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

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

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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

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POES AVHRR images and derived cloud products

A comparison of AWIPS images of the 1-km resolution POES AVHRR 0.86 µm visible channel, 12.0 µm IR channel, Cloud Top Temperature product, Cloud Type product, Cloud Top Height product, Cloud Optical Depth product, and Cloud Particle Effective Radius product (above) demonstrated that the Cloud Particle Effective Radius product was the best tool to use in locating the pattern of ship condensation trails (or “ship tracks”) that was located over the northern Gulf of Alaska on 19 November 2010.

The Cloud Type of the stratus cloud deck where the ship tracks were seen was primarily of the “supercooled” classification (green color enhancement), with Cloud Top Temperatures generally in the -3º to -4º C range and Cloud Top Heights of 2-3 km. According to the Cloud Particle Effective Radius product, the size of the particles within the ship track features was about 23-25 µm — somewhat smaller than the 27-31 µm in the surrounding undisturbed stratus cloud deck. This is evidence of the “Twomey effect”, where sulfate aerosols (found in the exhaust plumes of the diesel-burning ships) act as cloud condensation nuclei, which leads to greater numbers of smaller cloud droplets.

As part of the CIMSS involvement in GOES-R Proving Ground activities, POES AVHRR satellite images and products are currently being made available in an AWIPS format for interested NWS forecast offices to add to their local AWIPS workstations (via LDM subscription). For more information, see the  AVHRR Imagery and Products in D-2D site. A VISIT training lesson has also been created for these AVHRR images and products.

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GOES-13 0.63 µm visible channel images

A strong cyclone was located over the northern portion of Hudson Bay, Canada on 18 November 2010 — and McIDAS images of GOES-13 0.63 µm visible channel data (above) showed the formation of widespread cloud bands due to very cold arctic air flowing over the still-unfrozen waters of Hudson Bay.

The strong westerly component of the surface winds was apparently helping to cause some of the land-fast ice along the far western shoreline to begin to drift eastward into the open waters of Hudson Bay. This was more easily seen on a zoomed-in version of the GOES-13 visible images. According to lower-tropospheric GOES-13 atmospheric motion vectors, the speed of motion of the cloud band features was generally in the 25-35 knot range.

A closer view using AWIPS images of 1-km resolution MODIS 0.65 µm visible channel data (below) revealed better detail in the cloud band structure — but also suggested the initiation of ice formation along the western and southwestern near-shore waters of Hudson Bay.

MODIS 0.65 µm visible channel images

To verify that the brighter near-shore features seen on the MODIS visible images were indeed ice forming in Hudson Bay, a pair of false-color Red/Green/Blue (RGB) images were created using the MODIS 0.65 µm visible channel data as the Red component and the MODIS 2.1 µm “snow/ice channel” data as the Green and Blue components of the image. Snow cover (which was generally 3-5 inches at the first-order reporting stations in the region) and thick ice show up as darker red features on the RGB images.

MODIS false-color Red/Green/Blue (RGB) images

An AWIPS image of the POES AVHRR Cloud Type product (below) indicated that many of the cloud bands likely consisted of supercooled water droplets (green color enhancement), although a number of the cloud bands were beginning to show signs of glaciating farther downstream (as indicated by the yellow and orange color enhancements).

POES AVHRR Cloud Type product

The POES AVHRR Cloud Top Height product (below) showed that the tops of most of thee cloud bands were in the 3-4 km range.

POES AVHRR Cloud Top Height product

Another indication of the change from supercooled water droplets to more of a glaciated composition could be seen on the POES AVHRR Cloud Particle Effective Radius product (below), with an increasing presence of the larger size ice crystals showing up as increasingly darker blue colors farther downstream.

POES AVHRR Cloud Particle Effective Radius product

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