Himawari-9 Upper Level Water Vapor infrared (6.25 µm) imagery for the 8 hours ending 1300 UTC on 8 February 2023 show the nighttime blossoming of convection near the center of developing tropical cyclone Gabrielle to the east of Australia in the tropical southwest Pacific. The environment of the storm is favorable to strengthening: the Band 8 imagery above shows little dry air near the storm center, and a toggle with the Band 10 (7.34 µm) infrared imagery at 1200 UTC similarly shows little dry air.

MIMIC Total Precipitable Water fields (from this site) also show an atmosphere rich in moisture where Gabrielle is developing.

Analyses from the CIMSS Tropical Website, show an environment that supports development. SSTs in the region are warm, shear values are low, low-level convergence and upper level divergence are all strong, as shown in the toggle below. Indeed, many numerical models predict Gabrielle to strengthen as it moves southeastward through the Coral Sea between Australia to the west and New Caledonia to the east.

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Added: 10 February. RCM-1 overflew the western half of Gabrielle at 0800 UTC on 10 February, and the SAR Wind analysis (the colorbar ranges from 0-60 m/s), below, shows maximum wind speeds a bit stronger than 30 m/s.

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Predominantly clear skies and a waning Full Moon (the Full Moon was on 5 February) allowed for both Day Night Band imagery of the surface and Advanced Clear-Sky Processing for Ocean (ACSPO) sea-surface temperature (SST) fields using VIIRS data downloaded at the CIMSS Direct Broadcast antenna (and processed using CSPP and Polar2Grid software). The toggle above shows the Day Night Band image (there are some clouds over eastern Lake Superior, and ice is apparent near/around the Apostle Islands and along the Wisconsin and western Upper Peninsula shorelines) with and without an overlain SST analysis over Lake Superior. SST values over Superior range from 32^o F (white in the enhancement) to a brisk 37^o F (green in the enhancement). Of interest in the SST analysis is the surface front extending to the northeast of Keewenaw Point: Green values to the east are just above 37^o F; cyan values to the west are cooler than 36^o F.

Fewer clouds were obvious over Lake Michigan. Lake Michigan is shows warmer surface temperatures, near 40^o F (red enhancement) to the northeast of Chicago. Note the presence of ice over Green Bay.

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You can also view imagery over the Great Lakes at the CIMSS VIIRS Imagery Viewer that packages imagery from the CIMSS Direct Broadcast site.

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GOES-16 (GOES-East) Air Mass RGB images (above) included contours of RAP40 model PV 1.5 Pressure — and showed a Hurricane Force low pressure that had rapidly intensified as it moved northeastward off the US East Coast (surface analyses) on 06 February 2023. The PV 1.5 surface (which represents the dynamic tropopause) descended to the 500 hPa pressure level at 2200 UTC.

GOES-16 Mid-level Water Vapor (6.9 µm) images (below) included contours of RAP40 model surface wind speed. One prominent cloud feature was the development of what resembled a “scorpion tail” just south of the low pressure center — which is sometimes seen in storms that feature a sting jet.

GOES-16 “Red” Visible (0.64 µm) images (below) included the 1800 UTC analysis of surface pressure and fronts. Note the cloud-free area of water just east of the low center and just south of the occluded front, which exhibited a slightly brighter “milky/hazy” appearance — enhanced diffuse solar reflection off a highly-agitated ocean surface with large waves (generated by very strong winds) supported the existence of either a sting jet or a cold conveyor belt jet that had descended to the surface. A similar ocean surface signature has been observed in other cases where a sting jet was likely, such as here, here and here.

GCOM-W1 AMSR2 surface wind speeds at 1725 UTC on 06 February (below), from this site, showed hurricane force winds (darker brown to violet enhancement) in the same area where a hazy ocean surface appearance was evident in GOES-16 Visible imagery.

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Synthetic Aperture Radar (SAR) RADARSAT-2 observations continue in/around the South Pacific Ocean surrounding American Samoa. On 6 February, observations occurred at 0543 and 1640 UTC. The toggle below compares the 0540 UTC Band 13 Clean Window infrared (10.3 µm) image with overlain SAR winds (color-enhanced with the AWIPS ‘gridded data’ enhancement, with values from 0-50 knots). This was a day with wind speeds around 10-12 knots over the ocean, with exception of a few lines of stronger winds. As with previous events, the strongest winds in these SAR analyses are likely related to highly reflective ice in the clouds. (Here is a toggle of the wind analysis and the Normalized Radar Cross Section NRCS analysis taken from this website; ice shows up as very bright white feathery structures in the NRCS imagery).

The Shortwave Infrared (ABI Band 7, 3.9) mp4 animation, below, (from the CSPP Geosphere site; here’s the direct link) shows the persistent westward motion of cloud features.

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RADARSAT-2 also overflew American Samoa at 1643 UTC on 6 February. The toggle below shows the derived SAR wind speed and the GOES-18 Clean Window infrared (Band 13, 10.3 µm) imagery at that time. As with the morning pass, the strongest winds here are associated with ice crystals within the clouds, identifiable in the NRCS imagery (link to original imagery).

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