Observations of Low Clouds at Night via the VIIRS Day-Night Band

Viewing low-level clouds at night can be a problem when utilizing standard IR imagery available through GOES or most polar orbiters, especially when widespread high clouds are present.  However, Day-Night Band imagery available through the VIIRS instrument onboard the Suomi-NPP satellite offers this ability.  Well, I should add…when sufficient moonlight is present.  Last night was one such night.  First, let’s take a look at the 11-3.9 channel difference product (commonly known to forecasters as the “fog” product) with the standard yellow/gray/blue color curve (Image 1).

FogProduct_0717Z30Nov2015

Image 1.  Suomi-NPP VIIRS 11-3.9 um product, 0717 UTC 30 Nov 2015.

Some low/mid clouds (yellows to medium gray) can be distinguished from higher clouds (blues) in portions of the image, especially over parts of the interior South.  However, it’s difficult to tell the extent of the low clouds with much confidence.  By the way, the values/symbols in white are automated ground observations.  the number at the bottom of the circles represents visibility (statute miles), while the numbers to the left indicate cloud base heights (in hundreds of feet).

Now, take a look below at the VIIRS Day-Night Band Reflectance product.

DNBReflectance_0717Z30Nov2015

Image 2.  Suomi-NPP VIIRS Day-Night Band Reflectance product, 0717 UTC 30 Nov 2015

In the Day-Night Band Reflectance image above (Image 2), the extent of the clouds is more easily discernible.  However, it can be difficult to differentiate low clouds from high level clouds.  This is why I generally prefer the Day-Night Band Reflectance RGB for these purposes (Image 3).

DNBReflectanceRGB_0717Z30Nov2015

Image 3. Suomi-NPP VIIRS Day-Night Band RGB product, 0717 UTC 30 Nov 2015

Notice in Image 3 above, it’s much easier to distinguish the low clouds (grays/yellow-grays) from high clouds (blues) due to the color-coding provided in the RGB.  Now, a forecaster can easily see the extent of the low cloud deck across the Southeast, and can tell where this line ends from southern Louisiana, trough central Mississippi, Alabama and into northern Georgia.  If you look back at Image 1, it would have been impossible to tell where the terminus of low clouds was situated.  Sure, observations (shown in white) will provide some clue for forecasters, but in areas with a dearth of observations, there would be no way to know for sure.

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