Smoke and Fog in the VIIRS Day/Night Band

July 2nd, 2015 |
Suomi NPP VIIRS 0.70 µm visible Day/Night Band and 11.45 µm - 3.74 µm Brightness Temperature Difference images, and Ceilings and Visibilities, ~0800 UTC (click to enlarge)

Suomi NPP VIIRS 0.70 µm visible Day/Night Band and 11.45 µm – 3.74 µm IR Brightness Temperature Difference images, and Ceilings and Visibilities, ~0800 UTC (click to enlarge)

July’s first Full Moon occurred at 0219 UTC on 2 July (a second full moon occurs later this month on 31 July). Strong illumination from the moon showed river valley fog in several tributaries of the Mississippi River (for example, the Wisconsin River in southwest Wisconsin; the Upper Iowa River in Iowa) across the Upper Midwest. The Suomi NPP VIIRS Day/Night Band also shows a plume of Canadian wildfire smoke aloft, stretching from central Iowa northwestward to western Minnesota. This smoke (visible on 1 July in Aqua true-color imagery from the MODIS Today site) is not apparent in the IR Brightness Temperature Difference field, although the river valley fog certainly is. Smoke is transparent to most infrared channels and detection at night is very difficult if visible information such as that provided by the Day/Night Band is not present.

The VIIRS Day/Night Band also enabled detection of the dense plume of Canadian wildfire smoke as it moved off the US East Coast and over the adjacent offshore waters of the western Atlantic Ocean at 0614 UTC  (below). Again, note that the smoke aloft does not exhibit a signature on the corresponding VIIRS Infrared imagery.

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm Infrared images (click to enlarge)

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm Infrared images (click to enlarge)

Can you use the VIIRS Day/Night Band to know where heavy rain is falling?

August 19th, 2014 |
Suomi NPP VIIRS Day/Night Band at 0659 UTC and 0838 UTC, 19 August 2014 (click to enlarge)

Suomi NPP VIIRS Day/Night Band at 0659 UTC and 0838 UTC, 19 August 2014 (click to enlarge)

The VIIRS Day/Night Band image toggle above shows nighttime cloud cover over southern Wisconsin associated with a line of deep convection; note how some city lights are not seen (Madison WI KMSN at 0659 UTC, and Milwaukee WI KMKE at 0838 UTC). There are several reasons this may have happened. For example, the convection could have knocked out power over a large region (this did not happen). Scattering associated with the thick convective clouds may have attenuated the city light so much that it could not be detected.

The toggle below of the corresponding VIIRS 11.45 µm Infrared imagery shows very cold cloud tops (-60º to -70º C, near the tropopause) over Madison at 0659 UTC (the observation at 0653 UTC at the Madison airport was Heavy Rain with a Thunderstorm) and over Milwaukee at 0838 UTC (when the Milwaukee airport was having Moderate Rain; they received a half-inch of rain between 0753 and 0853 UTC). The combination of the thick convective cloud and especially the heavy rain is very likely why city lights cannot be seen at certain times, as liquid water is an excellent absorber of visible light. This radar image (from this story) shows the areal extent of the heavy rain at 0745 UTC on 19 August.

Suomi NPP VIIRS 11.45 µm Infrared imagery  at 0659 UTC and 0838 UTC, 19 August 2014 (click to enlarge)

Suomi NPP VIIRS 11.45 µm Infrared imagery at 0659 UTC and 0838 UTC, 19 August 2014 (click to enlarge)

Lunar Eclipse Effects on VIIRS Day/Night Band Imagery

April 15th, 2014 |
Suomi NPP VIIRS Day Night Band images, times as indicated

Suomi NPP VIIRS Day Night Band images, times as indicated

A Total Lunar Eclipse occurred over North America early in the morning of 15 April 2014. The resultant lack of lunar illumination had a profound impact on the “visible image at night” quality of the VIIRS 0.7 µm Day/Night Band imagery during the Suomi NPP satellite overpass at around 0800 UTC over the midsection of the United States. The animation above shows excellent illumination from the Full Moon at 0619 UTC (2:19 AM Eastern time) along the East Coast, and also at 0940 UTC (2:40 AM Pacific time) along the West Coast; however, the image at 0800 UTC (3:00 AM Central time), in the middle of eclipse totality, resembles Day/Night Band images seen during a New Moon. The three images can also be combined into a collage, as shown below, to illustrate the changes in the Day/Night band that occur as lunar intensity changes.

