/S sensor as easily as the higher-frequency AMSR2, thick clouds still do it.

I don't have time at the moment to show a series of images that makes it visually clear how to see the impact of clouds on the concentration maps, but suffice it to say that if you have a MODIS image of well-defined cloud bands over heavily ponded ice, and open a browser window side-by-side with a CT or Bremen map of the same day*, it will be obvious what's going on, with only cursory inspection.

*BEWARE of the delay in CT maps - the labeled date is incorrect, and the actual date lags by 2-3 days.

ghoti

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Re: 2015 sea ice area and extent data
« Reply #601 on: July 09, 2015, 04:47:31 PM »
I think the fact that these sensors continue to show ice forming and melting in the Great Lakes in July is proof enough that clouds are fooling them (and us). The temperatures range from 20C to 30C  when they show this new "ice".

Wipneus

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Re: 2015 sea ice area and extent data
« Reply #602 on: July 09, 2015, 05:07:22 PM »
From yesterday's NSIDC data update I calculate the following CT updates (the days in this list are the days of data released, three days behind ice dates):

Thu       6.650652
Fri -192.9  6.457797
Sat -89.5  6.368337

So a sub-century again on Saturday. Regionally the declines are much more modest: Baffin (-19k6), CAA (-19k), CAB (-18k) and Laptev (-11k9). The extent update in Chukchi (+28k1) is remarkable with a modest area decline of +7k8.

In the attached NSIDC delta map, pixels with larger concentration changes than 7% or colored pinkish (down) or light blue (up). Solid red and blue are where the pixel concentration crosses the 15% limit (for extent).

Peter Ellis

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Re: 2015 sea ice area and extent data
« Reply #603 on: July 09, 2015, 05:12:49 PM »
I doubt that draining melt ponds would cause the concentration to go up that much in any one place. A melt pond that has been there long enough to lose its fresh water would have also been there long enough to have a good shot at turning into an outright melt hole, especially on first-year ice.
Do you have a bath?  If so, how big is the plughole? You can drain an acre of melt pond through a 1 metre hole.

I would argue that a more plausible explanation for these upticks is clouds. Though thin clouds don't affect the lower-frequency SSMI/S sensor as easily as the higher-frequency AMSR2, thick clouds still do it.
Clouds, sun angle, satellite angle, temperature and other factors, as I said a few hours ago in the "2015 melting season" thread.

Wipneus

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Re: 2015 sea ice area and extent data
« Reply #604 on: July 09, 2015, 05:17:06 PM »
I think the fact that these sensors continue to show ice forming and melting in the Great Lakes in July is proof enough that clouds are fooling them (and us). The temperatures range from 20C to 30C  when they show this new "ice".

You and I are having different ideas what proofs are.
The reason for false ice can be seen on all coasts and is called "land spillover". It is a well known effect caused by the large field of view of the SSMIS sensor and can be seen on all the coasts.

The false ice is of low concentration so the influence on area is also small (but not zero). On extent it is much larger (due to the 15% is counted as 100% effect), but wisely NSIDC does not include lake ice in their extent figures.

The AMSR2 sensor is superior in this respect.

Nightvid Cole

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Re: 2015 sea ice area and extent data
« Reply #605 on: July 09, 2015, 06:03:52 PM »
I doubt that draining melt ponds would cause the concentration to go up that much in any one place. A melt pond that has been there long enough to lose its fresh water would have also been there long enough to have a good shot at turning into an outright melt hole, especially on first-year ice.
Do you have a bath?  If so, how big is the plughole? You can drain an acre of melt pond through a 1 metre hole.


Ok, but if the effective freeboard of the bottom of the melt pond is negative, then even with a drain, some water will remain in the pond at gravitational fluid (isostatic) equilibrium.

ktonine

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Re: 2015 sea ice area and extent data
« Reply #606 on: July 09, 2015, 06:05:29 PM »
I think the fact that these sensors continue to show ice forming and melting in the Great Lakes in July is proof enough that clouds are fooling them (and us). The temperatures range from 20C to 30C  when they show this new "ice".

You and I are having different ideas what proofs are.
The reason for false ice can be seen on all coasts and is called "land spillover". It is a well known effect caused by the large field of view of the SSMIS sensor and can be seen on all the coasts.

The false ice is of low concentration so the influence on area is also small (but not zero). On extent it is much larger (due to the 15% is counted as 100% effect), but wisely NSIDC does not include lake ice in their extent figures.

The AMSR2 sensor is superior in this respect.

Instruments have uncertainties.  Some of this may be seen as bias and some as random fluctuations.  Understanding measurement uncertainty makes many of these seemingly 'strange' data values become nothing more than - well, what did you expect?

