October 25th, 2013
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 roughtly 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
October 17th, 2013
MTSAT-2 visible channel (left) and shortwave IR channel (right) images (click to play animation)
Numerous bush fires began to burn in state of New South Wales near Sydney, Australia on 16-17 October 2013. On a comparison of MTSAT-2 visible channel and shortwave IR channel images (above; click image to play animation) some of the southeastward-drifting smoke plumes were evident on the visible images, while numerous fire “hot spots” (dark black pixels) could be seen on the shortwave IR images. Toward the end of the animation (06:32 UTC on 17 October), the hot fire pixels grew in areal coverage as winds increased in association with a cold frontal passage.
A 48-hour plot of surface data for Sydney Airport (below) showed that smoke reduced the surface visibility to 4-5 miles for several hours on 17 October. Prior to the passage of the cold front, surface air temperatures were unseasonably hot (in the middle 90s F), with wind gusts as high as 38 knots. In addition, note the sharp drop in dew point temperature to -2º F at 04 UTC on 17 October.
Plot of Sydney Airport surface meteorolofical data
A 250-meter resolution Aqua MODIS true color image from the NASA EOSDIS Worldview site (below) offers a closer view of the smoke plumes in the Sydney area.
Aqua MODIS true color image
October 5th, 2013
GOES-13 0.63 µm visible channel images (click to play animation)
The “De Luz” wildfire began to burn at the Camp Pendleton Marine Corps Base in southern California around 19:45 UTC or 12:45 PM on 05 October 2013. The fire burned over 2200 acres and forced evacuations of some housing and a hospital (InciWeb). McIDAS images of GOES-13 (GOES-East) 0.63 µm visible channel data (above; click image to play animation) showed the smoke plume as it drifted quickly southwestward due to strong Santa Ana winds.
A 2-panel comparison of GOES-13 0.63 µm visible channel and 3.9 µm shortwave IR channel images (below; click image to play animation) revealed that the fire “hot spot” (red color enhancement on the shortwave IR images) was detected about 30 minutes prior to the time when a smoke plume became obvious on the visible images.
GOES-13 0.63 µm visible (left panel) and 3.9 µm shortwave IR (right panel) images (click to play animation)
The fire growth slowed that evening as temperatures began to cool down and the strong Santa Ana winds gradually subsided, but fire continued to burn into the night — a small fire hot spot (yellow to orange color enhancement) was still evident on a Suomi NPP VIIRS 3.74 µm shortwave IR channel image at 09:18 UTC or 2:18 AM local time (below).
September 9th, 2013
GOES-15 3.9 µm shortwave IR images (click image to play animation)
The Morgan Fire began burning in the Mount Diablo State Park northeast of San Francisco during the afternoon hours on 08 September 2013, which caused the evacuation of dozens of homes near the fire. McIDAS images of 4-km resolution GOES-15 3.9 µm shortwave IR data (above; click image to play animation; also available as a .mp4 file) revealed that the fire “hot spot” (red color enhancement) first appeared on the 21:30 UTC (2:30 PM local time) image. The fire appeared to “settle down” for a few hours after sunset, but then there was an apparent flare-up of the fire from 04:00 – 06:30 UTC.
One thing to note on the GOES-15 shortwave IR animation above is that there was a significant Image Navigation and Registration (INR) anomaly during the 08:45 – 10:41 UTC period. An AWIPS 1-km resolution Suomi NPP VIIRS 3.74 µm shortwave IR image was available at 09:24 UTC (during the GOES-15 INR anomaly) — a comparison of this VIIRS image with the closest available GOES-15 shortwave IR image (below) demonstrates the value of higher spatial resolution VIIRS data to aid in a more accurate determination of the actual fire size and location.
Suomi NPP VIIRS 3.74 µm and GOES-15 3.9 µm shortwave IR images
Hot spots associated with the ongoing Rim Fire — so far the 3rd-largest fire on record in California — can also be seen about 100 miles to the east-southeast.