RADARSAT-2 and RCM-3 satellites had nearly simultaneous overpasses over Typhoon Koinu on 3 October 2023, as shown below. (See this blog post from 2 October for more on Koinu) Both SAR analyses showed the strongest winds (nearly 120 knots) in the southern eyewall of the storm, with long interesting wind minima features (close to 65 knots, dark cyan/green in the enhancement, surrounded by stronger winds, exceeding 80 knots, yellow in the enhancement), threading in towards the storm’s eyewall.

How do the SAR Wind fields compare to Himawari-9 infrared imagery? That is shown in two toggles below (0945 UTC was shortly after sunset over the storm, so visible data aren’t used here). The eye is distinct in the SAR imagery, as it was on 2 October. There is some warming in the center of the storm, and cold cloud tops are apparent in the eyewall — especially over the region of strongest SAR-diagnosed winds. An hour-long animation of the Himawari-9 target scene is below the two toggles.

This RCM-3 SAR wind analysis (from this website) uses the same color scale as this example from this blog post. The side-by-side comparison, below, shows that peak winds have increased since 2122 UTC 01 October, and the areal extent of the winds have also broadened.

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Target Sector (2.5-minute interval) JMA Himawari-9 AHI Visible and Infrared images (above) showed Typhoon Koina as it intensfied to a Category 3 storm by 0000 UTC on 02 October 2023 (SATCON). Cloud-top infrared brightness temperatures were as cold as -90C.

Just prior to the start of the Himawari-9 animation, an image of RCM-3 Synthetic Aperture Radar (SAR) winds at 2122 UTC on 01 October (source) is shown below — which depicted a narrow but fully closed eyewall.

Later in the day, Himawari-9 Infrared Window (11.2 µm) images with an overlay of deep-layer wind shear at 2100 UTC — from the CIMSS Tropical Cyclones site (below) indicated that Koinu was moving through an environment where the shear (around 15 knots) was favorable. In addition, Koinu was traversing warm water (Ocean Heat Content | Sea Surface Temperature).

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NOAA-20 viewed the (mostly clear) Great Lakes twice early in the morning on 2 October 2023, as shown above. Day Night Band imagery shows convection moving from eastern Lake Superior (lightning strokes are apparent as bright smears in the images) into Ontario. (Day Night Band imagery shows great detail owing to a waning gibbous moon with >90% illuminated! [source]) The Advanced Clear-Sky Processing for Oceans (ACSPO) “Sea” surface temperature algorithm applied to VIIRS data shows surface water temperatures peaking at around 70^oF in Lakes Erie and Michigan (yellow in the enhancement used). Lake-wide temperatures are a bit above normal for early October (Lake Michigan ; Lake Erie ; both from this website), a testament to the recent warm weather in the Great Lakes states. VIIRS Day Night Band imagery also reveals river valley fog over Pennsylvania (and elsewhere).

GOES-16 also viewed the convection over Lake Superior. Minimum Flash Area imagery, overlain below on top GOES-16 Band 13 imagery (Click here for an animation of just Band 13 imagery) showed persistent lightning; some of the strokes within the cirrus were quite large in area (purple in the enhancement used).

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The image below compares the 5-minute accumulation of Minimum Flash Area at 0652 UTC with the Day Night Band image — that was scanning over Lake Superior at about that time (as determined from the NOAA-20 orbits at this SSEC site). (Here’s a toggle of the same images shown in the comparison below). The light smears in the Day Night Band image might correspond to two small MFA signals displaced north of the signal in the NOAA-20 image, as might be expected because of a parallax shift. The Day Night Band/GLM MFA toggle for the later overpass, at 0831 UTC, does not show a distinct GLM signal were the Day Night Band shows a bright emission.

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VIIRS imagery and products are available from CIMSS via an LDM feed. You can also view the imagery at the CIMSS VIIRS Imagery Viewer (or at the Direct Broadcast site here).

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Sentinel-1A overflew the country of Savai’i and the south Pacific to its south on 1 October 2023 (Click here to view the scene at the OceanDataLab website; data are also available at NOAA/STAR’s SAR data website). A benefit of the SAR data is its remarkable horizontal resolution, manifest above as very narrow regions of strongest winds (nearly 30 knots, red in the enhancement used) within a broad region of 20-knot winds (green and yellow in the enhancement used). The southern edge of this scene is 16.8^oS, and the arcs of stronger wind start at about 14.3^oS. Metop-B and Metop-C sampled the region on 1 October, as shown below, two Advanced Scatterometer (ASCAT) views from the ascending passes (near 0900 UTC) and descending passes (near 2000 UTC).

The ca. 2000 UTC ASCAT observations shown below don’t really sample the region (between 14^o and 17^o S) sampled by SAR data above. One might be tempted to draw a line from the eastern to western swaths of strongest winds to anticipate winds of 20-25 knots to the south of Samoa, that is, an expansion northward of the observed winds at ca. 0900 UTC shown above. Peak ASCAT winds are 25 knots however, in contrast to the 30-knot SAR observations.

Ascending (morning) and Descending (afternoon) passes on 1 and 2 October, below, show the expansion of strong winds from south of Samoa to north of Samoa. Note also at the end the appearance of weaker winds downwind of Savai’i, Opolu and Tutuila due to the islands blocking the wind.

Consider the horizontal scale of the wind observations you are using as you interpret them.

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