Given that the uncertainties in these measurements are relatively large, fluctuations - whether positive or negative - should just be ignored.  A single daily value should be ignored.  It's hard not to trumpet a double-century, but it has as much error in the measurement as the +46k number.  We DON'T KNOW the true value in either case.  We only know that the true value lies around the stated value plus or minus the measurement uncertainty - and even that is typically only true 95% of the time.

If we could place the earth into a stasis field and let the satellite circle around 1000 times and provide us 1000 measurements of the exact same arctic we would hope to see a normal distribution around the true value.  We would NOT get 1000 measurements with the same value - even though the sensors would be looking at the exact same earth each time.

Wipneus

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Re: 2015 sea ice area and extent data
« Reply #607 on: July 09, 2015, 07:07:44 PM »

Instruments have uncertainties.  Some of this may be seen as bias and some as random fluctuations.  Understanding measurement uncertainty makes many of these seemingly 'strange' data values become nothing more than - well, what did you expect?


Really? I haven't seen anything strange despite the assertions made here. Near surface temperatures (T2m) are still hovering around zero and occasionally dipping below. Under such circumstances the ice surface dries quickly and that is having a exaggerated effect on NSIDC concentration. Call it measurement uncertainty, it is a known feature of the product.

Melt ponds? Most of what you see in the day to day changes is IMO the whetting/drying of the surface.

BTW the ADS-NIPR AMSR2 Jaxa thickness/melting map showed a simultaneous drop in melting area at the same day (ice date) that CT-area had the uptick. Different sensors, different microwave bands, different methods. That bump was real, no need for calling something strange, do not miss the opportunity to learn.
« Last Edit: July 09, 2015, 07:13:19 PM by Wipneus »

ktonine

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Re: 2015 sea ice area and extent data
« Reply #608 on: July 09, 2015, 07:10:20 PM »

Instruments have uncertainties.  Some of this may be seen as bias and some as random fluctuations.  Understanding measurement uncertainty makes many of these seemingly 'strange' data values become nothing more than - well, what did you expect?


Really? I haven't seen anything strange despite the assertions made here. ....

I haven't seen anything strange either - that's why strange is in quotation marks.

ktonine

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Re: 2015 sea ice area and extent data
« Reply #609 on: July 09, 2015, 08:26:05 PM »
Just to remember what we're dealing with:

Quote
In terms of absolute uncertainty of the algorithms, daily concentration errors of 5-10% are found within the ice pack during cold winter conditions. However, errors can be much higher in non-optimal conditions, such as near the ice edge and during surface melt conditions, and vary both spatially and temporally due to local conditions. Because of the low spatial resolution of the passive microwave sensors (footprints as large as 70 km for some frequencies), daily concentrations near ice edge may have errors of 50% or more; the low spatial resolution essentially limits the precision of the ice edge determination to ~25-50 km. Near the ice edge, concentration may be overestimated or underestimated depending on the sensor footprint location relative to the edge and the character of the ice at the edge (thin vs. thick, consolidated vs. loose). Within the ice pack, the passive microwave algorithms tend to underestimate concentration, especially in regions of thin ice and melting ice. Under strong surface melt conditions (including melt pond formation), concentrations may be underestimated by 20-30%. Extreme cold surface temperatures and atmospheric emission (primarily from thick clouds) tend to result in less extreme underestimations.

Obs4MIPs Sea Ice Concentration CDR Technical Note v3.1

seaicesailor

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Re: 2015 sea ice area and extent data
« Reply #610 on: July 09, 2015, 09:01:23 PM »

Instruments have uncertainties.  Some of this may be seen as bias and some as random fluctuations.  Understanding measurement uncertainty makes many of these seemingly 'strange' data values become nothing more than - well, what did you expect?


Really? I haven't seen anything strange despite the assertions made here. Near surface temperatures (T2m) are still hovering around zero and occasionally dipping below. Under such circumstances the ice surface dries quickly and that is having a exaggerated effect on NSIDC concentration. Call it measurement uncertainty, it is a known feature of the product.

Melt ponds? Most of what you see in the day to day changes is IMO the whetting/drying of the surface.

BTW the ADS-NIPR AMSR2 Jaxa thickness/melting map showed a simultaneous drop in melting area at the same day (ice date) that CT-area had the uptick. Different sensors, different microwave bands, different methods. That bump was real, no need for calling something strange, do not miss the opportunity to learn.
Chukchi extent increase is indeed strange. I wouldn't call it normal given current conditions.  8)

plinius

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Re: 2015 sea ice area and extent data
« Reply #611 on: July 09, 2015, 09:18:55 PM »
Indeed - would also like to point out that on the obuoy cameras you can even see a bit of frost extending into the little melt ponds... it's not always surface melt 24h/day, in particular not so far off the pole.

ktonine

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Re: 2015 sea ice area and extent data
« Reply #612 on: July 09, 2015, 09:42:37 PM »
Chukchi extent increase is indeed strange. I wouldn't call it normal given current conditions.  8)

The Chukchi has roughly extent of approximately 200k.  We see an uptick of 28k.  Is this strange or normal?

