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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As noted previously, NOAA/STAR has arranged for special Synthetic Aperture Radar (SAR) wind observations over American Samoa during the month of February. Sentinel-1A SAR also provides occasional observations in and around the Samoan islands. The toggle above compares (using imagery from this website) derived wind speed and Normalized Radar Cross Section (NRCS) at 0600 UTC on 5 February 2023; a corresponding GOES-18 Clean Window infrared image is shown below, with the western-most end of a convective feature sampled (Here is the image without the sampling information). The coldest brightness temperatures with that convective complex are in the -61 to -66^oC range. (This animation from 0500 to 0750 UTC shows the convection decaying with time).

Outflow winds with this system are in the 15-20 knot range (bright cyan colors). The strongest derived winds — along 15.4o N in the wind analysis — are a result of ice in the cloud affecting the cloud signal. The NRCS for that region has the feathery characteristic of highly reflective ice crystals.

Sentinel-1A continued north, and sampled the waters surrounding Samoa, as shown in the toggle of derived windspeed and NRCS below. The island of Savai’i has an affect on the winds!

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When comparing SAR surface winds and GOES-18 (or any geostationary satellite) imagery, you must make sure to consider the effects of parallax. The satellite information may be parallax-shifted. Thus, the coldest brightness temperatures in the ABI imagery may not align with the region where ice crystals are affecting the SAR signal.

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Numerous wildfires were burning near Concepción, Chile during the 02 February – 04 February 2023 period (media report) — GOES-16 (GOES-East) CIMSS Natural Color RGB images combined with the Fire Power derived product (above) showed the diurnal variation of fire thermal signatures along with the dense daytime smoke plumes. The maximum Fire Power values exceeded 2500 MW with some of the larger fires. Surface reports in the vicinity of the wildfires were sparse and intermittent, but did indicate reductions in visibility due to the dense smoke.

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According to NOAA’s Great Lakes Surface Environmental Analysis (GLSEA) the Great Lakes were 11.6% ice covered on February 2nd when an arctic air mass was descending southward over the region. The cold air boosted ice coverage to 14.7% on February 3rd, less than half of the average extent (1973-2022) of 34% for February 3rd. This meant ample open waters for lake-effect clouds and snow, per the 5-hour GOES East animation below.

Ice extent on the Great Lakes is highly variable. In the past five years ice coverage on February 3rd has been as low as 5.3% in 2020 and as high as 40.9% in 2019. Records have been kept since 1973. (Check current conditions via https://coastwatch.glerl.noaa.gov/ice.html)

In spite of wide year-to-year variability, studies show a downward trend in Great Lakes ice coverage since records began.

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