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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Daynight band imagery from the CIMSS Direct Broadcast antenna (processed by CSPP software) early on 6 February 2023 showed a curious region of darker reflectance over western Hudson Bay (VIIRS imagery is available from this temporary file; microwave imagery from ATMS at the same time is available from this temporary file). The toggle below, between the VIIRS I04 (3.74 µm) brightness temperature and the Dynamic Day Night Band imagery, shows that the darker ice is a warmer temperature, i.e., a darker shade of grey.

Products created from VIIRS and ATMS imagery on board NOAA-20 also show the curious ice difference over western Hudson Bay, For example, the Ice Surface Temperature, shown below with the VIIRS cloud mask, shows warmer ice surface temperatures over western Hudson Bay.

Microwave imagery from the Advanced Technology Microwave Sounder (ATMS) at the same time (for window channels) also shows warmer brightness temperatures over western Hudson Bay. An interesting question is why — and why do these brightness temperatures also mean less reflectance?

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Visible (daytime) imagery from the previous day from the 1746 UTC NOAA-20 (from here) overpass hints at a possible reason. The 1.61 µm imagery over the western part of the Hudson Bay is a bit darker — suggestive of less reflectance either because of snow age, or because of past freezing rain/ice pellet accumulation. The difference apparent in the Day Night band is also apparent in the VIIRS Day time visible imagery toggling below (and here). The difference in the surface inferred by the changes in the 1.61 µm imagery would affect the emission of infrared imagery on the following night, leading to the differences in brightness temperatures observed.

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Brad Vrolijk, a forecaster in eastern Canada, notes via email that a more plausible explanation is persistent northwesterly winds in the region. The animation of 0900 UTC surface analyses (from 24 January to 6 February) below does show a surface pressure analysis consistent with northwesterly winds around persistent Low Pressure over that would result in a scouring of snow from the western part of the bay, and that would affect the reflectance.

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The NOAA/STAR Water Surface Conditions / Synthetic Aperture Radar (WSC/SAR) Team sent the following composite SAR Ice image over Hudson Bay (Thanks!!): the brighter region over western Hudson Bay shows rougher ice there.

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VIIRS ice products also include Sea Ice Thickness and Sea Ice Age, as shown below from 5 February. Note that these two products are controlled in part by the Ice Surface Temperature, shown above. The darker ice over western Hudson Bay is both thinner (dark blue v. cyan to the east) and younger (green v. yellow to the east). (Images courtesy Rich Dworak, CIMSS).

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SMOS satellite observations of sea ice thickness also show thinner ice over western Hudson Bay. The image below is taken from this website.

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Added, March 2023: The National Sea Ice Data Center (NSIDC) had a news article on this feature (link) as part of a discussion on the Kivalliq polynya.

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