PyroCb

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

The Yarnell Hill Fire (InciWeb) was a relatively small wildfire that was started by lightning from a dry thunderstorm southwest of Prescott, Arizona on 28 June 2013. However, fire conditions became more favorable for rapid growth on 30 June, as surface air  temperatures rose above 100 F across the area with low relative humidity values. During the afternoon hours, GOES-15 0.63 µm visible channel images (above; click image to play animation; also available as a QuickTime movie) showed that a line of thunderstorms developed over northwestern Arizona, and moved toward the southwest (the red circle highlights the general area of the Yarnell fire). It is likely that strong surface winds associated with a thunderstorm outflow boundary (nearby surface mesonet data) caused rapid growth and an abrupt change in direction of the fire, which tragically killed 19 firefighters who attempted to shelter in place (for additional details, see the Wildfire Today site).

On the GOES-15 visible imagery, a smoke plume became more obvious after 16:45 UTC, with the first formation of pyrocumulus clouds evident at 21:00 and 21:30 UTC. As the cloud shield of the thunderstorm line moved over the fire, the images revealed the development of a pyrocumulonimbus (pyroCb) cloud which exhibited a pronounced overshooting top at 23:45 UTC.

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

Taking a look at the period of pyroCb formation, the overshooting plume and pronounced overshooting top could be seen spreading southward (due to northerly winds aloft, as shown on Flagstaff AZ rawinsonde data) on the GOES-15 0.63 µm visible images (above) — and this pyroCb plume and overshooting top appeared warmer/darker on GOES-15 3.9 µm shortwave IR images (below), which indicated that the plume was comprised of smaller particles which were more efficient reflectors of solar radiation.

GOES-15 3.9 µm shortwave IR images (click image to play animation)

On GOES-15 10.7 µm longwave IR images (below), the coldest cloud top IR brightness temperature of -65 C (darker red color enhancement) was associated with the overshooting top at 23:45 UTC.

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

Shown below is a comparison of the 23:45 UTC images of GOES-15 visible, shortwave IR, and longwave IR images.

Comparison of 23:45 UTC GOES-15 visible, shortwave IR, and longwave IR images

A comparison of Suomi NPP 0.64 µm visible channel and 11.45 µm IR channel images at 21:22 UTC (below) showed the Yarnell fire “hot spot”  (dark black pixels), with some pyrocumulus clouds beginning to form to the east/northeast of the fire source (due to strong southwesterly winds in the boundary layer).

Suomi NPP VIIRS 0.64 µm visible and 11.45 µm longwave IR images

The corresponding Suomi NPP VIIRS true-color image is shown below, visualized using the SSEC Web Map Server. Again, the smoke plume from the fire can be seen, along with the development of pyrocumulus clouds to the east/northeast.

Suomi NPP VIIRS true-color image

Flagstaff, Arizona WSR-88D radar base reflectivity and volume of 13.5 dBZ reflectivity isosurface (click image to play animation)

Volumetric displays of Flagstaff, Arizona WSR-88D radar base reflectivity and spectrum width are shown above and below, respectively (radar data visualized using GR2Analyst software, courtesy of Jordan Gerth, CIMSS). The viewing perspective is looking from the northwest, so Prescott is located in the left corner of the data cube, and the Yarnell fire with its growing pyrocumulus (pyroCu) and pyrocumulonimbus (pyroCb) cloud is located in the right corner of the data cube.

The blue-shaded isosurface of the 13.5 dBZ base reflectivity (from 22:42 to 23:48 UTC) showed the upward pulsing of the pyroCu and finally the pyroCb, which grew upward past an altitude of 40,000 feet on the final image (23:48 UTC, about the time that the prominent overshooting top was seen on the 23:45 UTC GOES-15 visible image).

The Doppler radar spectrum width is shown from 23:34 to 23:48 UTC. This parameter represents the amount of variance in the velocity field. Note the higher spectrum width values (darker orange shading) associated with the growth of the pyroCb cloud over the Yarnell fire — this was likely a result of the variety of particles (biomass burning particles, supercooled water droplets, ice crystals) moving upward at different velocities because of their differing size and shape characteristics.