Collage of three successive Suomi NPP VIIRS Day Night Band images, times as indicated

Collage of three successive Suomi NPP VIIRS Day Night Band images, times as indicated

A comparison of three similar Day/Night Band images from the day before is here.

Even though the 08:00 UTC VIIRS Day/Night Band image exhibited a comparatively dull and “washed-out” appearance (due to very little illumination from eclipsed moonlight), there were still features of interest that could be seen. For example, a closer look over the southeastern US using the 0.7 µm Day/Night Band and 11.45 µm IR channel images (below) showed the effect of lightning activity associated with a pre-cold-frontal squall line over the Florida panhandle and the adjacent offshore waters of the Gulf of Mexico: a pair of long, narrow bright streaks (caused by cloud illumination from intense lightning activity as the sensor was rapidly scanning from northwest to southeast), abruptly followed by dark black streaks that eventually faded away. These dark black streaks represent post-saturation “recovery periods” after the sensor scanned the extremely bright cloud features. Incidentally, the coldest 11.45 µm IR brightness temperature over the Florida panhandle was -80º C (very near the area of dense cloud-to-ground lightning strikes).

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images, with surface frontal analysis and 1-hour cloud-to-ground lightning strikes

Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm IR channel images, with surface frontal analysis and 1-hour cloud-to-ground lightning strikes

Detecting snow cover at night with VIIRS Day/Night Band imagery

January 24th, 2014 |
Suomi NPP VIIRS 0.7 µm Day/Night Band image

Suomi NPP VIIRS 0.7 µm Day/Night Band image

Due to illumination by moonlight — the Moon was in the waning crescent phase, at 40% of full — a broad area of snow cover could be seen over parts of the High Plains and Foothills regions of the US on an AWIPS image of Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) data at 08:15 UTC or 1:15 AM local time on 24 January 2014 (above). This was what remained of the general 1-4 inches of new snow that fell over that area, aided by upslope flow in the wake of a southward-moving cold frontal passage on 23 January.

A comparison of this Day/Night Band image with the corresponding VIIRS 11.45-3.74 µm IR brightness temperature difference (BTD) “Fog/stratus product” and 11.45 µm IR channel images (below) confirmed that this feature seen on the DNB image was not an area of fog or low-level clouds (although some patches of cold high-altitude clouds were seen from far northern Colorado into Wyoming and Nebraska). On the 11.45 µm IR image, some areas in eastern Colorado exhibited IR brightness temperature values of -30º C or colder (yellow color enhancement) — these were likely locations where the snow cover was the deepest, allowing faster radiational cooling of the surface air layer.

Suomi NPP VIIRS 0.7 µm Day/Night Band, IR BTD "fog/stratus product", and 11.45 µm IR channel images

Suomi NPP VIIRS 0.7 µm Day/Night Band, IR BTD “fog/stratus product”, and 11.45 µm IR channel images

A Suomi NPP VIIRS 0.64 µm visible channel image at 19:42 UTC or 12:42 PM on the following afternoon (below) showed that while the areal coverage of the snow cover had decreased with daytime heating, what snow cover did remain was acting to hold surface air temperatures down at least 10-15º F compared to adjacent bare-ground locations.

Suomi NPP VIIRS 0.64 µm visible channel image, with METAR surface reports

Suomi NPP VIIRS 0.64 µm visible channel image, with METAR surface reports