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anonymous

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Re: What the Buoys are telling
« Reply #100 on: July 20, 2013, 04:07:09 PM »
I've focused the mass balance buoys, but now the forecasts see a major storm system spending some days in the Beaufort Sea. Could someone help me out with a short introduction which buoys have an eye on the halocline in this region, if at all?

Jim Hunt

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Re: What the Buoys are telling
« Reply #101 on: July 20, 2013, 04:35:34 PM »
Could someone help me out with a short introduction which buoys have an eye on the halocline in this region, if at all?


Here's the ITP map:



ITP's 62, 64 and 65 are still acquiring profiles in the Beaufort. Here's what ITP 65 (which is colocated with OBuoy 7) reveals:

« Last Edit: July 20, 2013, 05:49:30 PM by Jim Hunt »
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ghoti

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Re: What the Buoys are telling
« Reply #102 on: July 20, 2013, 05:34:21 PM »
Notice the snow depth data from buoy 2013E, the NPEO webcam #2 and look at the photo of the spot. It looks to me as if the snow height sensor is in the photo on the left side of the large buoy structure. Also note that clearly the snow sensor is positioned in the spot where the winds created the most snow drift accumulation in the area. Details like this make it really hard to generalize buoy data to the system as a whole



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Re: What the Buoys are telling
« Reply #103 on: July 20, 2013, 06:01:23 PM »
Notice the snow depth data from buoy 2013E, the NPEO webcam #2 and look at the photo of the spot. It looks to me as if the snow height sensor is in the photo on the left side of the large buoy structure. Also note that clearly the snow sensor is positioned in the spot where the winds created the most snow drift accumulation in the area. Details like this make it really hard to generalize buoy data to the system as a whole


The observer effect?

Notice, too, how what seems to be a fallen pole lying between the first and second marker on the left in the 07.31 image posted by ghoti seems to have drifted away by 13.37. (It's probably not what appears to be lying at 90 degrees to the left of the fourth marker from the left, as whatever that is appears in earlier photographs.)


anonymous

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Re: What the Buoys are telling
« Reply #104 on: July 20, 2013, 07:57:23 PM »
Thanks Jim, that'll give me a starting point. Looks like a long road from 'salt melts ice' to an interpretation of the profiles before and after the storm. It starts with why there are 500 days in 2012? At least there are enough buoys, so any effect should be visible on at least more than one.

I remember some research in the Beaufort trying visualize movement of the halocline in 3D. IIRC the shape of the sea bottom had supported some unusual behavior. Going to check old bookmarks files, since Google is not helpful....

SteveMDFP

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Re: What the Buoys are telling
« Reply #105 on: July 20, 2013, 08:18:32 PM »


Jim,

Thanks SO MUCH for lending your expertise in helping people understand the data.  You should, perhaps, be a professor somewhere.

Anyway, I've stared and pondered at a bunch of these temp/salinity profiles, trying to interpret them.  Here's a question.  In the final days of the salinity profile shown, we see some INCREASE in surface salinity, despite obvious melting of ice.  Could the interpretation be that wind-driven Ekman pumping is bringing salty water up from depth, faster than ice melt is diluting the surface sea water?

It seems to me that a direct consequence of the widespread "slushification" of the Arctic is that all the ice is now far more mobile in the presence of wind, eliminating the fresh water lens that usually sits under melting ice, protecting it from salt and warmth of the waters below.  Thus, wind-related effects on melting could be as important as anything short of insolation.

ktonine

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Re: What the Buoys are telling
« Reply #106 on: July 20, 2013, 09:19:46 PM »
It seems to me that a direct consequence of the widespread "slushification" of the Arctic is that all the ice is now far more mobile in the presence of wind, eliminating the fresh water lens that usually sits under melting ice, protecting it from salt and warmth of the waters below. 

