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)

High-resolution Imagery of Stratus along the West Coast

October 10th, 2014
Suomi NPP 0.7 µm Day Night Band imagery at 0907 and 1048 UTC over central California with surface observations of ceilings and visibilities (click to enlarge)

Suomi NPP 0.7 µm Day Night Band imagery at 0907 and 1048 UTC over central California with surface observations of ceilings and visibilities (click to enlarge)

The Suomi NPP VIIRS Day Night Band during a Full (or near-Full) Moon yields striking visible imagery at night because of abundant reflected lunar illumination. Sequential orbits along the west coast of the Pacific on the morning of 10 October showed the penetration of coastal stratus and fog inland at two times (click here for the images above without the surface observations). A similar case study from 9 October is shown here. The slow inland penetration of stratus/fog is captured by the scenes: tendrils of fog extend up small valleys along the edge of the Salinas Valley, for example, and the fog extends farther down the valley at 1048 UTC. Similar expansion of fog occurs over Sonoma Valley north of San Francisco Bay.

Careful inspection of the imagery shows parallel lines along the western edge at 0907 UTC and along the eastern edge at 1048 UTC. In addition, city lights and topographic features are displaced somewhat along the eastern edge of the 1048 UTC image. These are all artifacts of the VIIRS instrument viewing geometry (that is, parallax) and post-processing that is necessary near the edges to maintain high-resolution imagery there.

A toggle between the corresponding VIIRS 11.45 µm – 3.74 µm infrared brightness temperature difference images (commonly referred to as the “fog/stratus product”), below, similarly shows gradual expansion of water-based clouds between 0907 and 1048 UTC. There are also image features, color enhanced as black, that suggest very thin cirrus is moving over the coast. These clouds are thin enough that they cannot be discerned in the Day Night Band imagery, but their presence nevertheless inhibits the detection of low clouds in places, such as over the southernmost part of the Salinas Valley at 1048 UTC.

Suomi NPP Brightness Temperature Difference (11.45 µm - 3.74 µm) Imagery at 0907 and 1048 UTC over central California (click to enlarge)

Suomi NPP Brightness Temperature Difference (11.45 µm – 3.74 µm) Imagery at 0907 and 1048 UTC over central California (click to enlarge)

Suomi NPP 0.7 µm visible Day Night Band imagery at 0907 and 1048 UTC over Washington State with surface observations of ceilings and visibilities (click to enlarge)

Suomi NPP 0.7 µm visible Day Night Band imagery at 0907 and 1048 UTC over Washington State with surface observations of ceilings and visibilities (click to enlarge)

The image toggle above shows similar features over Washington State. Fog/stratus tendrils move up river valleys in the ~90 minutes between the two polar-orbiting satellite passes, and areas of fog increase in size. (click here for the same images without observations). Because the first image is very near the edge of the VIIRS instrument scan swath, there is also a shift in city lights and some geographic features, again an artifact of scanning geometry (parallax) and the post-processing to maintain high-resolution imagery at the scan edges.

Suomi NPP Brightness Temperature Difference (11.45 µm - 3.74 µm) Imagery at 0907 and 1048 UTC over Washington State (click to enlarge)

Suomi NPP Brightness Temperature Difference (11.45 µm – 3.74 µm) Imagery at 0907 and 1048 UTC over Washington State (click to enlarge)

The IR brightness temperature difference product over Washington, above, also shows evidence of a slow increase in the areal coverage of fog/stratus near the coast. The effects of limb brightening are also present in the first image. When a satellite scans near the edge of its domain, the path from the point on the Earth to the satellite traverses more of the upper atmosphere, and a colder sensed temperature results. This effect is wavelength-dependent. For example, at one point (47º N, 125º W) in the stratus (with fairly uniform temperature) off the west coast of the Washington, in the stratus (which should have a fairly constant temperature), brightness temperatures were about 1º C cooler in 11.35 µm imagery, but closer to 2.5º C cooler in the 3.74 µm imagery. Hence, the brightness temperature difference signal is larger at 0906 UTC.

Both brightness temperature difference fields show signals over dry land that are related to emissivity differences in the soils. These occur over central Washington, above and over Nevada in the images centered over California.

GOES-14 SRSOR: Dissipation of river valley fog

August 20th, 2014
GOES-14 0.63 µm visible channel images, at 15 vs 5 vs 1-minute intervals (click to play Animated GIF)

GOES-14 0.63 µm visible channel images, at 15 vs 5 vs 1-minute intervals (click to play Animated GIF)

The GOES-14 satellite was in Super Rapid Scan Operations for GOES-R (SRSOR) mode, providing images at 1-minute intervals over the central US on 20 August 2014; an animation of 0.63 µm visible channel images (Animated GIF | MP4 movie | YouTube) showed the dissipation of river valley fog that had formed during the previous night over the Mississippi River and adjacent portions of southwestern Wisconsin, southeastern Minnesota, and northeastern Iowa. The 3 panels show images every 15 minutes (today’s current routine schedule), every 5 minutes (available during Rapid Scan Operations), and every 1 minute (which will be available from the ABI instrument on the next-generation GOES-R satellite).

Along the Wisconsin River valley, fog restricted the surface visibility to 0.15 mile at Prairie Du Chien KPDC and Boscobel KOVS, and 0.25 mile at Lone Rock KLNR (images with map and station location overlays).

GOES-14 SRSOR: Fog Dissipation over Pennsyvlania

August 18th, 2014
GOES-14 0.63 µm visible channel images (click to play animation)

GOES-14 0.63 µm visible channel images (click to play animation)

GOES-14 is in special SRSO-R operations that provides 1-minute imagery, allowing a compelling look at the dissipation of valley fog over Pennsylvania this morning. This animation is also available as a YouTube video, or as an mp4.

A description of the detection of the fog development overnight can be found on the CIMSS Fog Blog.