No.  The daily numbers have uncertainties that are likely 25% to 40% over the Chukchi at this point in the melt season.  I.e., the +28k probably has an uncertainty of anywhere from +/- 50k to +/- 80k.  So the 'true' value may well be -50k. 

Or the value from the day before may have been incorrect by -28k.  What we today is the 'true' value - meaning the number had to go up by +28k to correct for the previous number.

Bearing all this in mind a single day value increasing by 28k just simply is NOT surprising.  It doesn't matter whether you expect it to decline or not.  Math, statistics, and uncertainties simply dictate that you HAVE to see values like this.

Now, if we saw this for 5 days in a row - then I'd call it surprising.


Rubikscube

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Re: 2015 sea ice area and extent data
« Reply #613 on: July 09, 2015, 10:30:37 PM »
The Chukchi has roughly extent of approximately 200k.  We see an uptick of 28k.  Is this strange or normal?

No.  The daily numbers have uncertainties that are likely 25% to 40% over the Chukchi at this point in the melt season.  I.e., the +28k probably has an uncertainty of anywhere from +/- 50k to +/- 80k.  So the 'true' value may well be -50k. 

Or the value from the day before may have been incorrect by -28k.  What we today is the 'true' value - meaning the number had to go up by +28k to correct for the previous number.

Bearing all this in mind a single day value increasing by 28k just simply is NOT surprising.  It doesn't matter whether you expect it to decline or not.  Math, statistics, and uncertainties simply dictate that you HAVE to see values like this.

Now, if we saw this for 5 days in a row - then I'd call it surprising.

Totally agree.

What I find most puzzling right now is that surface melt at the O-buoys and the North pole cam doesn't seem to correlate at all with the respective 850 hPa temps. The melt ponds at the pole apparently stopped expanding just when the heat should really start to pound the ice. I was expecting to see several more drops in the 150k range, but I'm not so sure anymore.

Wipneus

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Re: 2015 sea ice area and extent data
« Reply #614 on: July 10, 2015, 08:02:11 AM »

Chukchi extent increase is indeed strange. I wouldn't call it normal given current conditions.  8)

It is only strange because you do not think. Try it.

Hints:
- think what extent means
- think why extent is not area
- think why grid size matters for extent

If you cannot figure it out, or someone else explains it to you, I will explain later today or tomorrow.

seaicesailor

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Re: 2015 sea ice area and extent data
« Reply #615 on: July 10, 2015, 08:59:53 AM »

Chukchi extent increase is indeed strange. I wouldn't call it normal given current conditions.  8)

It is only strange because you do not think. Try it.


Wow thank you! That is right, I do think as a norm. Maybe I could read some literature too.  :)



Neven

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Re: 2015 sea ice area and extent data
« Reply #616 on: July 10, 2015, 12:27:12 PM »
For everyone who doesn't like to think (like me), just say to yourself: it all evens out, it all evens out, it all evens out.  ;D
Il faut comparer, comparer, comparer, et cultiver notre jardin

anthropocene

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Re: 2015 sea ice area and extent data
« Reply #617 on: July 10, 2015, 04:47:14 PM »
The Chukchi has roughly extent of approximately 200k.  We see an uptick of 28k.  Is this strange or normal?

No.  The daily numbers have uncertainties that are likely 25% to 40% over the Chukchi at this point in the melt season.  I.e., the +28k probably has an uncertainty of anywhere from +/- 50k to +/- 80k.  So the 'true' value may well be -50k. 

Or the value from the day before may have been incorrect by -28k.  What we today is the 'true' value - meaning the number had to go up by +28k to correct for the previous number.

Bearing all this in mind a single day value increasing by 28k just simply is NOT surprising.  It doesn't matter whether you expect it to decline or not.  Math, statistics, and uncertainties simply dictate that you HAVE to see values like this.

Now, if we saw this for 5 days in a row - then I'd call it surprising.

Totally agree.

What I find most puzzling right now is that surface melt at the O-buoys and the North pole cam doesn't seem to correlate at all with the respective 850 hPa temps. The melt ponds at the pole apparently stopped expanding just when the heat should really start to pound the ice. I was expecting to see several more drops in the 150k range, but I'm not so sure anymore.
850hPa is approx. 1500m altitude. For melting sea-ice that heat may as well be on the moon. I'll ask a question: 850hPa is at temp x. Is the surface at higher, lower or the same temp as x?7

realitybytes

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Re: 2015 sea ice area and extent data
« Reply #618 on: July 10, 2015, 05:25:02 PM »
The Chukchi has roughly extent of approximately 200k.  We see an uptick of 28k.  Is this strange or normal?

No.  The daily numbers have uncertainties that are likely 25% to 40% over the Chukchi at this point in the melt season.  I.e., the +28k probably has an uncertainty of anywhere from +/- 50k to +/- 80k.  So the 'true' value may well be -50k. 