Typically in the CAB the freshwater lens will actually be *warmer* than the sea water:  0C vs -1.8C.  The lens doesn't protect the ice from warmer waters - the lens *is* the warmer water. So draining of melt ponds accelerates bottom melt.  Now that so much of the CAB is floe-filled with significant open water it resembles the Marginal Ice Zones we find on the periphery.  While this would dilute the effect of the freshwater lens, it adds mechanical and lateral melt into the mix plus the lower albedo of the open water. 

Others disagree, but I'm of the opinion these additional factors more than compensate for the loss of a true freshwater lens.  We do not have a buoy in the low concentration CAB north of 85 degrees between Franz Josef Land and the pole - I'd really, really like to see data from a floe in that area.


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Re: What the Buoys are telling
« Reply #107 on: July 20, 2013, 10:20:23 PM »
Arcticio - Now that you mention it I notice that the last profile for ITP 65 was on June 29th. It looked to be lying at a very strange angle before OBuoy 7 itself keeled over. Perhaps it's already gone to meet it's maker?

Steve - You're very kind, but I'm no expert when it comes to understanding the whys and wherefores of melting sea ice. I'm just watching and endeavouring to learn. Your guess is as good as mine, if not better!

Ktonine - Doesn't ITP 57 fit your bill, at 87.3277° N, 94.1396° E? Presumably you were you hoping for an IMB buoy in the vicinity also?
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ktonine

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Re: What the Buoys are telling
« Reply #108 on: July 20, 2013, 10:48:11 PM »
Jim - I'd like to see an ice profile from the area.  2012J (?) is also in the area- but lost its sensors.

I'd like to see if the melt profile is comparable to one from the peripheral MIZs.

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Re: What the Buoys are telling
« Reply #109 on: July 21, 2013, 12:04:58 AM »
2012J (?) is also in the area- but lost its sensors.


As luck would have it the skies were clear(ish) today above 2012J. It's only lost its bottom sounder, and the thermistors are still going. The whole floe seems now to be at or above the temperature of the water below it. Hence I haven't the faintest idea where the bottom might be at present!
« Last Edit: July 21, 2013, 12:15:05 AM by Jim Hunt »
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ktonine

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Re: What the Buoys are telling
« Reply #110 on: July 21, 2013, 03:13:00 AM »
Jim, I've looked at the downloaded data file for 2012J, but can only guess at how to determine ice thickness from the thermistor data.

The most recent entry had this for the first 15 thermistors:
T1      0.65
T2      0.89
T3      0.38
T4      0.13
T5      -0.02
T6      -0.38
T7      -0.69
T8      -1
T9      -1.32
T10    -1.51
T11    -1.69
T12    -1.69
T13    -1.76
T14    -1.71
T15    -1.76

My guess is the ice surface is very close to T5 and the ice bottom between T10 & T11. Back around June 1st the ice bottom would have been around T24. At 10cm per thermistor, that's *a lot* of melt.

If this is representative of the general floes in the area, then we've seen close to a meter and a half of melt already.  The CAB has truly become a Marginal Ice Zone.




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Re: What the Buoys are telling
« Reply #111 on: July 21, 2013, 05:16:35 AM »
It starts with why there are 500 days in 2012?


This is likely the calendar system they are using for several buoys (day number since Jan 1st 2012). Can't use 'the day of operation of the buoy' since they have to be coordinated. The standard Gregorian Calendar has once again proven difficult to use in scientific context. It's the same with clock, much shorter to say and compute '2 2/3 days since the experiment begin' than say 'two days and 16 hours since the experiment began.' And 'second', the standard unit of time, is just too short or might even be falsely accurate for some measurements, 4.3200000*107 seconds (excactly 500 days (not counting in leap seconds)) is also somewhat cumbersome to write down and comprehend.
« Last Edit: July 21, 2013, 05:57:15 AM by Pmt111500 »
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Re: What the Buoys are telling
« Reply #112 on: July 21, 2013, 05:22:41 AM »
looks like the the right hand side rod at #1 cam is moving now too. how long are these rods? a meter?

on cam #2 the rods have been sinking lower as well as moving. It looks to me (guesstimate) 1 or 2 cm over the past ~2 days for sinking on some of those rods...that would mean possibly 20 cm loss in total thickness of the ice? Or perhaps more likely localized contortion of ice sheet, maybe pushing that section lower....but why would that be with so much open water and so little tension? Maybe the ice sheet is slow motion imploding?...internal tension. Who knows what it looks like underneath there? could be a dentists dream.

