GOES-13 0.63 µm visible channel images (click image to play animation)

Each year, about every 6 months, the Earth-Sun-Satellite geometry is such that the GOES Imager can look right at the Sun. In the past, there were ‘keep-out zones’ in which the satellites did not image because it was known to be looking at the Sun during those times. The imagery above, from GOES-13, shows visible light in the night-time imagery. (Click here for a similar GOES-15 animation). Stray light values typically peak around 0500 UTC for GOES-East and around 0900 UTC for GOES-West.

In addition, imagery was not possible during the so-called ‘eclipse season’ because the satellites lacked sufficient batteries to power the instruments as they passed through Earth’s shadow. Now, an improved battery system on the current generation GOES-13/14/15 satellites allows for imaging to proceed while the satellite is in the Earth’s shadow.

This new scheduling, however, introduces issues. The GOES Imager is calibrated by periodic looks into deep space, regions from which only very small amounts of radiation (at 3.9, 6.5, 10.7 and 13.3 µm) are being emitted. These ‘space looks’ are on either side of the full-disk GOES Image. During the ‘eclipse season’, that space look can include part of the solar energy, meaning the very small amount of radiation that the satellite is designed to detect is actually potentially significant. Thus, the calibration of the image can be affected. NOAA NESDIS does operationally correct images with ‘stray light’, but this correction does not consider the impact of a corrupted space view. The GOES-13 stray light corrections were implemented in 2012, as discussed here on this blog.

In addition to the calibration images, solar radiation can also be scattered off clouds towards the imager. So, instead of detecting only emitted radiation at night, the GOES Imager is detecting emitted terrestrial radiation in addition to scattered/reflected solar radiation. This solar radiation contaminates the signal, and results in ‘too much’ radiance being detected, resulting in warmer-than-actual inferred blackbody/brightness temperatures.

GOES-13 imagery from infrared channels (click image to enlarge)

When Stray Light issues occur, the most noticeable effects are in the 3.9 µm channel (Above loop, bottom left) and in products that use the 3.9 µm channel, such as the brightness temperature difference (Above loop, top left). In other words, this calibration issue can affect derived products that use 3.9 µm data at night. The image below shows how the 3.9 µm imagery can change when Stray Light is an issue. Compare the 0415 UTC image, on the left, when Stray Light did not contaminate the space look, with the 0502 UTC image on the right, when Stray Light was an issue.

GOES-13 3.9 µm imagery

NESDIS is considering methods of mitigating the stray light issues that occasionally occur in the GOES Imager.

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GOES-13 0.63 µm visible channel (top) and 3.9 µm shortwave IR channel (bottom) images (click to play animation)

McIDAS images of GOES-13 0.63 µm visible channel and 3.9 µm shortwave IR channel data (above; click image to play animation) showed the large smoke plumes and fire “hot spots” (dark black pixels on the shortwave IR imagery) associated with the Germann Road Fire in northwestern Wisconsin and the Green Valley Fire in Minnesota on 14 May 2013. The Germann Road Fire burned 8495 acres, making it the largest wildfire in northern Wisconsin in 33 years. In Minnesota, the Green Valley fire burned 7100 acres.

Items of interest to note on the GOES-13 imagery: (1) the presence of a well-defined lake breeze (lighter gray color enhancement on the IR images) which extended quite a distance inland from the colder waters of Lake Superior (which still exhibited Sea Surface Temperature values in the middle to upper 30s F); (2) the change in wind direction from southwesterly to westerly/northwesterly as a frontal boundary moved eastward across the region; (3) the apparent “flare-up” of the Germann Road Fire as the frontal boundary arrived around 00:45 UTC — the size of the cluster of black “hot spot” pixels increased on the shortwave IR image, concurrent with the rapid growth of an area of pyrocumulus clouds; (4) the eastward motion of the thin lake ice that remained on Mille Lacs in Minnesota (the large lake just south of the Green Valley smoke plume).

2 days after the fire, the burn scar was apparent on an Aqua MODIS false-color Red/Green/Blue (RGB) image (below), viewed using the SSEC Web Map Server. Note the “right turn”on the northern end of the burn scar, caused by a change from southwesterly winds to strong westerly winds in the wake of a frontal passage (which altered the direction of the fire’s progress).

Aqua MODIS false-color image showing wildfire location and burn scar

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GOES-13 0.63 µm visible channel images (click image to play animation)

Strong northwesterly winds along the California coast interacted with the complex terrain and orientation of Monterey Bay to promote the formation of a cyclonic coastal eddy (known locally as an “Otter Eddy”) early in the day on 13 May 2013. McIDAS images of GOES-15 0.63 µm visible channel data (above; click image to play animation) showed the evolution of the eddy feature, which gradually dissipated by the early afternoon hours. “MRY” denotes the location of Monterey.

Farther to the north, an interesting type of “bow shock wave” formed downwind of Point Reyes (labelled “PR” on the images). Better detail of this feature could be seen in an AWIPS image of Suomi NPP VIIRS 0.64 µm visible channel data (below). At the time of this image, surface winds at the offshore buoy just to the north of Point Reyes were gusting to 33 knots (38 mph).

Suomi NPP VIIRS 0.64 µm visible channel image

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Morphed Total Precipitable Water (click image to play animation)

The “cutoff low” system that had been slowly moving across the country for the past week spawned heavy rains which caused flooding in parts of the New York City (NYC) metropolitan area on the morning of 08 May 2013. The image above, of MIMIC Total Precipitable Water, showed a plume of moisture-rich air moving northwestward from the tropical Atlantic towards New York (in advance of the surface frontal system associated with the cutoff low). This region of enhanced precipitable water was seen on the previous day as well. The blended Total Precipitable Water Product (as described here) also showed a plume of higher-than-normal precipitable water air moving over New York City — values of 170+% of normal are over New York City, with a value exceeding 200% (in yellow) sits over the Atlantic Ocean.

GOES-13 6.5 µm water vapor imagery (click image to play animation)

High values of Total Precipitable Water were being been entrained by the circulation of the upper-level low, as shown in the animation of GOES-13 water vapor channel images above. The cyclonic circulation had drawn the moisture north and west into the NYC metropolitan region, and convection developing in the cyclonic flow was responsible for the heavy rainfall. A Suomi/NPP VIIRS 11.45 µm IR image, below, overlain with model-based 500-mb geopotential height fields, showed the strong convection and the cyclonic flow moving into New York. It is interesting to note that the southern tail end of the convection sat right over the Gulf Stream.

Suomi/NPP VIIRS 11.45 µm imagery

The GOES-13 satellite had been placed into Rapid Scan Operations (RSO) mode, providing images as frequently as every 5-10 minutes. Discrete convective cells with cloud-top IR brightness temperatures colder than -60º C (darker red color enhancement) can be seen developing and moving northwestward over the NYC area on 4-km resolution GOES-13 10.7 µm IR channel images (below).

GOES-13 10.7 µm IR channel images (click image to play animation)

A closer view using 1-km resolution POES AVHRR 0.63 µm visible channel and 10.8 µm IR channel images at 10:09 UTC or 6:09 AM local time (below) revealed the texture and shadowing of overshooting tops on the visible image, with cloud-top IR brightness temperature values as cold as -67º C (dark red color enhancement).

POES AVHRR 0.64 µm visible channel and 10.8 µm IR channel images

GOES-13 0.63 µm visible channel imagery (below) showed the different bands of convection that developed offshore and moved inland across the NYC metropolitan area.

GOES-13 Visible Imagery (0.63 µm) (click image to play animation)

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