Or the value from the day before may have been incorrect by -28k.  What we today is the 'true' value - meaning the number had to go up by +28k to correct for the previous number.

Bearing all this in mind a single day value increasing by 28k just simply is NOT surprising.  It doesn't matter whether you expect it to decline or not.  Math, statistics, and uncertainties simply dictate that you HAVE to see values like this.

Now, if we saw this for 5 days in a row - then I'd call it surprising.

Totally agree.

What I find most puzzling right now is that surface melt at the O-buoys and the North pole cam doesn't seem to correlate at all with the respective 850 hPa temps. The melt ponds at the pole apparently stopped expanding just when the heat should really start to pound the ice. I was expecting to see several more drops in the 150k range, but I'm not so sure anymore.
850hPa is approx. 1500m altitude. For melting sea-ice that heat may as well be on the moon. I'll ask a question: 850hPa is at temp x. Is the surface at higher, lower or the same temp as x?7
Yes. ;)

Wipneus

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Re: 2015 sea ice area and extent data
« Reply #619 on: July 10, 2015, 06:55:03 PM »
From yesterday's NSIDC data update I calculate the following CT updates (the days in this list are the days of data released, three days behind ice dates):

Fri       6.458558
Sat -89.5  6.369031
Sun -113.4  6.255653

Sunday's century is brought to you by Baffin (-36k7). Also brought by Laptev (-22k), Greenlands Sea (-17k2), ESS (-16k3) and CAA (-15k5).

Today's puzzle is the ESS. The (CT) area drops by -16k, but (NSIDC) extent .... has an increase of a whopping +45k7 !

You can post your explanation below. I am not sure what the answer is myself, but "cannot be" is almost certainly incorrect.

In the attached NSIDC delta map, pixels with larger concentration changes than 7% or colored pinkish (down) or light blue (up). Solid red and blue are where the pixel concentration crosses the 15% limit (for extent).

nuwandaraalwis

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Re: 2015 sea ice area and extent data
« Reply #620 on: July 10, 2015, 08:17:45 PM »
So great predictions. Seems you have internal connection with CT today group ;) lol

plinius

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Re: 2015 sea ice area and extent data
« Reply #621 on: July 10, 2015, 08:29:24 PM »
Nasty week, lost >10% of the remaining area...

seaicesailor

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Re: 2015 sea ice area and extent data
« Reply #622 on: July 10, 2015, 11:58:07 PM »

I am not sure what the answer is myself, ...


Now, that is strange
 

Rubikscube

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Re: 2015 sea ice area and extent data
« Reply #623 on: July 11, 2015, 01:25:09 AM »
850hPa is approx. 1500m altitude. For melting sea-ice that heat may as well be on the moon. I'll ask a question: 850hPa is at temp x. Is the surface at higher, lower or the same temp as x?7

Duh? I dare to claim that the amount of heat in the 850 hPa layer is more relevant to the sea ice melt than the amount of heat on the moon. As for your question, the answer depends on whether the underlying surface is ocean, ice or land as well as a whole range of other conditions such as season, cloudiness, altitude and the list goes on. Thats in fact the entire point of the 850 hPa temps; that they are relatively unaffected by the local surface conditions, and thus can be used to distinguish warm and cold air masses. So when an extremely warm air mass engulfs virtually the entire CAB one should expect melt rates at the surface to pick up, and I suspect that the recent failure to do so may be related to cloud cover.

anthropocene

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Re: 2015 sea ice area and extent data
« Reply #624 on: July 11, 2015, 09:31:51 AM »
850hPa is approx. 1500m altitude. For melting sea-ice that heat may as well be on the moon. I'll ask a question: 850hPa is at temp x. Is the surface at higher, lower or the same temp as x?7

Duh? I dare to claim that the amount of heat in the 850 hPa layer is more relevant to the sea ice melt than the amount of heat on the moon. As for your question, the answer depends on whether the underlying surface is ocean, ice or land as well as a whole range of other conditions such as season, cloudiness, altitude and the list goes on. Thats in fact the entire point of the 850 hPa temps; that they are relatively unaffected by the local surface conditions, and thus can be used to distinguish warm and cold air masses. So when an extremely warm air mass engulfs virtually the entire CAB one should expect melt rates at the surface to pick up, and I suspect that the recent failure to do so may be related to cloud cover.

Thank you Rubikscube - I couldn't have put it better myself. To paraphrase - high temperatures at 850hPa MAY provide heat energy to the surface, there again they may not. Some on here seem to think that this is a given and so massive ice melt should automatically be the outcome of high 850hPa temps. When the measurements of the ice do not match this assumption then the measurements are  treated as suspicious. I'm just pointing out that a fault in this assumption is more likely to be the cause of the mismatch between expectation and actual behaviour than any measurement issues. 
(of course you are correct about the moon and 850hPa - I mentioned it to provoke thought about what is the difference between 850hPa and the moon and when this is at its greatest. ).

