1-minute Mesoscale Domain Sector GOES-17 (GOES-West) Night Fog brightness temperature difference (BTD) and “Red” Visible (0.64 µm) images (above) showed the nighttime and daytime signature of a fog/stratus layer that had persisted throughout much of the Snake River Basin in southern Idaho on 29 December 2020. Some sites along the Interstate 84 corridor were reporting freezing fog — and the surface visibility was restricted to less than 1/4 mile at times in Boise (KBOI).

During the preceding nighttime hours, the extent of fog/stratus cloud within the Basin was well-illuminated by a nearly-full Moon (which was in the Waxing Gibbous phase, at 99% of Full) as seen in a Suomi NPP VIIRS image at 0856 UTC (below).

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GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) revealed the formation of a mesovortex in northern Lake Superior on 28 December 2020. Mid-lake convergence — as depicted by RAP40 model surface winds — contributed to the development of this feature.

ASCAT surface scatterometer winds from Metop-A at 1537 UTC (below) provided a good view of the cyclonic flow of the mesovortex in its early stages, before it became organized enough to become obvious on satellite imagery.

A toggle between ASCAT winds from Metop-A and Metop-B (source) is shown below.

VIIRS True Color and False Color RGB images from Suomi NPP and NOAA-20 (below) showed a higher resolution view of the mesovortex; the shades of cyan in the False Color images suggested that the tops of some of the cloud bands were becoming glaciated.

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False color images from VIIRS as shown above combine bands M11, I2 and I1: 2.25 µm, 0.865 µm and 0.64 µm. Inclusion of the near-IR channel at 2.25 µm causes a color change – less red (blue and green make cyan) – in regions where ice crystals exist, because ice crystals absorb, rather than reflect, solar energy at that wavelength. A similar occurrence happens at 1.61 µm wavelengths.

Accordingly, the Day Cloud Phase Distinction RGB, shown below, highlights ice crystals in clouds (they are yellow or orange because there is less green in the RGB where ice crystals are present;  before sunrise, and after sunset, the RGB is only red:  green and blue contributions depend on solar reflectance). Solar reflectance is small at all wavelengths at this time of year over Lake Superior, but a definite color difference in the clouds of the vortex is apparent. Snow showers/squalls are more likely where the Day Cloud Phase Distinction RGB suggests ice crystals in the clouds, as shown in this blog post (and this one!).

Mesoscale vortices over warm lakes owe their existence to sensible and latent heat fluxes from the (relatively) warm water into the colder atmosphere aloft. (Click here for a presentation of such an event over southern Lake Michigan). When the vortex moved over Ontario and lost the lake fluxes, it dissipated. Visible imagery from the morning of 29 December 2020, below, showed no circulation.

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A sequence of GOES-16 (GOES-East) “Red” Visible (0.64 µm) images covering the 14 December – 26 December 2020 period (above) showed that after Iceberg A68a encountered the more shallow continental shelf waters southwest of South Georgia island earlier in the month (NOAA-20 True Color RGB images, 02-13 December), the large iceberg then began to move south-southeastward, shear apart and release 3 smaller orphan icebergs (in addition to widespread small ice floes).

A toggle between NOAA-20 VIIRS True Color RGB images on 23 December and 25 December viewed using RealEarth (below) provided a higher-resolution view of the numerous small ice floes that resulted from the iceberg fracturing process.

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GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) indicated that a strong arctic cold front (surface analyses) had plunged southward across Mexico, through the Chivela Pass, and emerged as a Tehuano (or “Tehuantepecer“) gap wind into the Gulf of Tehuantepec on 24 December 2020. Along the Gulf of Mexico coast, a few sites in Mexico reported blowing dust and/or blowing sand with onshore winds gusting to 40 knots.

GOES-16 True Color RGB images created using Geo2Grid (below) showed the hazy signature of blowing dust/sand as it was transported off the south coast of Mexico and spread out across the Gulf of Tehuantepec.

===== 25 December Update =====

On the following day, GOES-16 Visible images (above) showed that the leading edge of the gap wind flow — marked by a broad arc cloud — was approaching the ITCZ / Monsoon Trough in the Pacific Ocean. Ship reports of 30-35 knot winds were seen within the offshore flow — and ASCAT surface scatterometer winds revealed speeds as high as 44 knots.

GOES-16 True Color RGB images (below) showed the hazy signature of blowing dust from Mexico as it spread out across the Pacific Ocean.

Aided by enhanced forward scattering during the morning hours, True Color RGB images from GOES-17 (GOES-West) showed the hazy signature of airborne dust from Mexico a bit better (below).

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