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Author Topic: Air temperature at different heights above the sea-ice - what impact?  (Read 4593 times)

anthropocene

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Hi All,

        This discussion started on the 2015 Melting thread. It's not directly related to this melt season so I've split it out into a different thread. Neven, feel free to put this elsewhere if you see fit (e.g. science) but I think it has most relevance to the arctic sea ice.
   If I may be so bold to summarise the points made previously:
      1) During the melting season most of the posts forecasting warm weather in the arctic include maps showing heat well above ground levele.g. 800hPa or 850hPa.
      2) The majority of the time the air temp in the few metres above the ice will be at or below freezing (I think there are some differences as to why this happens but it doesn't change the temperature).
      3) Sometimes the ground-level temperature could be significantly above 0degC. This situation is most likely to be caused by strong warm winds blowing across the ice
 
Getting back to one of my original points: It is suggested that the temperature at 800hPa is used because that gives an indication of the amount of energy being provided to melt the ice. My question to that is, what mechanisms are there, for the heat at that height to get to ground level where it can melt the sea-ice? 

seaicesailor

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Re: Air temperature at different heights above the sea-ice - what impact?
« Reply #1 on: April 03, 2015, 01:56:48 AM »
Turbulent mixing within the planetary boundary layer that enhances heat transfer and disrupts thermal inversion

anthropocene

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Re: Air temperature at different heights above the sea-ice - what impact?
« Reply #2 on: April 03, 2015, 10:12:04 AM »
Turbulent mixing within the planetary boundary layer that enhances heat transfer and disrupts thermal inversion

So given my (most probably over simplistic) view of the situation: A huge dense cold mass of air sitting on the ice for up to thousands of square kms and for arguments sake 200m thick is hit with warm southerly winds at say 10mph. What is the approximate rate of disruption of the temperature inversion over the ice? Is it hundreds of kms a day or just several? I can appreciate that temperature inversions on land are most stable in valleys where the cold air is protected from disruption from the side by the sides of the valley.  The temp. inversion on the sea-ice does not have this protection. However, the size of the block of cold air is significantly larger than usually seen on land and it is sitting on a  layer of ice which provides uniform conditions which tend to keep the temperature inversion in place.  Over the major ice fields the disruption can occur because the cold air tends to flow downhill from the high central parts. Again this is not the case over the arctic sea ice. I'm genuinely interested in how this situation plays out in the arctic. 

Neven

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Re: Air temperature at different heights above the sea-ice - what impact?
« Reply #3 on: April 03, 2015, 11:31:46 AM »
Quote
Neven, feel free to put this elsewhere if you see fit (e.g. science) but I think it has most relevance to the arctic sea ice.

Perfectly placed, anthropocene. Thanks for opening a separate topic.
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Nightvid Cole

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Re: Air temperature at different heights above the sea-ice - what impact?
« Reply #4 on: April 03, 2015, 06:43:58 PM »
It depends on wind shear and the dew point, among other things. A big dome of high pressure like we saw in the first half of July 2011 will be associated with descending air. If this air has a dew point significantly above freezing, it can deliver a non-negligible amount of heat to the ice.

A low pressure system can cause shear and turbulence, but air temperatures tend to be lower.

seaicesailor

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Re: Air temperature at different heights above the sea-ice - what impact?
« Reply #5 on: April 03, 2015, 07:42:30 PM »
It depends on wind shear and the dew point, among other things. A big dome of high pressure like we saw in the first half of July 2011 will be associated with descending air. If this air has a dew point significantly above freezing, it can deliver a non-negligible amount of heat to the ice.

A low pressure system can cause shear and turbulence, but air temperatures tend to be lower.

Right. I was thinking on the wind shear case only. Low pressure systems sometimes pull mass of warm air  from south at very high speeds, then in the 850 mb map one can see these tongues of warm air surviving many miles into the Arctic. I have always assumed that these flows are turbulent and mix and transfer heat very well down to the surface.

seaicesailor

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Re: Air temperature at different heights above the sea-ice - what impact?
« Reply #6 on: April 03, 2015, 07:50:10 PM »
Turbulent mixing within the planetary boundary layer that enhances heat transfer and disrupts thermal inversion

So given my (most probably over simplistic) view of the situation: A huge dense cold mass of air sitting on the ice for up to thousands of square kms and for arguments sake 200m thick is hit with warm southerly winds at say 10mph. What is the approximate rate of disruption of the temperature inversion over the ice? Is it hundreds of kms a day or just several? (...).

I guess in the scenario I just was thinking, one can see these pulls of warm air penetrating, say, a few hundreds miles in a matter of two days or so ...

anthropocene

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Re: Air temperature at different heights above the sea-ice - what impact?
« Reply #7 on: April 04, 2015, 09:59:52 AM »
SIS and NVC,   Thanks for the responses. That amount of turbulent mixing is more than I was  intuitively thinking would happen. Sorry, some more questions  ;)  Again intuitively, the turbulent mixing is greater the less the density difference between the warm intruding air and the cold air in the temp. inversion block? So, one way to quickly disrupt the temp. inversion would be a sequence of intrusions, starting with air just a bit warmer than the temp. inversion (to maximise mixing) and then subsequent warm air intrusions gradually increasing in temperature? 
  Carrying this on in a thought experiment, there is a density difference at which any intruding air wouldn't have any impact on the temp. inversion block? That is, there is a temperature difference at which to all intents and purposes the intruding warm air would just rise up and over the cold air mass and not move, mix with or impact the temp. inversion? Any ideas what this temp. difference would be?

seaicesailor

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Re: Air temperature at different heights above the sea-ice - what impact?
« Reply #8 on: April 04, 2015, 12:57:25 PM »
Anthropocene,

There is this highly cited paper that reviews turbulent mixing in stratified flows:

http://goo.gl/l06wVF

In page 2 it shows a schematic of almost the same experiment you describe, and chapter 2 deals with the analysis of the problem. It would take us (at least me) some days to go thru it and understand some answers: what is the upper flow speed and the intensity of turbulent shear flow needed to disrupt a stably stratified boundary with given density difference ( in the case of air, caused by diiferent temperatures). I will try find time for it.
« Last Edit: April 04, 2015, 01:02:52 PM by seaicesailor »

seaicesailor

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Re: Air temperature at different heights above the sea-ice - what impact?
« Reply #9 on: April 04, 2015, 01:11:08 PM »
Actually it is very specialized article of, so it might take months  :-\

Nightvid Cole

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Re: Air temperature at different heights above the sea-ice - what impact?
« Reply #10 on: April 04, 2015, 09:25:28 PM »
SIS and NVC,   Thanks for the responses. That amount of turbulent mixing is more than I was  intuitively thinking would happen. Sorry, some more questions  ;)  Again intuitively, the turbulent mixing is greater the less the density difference between the warm intruding air and the cold air in the temp. inversion block? So, one way to quickly disrupt the temp. inversion would be a sequence of intrusions, starting with air just a bit warmer than the temp. inversion (to maximise mixing) and then subsequent warm air intrusions gradually increasing in temperature? 
  Carrying this on in a thought experiment, there is a density difference at which any intruding air wouldn't have any impact on the temp. inversion block? That is, there is a temperature difference at which to all intents and purposes the intruding warm air would just rise up and over the cold air mass and not move, mix with or impact the temp. inversion? Any ideas what this temp. difference would be?

This could only work until the surface air approaches the freezing point. Until the ice is gone the air immediately touching the surface cannot exceed the freezing point. It's a matter of how thick, not if, the inversion layer is there.