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.  

Deceptive Alaskan Weather

Since I lived in Anchorage for a summer two years ago while working for the weather service there, I also have a healthy interest in Alaskan weather, something that is not often discussed down here (except in the context of climate change in the arctic). So for today, we'll take a brief look at what's going on in Alaska.  Here is an infrared satellite image from earlier this morning:

Fig 1--GOES-W IR satellite image from 1030Z Nov 4th, 2010

This looks like a powerful extratropical cyclone, with its center of circulation just about centered on Anchorage.  And look at the size of this cyclone!  The colder IR temperatures, indicated by the yellows and the reds in the image and almost always associated with IR emissions from clouds, almost fill the entire state of Alaska and the Yukon.

This looks like a powerful storm, but just how severe is it at the surface?  It's difficult to get good obsevations in Alaska, mostly because there's a lot of open water and even on land the populated areas are very sparse.  One way we can try to get a more cohesive set of measurements is to use satellite-derived winds.  One simple way to do this is to identify certain cloud features on a satellite image and then see how far they move over a given time.  By estimating the elevation off the ground (or ocean) of these cloud features, we can get an estimate of the winds at that level.  However, since clouds have depth, they tend to cover multiple levels of height.  Therefore, the best we can do is guess at the wind speeds within a layer.  An example of one of these products can be seen here:

Fig 2--GOES W Pacific region IR-derived winds, 1445Z, Nov 4th, 2010.  Yellow indicates 700-1000mb, cyan indicates 400-699mb, and red indicates 100-399mb.

We can see that we don't have data everywhere.  This is because sometimes either the cloud structure is too dense to identify any clear cloud features or there just are no clouds in that region.  Another reason this happens, particularly over Alaska, is that Alaska is just too far north for a geostationary satellite like GOES to accurately calculate winds.  If you think about it, the GOES satellites are parked over the equator and you can see that the view of Alaska is rather squashed and distorted.  Trying to calculate any sort of wind vectors accurately over that distorted of an image can be problematic.  We can try to use polar-orbiting satellites to get better pictures, but their orbital passes are not as frequent as image updates from GOES.  I also don't know of a good place to get POES (the polar-orbiting satellite) data, though I'm continuing to look...

Anyhow, we can see that in the Gulf of Alaska that there are several low-level (yellow) wind barbs in the 30-35 knot range, which isn't too extreme but still could pack quite a punch.  It's clear that there's probably a lot of cloud cover and maybe precipitation associated with this cyclone as per the IR image (fig 1).  So how is Alaska reacting to this storm?

Fig 3--Warnings and advisories from the National Weather Service for the Alaska region from 8:45 AKDT on Nov 4th, 2010.  Pale purple indicates small craft advisory, medium purple indicates gale warning, dark purple indicates storm warning and green indicates a winter weather advisory.

Those aren't as intense of advisories as you might expect with a swirling cyclone of doom that this particular storm happens to look like.  The worst is a storm warning in the marine zone between Yakutat and Prince William Sound, where they're expecting winds to be decreasing throughout the day from 50 kts to 30 kts.  Otherwise, based on those satellite-derived winds we saw, the gale warnings throughout the Gulf of Alaska make sense as a gale warning covers winds of 35-40 kts.  Makes for a pretty straightforward forecast, at least out at sea.  But what about precipitation or areas inland?  Why no warnings there?

Here's the current radar (1742Z) from Anchorage (PAHG):

Fig 4--PAHG BREF1 Radar image from 1742Z, Nov 4th, 2010

And here's the NWS Anchorage WFO's forecast for Anchorage today:

.TODAY...RAIN AND SNOW SHOWERS CHANGING TO ALL SNOW. SNOW
ACCUMULATION 2 TO 4 INCHES. HIGHS IN THE LOWER TO MID 30S.
SOUTH WIND TO 15 MPH WITH LOCAL GUSTS TO 25 MPH.

So we can see that they are indeed calling for rain and snow and some wind.  Why isn't this such a big deal?  Two big reasons--

1) Most of Alaska's major populated areas are not located in areas immediately facing the open Pacific Ocean.  In fact, The coastal mountain ranges including the Aleutians, the Kenai, the Chugach and the Wrangell-St. Elias Mountains all provide some measures of shelter to even "coastal" cities and towns.  Most are on some sort of gulf or inlet that actually puts the city pretty far inland from the open water.  Such mountain ranges can also force a lot of lift and consequently lots of precipitation on the windward, ocean-ward side and leave much drier conditions on the leeward side.

2) It's Alaska.  2-4 inches of snow is normal.  Nothing to be alarmed about...

So, there you have it.  It should be clear that just because a storm looks impressive on satellite (or radar..) doesn't meant that the storm necessarily must be extraordinarily crazy or record breaking.  Alaska has its share of impressive-looking storms and some do arrive with more impressive precipitation and winds than most.  Therefore forecasters up there must be very vigilant and proactive to get warnings out there.  However, like we saw with the satellite-derived winds and the squashed images, getting good weather data in Alaska is rather difficult.  It just makes forecasting that much harder, even if the weather can get somewhat beautiful (fig 1).