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Re: 2015 sea ice area and extent data
« Reply #625 on: July 11, 2015, 10:28:51 AM »
Today's puzzle is the ESS. The (CT) area drops by -16k, but (NSIDC) extent .... has an increase of a whopping +45k7 !

That looks like a diffusion / shift by wind for the ice; nullschool shows slow wind towards ess, but faster winds in ess towards the coast.

Could this be simple divergence of fragmented ice? -- unusual fluctuations in *area* would be unphysical, but extent is just the shuffling of the ice fragments; especially with the purple "lost concentration %" segment nearby, it looks like that ice just got shuffled into pixels which had a little under 15% ice last pass.

seaicesailor

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Re: 2015 sea ice area and extent data
« Reply #626 on: July 11, 2015, 11:44:03 AM »
Today's puzzle is the ESS. The (CT) area drops by -16k, but (NSIDC) extent .... has an increase of a whopping +45k7 !

That looks like a diffusion / shift by wind for the ice; nullschool shows slow wind towards ess, but faster winds in ess towards the coast.

Could this be simple divergence of fragmented ice? -- unusual fluctuations in *area* would be unphysical, but extent is just the shuffling of the ice fragments; especially with the purple "lost concentration %" segment nearby, it looks like that ice just got shuffled into pixels which had a little under 15% ice last pass.

Divergent drift toward the coast sounds reasonable since it would explain both concentration decrease and extent increase. However, note that the edge advance happens along a length of +1000 Km. A total of 45k km2 of extent increase means an average of 45000 / 1000 / 24 / 3600 * 100000 = 50 cm/s of mean drift during the day, which is very high!. HYCOM model however shows this



This model doesn't usually show drifts in the opposite direction than real.

So the reason must be another one. Another bump on the road, at least a nice exercise to keep us thinking a bit.

PS. Apparently it is been colder than previous days too, google "Wrangler island weather". Maybe just as much as to stop surface melting and cause some refreezing, just as much as to make some grid elements with <15% concentration become >15%? The edge is so spread that the number of grid elements on the verge of 15% may be pretty large.
« Last Edit: July 11, 2015, 11:51:24 AM by seaicesailor »

seaicesailor

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Re: 2015 sea ice area and extent data
« Reply #627 on: July 11, 2015, 12:45:24 PM »

Chukchi extent increase is indeed strange. I wouldn't call it normal given current conditions.  8)

It is only strange because you do not think. Try it.

Hints:
- think what extent means
- think why extent is not area
- think why grid size matters for extent

If you cannot figure it out, or someone else explains it to you, I will explain later today or tomorrow.

It took me two days, sorry. I figured out a possible reason for these fluctuations. If not correct, yes please, explain it to me.
When compaction is low (and this year beats records) NSIDC extent numbers are particularly noisy, because the edge is less compact too, (wider, more spread) and there can be a very large number of grid elements close to 15% of concentration.

Jim Pettit

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Re: 2015 sea ice area and extent data
« Reply #628 on: July 11, 2015, 02:44:41 PM »
ADS-NIPR Extent:
8,660,819 km2 (10 July)
Down 5,281,241 km2 (37.88%) from 2015 maximum of 13,942,060 km2 on 15 February.
5,483,364 km2 above record minimum extent of 3,177,455 km2 (16 September 2012).
Down 89,966 km2 from previous day.
Down 680,440 km2 over past seven days (daily average: -97,206 km2).
Down 821,497 km2 for the month of July (daily average: -82,150 km2).
446,956 km2 below 2000s average for this date.
434,103 km2 above 2010s average for this date.
524,857 km2 above 2014 value for this date.
684,247 km2 above 2012 value for this date.
8th lowest July to-date average.
8th lowest value for the date.
64 days this year (33.51% year-to-date) have recorded the lowest daily extent.
32 days (16.75%) have recorded the second lowest.
32 days (16.75%) have recorded the third lowest.
128 days (67.02%) in total have been among the three lowest on record.


CT Area:
6,369,842 km2 (10 July [Day 0.5205])
Down 6,904,714 km2 (52.01%) from 2015 maximum of 13,274,555 km2 on 17 February [Day 0.1288].
4,135,832 km2 above record minimum area of 2,234,010 km2 (14 September 2012).
Down 88,717 km2 from previous day.
Down 733,932 km2 over past seven days (daily average: -104,847 km2).
Down 1,084,979 km2 for the month of July (daily average: -108,498 km2).
467,909 km2 below 2000s average for this date.
117,015 km2 above 2010s average for this date.
222,131 km2 below 2014 value for this date.
454,264 km2 above 2012 value for this date.
5th lowest July to-date average.
5th lowest value for the date.
7 days this year (3.66% year-to-date) have recorded the lowest daily area.
19 days (9.95%) have recorded the second lowest.
29 days (15.18%) have recorded the third lowest.
55 days in total (28.8%) have been among the lowest three on record.

