Pyrocumulonimbus cloud over northern Paraguay

September 29th, 2021 |

GOES-16 Visible (0.64 µm, top), Shortwave Infrared (3.9 µm, center) and Infrared Window (10.35 µm, bottom) images [click to play animation | MP4]

Fires burning in far northern Paraguay on 29 September 2021 created a pyrocumulonimbus or pyroCb cloud — GOES-16 (GOES-East) “Red” Visible (0.64 µm), Shortwave Infrared (3.9 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed the pyroCB cloud, fire thermal anomalies or “hot spots” (clusters of red pixels) and cold cloud-top infrared brightness temperatures, respectively. The minimum 10.35 µm temperature was -47.6ºC at 1840 UTC. Note the relatively warm (darker gray) appearance of the pyroCb cloud in the 3.9 µm images — this is a characteristic signature of pyroCb cloud tops, driven by the smoke-induced shift toward smaller ice particles (which act as more efficient reflectors of incoming solar radiation).

A Suomi-NPP VIIRS Infrared Window (11.45 µm) image at 1751 UTC as viewed using RealEarth (below) revealed cloud-top infrared brightness temperatures in the -60s C (shades of red). Surface temperatures at nearby sites had reached 38ºC (100ºF) by 18 UTC. 

Suomi-NPP VIIRS Infrared Window (11.45 µm) image at 1751 UTC [click to enlarge]

South American pyrocumulonimbus clouds are fairly uncommon — since the first documented case in 2018, only 7 other pyroCbs have been identified over that continent. 

Thanks to Mike Fromm, NRL, for alerting us to this latest pyroCb case. Additional information is available from Metsul.com.

Hurricane Sam reaches Category 4 intensity

September 25th, 2021 |

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

1–minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed Hurricane Sam as it intensified from a Category 3 to a Category 4 storm (ADT | SATCON) in the central Atlantic Ocean on 25 September 2021. The eye became cloud-filled during the middle portion of the day, but Visible images revealed the presence of mesovortices within the eye both early and late in the day.

A DMSP-16 SSMIS Microwave (85 GHz) image at 1918 UTC from the CIMSS Tropical Cyclones site (below) displayed a fully closed eyewall, with several spiral bands wrapping inward toward the storm center.

DMSP-16 SSMIS Microwave (85 GHz) image at 1918 UTC [click to enlarge]

GOES-16 Infrared images with an overlay of deep-layer wind shear at 2200 UTC (below) indicated that Sam was in an environment of low shear — which favored intensification as the hurricane moved across relatively warm water (SST | OHC).

GOES-16 Infrared images, with an overlay of deep-layer wind shear at 2200 UTC [click to enlarge]

During the following nighttime hours, ample illumination from the Moon — which was in the Waning Gibbous phase, at 81% of Full — provided a “visible image at night” using the Suomi NPP VIIRS Day/Night Band (0.7 µm) (below).

Suomi NPP VIIRS Day/Night Band (0.7 µm) image [click to enlarge]

===== 26 September Update =====

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4

On the following day, Sam exhibited a similar appearance on 1-minute GOES-16 Infrared and Visible images (above), with a small 7-15 mile diameter eye (containing mesovortices, as seen in Visible imagery). Both Infrared and Visible images revealed repeated pulses of gravity waves propagating away from the storm center. Sam’s intensity peaked at 135 knots late in the day (NHC advisory).

1-minute GOES-16 Visible images with plots of corresponding GLM Flashes (below) showed that Sam exhibited an Enveloped Eyewall Lightning signature (reference).

1-minute GOES-16 “Red” Visible (0.64 µm) images, with 1-minute GLM Flashes plotted in red [click to play animation | MP4]

Rope cloud feeding into Tropical Invest 98L

September 22nd, 2021 |

GOES-16 True Color RGB images [click to play animation | MP4]

GOES-16 (GOES-East) True Color RGB images created using Geo2Grid (above) showed a long rope cloud feeding into the western portion of Tropical Invest 98L in the eastern Atlantic Ocean on 22 September 2021. The rope cloud was located near the African Monsoon Trough (12 UTC surface analysis), and north of the rope cloud the hazy appearance of a dust-laden Saharan Air Layer was apparent — so this rope cloud feature likely marked the boundary between dry SAL air to the north and moist tropical air to the south (as seen in the MIMIC Total Precipitable Water product). In addition, Metop-A ASCAT surface scatterometer winds at 0956 UTC and 2104 UTC showed speed and/or directional convergence in the vicinity of the rope cloud (between 10-15 N latitude). Also of interest was the effect of the Cabo Verde islands on marine boundary layer clouds beneath the SAL. On the following day, Invest 98L intensified to become Tropical Storm Sam, as discussed in this blog post.

The hazy Saharan Air Layer was also seen in a Suomi NPP VIIRS True Color image at 1439 UTC, as visualized using RealEarth (below). The west-to-east oriented rope cloud was located around 11-12 N latitude.

VIIRS True Color RGB image from Suomi NPP [click to enlarge]

The “Saharan Air Layer” (SAL) Split Window Difference product from the CIMSS Tropical Cyclones site (below) depicted the westward advance of the dusty SAL air off northwestern Africa.

GOES-16 “Saharan Air Layer” Split Window Difference product [click to play animation]

Sheared Tropical Storm Peter

September 20th, 2021 |
Suomi NPP Day Night Band visible (0.7 µm) imagery and GOES-16 infrared (10.3 µm) imagery, 0620 UTC on 20 September 2021 (Click to enlarge)

Suomi NPP Day Night imagery, in a toggle above with infrared imagery from GOES-16, (both displayed in RealEarth) depicts a classic sheared Tropical Cyclone structure to the east-northeast of the Caribbean Sea. The low-level circulation (very apparent in the nighttime visible imagery under the illumination of a near-full moon) is displaced to the west of the deep convection near the edge of this Suomi NPP scan. Indeed, the low-level circulation is difficult to discern in the single enhanced infrared image (animation better reveals its structure however). An earlier (0527 UTC) overpass from NOAA-20, below (VIIRS imagery courtesy William Straka, CIMSS), also shows the circulation to the west of the main convection. It would be a challenge to locate correctly the storm center based solely on the VIIRS infrared imagery!

NOAA-20 VIIRS I05 (11.5 µm) and Day Night Band Visible (0.7 µm) imagery at 0527 UTC on 20 September 2021 (Click to enlarge)

The shear that is disrupting TS Peter’s structure is shown below (imagery from the SSEC Tropical website), overlain on top of 1-km visible imagery at 1300 UTC.

GOES-16 Visible (0.64 µm) Imagery, 1300 UTC, along with an analysis of 200-850 mb wind shear (Click to enlarge)

For more information on struggling Tropical Storm Peter, visit the website of the National Hurricane Center.