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Yet another example of the IR2 and Fog Products' usefulness
In a previous electronic poster, it was
demonstrated that the Fog Product and 3.9 micron IR imagery both did a better
job of depicting nighttime low cloudiness than did the conventional 10.7 micron IR
product. The following images, taken from a case earlier this month (August '97),
provide more proof of the advantages of these products.
Today's case centers on an isolated area of low clouds which covered a portion of north Georgia during the pre-dawn hours of
August 16th. The following are the METAR observations for Atlanta (ATL), Anniston (ANB), and
Columbus (CSG) at 08Z and 09Z:
METAR KATL 160756Z 25007KT 6SM BR BKN003 24/23 A3005 RMK AO2 SLP 165 T02390233
METAR KATL 160856Z 26006KT 5SM BR OVC003 24/23 A3007 RMK AO2 SLP 172 T02390233 53006
METAR KANB 160751Z 00000KT 5SM BR SKC 23/23 A3004 RMK SLP 168=
METAR KANB 160855Z 00000KT 5SM BR SKC 23/23 A3006 RMK SLP 174 53007=
METAR KCSG 160753Z 25007KT 10SM CLR 24/23 A3004 RMK AO2 SLP 166 T02440233
METAR KCSG 160853Z 26003KT 10SM CLR 24/23 A3005 RMK AO2 SLP 171 T02440233 53001
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First,
let's look at what the conventional 10.7 micron IR image at 0815Z
has to tell us about the cloud situation over Atlanta:
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Not much!! There are some lighter patches over east central Alabama
that at first glance might resemble low clouds, but in fact these are thin cirrus
clouds that have rotated into the area from the south and southeast. Over the Atlanta
area, almost no contrast is apparent in the brightness temperatures, making it
impossible to discern the presence of any low clouds.
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Now,
let's look at the 3.9 micron (IR2) image, also taken at 0815Z:
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A little better.
Now, we start to see some differentiation in the brightness temperatures
across the Atlanta metro area. Admittedly, the enhancement used here is not the
best, but we can still pick out an area of cooler brightness temperatures between
ATL and ANB, representing the low clouds forming over the region. But maybe
there's something even better out there...
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As a final thought, let's take a look at what the Fog
Product can offer us at 0815Z:
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Aha!
With the Fog Product, we're clearly able to see a distinct patch of low cloudiness over north Georgia. In time, this patch grew to encompass
a good chunk of western Georgia and east central Alabama.
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Remember...The 3.9 micron channel has different radiative characteristics than the 10.7 micron channel
does...so the IR2 images can often pick up subtle details in the low clouds that the IR products completely miss. Look here for a nice tutorial on the 3.9 micron channel.
Basically, the Fog Product is simply a product of the differences between the brightness temperatures at 10.7 microns, and those at 3.9 microns. As demonstrated
above, and in the prior electronic poster on the Fog Product, this makes it an excellent
tool for detecting nighttime low clouds and fog.
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