Flagstaff, Arizona WSR-88D radar spectrum width (click image to play animation)

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

McIDAS images of 1-km resolution GOES-15 (GOES-West) and GOES-13 (GOES-East) 0.63 µm visible channel data (above; click image to play animation) showed a pyrocumulonimbus (pyroCb) cloud from the West Fork fire complex rapidly penetrating the tops of surrounding orographically-induced cumulonimbus (Cb) clouds, beginning around 20:10 UTC or 2:10 PM local time on 28 June 2013. This pyroCb cloud was located just northwest of Wold Creek Pass (station identifier KCPW). The GOES-13 satellite had been placed into Rapid Scan Operations, providing twice as many images during this short time period compared to GOES-15.

A timely overpass of the Suomi NPP satellite allowed a comparison of AWIPS images of 1-km resolution VIIRS 0.64 µm visible, 1.61 µm near-IR, 3.74 µm shortwave IR, and 11.45 µm longwave IR channel data at 20:17 UTC (below). Some important observations include: (1) as seen on the GOES visible imagery, the penetrating pyroCb cloud had a darker gray appearance compared to the adjacent brighter white Cb cloud tops; (2) a number of negative and positive cloud-to-ground lightning strikes were associated with the adjacent Cb clouds; (3) the pyroCb cloud appeared much warmer (darker) than the surrounding convective clouds, due to solar reflection off of much smaller cloud particles — the warmest 3.74 µm IR brightness temperature was +31º C; (4) the pyroCb cloud exhibited much colder 11.45 µm IR brightness temperatures than the surrounding Cb clouds, with a minimum of -71º C.

Suomi NPP VIIRS 0.64 µm visible, 1.61 µm near-IR, 3.74 µm shortwave IR, and 11.45 µm longwave IR channel images

GOES-13 Visible and Shortwave IR Images (Click to Play Animation)

The Silver Fire continued to burn on June 27, 2013 in Southwestern New Mexico.  GOES-13  0.63 micrometer visible imagery (above, left panel; click image to play) show a smoke plume erupting from the desert during the late morning and early afternoon hours of the 27th.  After the smoke plume is evident, a large pyrocumulonimbus forms over the hotspot and drifts south-southwestward along the mean flow.  Also, GOES-13 3.9 micrometer shortwave IR imagery (above, right panel; click image to play) showed a well defined hot spot before and during the formation and growth of the pyrocumulonimbus cloud.  Note the proliferation of red pixels during the late morning and early afternoon hours in the IR imagery.  This is due to the ground temperatures reaching extreme levels in parts of New Mexico and not due to fires.

Corresponding GOES-13 10.7 micrometer longwave IR imagery (below; click image to play) showed the development of a pyrocumulonimbus cloud during the late afternoon hours of June 27, 2013.  This animation showed cloud tops reaching -50 degrees Celsius during the peak of the pyrocumulonimbus growth.  The debris from the pyroCB later drift southward and reach temperatures as cold as -60 degrees C.

GOES-13 Visible and Shortwave IR Images (Click to Play Animation)

The West Fork Fire continued to burn on June 26, 2013 in South Central Colorado.  This fire had its origins three weeks earlier on June 5, 2013.  GOES-13  0.63 micrometer visible imagery (above, left panel; click image to play) show a smoke plume erupting from the mountain fire during the early afternoon hours of the 26th.  After the smoke plume is evident, a large pyrocumulonimbus forms over the hotspot and drifts south-southeastward along the mean flow.  Also, GOES-13 3.9 micrometer shortwave IR imagery (above, right panel; click image to play) showed a well defined hot spot before the formation and growth of the pyrocumulonimbus cloud.

Corresponding GOES-13 10.7 micrometer longwave IR imagery (below; click image to play) showed the development of a pyrocumulonimbus cloud during the late afternoon hours of June 26, 2013.  This animation showed cloud tops reaching -60 degrees Celsius during the peak of the pyrocumulonimbus growth.

GOES-13 Longwave IR Images (Click to Play Animation)