I’ve written about the operational utility of Day-Night Band (DNB) RGB imagery several times in the SPoRT blog, and here I’m going to take the chance to do that again. First, just some brief background information in case you’re not familiar with this type of imagery. The DNB RGB is composed of a long wave IR channel (~10.8 µm), which is assigned to the blue color component of the RGB recipe, while the DNB channel (0.7 µm) is assigned equally to the red/green colors of the RGB. SPoRT produces two DNB RGB products: Radiance and Reflectance. I typically prefer the Radiance RGB for operational uses since it is composed of the raw data (emitted and reflected light) from the sensor. Sure, cities are quite bright in the imagery, but the cloud features also stand out better compared to the reflectance product, where the data are normalized by the available amount of moonlight. Below are a few observations from the Suomi-NPP VIIRS instrument during this most recent full moon cycle.
First, take a look at the images below from the SE half of the CONUS on the early morning of December 7th. The top image (Image 1) is a Nighttime Microphysics RGB at approx. 0736 UTC, while Image 2 is a DNB Radiance RGB valid at the same time. While this type of imagery is far superior to legacy IR imagery (even enhanced with fanciful color curves), there are proper operational forecasting/analysis applications that one has to consider. The Nighttime Microphysics RGB is generally more useful for distinguishing different cloud types (e.g., low stratus vs fog, thin cirrus vs thick cirrus, etc). After at least a year of viewing the DNB imagery, I think perhaps the best application of these types of imagery (at least with respect to operational forecasting) lies in the ability to view low clouds through cirrus at night. No other imagery available to forecasters offers this capability currently. Take for example these first two images below and pay close attention to the cloudy regions stretching from the central Plains into the lower Mississippi Valley.
Notice that in the Nighttime Microphysics RGB the expansive deck of low stratus across much of Kansas, southwestern Missouri and Oklahoma is almost entirely obscured by the cold cirrus clouds. Of course, this is only realized upon looking at the DNB imagery. Details in the low stratus can also be observed in the DNB imagery, such as the cloud banding stretching SW-NE across much of northern Louisiana and Mississippi. Since the cloud bases in this imagery were mostly at MVFR and IFR levels with respect to aviation forecast concerns, knowledge about the details and characteristics of the low clouds are very important.
The next series of images from the New England region in the early morning hours of December 9th again demonstrates this application of the DNB imagery.
In the images above, notice that the extensive low cloud deck across the region that spans from Maine to at least as far south as northeastern North Carolina cannot readily be observed either in the legacy IR (10.8 µm ) imagery or in the Nighttime Microphysics RGB. However, more details about the low clouds can be discerned from the DNB imagery. Sure, cirrus clouds are optically thick enough to prevent viewing of any low clouds in the NY metro area. Nevertheless, the advantages of the DNB imagery for detecting low clouds beneath thin cirrus can clearly be seen. Again, as expressed earlier, this type of imagery certainly offers application for aviation forecasting, in particular.
Lastly, here are some observations from just this morning (Dec 10th) over the Rio Grande Valley region.
In the VIIRS IR image (Image 6) just as in previous IR imagery the cirrus clouds obscure the presence of any clouds beneath. However, the patchy low clouds in eastern New Mexico can be much more easily seen in the DNB imagery. In the area between Midland, TX (KMAF) and Fort Stockton (KFST), a forecaster might have made the assumption that the low clouds were continuous based on the observations alone and without the aid of the DNB imagery. Yet, what becomes noticeable in the DNB imagery is that a gap exists in the low cloud deck.
Of course, with all of this said, the availability of the imagery severely limits its application for operational forecasting and analysis. Generally, only one or two passes are available over a given location on any night. Also, due to moonlight limitations, the imagery are only available for about half of the month…at best. I can only lament that the DNB imagery will not be available on a geo-stationary platform (at least anytime soon). Nevertheless, understanding the limitations of the imagery while also appreciating its advantages can offer operational utility when applied properly to a forecast challenge.