The longwave infrared (10.7 µm) and shortwave infrared (3.9 µm) channels on GOES-13 have been shown in the past to have poor co-registration, meaning that the sensors are not viewing the same pixel at the same time. This error can lead to false signals in (for example) the IR Brightness Temperature Difference product that has historically been used to detect fog and low stratus. The error can propagate to other products as well, such as GOES-R IFR Probabilities (a data fusion product used to detect fog). The error is most obvious along north-south shorelines.

The figure below (Courtesy Tony Schreiner, SSEC/CIMSS) shows differences averaged over 6 pixels near the eastern shore of northern Lake Superior.  The orange line (labeled ‘Original’) represents differences arising from the operational algorithm used before November 2014.  Note that at 0700 UTC for this date and (clear) location (0600 UTC Surface Map) the brightness temperature difference nevertheless showed a negative value because the 10.7 µm pixel was over the cold lake and the 3.9 µm pixel was over warmer land. Shoreline on the western side of the lake would have a positive value: there, the 10.7 µm pixel would be over warm land and the 3.9 µm pixel (co-registered too far to the east) would be over cold water. This positive signal is consistent with fog detection.

In November 2014, NESDIS implemented a fix to mitigate the co-registration issue (Link). One-pixel shifts were applied to the shortwave infrared (3.9 µm) data to force a better alignment. The red line in the figure above (labeled ‘Current Ops’) shows brightness temperature differences that occurred when that fix was used; a one-pixel shift occurred, usually around 0700 and 1700 UTC, and that shift reduced the average error. However, it also introduced a phenomena of fog signals appearing (or disappearing) quickly as the shift occurred. The animation below shows high clouds clearing in a small region over the Lake Michigan shoreline east of Green Bay; a fog signal appears suddenly at 0700 UTC. Similarly, in this toggle over Baja California, fog is indicated on the western coastline of Baja at 0630 UTC (before the pixel shift) and on the eastern/southern coastline of Baja at 0700 UTC (after the pixel shift). Imagery over the St. Lawrence shows fog/low clouds along the south shore of Anticosti Island in the mouth of the St. Lawrence at 0630 UTC; at 0700 UTC, that indication of fog is gone, but it has appeared along the western shore of the St. Lawrence River.

The red line in the figure above also shows a shift at 1700 UTC, and that shift is apparent in data as well, as shown in the brightness temperature difference product, below, north of Lake Superior. The apparent shift occurs in the 1700 UTC image.

Between the 1515 and 1545 UTC imagery on 9 February 2015, a software change was implemented by NESDIS (link), and that now operational software is represented by the green line (labeled ‘Resample’) in the figure above. Rather than a step function change, a smoothly varying change is applied to the co-registration over the course of the day. This has reduced the obvious changes in brightness temperature difference fields that occurred between 0645 and 0700 and between 1645 and 1700 UTC. Consider the two animations below (Courtesy Jim Nelson, SSEC/CIMSS). In both, the former operational technique (the red line in the figure above) is on the left and the current operational technique (the green line in the figure above) is on the right. The operational change has certainly eliminated the jump that was occurring at 0700 and 1700 UTC.

Note that even the green line in the figure up top shows errors approaching 1 C at times during the day (and that may change over the course of the year). It is therefore still possible to find cases in which the brightness temperature difference field from GOES erroneously indicates fog or low stratus. The toggle below shows data from 10 February, after the operational change. A fog/stratus signal is indicated by the GOES Brightness Temperature Difference product along the eastern shore of Lake Michigan; however, there is no signature of fog/stratus on the VIIRS Day Night Band and brightness temperature difference (11.35µm – 3.74µm) imagery from Suomi NPP. As always, a positive indication of a phenomena in data should always be verified with other data types.

[Added, Friday the 13th]: The co-registration error between the longwave and shortwave infrared bands on the GOES-13 Imager is larger than on any of the other Imagers from GOES-8 through GOES-15. For more information, see here and here.

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A sequence of daily Suomi NPP VIIRS Red/Green/Blue (RGB) true-color image composites from the SSEC RealEarth web map server site (above) showed the northeastward transport of African dust across the Mediterranean Sea during the 31 January – 02 February 2015 period. On 02 February, orange snow was observed in Saratov, Russia (news story), a city about 580 miles or 936 km northeast of Stavropol (which is located in the far upper right corner of the VIIRS images).

