NOAA-18 10.8 µm IR image

A McIDAS image of the NOAA-18 10.8 µm IR channel (above) showed a region of very cold surface temperatures (darker blue to violet color enhancement) over the interior of Alaska on 19 November 2009, especially in the vicinity of Chandalar Lake (station identifier PALR) and Anuktuvuk Pass (station identifier PAKP). In addition, note the appearance of the warm signature of large cracks or “leads” in the ice over the Arctic Ocean (orange to red color enhancement), to the north and northeast of Kuparuk (station identifier PAKU).

A closer view with an overlay of the surface air temperatures (below) revealed a number of narrow fingers of very cold air — this represented the drainage of the coldest air into mountain valleys along the southern portion of the Brooks Range. The coldest IR brightness temperature in that area was -44º F, which happened to match the coldest surface air temperature from first-order weather stations of -44º F at Bettles (located near the center of the image). Bettles reported record low daily minimum temperatures of -45º F on 17 November, -46º F on 18 November, -47º F on 19 November, and -46º F on 20 November (the high temperature was only -40º F on that day!) — the normal high/low temperatures for Bettles during this period are +3º F and -10º F. This stretch of record cold temperatures followed a record 2-day snowfall of 23.7 inches on 11-12 November (the greatest 2-day snowfall on record for Bettles during the month of November).

However, note that the IR image also suggested the presence of a deck of clouds to the east of the very cold valley signatures — and surface air temperatures were significantly warmer under this cloud deck.

NOAA-18 10.8 µm IR image (with surface air temperatures)

AWIPS images of the AVHRR Cloud Type, Cloud Top Temperature, and Cloud Top Height products (below) indicated that the patch of clouds to the east and southeast of Bettles (station identifier PABT) was composed of supercooled water droplets (cyan color enhancement), with cloud top temperatures in the -30 to -38º C range and cloud top heights in the 3-5 km range. Note that the cloud product algorithms showed values of cloud properties over the region surrounding Bettles (even though it was clear there) — the very cold surface temperatures of -40 C and colder tricked the algorithms into thinking that there were high cirrus clouds over that particular area.

AVHRR Cloud Type, Cloud Top Temperature, and Cloud Top Height products

GOES-11 10.7 µm IR images (below) gave some subtle indication that this cloud deck was moving slowly northward across the region to the east of Bettles (note that north is toward the upper right corner, due to the North America projection of these particular AWIPS images).

GOES-11 10.7 µm IR images

MODIS visible, 2.1 µm near-IR, and 3.7 µm shortwave IR channels

AWIPS images of the MODIS visible channel, the 2.1 µm near-IR “snow/ice” channel, and the 3.7 µm shortwave IR channel (above) displayed a swath of snow cover on the ground in far northern Quebec, Canada on 04 October 2009. The Environment Canada snow cover analysis at 06 UTC placed a maximum of 21 cm (8 inches) in that area. The area of snow cover appeared bright on the visible image, and darker on the 2.1 µm near-IR snow/ice image (due to the strong absorption of snow at that wavelength) — however, there was a patch of supercooled water cloud over the northern portion of the snow cover, which appeared brighter white on the snow/ice image and darker (warmer) on the shortwave IR image (due to increased solar reflection off the supercooled water droplets).

A comparison of the MODIS visible image and the corresponding MODIS false color Red/Green/Blue (RGB) image constructed using the visible and the near-IR snow/ice channels (below) shows the value of RGB imagery for helping to distinguish between snow cover (which appears darker red on the false color image) and supercooled water droplet clouds (which appear as cyan to white shades on the false color image). Note the semi-transparent nature of this particular cloud deck: surface features (such as rivers, and the edges of the snow cover) can be seen through the thin cloud feature. Farther to the south, glaciated clouds that are composed primarily of ice crystals also appear as varying shades of red on the false color image. The ability to display these types of false-color RGB images will hopefully be available to forecasters using the next generation  of AWIPS-2 software.

MODIS visible and false color RGB images

The MODIS Land Surface Temperature product (below) indicated that LST values were only in the 30s F (darker green color enhancement) in the region of snow cover, compared to much warmer 40s and 50s F (lighter green to yellow color enhancement) in the surrounding bare ground areas.

MODIS Land Surface Temperature product