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Re: What the Buoys are telling
« Reply #113 on: July 21, 2013, 05:23:41 AM »

ktonine

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Re: What the Buoys are telling
« Reply #114 on: July 21, 2013, 11:48:53 AM »
The ablation stakes are typically 3 meters in length.

I would expect this pond to drain soon - unless it's already at sea level. 

Phil.

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Re: What the Buoys are telling
« Reply #115 on: July 21, 2013, 02:16:47 PM »
The ablation stakes are typically 3 meters in length.

I would expect this pond to drain soon - unless it's already at sea level.

Looking at cam#1 it looks like about a foot of surface melt in July based on the poles, two bars to 5 bars.

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Re: What the Buoys are telling
« Reply #116 on: July 21, 2013, 03:45:57 PM »
The state of IMB 2013B has changed dramatically overnight. According to the bottom sounder the thickness has suddenly increased. The thermistors reveal this:
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Re: What the Buoys are telling
« Reply #117 on: July 21, 2013, 04:28:57 PM »
The thickness has increased, but the thermistors say the temperature has jumped?

I suspect what's happened is the buoy's floated free and risen within the hole.  That will bring the bottom sonar closer to the underside of the ice - in the absence of recalibration that will show up as the ice thickening downwards towards the sensor.   What does the top sonar say?

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Re: What the Buoys are telling
« Reply #118 on: July 21, 2013, 04:41:06 PM »
Looking at the thermistor traces, as of the 15th, the ice ran from position ~9 to ~23 inclusive.  24 and below were at the saline water temperature  of ~-1.8, and the ice was grading from ~-1.8 at the bottom to 0 at the top (saline icemelt temp up to freshwater snowmelt temp)

As of the 21st, the bottom surface looks to be around 25 (everything below this is in the -1.8 water), but everything above this is now at zero or higher.  I wonder if the snowmelt pond is actually draining around the thermistor string and thus pulling it up to zero.

Peter Ellis

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Re: What the Buoys are telling
« Reply #119 on: July 21, 2013, 04:46:46 PM »
In fact, if the snowmelt has drained through around the thermistor string, it will form a less-dense "lens" underneath the floe.  I believe that sometimes you can get a false sonar return from the boundary between the fresh and salt water.  In that case, what the thermistor string is now reading is the drainage hole (all at zero), a further layer of fresh water ~20cm thick underneath the ice bottom, and then the salty water below that. That's consistent with the ~20-30cm thickening shown by the lower sonar sensor.

Jim Hunt

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Re: What the Buoys are telling
« Reply #120 on: July 21, 2013, 04:54:58 PM »
I wonder if the snowmelt pond is actually draining around the thermistor string and thus pulling it up to zero.


That was my best guess also. I'm even wondering if there's been a mix-up over which camera's pointing at which IMB!

What does the top sonar say


The top sonar says nothing's changed. My schematic above showed both sonars on the same pole, but I seem to remember seeing photos suggesting they could be separate in some installations.

[Edit - Here's the picture]



Sometimes you can get a false sonar return


That would explain a lot! 
« Last Edit: July 21, 2013, 07:36:11 PM by Jim Hunt »
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Apocalypse4Real

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Re: What the Buoys are telling
« Reply #121 on: July 21, 2013, 05:21:07 PM »
Temps seem up at NPEO cam 1 - is this rain - or fog condensation?

The last three frames show increasing droplets on the lense.