Wipneus

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Re: 2015 sea ice area and extent data
« Reply #629 on: July 11, 2015, 06:15:17 PM »
From yesterday's NSIDC data update I calculate the following CT updates (the days in this list are the days of data released, three days behind ice dates):

Sat       6.369842
Sun -113.4  6.256430
Mon -100.0  6.156442

So Monday the chance of a century is exactly 50%. Supported by Hudson (-29k7), Laptev (-21k9), ESS (-16k3), CAB (-12k9) and Greenland Sea (-12k4). Baffin showed a small uptick (+11k7).

In the attached NSIDC delta map, pixels with larger concentration changes than 7% or colored pinkish (down) or light blue (up). Solid red and blue are where the pixel concentration crosses the 15% limit (for extent).

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Re: 2015 sea ice area and extent data
« Reply #630 on: July 11, 2015, 07:32:35 PM »


It took me two days, sorry. I figured out a possible reason for these fluctuations. If not correct, yes please, explain it to me.
When compaction is low (and this year beats records) NSIDC extent numbers are particularly noisy, because the edge is less compact too, (wider, more spread) and there can be a very large number of grid elements close to 15% of concentration.

Yes, maybe I can offer another way to look at it.

Assuming an exact algorithm, in an idealized sea ice measurement:

- infinite small grid cell and microwave field of view width;

  extent and are a are the same and compactness is always 100%.

When the grid is not infinitely fine, or the microwave field of view is wider than zero, extent will be larger than area depending how the ice is distributed: compact or a loose flotilla. But it will depend in the first place on the grid cell size/ microwave field of view.

NSIDC has the larger grid cell size 25km and an even bigger field of view of the microwave sensor (50km or so). That is one reason why NSIDC may behave strange now and than, stranger than the Uni Hamburg AMSR2 product that I use in the "home brew" thread. (The other reason is of course that exact algorithms do not exist.)

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Re: 2015 sea ice area and extent data
« Reply #631 on: July 11, 2015, 09:22:56 PM »
Thank you

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Re: 2015 sea ice area and extent data
« Reply #632 on: July 12, 2015, 04:44:29 PM »
Update for the week to July 12th

The current 5 day mean is on 9,230,800km2 while the 1 day extent is at 9,013,000km2.
The daily anomaly (compared to 81-10) is at -854,100km2, an increase from -788,180km2 last week. The anomaly compared to the 07, 11 and 12 average is at +799,267km2, an increase from +618,200km2 last week. We're currently 9th lowest on record, down from 8th last week.



The average daily change over the last 7 days was -89.9k/day, compared to the long term average of -80.5k/day, and the 07, 11 and 12 average of -115.8k/day.
The average long term change over the next week is -85.9k/day, with the 07, 11, and 12 average being -107.9k/day.



The loss so far this July is the 15th largest on record. To achieve the largest monthly drop, a daily loss of at least 128.3k/day is required, while the smallest drop requires a loss of less than 53.0k/day and an average drop requires a loss of 85.8k/day.


Wipneus

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Re: 2015 sea ice area and extent data
« Reply #633 on: July 12, 2015, 04:54:42 PM »
From today's NSIDC data update I calculate the following CT updates (the days in this list are the days of data released, three days behind ice dates):

Sat       6.369842
Sun -113.4  6.256430
Mon -100.3  6.156095
Tue -118.6  6.037464

The respected members of this forum will be pleased to see the thirdfourth clean century in a row without wild excursions in either direction. That is on expense of the NSIDC extent of course, you cannot have it all.
Today's decline come from mostly from the CAB (-62k9). Much smaller (~ -10k) are declines in ESS, Hudson, Laptev, Beaufort, Baffin and "Lakes". The CAA increased by +14k.

Today's region that increased in extent and decreased in area is Hudson ( +27k9, -12k6)

In the attached NSIDC delta map, pixels with larger concentration changes than 7% or colored pinkish (down) or light blue (up). Solid red and blue are where the pixel concentration crosses the 15% limit (for extent).
« Last Edit: July 13, 2015, 04:29:04 PM by Wipneus »

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Re: 2015 sea ice area and extent data
« Reply #634 on: July 13, 2015, 02:22:34 PM »
ADS-NIPR Extent:
8,461,055 km2 (12 July)
Down 5,481,005 km2 (39.31%) from 2015 maximum of 13,942,060 km2 on 15 February.
5,283,600 km2 above record minimum extent of 3,177,455 km2 (16 September 2012).
Down 86,828 km2 from previous day.
Down 748,022 km2 over past seven days (daily average: -106,860 km2).
Down 1,021,261 km2 for the month of July (daily average: -85,105 km2).
482,504 km2 below 2000s average for this date.
438,276 km2 above 2010s average for this date.
511,351 km2 above 2014 value for this date.
688,000 km2 above 2012 value for this date.
8th lowest July to-date average.
8th lowest value for the date.
64 days this year (33.16% year-to-date) have recorded the lowest daily extent.
32 days (16.58%) have recorded the second lowest.
32 days (16.58%) have recorded the third lowest.
128 days (66.32%) in total have been among the three lowest on record.