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Due to a tight pressure gradient between a high over the Yukon and a low over the Gulf of Alaska (surface analysis), strong offshore winds (with gusts as high as 78 mph) were lofting glacial silt from the northern portion of the Alaska Panhandle region and carrying it westward over the Gulf of Alaska on 01 February 2015. Hints of the narrow light grey plumes could be seen streaming southwestward then westward on GOES-15 (GOES-West) 0.63 µm visible channel images (above; click to play animation).

A closer look using a comparison of Suomi NPP VIIRS 0.7 µm Day/Night Band (DNB) and 0.64 µm visible channel images (below) showed that the areal extent of the airborne aerosols was much more evident on the DNB image (in part due to it’s more broad spectral response). However, other more intricate patterns were seen on the DNB image in the general vicinity of Middleton Island (station identifier PAMD) that did not appear to match the character of the airborne glacial silt features being blown westward from the Alaska Panhandle region.

A Suomi NPP VIIRS true-color Red/Green/Blue (RGB) image from the SSEC RealEarth web map server (below) offers a clue to help explain the meandering features that stretched from the coast east of Prince William Sound toward the Middleton Island area: strands of phytoplankton, fed by nutrients in the river waters draining from the interior into the Gulf of Alaska. Sun glint along the edge of the VIIRS scan may have helped to highlight these features in the DNB image above. In fact, these water features were less obvious — and the airborne glacial silt more obvious — in a subsequent VIIRS DNB vs Visible image at 23:20 UTC.

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GOES-13 6.5 µm water vapor channel images (above; click to play animation) showed dry air (brighter yellow to orange color enhancement) moving across the Mid-Atlantic and Southeast regions of the eastern US in the wake of a strong cold frontal passage on the morning of 30 January 2015. There were also numerous pilot reports of turbulence, at both low altitudes (plotted in red) and high altitudes (plotted in cyan).

The most obvious feature seen on the GOES-13 water vapor images was the “rippled” signature of mountain waves, which extended far to the lee (southeast) of the Appalachian Mountains (the topographical obstacle to the strong northwesterly boundary layer flow that was causing the waves to initially form). A comparison of 4-km resolution GOES-13 6.5 µm water vapor and 1-km resolution Aqua MODIS 6.7 µm water vapor images (below) demonstrated the benefit of higher spatial resolution for diagnosing the areal coverage of such small-scale mountain waves. Of special note is the pilot report of “severe to extreme” turbulence at 4000 feet over South Carolina.

A comparison of the MODIS 6.7 µm water vapor channel image with the corresponding MODIS 0.65 µm visible channel image (below) showed that the severe to extreme reports in North and South Carolina were examples of Clear Air Turbulence (CAT), since no clouds were apparent in those areas at the time.

Regarding the numerous high-altitude pilot reports of moderate to severe turbulence, the NAM80 model depicted a 120-knot jet streak over South Carolina at 12:00 UTC, with another 120-knot jet streak approaching from the middle Mississippi Valley region (below). Note that there was strong wind speed shear to the north of the jet stream axis, which is where the bulk of the pilot reports of turbulence were located. Quite often there is an obvious moist-to dry gradient water vapor signature along or just poleward of a strong jet streak axis — but such a signature was not seen with this particular event.

In response to some of these pilot reports, at 16 UTC a SIGMET (SIGnificant METeorological advisory) was issued for occasional severe turbulence due to jet stream wind shear (below).

4-panel images showing the three GOES-13 Sounder water vapor channels (6.5 µm, 7.0 µm, and 7.4 µm) along with the conventional GOES-13 Imager 6.5 µm water vapor channel (below; click to play animation) showed how each channel helped to identify where the pockets of middle-tropospheric dry air were located.

The GOES-13 water vapor channel weighting functions plotted using data from the 12 UTC rawinsonde reports from Roanoke/Blacksburg, Virginia and Greensboro, North Carolina are shown below. Due to the very dry middle to upper troposphere, the water vapor channels were able to sense features farther down into the atmosphere than is usually the case — this is illustrated by the relatively low altitude of the water vapor weighting function peaks.

Compare the 2 examples above with the altitude peaks of the various GOES-13 Sounder and Imager water vapor channels under “normal” conditions, plotted using the US Standard Atmosphere as the sounding profile (below).

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