The GOES Imager has five
channels
Most meteorologists are familiar with the visible and IR
imagery (.65 and 10.7 um, respectively). Using these more
familiar channels, let's look at an image made by subtracting the
12 um temperatures from the 10.7 um. This image is displayed using a
grey scale enhancement.
The darker values are negative differences and mean that the 12 um
channel is warmer than the 10.7 um. The 12 um channel is not a
transparent window, but is affected by water vapor attentuation in the
lower layers of the atmosphere. 12um brightness temperatures can be
warmer than 10.7 um in areas where surface inversions are present.
Negative differences can also occur along cloud edges due to sub-pixel
effects at 12um.
Note the large bright areas over Texas, Colorado, and Kansas. In
these areas, pixels are warmer in 10.7 um than 12 um by more than 10
degrees celsius. For this cloud area, this difference indicates thin
cirrus. The radiation from below the thin cirrus originates at a
lower (and warmer) level for 10.7 um than 12 um. This results in a
warmer temperature at 10.7 um than 12 um.
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10.7 minus 12.0
(click to enlarge)
Compare the above with the individual visible, IR and 12 um imager
channels. The visible image shows the very thin cirrus originating
over the Pacific, as well as across California and farther northeast.
Note also the appearance of the very thin cirrus to the east of the
Colorado Rockies. Notice the thicker (darker) area of cloudiness
embedded in the thin cirrus over southeast Colorado. Note that the
thicker clouds are more reflective in the visible image, but appear
grey in the 10.7/12.0 um difference image.
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Visible image
(click to enlarge)
For these thick cloud regions, the 10.7-12.0 difference is less than
for areas of thinner clouds. In general, increasing cloud thickness
results in decreasing 10.7-12.0 differences. The 12um channel shows
colder brightness temperatures than 10.7 um in the colored pixels
where the clouds are not opaque because the radiance at 12 um
originates from a higher/colder level than 10.7 um.
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12.0 micron image
(click to enlarge)
Compare the 12um image to the standard 10.7um. Notice that the
10.7um temperatures are warmer (in most cases) and show a different
structure within the cloud areas. In most instances, thin cirrus are
difficult to detect using the 10.7 um channel alone. For optimal
detection, we suggest using the visible, IR, and the 10.7 - 12 um
difference.
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10.7 micron image
(click to enlarge)
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Brian Motta
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