Tropical Storm Sean has developed in the mid-Atlantic and is moving northwest, but is predicted to dissipate by Sunday, 2023-10-15. RealEarth provides an opportunity to look back at the last 48 hours of Sean developing. A video from RealEarth shows GOES-16 true color and Band 13 (10.3 microns, “clean” infrared band) from 2023-10-11 at 1525Z to 2023-10-13 at 1525Z. While signs of convection are apparent on 2023-10-12, the storm structure appears dissipated by the daylight hours of 2023-10-13 when viewing at these wavelengths.

Investigating the finer details of Sean on 2023-10-13, Band 9 (6.95 microns, mid-level water vapor) is examined. A storm structure is hardly noticeable. What stands out in the Atlantic even more in the Band 9 animation is an unnamed disturbance to the East of Sean. (These are the red areas in the Band 9 animation.) The National Hurricane Center predicts this system of having a 10% chance of forming a cyclone in the next 48 hours.

The MIMIC Total Precipitable Water product, below, provides a look back at the last 24 hours of their TPW field. When examining TPW, a more cyclonic structure appears. The TPW field seems to show some actual rotation in Sean, which is harder to notice in the GOES animations.

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The Lightning Imaging Sensor (LIS) has been flying on the International Space Station since 2017 to extend LIS observations that began with the Tropical Rainfall Measurement Mission (TRMM). This provides for nearly global observations optical detection of lightning. On 11 October and 12 October, two passes of the LIS, shown below, observed Typhoon Bolaven: the ascending pass from 2124 to 2134 UTC on 11 October, and the descending pass from 0716 to 0728 UTC on 12 October. What kind of lightning was observed and where was it observed in relation to the Typhoon?

The observations at ca. 2130 UTC on 11 October 2023 are overlain on the AWIPS plot below. Three separate groups are apparent, and we’ll focus on the two more southerly clusters that include 23 separate flash observations.

The southwesternmost cluster has 6 flashes in a region between 20.15^o-20.33^oN and 142.76^o-142.91^oE. The northeastern cluster has 17 flashes near 21.87^oN 144.98^oE. Where are these flashes relative to the storm?

As indicated below, the lightning detected by LIS with this system is in the southwestern eyewall of the storm, and within a vigorous outer band. (For a recent study on the relationship between lightning and storm intensity, click here; an earlier study is here)

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What happened on 12 October? LIS observed 8 lightning flashes between 0718-0720 UTC in a region near 21.85^oN, 143.65^o E.

Where were the lightning observations in relation to the Typhoon? As at 2130 UTC on 11 October, lightning activity was detected right in the eyewall of the storm, as indicated below.

In both cases, LIS observed lightning within the eyewall of Bolaven. Such observations are consistent with the system’s strength.

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The RCM-1 (Radarsat Constellation Mission 1) satellite overflew STY Bolaven at 0840 UTC on 11 October. The Himawari-9 infrared (10.4 µm) presentation of the storm is excellent with a symmetric storm, distinct eye, and vigorous outer band convection. SAR winds show the strongest convection in the eastern half of the eyewall, where winds of 80 m/s (> 150 knots) are measured. (This website contains views of Bolaven from multiple SAR overpasses) The structure of the SAR winds (here‘s a zoomed-in view) shows a band of weaker winds outside the strongest winds on the east side of the storm. Is an eyewall replacement cycle occurring?

The SSEC/CIMSS Tropical Weather website includes a section on Microwave-based Probabilities of an Eyewall Replacement Cycle (M-PERC). The diagram below (click on it to see the entire figure including column headers) shows probabilities reached a maximum after 0300 on 11 October.

The MIMIC-TC animation below, however, shows winds consistently at/above 145 knots in the latter part of the animation, and no obvious signature of an ongoing eyewall replacement.

For more information on this storm, refer to the webpages of the Joint Typhoon Warning Center, the RSMC in Tokyo and the CIMSS Tropical Website. Bolaven is forecast to remain at sea as it recurves to the north and east. It is forecast to become a significant mid-latitude extra-tropical cyclone in the Gulf of Alaska by 18 October.

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Target Sector (2.5-minute interval) JMA Himawari-9 AHI Red Visible (0.64 µm) images (above) and Clean Infrared Window (10.4 µm) mages (below) showed Super Typhoon Bolaven as it reached Category 5 intensity west of the Northern Mariana Islands at 0000 UTC (SATCON) on 11 October 2023. Bolaven was an annular tropical cyclone, exhibiting a symmetric eye surrounded by a broad ring of dense and cold deep convection.

A closer view of Himawari-9 Visible and Infrared mages (below) revealed mesovortices within the eye, as well as the distinct stadium effect of the eye (with its edges sloping outward with height).

Microwave (85 GHz) images from DMSP-16 (at 0818 UTC) and AMSR2 (at 1627 UTC) from the CIMSS Tropical Cyclones site (below) displayed a compact and fully closed eyewall.

A noctrnal Suomi-NPP VIIRS Day/Night Band (0.7 µm) image valid at 1647 UTC (below) showed a pronounced packet of mesospheric airglow waves (reference) propagating southward away from the eye of Bolaven.

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