Satellite Temperatures

Seasonably cold weather has returned at last over interior Alaska, with Fairbanks seeing generally sub-zero temperatures in the past week and a chilly high temperature so far today of only -19°F.  The cooperative observer at Chicken reported temperatures below -40° the past three nights.

The map below shows a satellite-derived estimate of temperatures at 4am this morning over the southeastern interior, extending from Fairbanks-land (near the top left) to the Canadian border and western Yukon Territory on the right.  The cold conditions of the upper Tanana River valley and the steep valleys of the Fortymile Country and western Yukon are clearly evident, with estimated temperatures around -35°F to -45°F.  Ground truth observations provide confirmation: -38°F in Northway this morning, and below -40° in Dawson, but barely below 0°F in Eagle with a breeze blowing.

In recent months I've been working on a project to decode all of the wintertime historical land surface temperature data for Alaska from the MODIS instruments on the Aqua and Terra polar-orbiting satellites.  The map above is derived from the realtime MODIS feed, but the entire archive is available from NASA (back to 2000 for Terra and 2002 for Aqua).  The full archive is of course a lot of data and takes a long time for my little computer to decode, so I'm only about half way through; but the preliminary results are already quite interesting.

As an example of the kind of analysis that is possible with the data archive, the maps below show the number of days with estimated surface temperatures of -50°F or lower for several winters (November through March).  The numbers are conservative estimates, because I've used only "high quality" pixels and there may well have been times when ice fog, cloud cover, or other problems prevented good estimates; but the temperatures are unlikely to be too low and so the counts of days are probably not too high.

Click the images to enlarge.  The Old Crow Flats of northern Yukon stand out as remarkably and consistently cold.  The results also support Rick's opinion that there are places upriver from Umiat that are quite a bit colder than that notoriously cold place.

Nov 2002 - March 2003:

Nov 2003 - March 2004:

Nov 2004 - March 2005:

Nov 2005 - March 2006:

Nov 2006 - March 2007:

Satellite Temperature Estimates

Back in December I mentioned that I had found a way to begin working with land surface temperature estimates from the Suomi NPP satellite (launched in 2011); the modern radiometer on the satellite measures infrared radiation at a spatial resolution of 750m, which allows for nice high-resolution estimates of surface temperatures when clouds are absent.  I've recently also decoded similar data from the MODIS instrument on the TERRA and AQUA satellites, launched in 1999 and 2002 respectively.  The MODIS resolution is about 1km.

Here are a couple of images showing estimates of the minimum surface temperature observed by these satellites on January 18 over the Fairbanks area and points eastward into the hills; this is the day that temperatures bottomed out during the cold snap.  I've interpolated the data to a 750m grid in both cases for consistency (click to enlarge the images).  The similarities are striking, but greater spatial detail is evident in the S-NPP (lower) image, as expected.

It's gratifying to see that both maps show the cold conditions on the Salcha River in the bottom right; the Salcha River RAWS (SLRA2) reported -59°F, and the satellite data support this very cold reading.  On the other hand, the -65°F reported by the Upper Chena River HADS site (UCHA2) is not supported and was almost certainly in error.

Below is a comparison of the minimum temperatures (in °F) reported at each of the sites marked on the maps.  The agreement is, in my view, quite impressive, with only the Stuart Creek RAWS seeing a substantial difference if we exclude the Upper Chena HADS site.  At both Stuart Creek and at the Salcha RAWS, the estimates are better for the S-NPP, which probably reflects the superior resolution of the S-NPP instrument.

Station	Reported	S-NPP	MODIS
PAFA	-51	-48	-49
PAFB	-52	-51	-47
CHLA2	-52	-53	-53
PAEI	-53	-49	-48
TRSA2	-42	-46	-47
STUA2	-30	-42	-47
UCHA2	-65	-51	-48
AGLA2	-48	-45	-45
SLRA2	-59	-59	-55

Aurora

The SNPP day night imagery (VIIRS sensor) caught a nice image of the aurora yesterday evening traversing the region between Eagle, Fairbanks, and Point Hope. The city lights for nearly all population centers are easily identifiable.