CT Area:
6,156,546 km2 (12 July [Day 0.526])
Down 7,118,010 km2 (53.62%) from 2015 maximum of 13,274,555 km2 on 17 February [Day 0.1288].
3,922,536 km2 above record minimum area of 2,234,010 km2 (14 September 2012).
Down 100,730 km2 from previous day.
Down 721,897 km2 over past seven days (daily average: -103,128 km2).
Down 1,298,275 km2 for the month of July (daily average: -108,190 km2).
509,609 km2 below 2000s average for this date.
74,223 km2 above 2010s average for this date.
301,967 km2 below 2014 value for this date.
558,373 km2 above 2012 value for this date.
5th lowest July to-date average.
5th lowest value for the date.
7 days this year (3.63% year-to-date) have recorded the lowest daily area.
19 days (9.84%) have recorded the second lowest.
29 days (15.03%) have recorded the third lowest.
55 days in total (28.5%) have been among the lowest three on record.

Wipneus

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Re: 2015 sea ice area and extent data
« Reply #635 on: July 13, 2015, 04:12:01 PM »
From today's NSIDC data update I calculate the following CT updates (the days in this list are the days of data released, three days behind ice dates):

Mon       6.156546
Tue -118.9  6.037617
Wed -100.1  5.937527

So the fifth near century in a row. Drops in ESS, Beaufort, CAB, Greenland Sea and Laptev, about -20k each. An increase in the CAA (+17k).

Ghoti: Yes, there was a large drop in lake ice extent (-25k), but by area not so much (-7k).

In the attached NSIDC delta map, pixels with larger concentration changes than 7% or colored pinkish (down) or light blue (up). Solid red and blue are where the pixel concentration crosses the 15% limit (for extent).

seaicesailor

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Re: 2015 sea ice area and extent data
« Reply #636 on: July 13, 2015, 04:29:54 PM »

FWIW, NSIDC Extent fell 1,023,000 km2 for the last 10 days, averaging 100K per day.


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Re: 2015 sea ice area and extent data
« Reply #637 on: July 13, 2015, 04:37:58 PM »
 :D Thanks for the mention Wipneus!

I also wonder about the numbers for the entire CAA given that the distances across most of the straits are less than the distances from shore to shore of the Great Lakes. Seems as if land edge problems might swamp the values for gain and loss of ice in the CAA.

Clearly it doesn't make much difference in the overall scheme of things since the total area there is small relative to the total ice area.

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Re: 2015 sea ice area and extent data
« Reply #638 on: July 13, 2015, 05:20:15 PM »
Honestly I think the curve for the CAA is pretty realistic, if you figure in melt ponds: Fast ice --> fast drop when melt ponds form --> recovery in the statistics when the ice surface collapses and melt ponds start to disappear --> melt-out.
http://arctic.atmos.uiuc.edu/cryosphere/IMAGES/recent365.anom.region.12.html

We are now in phase III for most of the archipelago.

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Re: 2015 sea ice area and extent data
« Reply #639 on: July 13, 2015, 05:47:27 PM »
:D Thanks for the mention Wipneus!

I also wonder about the numbers for the entire CAA given that the distances across most of the straits are less than the distances from shore to shore of the Great Lakes. Seems as if land edge problems might swamp the values for gain and loss of ice in the CAA.


Indeed. Again this affects extent more than area, because the concentration of the false ice is less than 100% and it is mostly important when there is little real ice.

If you look at the regional extent graphs for NSIDC and Uni Hamburgs higher resolution SIC, you see that at minimum NSIDC saw more than two times as much ice extent in the CAA. The UH 2012 data where obtained by SSMIS, the AMSR2 instrument would have been a lot sharper still!

NSIDC:
https://sites.google.com/site/arctischepinguin/home/sea-ice-extent-area/grf/nsidc-nt-regional-extent-overview.png
UH Hamburg:
https://sites.google.com/site/arctischepinguin/home/amsr2/grf/amsr2-extent-regional.png




There was a little discuusion on the "home brew" thread last winter about ice in the Baltic that has the same problem:
http://forum.arctic-sea-ice.net/index.php/topic,382.0.html



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Re: 2015 sea ice area and extent data
« Reply #640 on: July 14, 2015, 04:22:57 PM »
From today's NSIDC data update I calculate the following CT updates (the days in this list are the days of data released, three days behind ice dates):

Tue       6.037754
Wed -100.2  5.937507
Thu -70.0  5.867492

So a sub-century on Thursday, lake ice is a factor as it jumped +20k1 (or CT's decision to include it in its sea ice area calculation).
Otherwise it is again the ESS and Laptev regions that do this: -38k5 and -36k7. Hudson declined -23k1, Beaufort -10k but CAA increased (again), now +29k7.

