Another PyroCb in Alaska

On 20 June according to the Alaska Interagency Coordination Center the Blair Fire started in the interior of Alaska (south of Fairbanks) at 64.3º N 147.3º W. On 24 June at 20:15 UTC a pyroCb formed from this fire. GOES-15 detected the smoke plume and pyroCb cloud, as well as the fire hot spot. Starting at 19:00 UTC on 24 June, the animation below shows visible (.63 μm) on the left and shortwave IR (3.9 μm) on the right (click image to play animation). In the shortwave IR images the darker black to red pixels indicate very hot IR brightness temperatures exhibited by the fire source region.

GOES-15 0.63 μm visible (left) and 3.9 μm shortwave IR (right) images (click to play animation)

GOES-15 0.63 μm visible (left) and 3.9 μm shortwave IR (right) images (click to play animation)

Also, using GOES-15 10.7 μm IR channel imagery the cloud-top IR brightness temperature could be determined. The animation below, starting at 19:00 UTC on 24 June, shows the brightness temperature of this pyroCb reaching -53.6ºC (lime green/yellow color enhancement) around 21:45 UTC.

GOES-15 10.7 μm IR images (click to play animation)

GOES-15 10.7 μm IR images (click to play animation)

A timely overpass of the Suomi NPP satellite provided a good view of the pyroCb cloud very soon after its formation — a comparison of 20:18 UTC VIIRS 0.64 µm visible channel, 3.74 µm shortwave IR channel, and 11.45 µm IR channel images is shown below. On the visible image, the tall pyroCb cloud casts a shadow onto the dense layer of smoke below; the fire hot spot (black to yellow to red pixels) can be seen along the southeastern edge of the cloud; the minimum cloud-top IR brightness temperature was -56º C, which corresponded to an altitude of approximately 10.8 km (using data from the 00 UTC Fairbanks sounding). At the time of the VIIRS images, smoke was reducing the surface visibility at Fairbanks (PAFA) to 3/4 of a mile, and to the west-northwest at Tanana (PATA) the occasional fall of wildfire ash was being reported at the surface.

Suomi NPP VIIRS 0.64 µm visible, 3.74 µm shortwave IR, and 11.45 µm IR images

Suomi NPP VIIRS 0.64 µm visible, 3.74 µm shortwave IR, and 11.45 µm IR images

A sequence of daily Suomi NPP VIIRS true-color image composites from 18 June to 24 June (below) showed a dramatic increase in the areal coverage of dense wildfire smoke over Alaska during that period. A post on the Wildfire Today site mentioned the high number of lightning strikes that had occurred over the region in recent days, with a record  of over 14,000 lightning strikes in a single day on 21 June. A closer view of the 24 June VIIRS true-color image with an overlay of AVHRR-detected fire hot spots can be seen here. Due to the dense smoke , the Air Quality Index at Fairbanks on 24 June was 242 (on a scale of 0-500), placing it in the “Very Unhealthy” category.

Daily composites of Suomi NPP VIIRS true-color images during the 18 June - 24 June 2015 period

Daily composites of Suomi NPP VIIRS true-color images during the 18 June – 24 June 2015 period

Regarding the “smoke vortex” seen over northern Alaska on the 24 June VIIRS images above, GOES-15 0.63 µm visible channel images (below; click to play animation) showed the development and motion of this feature during the day. A pair of fires seemed to intensify in the southern foothills of the Brooks Range, north-northeast of Fort Yukon (PFYU), as the vortex passed — perhaps this was a boundary layer feature that helped to increase surface winds in its wake.

GOES-15 0.63 µm visible channel images (click to play animation)

GOES-15 0.63 µm visible channel images (click to play animation)

On 24 June OMPS Aerosol Index (AI) images were useful to see the transport of the smoke. From the AI image (below; courtesy of Colin Seftor and Rene), it can be seen that the smoke was being transported eastward. Region 1 has a max value of 3.6 which is consistent with the time and location of the max value of 7.5 on 23 June. These values both trace back to the region where five pyroCbs developed in Alaska between 21:45 UTC on 22 June and 07:30 UTC 23 June. Region 2 has a max value of 2.9 and is from the northern British Columbia pyrocb. Region 3 has a max value of 7.2 and is from the pyroCb produced by the Blair fire that is discussed above. Region 4 has the highest value on the map at 7.8 but can’t be traced directly back to a source, most likely from the numerous fires burning close by.

OMPS Aerosol Index image on 24 June (click to enlarge)

OMPS Aerosol Index image on 24 June (click to enlarge)

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