I’ve blogged about this particular use of VIIRS Day-Night Band imagery before, but wanted to take a moment to showcase this excellent example from last night. In the early morning hours of Thursday, Oct 9th, fog began developing in the valleys of Jackson County, AL (denoted by the yellow circle), as it often does on nights with high relative humidity and light winds. Of course, save for a scant observation or two, a forecaster would not have known because of the presence of high, thin cirrus clouds that obscured the fog below. Take a look at this GOES-IR image from early that morning (Image 1).
Since this is an IR image, temperatures from the colder cirrus clouds saturate the instrument signal across northeastern portions of Alabama, where the fog occurred. Clearly, fog could not be discerned in this type of imagery due to the presence of cirrus. Next, let’s take a look at a Nighttime Microphysics RGB image from the Suomi-NPP VIIRS instrument valid at about the same time (Image 2).
Even here (in what has become favorite imagery of mine) the fog is not clearly detectable. Perhaps a faint hint of fog, indicated by a narrow sliver of whitish-gray can be seen in the far northwest corner of Jackson County. Nevertheless, since this RGB recipe utilizes different IR components, the cirrus once again obscures the fog below. Now, let’s take a look at SPoRT”s Day-Night Band Radiance RGB valid at the same time (Image 3).
The fog is much more clearly observed in this imagery since thin cirrus are mostly translucent in the VIIRS nighttime visible band. This image, as stated, is an RGB and also employs a longwave IR channel (~11 µm), giving the cirrus a blue appearance.
Visibility observations are only available from one site in this area, at the Scottsboro, AL airport, which is located near the center of the circle. At 0800 UTC, the visibility at the location was 7 SM, but dropped to 1/4 SM by 0900 UTC. However, that is just one small point location. Forecasters really need a better understanding of the extent of the fog for special weather statements or dense fog advisories that address the threat. After viewing the Day-Night Band imagery for well over a year now, I think the ability to see fog and other phenomena through cirrus at night is one of the best applications of the DNB imagery, at least from an operational forecast perspective.