Suomi NPP VIIRS 0.7 µm Day/Night Band image, with overlays of GFS surface winds, surface observations, and fronts

Just as we have documented in previous blog posts (on 25 April 2013, and 04 June 2009) using visible imagery during daylight hours, Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) imagery can also be used to help diagnose areas of light winds over water at night (using reflected moonlight). The VIIRS DNB image at 08:20 UTC or 3:20 AM local time on 25 October 2013 (above) showed a broad area of bright moon glint over the western Gulf of Mexico, off the coasts of Texas and Louisiana. However, located within this area of bright moon glint was a very dark band, oriented roughly west-to-east. An overlay of the 06 UTC GFS40 surface winds suggested that there should be a band of calm winds in this area, ahead of a slow-moving cold frontal boundary that was sagging southward across the Gulf Coast region. Surface observations and buoy reports confirmed that the winds within and near this dark DNB feature were either calm or very light. As a result, wind-driven wave heights there were minimal, allowing for an effectively “flat” water surface.

So why did this large area of smooth water appear darker on the VIIRS DNB imagery? The answer lies in the fact that with imagery from polar-orbiting satellites such as Suomi NPP, there is often a significant amount of sun glint or moon glint off the rough water surfaces below the satellite overpass; due to the varying scattering angles of the light reflected off of wind-driven waves, these areas of rough water surfaces appear brighter on DNB imagery. However, in an area of calm winds, the water surface becomes very flat; this flat water surface then reflects incoming moonlight like a mirror (with all the light being reflected back in one direction — but in this case, that one direction was not directly back toward the satellite).

One other feature of interest on the DNB image was the very bright “streak” located well south of the Mississippi River delta. A comparison of the DNB image with the corresponding VIIRS 3.74 µm shortwave IR image (below) indicated that a “hot spot” (yellow to red color enhancement) was co-located with the bright DNB feature — this suggests that a large natural gas flare was occurring at one of the drilling platforms at that particular time. Numerous smaller bright spots could be seen scattered across the Gulf of Mexico on the DNB image, signatures of lights from ships and other offshore drilling platforms. On the shortwave IR image, there were also hot spots seen in Texas and Louisiana, which could have been due to small fires.

Suomi NPP VIIRS 0.7 µm Day/Night Band and 3.74 µm shortwave IR images

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Suomi NPP VIIRS 0.7 µm Day/Night Band images

McIDAS-V “before” (22 October 2013) and “after” (23 October 2013) night-time images of Suomi NPP VIIRS 0.7 µm Day/Night Band data (above; courtesy of William Straka, CIMSS) showed the coverage of power outages across much of Syria, caused by a rebel attack on a gas pipeline (near Damascus International Airport) that supplied the Tashrin power generating station in the southern part of the country. As can be seen in the “after” image on 23 October, full or partial power outages affected all provinces of Syria — in particular, parts of the Euphrates River valley, the northern Mediterranean coast near Latakia, and areas south of the capitol city of Damascus (map of Syria).

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Suomi NPP VIIRS IR brightness temperature difference “Fog/stratus product”

A strong upper-level ridge of high pressure coupled with a moist, stagnant boundary layer led to the formation of widespread areas of fog for several days across much of the interior lowlands of western British Columbia and Washington State, along with valley fog in the Rocky Mountains farther to the east (as was documented during 21 October 2013 on the GOES-R Fog Product Examples blog). On 22 October 2013, two consecutive Suomi NPP VIIRS IR brightness temperature difference (BTD) “fog/stratus product” images (above) at 09:20 UTC (2:20 AM local time) and 10:58 UTC (3:58 AM local time) showed little change to the areal coverage of the fog/stratus located over the Puget Sound region and adjacent interior lowlands, but a significant growth of narrow fingers of valley fog from eastern British Columbia southward into far northern Washington, Idaho, and Montana.

A comparison of the 10:58 UTC VIIRS 0.7 µm Day/Night Band (DNB) image with the corresponding IR BTD fog/stratus product image (below) showcased the “visible image at night” capability of the Day/Night Band, given sufficient illumination by the Moon (which was in the Waning Gibbous phase, at 84% of Full). Not only are the fog (and other cloud) features evident in the DNB image, but the snow-covered higher elevations of the Rocky Mountains could also be seen. A number of ship tracks also appear over the Pacific Ocean in the far western portion of the satellite scene.

Suomi NPP VIIRS 0.7 um Day/Night Band and IR BTD “Fog/stratus product” images

A post-sunrise sequence of GOES-15 (GOES-West) 0.63 µm visible channel images (below; click image to play animation) revealed that many of the fog features were very persistent and slow to dissipate — a strong boundary layer temperature inversion and light winds inhibited the rate of fog burn-off in those areas.

GOES-15 0.63 um visible channel images (click to play animation)

A comparison of two different AWIPS II false-color Red/Green/Blue (RGB) images (below) using VIIRS data at 20:44 UTC (1:44 PM local time) showed the value of using the 1.61 µm “snow/ice” channel to discriminate between snow cover (which appears as varying shades of red) and any fog, stratus, or other cloud features composed of water droplets (which appear as varying shades of white) in the area.

One other feature of interest can be seen on the VIIRS RGB images: in the far upper right corner, a pair of narrow aircraft dissipation trails (or “distrails”) can be seen within the broader band of mid-level supercooled water droplet clouds. As aircraft ascended (or descended) through the supercooled water droplet cloud layer, particles in the jet engine exhaust acted as ice condensation nuclei, causing thin streaks of the cloud to glaciate (hence their red appearance on the RGB image using the 1.61 µm VIIRS data) along the aircraft path. From the ground, these aircraft dissipation trails often appear as dramatic-looking “cirrus fall streaks”, as the larger, heavier ice crystals begin to descend from the supercooled cloud layer.

Suomi NPP VIIRS false-color RGB images (using Bands I1/I2/I5, and Bands I1/I3/I3)

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MTSAT-2 10.8 µm IR channel images (click to play animation)

Super Typhoon Francisco became the third Category 5 tropical cyclone of 2013 on 19 October 2013, as it intesified over the West Pacific Ocean northwest of Guam. 4-km resolution MTSAT-2 10.8 µm IR channel images (above; click image to play animation) showed the evolution and track of the eye of Francisco during the 17-19 October period (the island of Guam is in the lower right corner of the images). Note the trochoidal motion or “wobble” that is exhibited by the eye of the tropical cyclone as it tracked northwestward – this is caused by changes within the inner core structure of the storm, such as convective asymmetries.

1-km resolution MTSAT-2 0.73 µm visible channel images (below; click image to play animation) revealed better details of the eye and eyewall structure during the daylight portion of 18-19 October. The lowering October sun angle tended to more brightly illumimate the sloped surface of the northern quadrant of the eye.

MTSAT-2 0.73 µm visible channel images (click to play animation)

A McIDAS-V comparison of 375-meter resolution Suomi NPP VIIRS 0.7 µm Day/Night Band and 11.45 µm thermal IR channel images at 15:48 UTC on 18 October (below; images courtesy of William Straka, CIMSS) showed a good example of the so-called “stadium effect”: the eye diameter appeared larger on the VIIRS IR image than on the corresponding “visible image at night” from the VIIRS Day/Night Band, because the clouds along the edges of the eye were steeply sloping outward with height.

Suomi NPP VIIRS 0.64 µm visible channel and 11.45 µm IR channel images

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