Wednesday, December 22, 2010

Cold air on IR in Canada

As of yesterday it's now officially the season of winter.  Thanks to the delayed response in our climate system, the coldest time of the year comes after the day with the least solar radiation (the winter solstice). So it's on with the cold weather.

Of course, when associating "winter" and "Canada" we naturally assume things are bitterly cold.  How cold is it?  Let's look at this infrared satellite image from this morning:
Fig 1 -- GOES-E infrared satellite image from 1745Z, Dec. 22, 2010.  From the HOOT website.
Normally we see cloud tops in the green-yellow-red range of colors on these particular infrared images.  According to the color bar, this corresponds to temperatures in the -20 to -70 degree Celsius range.  Infrared satellite works by measuring the longwave radiation emitted from the Earth's atmosphere.  It turns out that objects radiate away energy in proportion to how hot or cold they are.  The warmer the object, the more intense the radiation it emits.  By measuring how strong the longwave emissions area, we can infer the temperature at which they were emitted.  Since clouds are higher than the ground and temperature (usually!) decreases with height, they tend to be colder than the ground and emit less-energetic radiation than the ground itself.  Thus we can separate "cold" returns from clouds and "warm" returns from the ground. (It also helps that clouds are moving...)

But note in the above image how most of Canada is also in the green and yellow color range.  In looking at the loop of satellite images, we can see that these areas aren't really moving.  Is all of Canada stuck under a very cold stratus deck?  Not at all!  That's actually the radiation coming from the surface.  The surface is so cold that it's as cold as a lot of the clouds we're seeing in association with storms around the edges of the continent.  Looking at the color bar again, we see that those colors should correspond to temperatures in the -20 to -30 degrees Celsius range at the surface.  Is this accurate?
Fig 2 -- "Current Conditions" around Canada at around 18Z, Dec. 22 2010.  From the Environment Canada website.
Environment Canada's "current conditions" doesn't give a specific time, but I pulled this image around 18Z today so I assume if's from sometime around then.  Note the surface observations throughout western Canada are in the -10 to -20 degree Celsius range (the fact that Canada uses metric temperature units makes these comparisons so much easier...).  So the IR satellite image is close, though it does seem to be a few degrees too cold.  Regardless, that's pretty cold air.

Interestingly enough, though, this cold air at the surface does not translate to "cold" air aloft.  Here's the latest hemispheric analysis:
Fig 3 -- Hemispheric analysis of MSLP (contoured) and 1000-500mb thickness (shaded) at 12Z, Dec. 22, 2010.  From the HOOT website.
There's a tongue of relatively large thickness values stretching all the way from the central plains up to northern Greenland.  Now, thickness is directly proportional to the mean temperature in the layer (as opposed to geopotential heights which are more loosely connected to temperature) so we can infer that there are relatively warmer temperatures aloft in that region.  At least, warmer than in the two large troughs sitting off of the northwest and northeast coasts.

But this isn't totally unexpected.  We are under a very broad ridge in the central part of the country and you can see that translating to a very sprawling surface high pressure center across much of the plains on the image above.  Ridging aloft with high pressure below indicates large-scale subsidence. However, initially to have a ridge there had to be higher heights which (roughly, as stated above) correspond to "warmer" air since warmer air tends to occupy a greater volume than colder air, vertically expanding the troposphere and lifting heights.  Furthermore, the large scale subsidence inhibits cloud formation, and with snow-covered ground free to radiate to the open sky (particularly at night), the ground will cool off rapidly.  This already stable situation then becomes even more stable as we see colder air near the surface and "warmer" air aloft--a stable temperature profile.  All this makes for some pretty quiet conditions (and cold temperatures at the surface!).

There's a lot of hand waving in that argument above.  The details regarding the warm temperatures aloft changing heights could be debated in terms of quasi-geostrophic theory, but I'm not going to go into that now.  I just wanted to point out the cold temperatures visible in the IR image and how it translated to clear skies and cold temperatures for much of the middle of the continent.  

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