Laptev is the most remarkable region as regional extent dropped by a massive -83k9.

In the attached NSIDC delta map, pixels with larger concentration changes than 7% or colored pinkish (down) or light blue (up). Solid red and blue are where the pixel concentration crosses the 15% limit (for extent).

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Re: 2015 sea ice area and extent data
« Reply #641 on: July 14, 2015, 06:39:57 PM »
Using the NSIDC daily data, we've lost 905k in the last 7 days.

To put that into context, before 2007, only 8 different 7 day periods on the whole record saw a loss of greater than 900k, once in 1990, twice in 1991, twice in 1993, once in 1999 and twice in 2005.

In contrast, from 2007 to 2014 there have been 35 such losses

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Re: 2015 sea ice area and extent data
« Reply #642 on: July 14, 2015, 07:04:19 PM »
i.e. an average of ~4.5 per year.  So we need at least another three weeks similar to the current one for 2015 too keep up with the other post-2007 years.  Or do I misunderstand?

Edit:  If you're allowing overlapping periods, then I guess we'd only need another ~4 days of this rate of melt for 2015 to keep up with the other post-2007 years.

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Re: 2015 sea ice area and extent data
« Reply #643 on: July 14, 2015, 07:26:11 PM »
Yep, some overlap. Also, many get a boost from the land mask and algorithm changes that occur from June 30th to July 1st, which usually produces a loss of several 100k.



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Re: 2015 sea ice area and extent data
« Reply #644 on: July 14, 2015, 08:03:15 PM »
Taking a look at the 81-10 anomalies (for 1st of April to 15th of September) rather than the absolute losses, we get this.



The recent 905k drop has an anomaly of just 340k, which doesn't seem like too much compared to the >700k anomalies 2012 had at the start of June.

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Re: 2015 sea ice area and extent data
« Reply #645 on: July 15, 2015, 02:15:42 AM »
Gonna say this again.. Strip out Hudson and Baffin Bay, and 2015 is almost spot on 2012 area and extent numbers.

The qualitative difference between 2015 and the lowest three minimums is much less than the numbers might suggest.
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Re: 2015 sea ice area and extent data
« Reply #646 on: July 15, 2015, 06:44:12 AM »
Gonna say this again.. Strip out Hudson and Baffin Bay, and 2015 is almost spot on 2012 area and extent numbers.

The qualitative difference between 2015 and the lowest three minimums is much less than the numbers might suggest.

Those numbers are now being graphed and updated each day....

https://sites.google.com/site/arctischepinguin/home/amsr2/grf/basin-extent-multiprod.png

https://sites.google.com/site/arctischepinguin/home/amsr2/grf/basin-area-multiprod.png


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Re: 2015 sea ice area and extent data
« Reply #647 on: July 15, 2015, 07:45:38 AM »
Gonna say this again.. Strip out Hudson and Baffin Bay, and 2015 is almost spot on 2012 area and extent numbers.

The qualitative difference between 2015 and the lowest three minimums is much less than the numbers might suggest.

Those numbers are now being graphed and updated each day....

https://sites.google.com/site/arctischepinguin/home/amsr2/grf/basin-extent-multiprod.png

https://sites.google.com/site/arctischepinguin/home/amsr2/grf/basin-area-multiprod.png
Thanks for the affirmation, Bob :)

I hadn't realized wipneus was updating these daily.
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Wipneus

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Re: 2015 sea ice area and extent data
« Reply #648 on: July 15, 2015, 07:55:57 AM »
Gonna say this again.. Strip out Hudson and Baffin Bay, and 2015 is almost spot on 2012 area and extent numbers.

The qualitative difference between 2015 and the lowest three minimums is much less than the numbers might suggest.

Those numbers are now being graphed and updated each day....

https://sites.google.com/site/arctischepinguin/home/amsr2/grf/basin-extent-multiprod.png

https://sites.google.com/site/arctischepinguin/home/amsr2/grf/basin-area-multiprod.png

Yup, at least twice daily actually. In the graphs we can see that the numbers for the restricted Basin are also close to those of 2013 and 2014. To finish near 2012's minimum, 2015 will still have to show something special in the remainder of the season.

For the moment the fact is that some peripherals have a big problem meeting the "normal" melt levels/rates.


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Re: 2015 sea ice area and extent data
« Reply #649 on: July 15, 2015, 02:33:36 PM »
I wonder if (when?) these two graphs will show up on the ASIG Regional Graph page.
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