Four IR Channel from imager with stray light contamination

There are periodic, and predictable, errors within the raw signal on the GOES satellites that arise when sunlight hits the Satellite so that it emits radiation that is detected by the sensor, or when satellite structures reflect energy towards the sensors. There errors usually arise when the Sun is close to being viewed directly by the sensor near “Satellite Midnight”. NOAA/NESDIS has recently (22 February 2012) implemented a series of corrections to mitigate these errors on the GOES-13 Imager. Not only does this increase the number of useable images, but it makes derived products – cloud top pressure, for example – more accurate. Parameters pertinent to the correction are included within Block 0 of the GVAR signal. In McIDAS, these bits relating to the stray light status are included as part of the AREA line prefix.

An example of the error in the raw (or un-corrected) signal is shown at top, with data from the four infrared channels (3.9 6.5, 10.7 and 13.3 micrometers) shown. Note the comparative magnitude of the extra radiation: it is far stronger and more widespread in the 3.9 micrometer image because the sun emits so much more radiation at that wavelength. (The Imager band most affected is the visible band (click here to see two contaminated — and uncorrected — and one clean image), the images above are at night). Options to deal with the stray light errors included: (1) Send all imagery , regardless of solar position/contamination, and let users decide; (2) Cancel images if the sun is within 6 degrees (currently) or 10 degrees of the frame boundary; (3) Scan away from the sun – for example, scan only the Northern Hemisphere if the solar contamination is in the Southern Hemisphere during the Spring eclipse season; and (4) Apply an L1B algorithmic correction to minimize stray light in the images prior to GVAR broadcast. Option (4) has been implemented for GOES-13. Currently option (3) is being implemented for GOES-15.

3.9 micrometer images showing stray light contamination (left) and corrected version (right)

The figure above shows a 3.9 micrometer image with a significant amount of stray light contamination in the southwest part of the image. The corrected version is also shown. Note that the contamination extends throughout the picture – brightness temperatures are too warm even in regions away from the large contamination (over the central United States, for example; compare the brightness temperatures of the cloud tops in the scene). The contaminated 3.9 micrometer data are corrected using two sources of information. For regions outside 6 degrees, the known amount of additional stray light is subtracted from the signal. If the sun is within 6 degrees of the pixel and the stray light signal is overwhelming, signals from the longer wavelength channels are used in combination with the 3.9 micrometer signal to estimate the true 3.9 micrometer signal. Linear relationships between the IR channels will vary with geographical location. Other thermal channel data that contain much less stray light are used in each of 256 geographic bins as input into multiple linear regressions relating 3.9 micrometer data (or 6.5 micrometer data) to 10.7 and 13.3 micrometer data. The hybrid image that results is uniformly cooler with a clear signal in a region formerly overwhelmed by stray light. The algorithm was developed by ITT and implemented by NOAA/NESDIS.

Current plans call for correcting the GOES-15 Imager during the fall 2012 eclipse season.

This ftp site contains more information. The GOES Eclipse schedule is here. This is the ‘White Paper’ on Stray Light. Finally, click here for more information on GVAR.

Finally, here is the notification from SSD that the Stray Light Correction was implemented.

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GOES-15 (left) and GOES-13 (right) 0.63 µm visible channel images (click image to play animation)

Strong winds of 50-60 mph (with a peak gust of 74 mph at Fort Stanton, New Mexico) in the wake of a cold frontal passage caused widespread areas of blowing dust from New Mexico to Kansas on 28 February 2012. One notable feture that was apparent on both GOES-15 (GOES-West) and GOES-13 (GOES-East) 0.63 µm visible channel images (above; click image to play animation) was a long plume of blowing sand originating from the White Sands National Monument located in southern New Mexico. Note how the plumes of blowing dust/sand became easier to identify later in the day on the GOES-13 imagery, as the forward scattering angle increased during the afternoon hours.

A 250-meter resolution Aqua MODIS true color Red/Green/Blue (RGB) image from the SSEC MODIS Today site (below; viewed using Google Earth) revealed how the gypsum sand from White Sands appeared white in color (full-resolution view), in contrast to the light brown colored blowing dust that was seen across the Texas and Oklahoma panhandle regions into southwestern Kansas.

MODIS true color Red/Green/Blue (RGB) image (viewed using Google Earth)

The interaction of the strong winds with the terrain could be seen in a comparison of 1-km resolution MODIS 6.7 µm and 4-km resolution GOES-13 6.5 µm water vapor channel images (below), which revealed a complex pattern of mountain waves across the region.

MODIS 6.7 µm and GOES-13 6.5 µm water vapor channel images

The strong surface winds in tandem with very dry air were creating conditions favorable for wildfire activity — one such fire could be seen in the southern Texas panhandle region in a comparison of 1-km resolution MODIS 3.7 µm and GOES-13 3.9 µm shortwave IR images (below).

MODIS 3.7 µm and GOES-13 3.9 µm shortwave IR images

Additional information and imagery from this event can be found on the Wide World of SPoRT blog.

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Web Map Service mapping of radar returns, watches/warnings, and satellite-detected cloud features

Unseasonably strong thunderstorms in the Piedmont on the East Coast produced a variety of severe weather on February 24th. (Storm reports are here.) The image above was produced by the SSEC/CIMSS web map service at 2006 UTC on 24 February and includes radar reflectivities, watches/warnings, storm reports, and satellite-detected cloud features (black circles indicate overshooting tops; pink circles indicate convective initiation) over the Eastern United States.

0.86 micron imagery from AVHRR and auto-detection of Overshoots from GOES-13

GOES-13 infrared data can be used to detect overshooting tops (see here) that are well-correlated with severe weather at the surface. The loop above shows 0.86-micron imagery from AVHRR at 1914 UTC, with satellite-detected overshooting tops designated by the green thunderstorm icon. As seen here, the tops do overlap a thunderstorm that, at 1915 UTC, was likely producing severe weather. 12-micron brightness temperatures on this top were as cold as -77 C. The automated overshooting top detection algorithm also identified the storm over central South Carolina at 1815 UTC that was producing a tornado in central South Carolina.

GOES-13 11-micron enhanced imagery with auto-detected Thermal Couplet (Enhanced V) indicated by white arrow

More than an hour later, at 2040 UTC, automated satellite detection suggested the presence of a thermal couplet — that is, a warm trench downwind of an overshoot — as shown above. This storm was part of a complex of warned severe storms over eastern Georgia. This storm continued to display tornadic features as it moved northeastward into coastal eastern South Carolina.

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GOES-15 6.5 µm water vapor channel images (click image to play animation)

AWIPS images of GOES-15 6.5 µm water vapor channel data (above; click image to play animation) displayed a signal of strong subsidence around the eastern periphery of a middle-tropospheric ridge of high pressure located over the eastern North Pacific Ocean on 22 February 2012. Note the rapid warming of water vapor brightness temperatures just off the coast of California, reaching abnormally high values of 14º C (darker orange color enhancement) by 21:00 UTC. This rapid middle-tropospheric drying created the appearance of a very strong moisture gradient on the water vapor imagery, being adjacent to the clouds and higher levels of moisture associated with the polar jet stream that was moving southeastward across the Pacific Northwest region of the US.

The magnitude of the dry air within the middle to upper troposphere was very apparent on rawinsonde data from Oakland, California (below).

Oakland, California rawinsonde reports

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