Bruce Steele

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Re: What the Buoys are telling
« Reply #122 on: July 21, 2013, 05:25:57 PM »
Arcticio, re. Comment # 100. Like Jim said ITP # 62 and #64 are supplying good data. There is recent  surface water warming and freshening at both buoy's. Summer Pacific Warm Water overlays deeper Atlantic halocline and bottom waters.
For a good descriptive
http://psc.apl.washington.edu/HLD/ArcticChange12/2012_Lect7_Woodgate_UpperArcticCirculationHO.pdf

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Re: What the Buoys are telling
« Reply #123 on: July 21, 2013, 05:28:59 PM »
From http://imb.crrel.usace.army.mil/pdfs/SIMB.IGS.Final.pdf
Google cache: http://webcache.googleusercontent.com/search?q=cache:http://imb.crrel.usace.army.mil/pdfs/SIMB.IGS.Final.pdf

"Very interestingly, the profile appears to show rapid bottom ice growth on 4 June. This coincides with a jump in water temperature, which was likely caused by the release of fresh water from the dam in the town of Barrow several kilometers down the coast. We believe that this freshwater caused the development of a false bottom of the ice which did not decay until several days later. Figure 5 shows a series of temperature profiles from the lower ice and ocean every 8 hours from 2 to 5 June which helps support this explanation. The profiles in the ocean are vertically uniform at –1.88C prior to 4 June (yellow) when the profile begins to show warmer likely fresher water intruding under the bottom of the ice. The warmer water layer is about 0.5 m thick and the measured ice bottom is seen to jump downward to where the warmer and colder water interface.After the false bottom decays on 9 June, bottom melt resumes more expected behavior."

Edit:  from the same source

"Just as in the IMBs, the interpretation of thermistor data just below the ice–air interface is difficult after the ice becomes isothermal, due to the potential for preferential melt around the string and potential for ponding. "


I think we have a clear candidate here for preferential melt around the string, followed by drainage through and formation of a false bottom.  As for why we can't see it on the cam, possibly there was quite a thick layer of waterlogged snow rather than an actual pond.  The photos show a dramatic drop of ~10-15cm in the last 24-48 hours, going by the striped poles.

Jim Hunt

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Re: What the Buoys are telling
« Reply #124 on: July 21, 2013, 07:51:38 PM »
Bruce - Thanks for the most interesting link.

Peter - Likewise. Which reminded me that something similar happened under IMB 2012H on June 29th, contemporaneously with one of OBuoy #8's sudden gyrations. A sudden influx of warmer water under the floe (still visible on July 1st) caused one of the bottom sonar readings to be out of whack by 15 cm or so.

Speaking of OBuoy #8, it's now gyrated a bit more, to reveal some sorry looking sensors in the background:

 
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ghoti

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Re: What the Buoys are telling
« Reply #125 on: July 21, 2013, 08:52:37 PM »
Watching the webcam photos and the movies I get a strong impression that the visible melt ponds seem to fill and flow the most under very cloudy conditions. My expectation was we'd see more melt and thus more pond growth and filling under a bright sunny sky.

Am I getting the wrong impression? Is radiative loss to the clear sky balancing the solar gain? This certainly isn't my experience everywhere else when I've actually done those measurements but I've never worked on the ice anywhere and certainly not on the arctic ice.

None of the buoys give us irradiance data to attempt to match the surface melt data. Heat flux measurements are bottom of the ice flux as far as I can tell not top down. I think bottom melt is known to increase under cloud cover especially during low sun angle hours but I really expected surface melt to be largest under full sun conditions. Perhaps under cloud there is more heat from condensation on the surface.

Pmt111500

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Re: What the Buoys are telling
« Reply #126 on: July 21, 2013, 09:02:50 PM »
I really expected surface melt to be largest under full sun conditions.
maybe the sun's heat is so intense the snow evaporates straight to the sky? it's still 24/7 sunlight over large sections of the Arctic. well not anymore since there are so much clouds.
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ktonine

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Re: What the Buoys are telling
« Reply #127 on: July 21, 2013, 09:11:26 PM »
Clouds can be just as deadly to ice as sun.

