CIMSS Satellite Blog

NOAA-18 AVHRR 10.8 µm IR image

October 2008 turned out to be the 4th coldest October on record at Fairbanks, Alaska. About 150 miles (240 km) to the north of Fairbanks, a minimum surface temperature of -40º F (-40º C)  was reported at Chandalar Lake, Alaska on 25 October, followed by a low of -42º F (-41º C) on 26 October 2008:

PUBLIC INFORMATION STATEMENT
NATIONAL WEATHER SERVICE FAIRBANKS AK
1124 AM AKDT SUN OCT 26 2008

…VERY COLD TEMPERATURES OVERNIGHT IN THE NORTH EASTERN INTERIOR…

FOR THE SECOND NIGHT IN A ROW CHANDALAR LAKES OVERNIGHT TEMPERATURE WAS BELOW 40 DEGREES BELOW ZERO.

SATELLITE IMAGERY THIS MORNING INDICATED THAT SOME VALLEYS IN THE NORTH EASTERN BROOKS RANGE WERE APPROACHING 50 BELOW ZERO OVERNIGHT…BUT WITH NO OBSERVERS IN THE AREA WE HAVE NO OFFICIAL TEMPERATURE REPORTS FROM THERE.

THE PLACES THAT WERE 20 BELOW ZERO OR COLDER:

CHANDALAR LAKE…………………….-42
NORUTAK LAKE………………………..-30
BEAVER…………………………………..-29
COLDFOOT……………………………..-26
CHALKYITSIK……………………………-25
BETTLES AIRPORT……………………..-20

NOAA-18 AVHRR 10.8 µm IR imagery (above) showed numerous narrow fingers of very cold air draining into the valleys of the Brooks Range in northern and northeastern Alaska — many valleys exhibited IR brightness temperatures as cold as -40º C (darker blue color enhancement).

A magnified version of that same NOAA-18 AVHRR 10.8 µm IR image (below) revealed that some of the valleys located to the northeast of Chandalar Lake (station identifier PALR) were even colder, with IR brightness temperatures as low as -45º C or -49º F (violet color enhancement) — however, Eric Stevens (Science and Operations Officer at the Fairbanks AK NWS forecast office) informed us that the ASOS instruments at Arctic Village (station identifier PARC) were out of service at that time, so no surface temperature data was available to verify the cold values seen on satellite IR data. The coldest IR brightness temperature in the immediate PALR region was -44º C  (-47º F) at 12:38 UTC — a surface air temperature of of -39º C (-38º F) was reported at PALR about 2 hours after the time of the NOAA-18 IR image.

NOAA-18 AVHRR 10.8 µm IR image

While the corresponding GOES-11 10.7 µm IR images (below) did show a similar area of cold IR brightness temperatures near -40º C (darker blue color enhancement) from Chandalar Lake (PALR) to Arctic Center (PARC), the fine detail of the cold air drainage into the valleys was lost (the effective resolution of the “4 km” GOES IR pixels increase in size to about about 20 km over northern Alaska, due to the large satellite viewing angle). The coldest IR brightness temperatures seen on GOES data in that region at 12:30 UTC was -39º C (-38º F), compared to -45º C (-49º F) indicated by the AVHRR data at 12:38 UTC.

GOES-11 10.7 µm IR images

According to the USA Today tabulation of daily national temperature extremes, this is the earliest -40º F temperature reported in the US during the 1995-2008 period (the earliest -40º date was 05 November in 1999, with the latest -40º date being 31 December in 2002). While this was also the earliest recorded -40º temperature for Chandalar Lake, it did not threaten the monthly October record low temperature for the state of Alaska (which was -48º F or -44º C at Clear Water, set in 1975).

MODIS Visible, Snow/ice, Land Surface Temperature, and NDVI

AWIPS images of the MODIS visible channel, 2.1 µm near-IR “snow/ice channel”, Land Surface Temperature (LST) product, and Normalized Difference Vegetation Index (NDVI) product (above) showed an area of snow cover over parts of upstate New York, Vermont, New Hampshire, and Maine in the northeastern US, as well as Quebec and New Brunswick in southeastern Canada on 23 October 2008. Snowfall amounts over that region were generally in the 2-6 inch range (NOHRSC), with as much as 7 inches reported at Killington in Vermont. This snow cover was also very evident on true color imagery from the SSEC MODIS Today site.

Some items to point out on the AWIPS MODIS imagery above:

1. the snow cover appears as brighter shades of white on the visible image, as do the cirrus cloud features over the far northwestern corner of the image and the stratocumulus clouds found over the eastern and southeastern portions of the image
2. the snow cover appears darker on the 2.1 µm near-IR “snow/ice channel”, since snow (or ice) is a strong absorber of radiation at that wavelength
3. the LST values were about 10º F colder over the snow cover (upper 20s to low 30s F, darker blue colors) compared to adjacent bare ground areas
4. NDVI values over the snow cover were also significantly lower (0.1 to 0.2) compared to the adjacent areas that still had green vegetation in place

Even though the MODIS LST values over the snow cover were about 10º F lower,  the actual instrument shelter air temperatures reported across the region were only a few degrees F colder over the areas with snow on the ground (below), in part due to  the relatively high October sun angle.

MODIS LST product with METAR reports

As an aside, an examination of the patch of thick cirrus clouds in the far northwestern corner of the images helps to demonstrate the value and accuracy of the MODIS Cloud Top Temperature (CTT) product (below). The coldest  brightness temperature seen on the MODIS IR window channel image in the vicinity of the cirrus feature was -45º C (darker green enhancement), while the MODIS CTT product indicated temperatures as cold as -57.8º C (darker blue enhancement) over that same patch of cirrrus. Looking that the 12 UTC rawinsonde report from Maniwaki, Quebec (CWMW), assuming that the tops of the cirrus were in the 33,000-35,000 foot range, the air temperatures at those altitudes were around -55º to -58º C (closer to the coldest MODIS CTT value).

MODIS 11.0 µm IR window + Cloud Top Temperature product