The first -40º F (-40º C) temperatures of the season up in Alaska

November 30th, 2010
NOAA-15 AVHRR 10.8 µm IR image (with surface air temperatures)

NOAA-15 AVHRR 10.8 µm IR image (with surface air temperatures)

The first -40º F (-40º C) and colder surface air temperatures of the 2010-2011 winter season occurred on 30 November 2010 (the last day of the 2010 Atlantic Ocean Tropical Cyclone Season). A McIDAS image of NOAA-15 AVHRR 10.8 µm IR channel data (above) shows a signal of the coldest air (darker blue color enhancement) draining into the lower elevations of the river valleys and the Yukon Flats region. At Fort Yukon (station identifier PFYU), the daily maximum and minimum temperatures were -36º F (-38º C) and -42º F (-41º C), respectively. In contrast to the areas of very cold surface temperatures, cloud features exhibited much warmer IR brightness temperatures (green to yellow color enhancement).

Note the significantly warmer surface air temperatures at a few sites along the arctic coast of both Alaska and the Yukon Territory of Canada — this was due to a low cloud deck that was preventing the strong radiational cooling that was occurring farther inland. AWIPS images of the POES AVHRR Cloud Top Temperature (CTT) product and the POES AVHRR Cloud Top Height (CTH) product (below) showed that CTT values associated with this feature were in the -20º to -25º C range  (yellow to cyan color enhancement), with CTH values of 1-2 km (violet color enhancement).

POES AVHRR Cloud Top Temperature product

POES AVHRR Cloud Top Temperature product

POES AVHRR Cloud Top Height product

POES AVHRR Cloud Top Height product

Blowing dust from Mexico across Texas and New Mexico

November 28th, 2010
GOES-13 0.63 µm visible channel images

GOES-13 0.63 µm visible channel images

McIDAS images of GOES-13 0.63 µm visible channel data (above) revealed several plumes of blowing dust originating in northern Mexico, which then drifted northeastward across parts of Texas and New Mexico during the afternoon hours on 28 November 2010. At El Paso, Texas (located at the center of the image), winds gusted to 52 mph, and surface visibility was reduced to 2.5 miles due to  blowing dust. Farther to the east, winds gusted to 64 mph at Guadalupe National Park.

A comparison of the MODIS 0.65 µm visible channel image and the corresponding 11.0 µm – 12.0 µm IR brightness temperature difference image (below) showed a signal where the airborne blowing dust was the most concentrated (yellow enhancement).

MODIS 0.65 µm visible image + MODIS 11.0-12.0 µm IR difference image

MODIS 0.65 µm visible image + MODIS 11.0-12.0 µm IR difference image

AWIPS images of MODIS 0.65 µm visible channel data and MODIS 11.0 µm IR channel data (below) showed that many of the larger plumes exhibited a slightly cooler (lighter gray) appearance on the IR image.

MODIS 0.65 µm visible image + MODIS 11.0 µm IR image

MODIS 0.65 µm visible image + MODIS 11.0 µm IR image

Swaths of snow (and ice) across parts of Missouri, Illinois, Indiana, and Kentucky

November 26th, 2010
GOES-13 0.63 µm visible channel images

GOES-13 0.63 µm visible channel images

McIDAS images of GOES-13 0.63 µm visible channel data (above) showed the presence of long, narrow swaths of brighter white snow cover oriented from southwest to northeast across parts of Missouri and Illinois (as well as a broader swath of more significant snow cover across Kentucky) on the morning of 26 November 2010. Reports of snowfall within the narrow swaths — which occurred on the previous day — across Missouri and Illinois were generally 1 inch or less, with the highest amount being 2.0 inches at Seymour, Missouri (NOHRSC snowfall totals). The light snow cover was seen to melt rather quickly during the morning hours under full sunshine.

However, in addition to the light snowfall, a number of locations also reported freezing rain, with ice accumulations of about 1/8 inch. A comparison of AWIPS images of 1-km resolution MODIS 0.65 µm visible channel data with a MODIS false-color Red/Green/Blue (RGB) image (below, created using MODIS 0.65 µm data as the Red compoent and MODIS 2.1 µm “snow/ice channel” data as the Green and Blue components) helped to highlight the areas where there was also some light ice accrual (which, being generally transparent, would not show up on the visible channel image).

Since both snow and ice are very strong absorbers of energy at the 2.1 µm wavelength, the swaths of snow and/or ice on the ground showed up as the darker red-colored features on the RGB image — the darker the red color, the thicker the layer of snow/ice on the ground. Cloud features that were composed of large amounts of ice crystals also exhibited a lighter red appearance on the RGB image.

MODIS 0.65 µm visible image + MODIS false-color Red/Green/Blue (RGB) image

MODIS 0.65 µm visible image + MODIS false-color Red/Green/Blue (RGB) image

An image of the MODIS Land Surface Temperature (LST) product (below) showed that the swath of snow cover across southern Missouri was keeping LST values down into the 30s F (darker blue color enhancement), compared to LST values in the 40s in the bare ground areas adjacent to the snow cover.

MODIS Land Surface Temperature product

MODIS Land Surface Temperature product

Using MODIS imagery to locate areas of fog/stratus in the Canadian Rocky Mountains

November 23rd, 2010
MODIS 0.65 µm visible channel + MODIS false-color Red/Green/Blue (RGB) image

MODIS 0.65 µm visible channel + MODIS false-color Red/Green/Blue (RGB) image

A comparison of AWIPS images of the 1-km resolution MODIS 0.65 µm “visible channel” and a corresponding MODIS false-color Red/Green/Blue (RGB) composite (created using the 0.65 µm “visible channel” and the 2.1 µm near-IR “snow/ice” channels) showed the utility of such RGB images in helping to locate areas of flog/stratus that were otherwise difficult to detect using just the standard visible imagery (above). A  number of  fog/stratus features (colored cyan on the RGB image) could be seen within parts of river valleys as well as in some of the higher terrain of British Columbia, Canada on 23 November 2010.

Farther to the east in the province of Alberta, note the appearance on the MODIS visible image of brighter white snow cover across the Prairies (which covered much of the eastern third of the image scene), in contrast to the darker areas immediately to the north and to the west of the brighter snow cover. A comparison with the 1-km resolution MODIS Normalized Difference Vegetation Index (NDVI) product (below) indicated that the darker features were actually more densely forested — and the presence of more trees made those areas appear darker on the visible image, even though the depth of snow on the ground was about the same in both areas.

MODIS 0.65 µm visible channel image + MODIS Normalized Difference Vegetation Index product

MODIS 0.65 µm visible channel image + MODIS Normalized Difference Vegetation Index product