In clear skies incoming solar radiation that  is reflected off the surface usually goes up and out.  Under cloudy skies that which is reflected often gets bounced back down by the clouds for a second (or third) pass.  So the total energy absorbed can be higher under cloudy skies than under clear skies. This is usually the case when the snow/ice albedo is high. 

Cloud height, cloud temperature, cloud albedo, and surface albedo all play a part in the equation. 

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Re: What the Buoys are telling
« Reply #128 on: July 21, 2013, 09:32:52 PM »
Clouds can be just as deadly to ice as sun.

In clear skies incoming solar radiation that  is reflected off the surface usually goes up and out.  Under cloudy skies that which is reflected often gets bounced back down by the clouds for a second (or third) pass.  So the total energy absorbed can be higher under cloudy skies than under clear skies. This is usually the case when the snow/ice albedo is high. 

Cloud height, cloud temperature, cloud albedo, and surface albedo all play a part in the equation.

It's not just that, direct rays at a low angle are reflected from a wet surface. However cloud scatters incoming rays so more incident light is at steeper angles of incidence. Furthermore clouds back-radiate infra-red and can have a warming effect dependent on ice content and height, i.e. Francis & Hunter 2007, Changes in the fabric of the Arctic’s greenhouse blanket.

Jim Hunt

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Re: What the Buoys are telling
« Reply #129 on: July 22, 2013, 12:39:11 PM »
I note this morning that, as suggested by the image above, the top sounder on 2012H has stopped reporting.

I also note that the bottom sounder on 2013B reports bottom growth of another 22 cm! It's colocated with ITP 61, which is showing recent changes in both temperature and salinity near the surface:

 
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ktonine

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Re: What the Buoys are telling
« Reply #130 on: July 22, 2013, 03:29:43 PM »
Jim - the thermistor string data on 2013B has become impossible to interpret - 200 cm of thermistor data wandering around or just below freezing.   Unless there's a 2 meter deep pool there, it makes no sense whatsoever. And the sounder shows almost one-half meter of thickness increase over the last two days - yeah, right :)  What we might be seeing is an increase in weight that is pushing the bottom lower - has there been enough precipitation there over the last two days to do that?

Right now I wouldn't trust any implied thickness from that buoy.

ktonine

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Re: What the Buoys are telling
« Reply #131 on: July 22, 2013, 03:40:16 PM »
Jim - a better explanation is that it takes sound waves longer to travel through water than through ice.  So an increase in the water depth would appear as an increase in return time and an implied thickness change. The sounder is probably hitting higher water content as opposed to pure ice.

ghoti

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Re: What the Buoys are telling
« Reply #132 on: July 22, 2013, 04:51:31 PM »
One of the images from the 2012H webcam showed what looked like a top sounder knocked over lying on the surface next to one of those large yellow cylinders healed over at an extreme angle. The monitoring equipment is getting rearranged severely by all the movement of the broken ice. We may soon have only the webcam as a source of info from that site.

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Re: What the Buoys are telling
« Reply #133 on: July 22, 2013, 05:00:55 PM »



Judging from the foreground melt gauge, The ice has been top melting at a rate of about 0.5cm/hr.

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Re: What the Buoys are telling
« Reply #134 on: July 22, 2013, 05:15:42 PM »
Judging from the foreground melt gauge, The ice has been top melting at a rate of about 0.5cm/hr.

Whereas judging from the top sounder reports there's still 3 or 4 cm of snow left to melt!
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ghoti

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Re: What the Buoys are telling
« Reply #135 on: July 22, 2013, 07:18:50 PM »
Here's the view of the fallen instruments around Obuoy 8.


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Re: What the Buoys are telling
« Reply #136 on: July 22, 2013, 11:23:26 PM »
I do not think this is melt ponding.  I think this is cracked ice with open ocean showing.
  http://obuoy.datatransport.org/monitor#buoy8/camera

I have trouble getting a picture to show btw and I have looked at the tips.
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Re: What the Buoys are telling
« Reply #137 on: July 22, 2013, 11:28:46 PM »
Yes I think you right Helorine.
Have a ice day!

ktonine

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Re: What the Buoys are telling
« Reply #138 on: July 22, 2013, 11:53:31 PM »
If it is cracked ice - then the ice isn't very thick.

