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Saturday, September 21, 2013

NSIDC, PIOMASS and Cryosat

The NSIDC (The National Snow and Ice Data Centre) measures the extent (area) of the ice floating on the Arctic Ocean. With it's satellite, any area with more than 15% ice cover shows up as covered with ice so this will always be an over-estimate. 

PIOMASS (Pan-Arctic Ice Ocean Modeling and Assimilation System) models the volume of ice floating on the arctic ocean.  It is a model with limited observational inputs from ground truthing.

Cryosat is a European Space Agency satellite that measures the the freeboard of the floating ice and hence, combining this with the extent of the ice can calculate the volume of the ice floating on the Arctic ocean. As of 2013, they have been operating for three years and their satellite is estimated to be capable of a few decades more of observations.  Try as I may, I haven't yet being able to find a site which shows the results they have measured over the past three years.  There is a nice qualitative animation but I can't find any quantitative results.

Before we have a look at the results from these three sources, we need a pinch of physics and a modicum of meteorology. 

1) If you haven't caught up with how Coriolis works, click here. What is important to this discussion is that in the Northern Hemisphere, any horizontally moving object veers to the right and the effect is greater, the closer you are to the North pole.

2) Most* of the radiation from the sun passes through clear air without being absorbed.  When it hits the ground, it is absorbed or reflected, depending on the nature of the surface.  This heating from below is what powers the earth's weather.

* Some of the ultra-violet wave lengths are absorbed high in the atmosphere.

3) When the Arctic Ocean is covered in ice and snow, most of the incident energy from the sun is reflected back into space.  The air above the ice is therefore  not warmed.  The air itself above the Arctic, radiates heat as does any object with a temperature above zero degrees K (minus 273 degrees Centigrade) and cools.

4) As the air above the Arctic cools it contracts,  it's density increases and it descends.  When it reaches the ground it spreads out southward in all directions.  You can see this effect when you open your fridge on a humid day.  The humidity condenses into a little cloud which makes the flow of air visible.  You will notice that the air spreads out across the floor.  If you have bare feet you can feel the effect.

5) Since the air is moving horizontally southward across the land, coriolis veers it to the right.  Instead of a North wind (moving toward the South) you have a North East wind (moving toward the South West).  This is a typical high pressure clockwise weather pattern with generally clear skies. The skies are clear since as air descends it is compressed which warms it and it can hold more water vapour.  ie.  If there were water droplets in the air, they would evaporate as the air descends.

6)  This clockwise wind pushes on the ice and water of the Arctic inducing clockwise water flow.  The Beaufort gyre north of Alaska is  a good example of such a current.  Note what happens to the ice (and the layer of fresher water which floats on the surface of the Arctic Ocean).  Here it gets interesting.  If something is spinning, common experience tells you that it gets flung outwards.  However here Coriolis comes into play.  If something is moving clockwise, 'to-the-right' is into the centre.  Clockwise gyres tend to move surface objects and surface water into the centre of the gyre. You see the same effects in the garbage gyres of the world oceans.

7) To the contrary, counter clockwise currents tend to fling floating objects and floating water outwards.  Counter clockwise air currents are caused when air warms  at the surface, rises and pulls air inwards.  The air flowing in toward the rising air is veered to the right resulting in a counter clockwise flow.

Pulling all this together, we are still somewhat blindfolded in our observations of ice floating on the Arctic Ocean.  The NSIDC results are a model somewhat constrained by observation and the observations read any area of water with more than 15% ice as complete ice cover.  If the prevailing weather patterns are clockwise and the ice is pushed together, their result should be pretty accurate.  In addition, if they compare clockwise years with clockwise years, the trend they show will be a good indication of which way ice cover is evolving from year to year.

However, when prevailing conditions are counter clockwise, ice will tend to be scattered and show up as a greater area of ice.  They may apply a correction factor for this effect based on ground-truthing but it will be an estimate. 

What we are missing here is the three years of the ESA results from Cryosat and as far as I can find they haven't been published.  The Cryosat web site talks about how they measure freeboard but not how they measure ice extent.  It would seem to me that by comparing the strength of the ice signal from water and from ice for any patch of water, they should be able to get a much better measure of ice extent than the NSIDC satellite gets.  There would probably have to be a correction factor applied for the relative reflectivity of ice and water to the wave length they are using but that would be pretty simple to do.

Who knows.  NSIDC reports that this year is the sixth lowest since measurements have been made.  Cryosat was operating over 2012 and 2013 and so should be able to shed some more light on the relative amount of ice on Sept 15 for these two years.  Perhaps ice volume this year was not as high reletive to 2012 as reported.  Where is the data.

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