Change to the GOES-R ABI Band 7 (3.9 µm) Resampler
GOES-17 3.9 µm imagery around a fire at 23:30 UTC on 17 February 2019 with the former interpolation scheme (left), the updated interpolation scheme (center) and the difference field between the two (right). The yellow box shows the approximate fire location over Mexico. (Image courtesy Chris Schmidt, CIMSS)
GOES-R Advanced Baseline Imagery (ABI) detections must be interpolated from the detector grid on the satellite to a grid that is fixed and geographically referenced. This is accomplished by applying a truncated sinc function in both north-south and east-west directions to the data on the detector grid. Sinc functions include small negative tails adjacent to the large central maximum; for fifteen out of sixteen ABI bands, those subtractions are not detectable. For Band 7, however, the shortwave infrared band at 3.9 µm, the ABI band with the largest dynamic range (and 14 bits of information), the interpolation from detector space to the fixed grid pixel can introduce negative values of radiances and careful observers have seen Cold Pixels Around Fires, the so-called CPAF effect.
An improved interpolation for Band 7 only has been implemented (on 23 April for GOES-16 and on 18 April for GOES-17) in the GOES-R Ground System that reduces the negative tail in the Truncated Sinc function. In the single image above, from GOES-17 at 23:30 UTC on 17 February, the “old” truncated sinc function (denoted ‘Original’ in the image) has generated a falsely cold pixel — white in the greyscale enhancement — off the southeast corner of the warm pixels shown in black. The cold pixels are not present when the new, improved interpolation scheme is used. Note, however, that the Data Max annotated in the image has cooled by 2K with the improved interpolation; a fire is nevertheless obvious.
Consider the animation below, for example, (from this blog post on the Cranston fire), that used the ‘old’ interpolation scheme. Cold pixels (in white) occasionally appear around the periphery the fire (in red) in the center of the image. The new interpolation means that such cold pixels will no longer appear in the data.
GOES-16 ABI visible imagery (0.64 µm) and shortwave infrared imagery (3.9 µm) over the Cranston fire, 1842 UTC on 25 July 2018 to 0227 UTC on 26 July 2018 (Click to enlarge)
The image below shows a fire at 1641 UTC on 29 April 2019, after the CPAF change was implemented into the GOES-R Ground System (two different enhancements are shown). No artificial cold pixels are present. The hottest pixel is 405 K, which would have produced a CPAF under the original truncated sinc kernel.
GOES-16 3.9 µm Imagery at 16:41 UTC on 29 April 2019 (Image courtesy Chris Schmidt, CIMSS)(Click to enlarge)