This would match what I calculate from buoy 2012J - less than 1/2 meter thick.

There's still 6 to 7 weeks of the melt season left and we're looking at ice floes at 85, 86, 87 degrees north possibly melting out in situ?

Unbelievable. Really.

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Re: What the Buoys are telling
« Reply #139 on: July 22, 2013, 11:55:33 PM »
I have trouble getting a picture to show btw and I have looked at the tips.

I just right click on the image, save the .JPG to my own computer then attach it to the post. That's worked fine for me for months now.

This is more like 75 degrees north:

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Espen

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Re: What the Buoys are telling
« Reply #140 on: July 23, 2013, 12:02:04 AM »
If it is cracked ice - then the ice isn't very thick.

This would match what I calculate from buoy 2012J - less than 1/2 meter thick.

There's still 6 to 7 weeks of the melt season left and we're looking at ice floes at 85, 86, 87 degrees north possibly melting out in situ?

Unbelievable. Really.

To me that ice looks thicker than ½ meter more like  1 meter to 1 ½ meter.
Have a ice day!

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Re: What the Buoys are telling
« Reply #141 on: July 23, 2013, 12:54:55 AM »
Why are we trying to guess the thickness?  We have data from the colocated buoy, and it's 192cm thick [1].  All the guesses are useful for is proving just how bad the unaided human eye is at guesstimating distances from a small picture with no reference markers in frame.
http://imb.crrel.usace.army.mil/2012H.htm

[1] OK, the buoy appears to show the surface melt has stopped, and is thus possibly reading from the top of a melt pond.  Going by the melt rate before it hit the plateau (22 cm in about 10 days), we could maybe knock off another 20cm or so, for an actual thickness of ~170cm.

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Re: What the Buoys are telling
« Reply #142 on: July 23, 2013, 01:13:31 AM »
does anyone know how a single "thickness" is calculated when you have 75% 1.7m, 25% 0m? and a moderately large proportion of the ice somewhere in between?

Peter Ellis

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Re: What the Buoys are telling
« Reply #143 on: July 23, 2013, 01:36:07 AM »
The short answer is - however you like.  The long answer is that it will vary between sensors, and different ones will be better for different purposes.  Cryosat, for example, will be working from the radar return from its field of view, which has a resolution of about 250m.  I suspect the return profile will indicate the modal thickness per pixel (when everyone in a crowd shouts, democracy rules) rather than the max, min or median.  However, when they average together multiple readings, I would expect them to take the median of those readings, or some other form of average which is stable to individual outliers.

For the sonar buoy, it's effectively a point measurement so the question's void - the fact remains that it's the best estimate we have for the thickness at the webcam, given that it'll be measuring the thickness of a point at most a few feet away from the camera.

jdallen

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Re: What the Buoys are telling
« Reply #144 on: July 23, 2013, 02:01:29 AM »


A skua?


My word!  Certainly something of that general ilk, and in an extraordinarily unusual place.  It absolutely implies extensive open water reasonably close at hand. 
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Re: What the Buoys are telling
« Reply #145 on: July 23, 2013, 02:35:25 AM »
Watching the webcam photos and the movies I get a strong impression that the visible melt ponds seem to fill and flow the most under very cloudy conditions. My expectation was we'd see more melt and thus more pond growth and filling under a bright sunny sky.

Am I getting the wrong impression? Is radiative loss to the clear sky balancing the solar gain? This certainly isn't my experience everywhere else when I've actually done those measurements but I've never worked on the ice anywhere and certainly not on the arctic ice.

None of the buoys give us irradiance data to attempt to match the surface melt data. Heat flux measurements are bottom of the ice flux as far as I can tell not top down. I think bottom melt is known to increase under cloud cover especially during low sun angle hours but I really expected surface melt to be largest under full sun conditions. Perhaps under cloud there is more heat from condensation on the surface.

