Time-Resolved Observations of Precipitation structure and storm Intensity with a Constellation of Smallsats (TROPICS) Pathfinder imagery from 0545 UTC on 26 September, when super typhoon Mindulle was near peak intensity, is compared above to Himawari-8 visible(0.64 µm) imagery at about the same time. A separate image links small features in the Pathfinder image to small convective elements that are apparent in the Himawari imagery. Click here to view the TROPICS Pathfinder image with a NOAA-20 true-color image from 0426 UTC.

The pathfinder satellite that provided the microwave data used for the image above is the first in a series of a constellation of low-Earth orbiters; six additional satellites will be launched next year. These are very small satellites, with a size of 10 cm x 10 cm x 36 cm. They weigh in at 5.34 kg / 11.8 pounds! Pathfinder imagery was provided courtesy of the Science Team working with the data. Himawari-8 imagery are courtesy of JMA.

As noted above, NOAA-20 overflew Mindulle at about 0430 UTC. The Advanced Technology Microwave Sounder (ATMS) instrument on NOAA-20 sampled the storm, and imagery (88.2 GHz and 183.3 GHz) with a timestamp of 0435 UTC (from this archive) is shown below. The NOAA-20 orbits over the western Pacific on that day are shown here (from this site). Structures in the Pathfinder imagery at 0545 UTC can be identified in the 0435 UTC ATMS imagery below. A side-by-side comparison of the Pathfinder 205 GHz and NOAA-20 ATMS 183.3 GHz is shown at bottom.

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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]

===== 30 September Update =====

GOES-16 “Red” Visible (0.64 µm) images (credit: Tim Schmit, NOAA/NESDIS/ASPB) [click to play animation | MP4]

Sam was still a Category 4 hurricane on 30 September — and GOES-16 Visible images (above) showed that Sam exhibited a relatively compact eye, with hints of mesovortices within the eye.      

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Theresa was a short-lived subtropical storm in the Atlantic Ocean north of Bermuda late on the 24th and early on the 25th of September. When Suomi NPP overflew the storm early on the 25th (imagery from this source), the low-level circulation was devoid of any significant convection; convection was displaced to the northeast of the storm. The toggle below of True-color imagery from CSPP Geosphere shows the storm structures at 1800 UTC on the 24th and 25th. Convection was nearly wrapped around the subtropical storm on the 24th; it was mostly absent near the storm center on the 25th.

Note in the Day Night Band imagery at top that a dark region exists within the moonglint at around 29 N Latitude. Such features, discussed in this blog before (here and here, for example), suggest very light winds; an HY-2C overpass at 0330 UTC on 25 September, shown below (from this site; click here to see a similar image with wind flags), shows very light winds over/around 30 N.

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Tropical Storm Sam is poised to strengthen in the tropical Atlantic. The True-Color image above shows the cyclone just to the south of a region of dust that is apparent in the true-color imagery as a greyish/brown region extending from the storm towards Africa. A Saharan Air Layer (SAL) analysis, below (from this site), also shows this region of dry air to the east of the storm. Sam is not projected to be influenced by the dry air that is following it.

Both 200-850 mb shear and SSTs, shown in a toggle below, will not inhibit strengthening. Shear is weak, and waters are warm. An ASCAT pass from MetopC shortly before 1200 UTC on 23 September (from this NOAA OSPO site) is shown here. A slightly later pass from MetopB (from the OSI SAF site) is shown here.

The National Hurricane Center projects Sam to become a Major Hurricane. Refer to the National Hurricane Center for more information.

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