Wildfire in Alaska

May 1st, 2019 |

GOES-17 Shortwave Infrared (3.9 µm) and "Red" Visible (0.64 µm) images [click to play animation | MP4]

GOES-17 Shortwave Infrared (3.9 µm) and “Red” Visible (0.64 µm) images [click to play animation | MP4]

On 01 May, GOES-17 (GOES-West) Shortwave Infrared (3.9 µm) and “Red” Visible (0.64 µm) images (above) showed the thermal anomaly (or fire “hot spot”) and dispersion of smoke from the first moderate-size wildfire of 2019 in the Interior of Alaska — the Oregon Lakes Impact Area Fire about 7 miles southwest of Fort Greely. This fire grew from 30 acres to 4000 acres in a 24-hour period, aided by warm daytime temperatures with low relative humidity values and southwest winds late in the day on 30 April (surface data). The Oregon Lakes Impact Area Fire was burning in a remote area just west of the Delta River which was previously burned by the 2013 Mississippi Fire; that area also contained unexploded ordnance dropped by military aircraft during training exercises.

A toggle between Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images at 1216 UTC or 4:16 am local time (below) revealed the nighttime glow of the fire, along with a more accurate depiction of the size and location of the thermal anomaly.

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images [click to enlarge]

Although the color enhancements were different, a comparison of Shortwave Infrared images from Suomi NPP (3.74 µm) at 1216 UTC and GOES-17 (3.9 µm) at 1220 UTC (below) demonstrated the advantage of imagery from polar-orbiting satellites at high latitudes. In this example, the 375-meter resolution VIIRS image showed 2 distinct fire hot spots that were not apparent in the lower spatial resolution — 2 km at nadir, decreasing to about 4 km over Alaska — GOES-17 image.

Shortwave Infrared images from Suomi NPP (3.74 µm) and GOES-17 (3.9 µm) [click to enlarge]

Shortwave Infrared images from Suomi NPP (3.74 µm) and GOES-17 (3.9 µm) [click to enlarge]

A larger-scale view of GOES-17 Shortwave Infrared and Visible images from 02-04 UTC on 02 May (below) showed the fire as it exhibited its peak 3.9 µm infrared brightness temperature (51.3ºC or 324.5 K at 0210 UTC) and the smoke plume had drifted over 100 miles to the southeast, moving over Beaver Creek, Yukon (CYXQ). While most of the smoke was apparently lofted above the boundary layer, the surface visibility at Fort Greely PABI was briefly reduced to 6 miles at 09 UTC or 1am local time on 02 May. Note the lack of “false cold pixels” adjacent to the warmest 3.9 µm pixels — this is due to a recent change to the GOES-R ABI Band 7 resampler, as detailed in this blog post.

GOES-17 Shortwave Infrared (3.9 µm) and "Red" Visible (0.64 µm) images [click to play animation | MP4]

GOES-17 Shortwave Infrared (3.9 µm) and “Red” Visible (0.64 µm) images [click to play animation | MP4]

A comparison of Visible and Shortwave Infrared images from GOES-17 and GOES-15 (below) highlighted the improved fire detection and monitoring capability of the new GOES-R series. The higher spatial resolution (0.5 km vs 1.0 km at nadir for Visible, and 2 km vs 4 km at nadir for Shortwave Infrared) and more frequent image scans (10 minutes for GOES-17 Full Disk vs 15-30 minutes for GOES-15 CONUS sector) along with better Image Navigation and Registration (INR) were especially valuable at the higher latitudes of Alaska. For example, the subtle behavior of the fire’s smoke column vertical jump at 2350 UTC was only apparent in the GOES-17 Visible imagery.

GOES-17 Visible (0.64 µm, top left), GOES-15 Visible (0.63 µm, top right), GOES-17 Shortwave Infrared (3.9 µm, bottom left) and GOES-15 Shortwave Infrared (3.9 µm, bottom right) images [click to play animation | MP4]

GOES-17 Visible (0.64 µm, top left), GOES-15 Visible (0.63 µm, top right), GOES-17 Shortwave Infrared (3.9 µm, bottom left) and GOES-15 Shortwave Infrared (3.9 µm, bottom right) images [click to play animation | MP4]

Since the fire was also located within the GOES-17 Mesoscale Domain Sector #2, 1-minute imagery provided an even better depiction of the fire’s smoke column vertical jump and downstream smoke transport (below).

GOES-17 Visible (0.64 µm, top left), GOES-15 Visible (0.63 µm, top right), GOES-17 Shortwave Infrared (3.9 µm, bottom left) and GOES-15 Shortwave Infrared (3.9 µm, bottom right) images [click to play animation | MP4]

GOES-17 Visible (0.64 µm, top left), GOES-15 Visible (0.63 µm, top right), GOES-17 Shortwave Infrared (3.9 µm, bottom left) and GOES-15 Shortwave Infrared (3.9 µm, bottom right) images [click to play animation | MP4]

Change to the GOES-R ABI Band 7 (3.9 µm) Resampler

May 1st, 2019 |

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)

GOES-R Band 2 (“red visible”) Calibration Changes

May 1st, 2019 |

GOES-16 ABI Band 2 Visible Imagery (0.64 µm) at 1811 UTC on 21 April (before the calibration change) and at 1811 UTC on 25 April 2019 (after the calibration change) in and around Pima County (outlined in black) in southern Arizona (click to enlarge).

On 23 April 2019, a Ground Systems update resulted in a change to the ‘brightness’ (in the form of dimming) of the GOES-16 Band 2 (0.64 µm) visible imagery (as noted in this calibration events log).  The  calibration coefficients used for this band were determined in the lab before the launch. (Other calibration information is collected on-orbit.) GOES-R Advanced Baseline Imager (ABI) visible imagery have been compared to visible imagery from polar-orbiting satellites in the past several years (see this page), and GOES-R ABI Band 2 (0.64 µm) radiances were consistently larger than measurements from Suomi NPP and NOAA-20 using the Visible-Infrared Imaging Radiometer Suite (VIIRS).

ABI calibration coefficients for GOES-16 were modified on 23 April, using plausible values from pre-launch lab measurements. Thus, Band 2 radiances decreased by about 6.9%. This will have an impact on the computed albedo as shown above: the later date (after the calibation change) is slightly darker than the earlier (before the calibration change). This change means that Band 2 radiances and albedo are more closely aligned with values from other satellites. A similar toggle over Texas is shown below.

Note: A similar change for GOES-17 was implemented at 1540 UTC on 27 April 2019.

GOES-16 ABI Band 2 Visible Imagery (0.64 µm) at 1811 UTC on 21 April (before the calibration change) and at 1811 UTC on 25 April 2019 (after the calibration change) in and around Fayette County, TX (outlined in black), just west of metropolitan Houston (click to enlarge).