Yes, sun melts quite a lot, but so does a high dew point.

Even under clouds you have diffuse solar radiation, which is significant since Arctic clouds are usually fairly thin...

Nightvid Cole

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Re: What the Buoys are telling
« Reply #146 on: July 23, 2013, 02:36:42 AM »
Why are we trying to guess the thickness?  We have data from the colocated buoy, and it's 192cm thick [1].  All the guesses are useful for is proving just how bad the unaided human eye is at guesstimating distances from a small picture with no reference markers in frame.
http://imb.crrel.usace.army.mil/2012H.htm

[1] OK, the buoy appears to show the surface melt has stopped, and is thus possibly reading from the top of a melt pond.  Going by the melt rate before it hit the plateau (22 cm in about 10 days), we could maybe knock off another 20cm or so, for an actual thickness of ~170cm.


The discussion was about 2012J, not 2012H.

ktonine

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Re: What the Buoys are telling
« Reply #147 on: July 23, 2013, 04:30:26 AM »
We were talking initially about 2012J, but I think 2012H also shows that the ice is not as thick as it might seem.  The ice thickness profilers can give very deceptive readings at times. When there are thermistor strings co-located you can either verify the thickness sounder or spot problems.

2012H is a good example: if we look back to  late March we can see from the thermal gradient where the ice/snow interface is and the ice/ocean interface.  I chose to look at the 3/25/2013  12:00:00 AM data as it was close to the ice's coldest temperature and should allow us to see the largest gradients. (Line 4683 in the CSV datafile download)

Ice thickness is listed as 2.16 meters. 

The thermistor string data shows the snow/ice interface is between T6 & T7.
The thermistor string data shows the ice/ocean interface is between T24 & T27
Without doing any intensive math we can say the ice is approximately 1.9 meters thick based on the thermistor data (+/- 20cm).  This is in reasonable agreement with the sounder.

When we look at recent data, 7/22/2013  8:00:00 AM shows an Ice Thickness of 1.93 meters.

Looking at the thermistor temperatures, the ice top surface is at T12. The ice bottom appears to be between T21 & T26 with T23 the most likely value.  This is only 1.1 meters of thickness.  This is quite different than 1.93 meters.

Looking at the the March data we see that the bottom surface has only changed by 20 to 30 cm.  25 cm of bottom melt isn't unreasonable.   The thermistors show the top surface of the ice moving by 50 to 60 cm.  This is what accounts for the disagreement between the sounder and the thermistors.  Yet we know ice isn't likely to exist at temperatures above 0C - so unless the thermistors are all wrong we're not looking at ice until we get to T12.

The only reasonable explanation I can see is that the sounder is seeing 50 to 60 cm of water at the surface and counting it as part of the reported ice thickness.  The thermistor data gives hints that this might be the case when you look at the STDEV of individual thermistor readings over the past  few days.  There's a noticeable different between T1 -T4 and T5-T10 and another noticeable shift at T11-T13.  This could be explained by T1-T4 being in air, T5-T10 in water, and the ice surface moving towards T12 - T13.


The sounder then is telling us there is a potential for 1.9 meters of ice - if everything froze in place today.  The thermistors are telling us that there is only 1.1 meters of ice sitting under 60 cm of melt pond water.  Two compatible, but different stories.



MOwens

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Re: What the Buoys are telling
« Reply #148 on: July 23, 2013, 05:49:55 AM »
...the underside of the ice is not flat...perhaps that explains differences from estimated thickness averages and single location measurements.



MOwens

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Re: What the Buoys are telling
« Reply #149 on: July 23, 2013, 05:54:41 AM »
...also, on web cam#2, I noticed a few cases where something was rocking, as if under the influence of sea waves. It could have either been the camera rocking with the wind, or the ice itself moving with ocean swells. Now that #2 is underwater, I see that it's the ice because the water level acts as a measuring level...one side of rods comes up out of the water, and the other side sinks....