A neat circulation feature appeared this morning northwest of Fargo. #ndwx #mnwx pic.twitter.com/AhLRsVSTZQ

— NWS Grand Forks (@NWSGrandForks) November 6, 2018

One interesting aspect of this 06 November 2018 mesoscale vortex (which was embedded within a stratus cloud deck) was the fact that a signature of the feature was evident in imagery from 15 of the 16 ABI spectral bands on GOES-16 (below) — only the 6.2 µm Upper-level Water Vapor images lacked even a subtle signal. The appearance of the vortex on 1.37 µm Near-Infrared “Cirrus” imagery was possible because the atmospheric column was very dry over that region (MODIS Total Precipitable Water values of 2-4 mm or 0.08-0.16 inch, and 0.31 inch on the 12 UTC Aberdeen SD sounding), so there was very little attenuation of upwelling 1.37 µm radiation by middle/upper-tropospheric water vapor.

Closer views of GOES-16 “Red” Visible (0.64 µm), Near-Infrared “Snow/Ice” (1.61 µm) and Low-level Water Vapor (7.3 µm) images are shown below. The dry air aloft also shifted the altitude of the GOES-16 Water Vapor band weighting functions to lower altitudes — calculated using 12 UTC rawinsonde data from Aberdeen SD, the 7.3 µm Low-level Water Vapor weighting function peaked near 600 hPa, with significant contributions from as low as the 700 hPa level.

A sequence of Terra/Aqua MODIS and NOAA-20/Suomi NPP VIIRS legacy “fog/stratus” Brightness Temperature Difference (BTD) images (below) indicated that the vortex formed about 40 miles northwest of Devils Lake (KDVL) sometime between 0438 UTC and 0805 UTC (10:38 PM and 2:05 AM local time). An AWIPS-1 version of the animation is available here.

The stratus clouds surrounding the vortex could be further characterized using a variety of GOES-16 channel difference and derived products as shown below.

The Split Cloud Top Phase (11.2 µm – 8.4 µm) product (above) allowed this feature to be followed during darkness and daylight. It initially became apparent in GOES-16 imagery near Devils Lake shortly after 11 UTC or 5 AM local time, then traveled southeastward to the southeastern corner of the state before beginning to lose definition after 20 UTC or 2 PM local time. Positive values of this infrared BTD product (shades of blue to cyan) highlight water droplet clouds, while negative BTD values (shades of violet) indicate clouds composed of ice crystals.

The Cloud Particle Size Distribution derived product (above) uses Visible and Near-Infrared bands, so is only created during daylight hours — and only for solar zenith angles of 65º or less, which meant only for a few hours over much of North Dakota with the low sun angle of early November. The product showed a tongue of smaller cloud-top particles (darker blue to violet enhancement) wrapping cyclonically into the center of the feature during the day.

The Cloud Top Phase product (above) indicated that the vortex and surrounding stratus cloud deck were composed of supercooled water droplets (lighter green enhancement).

The Cloud Top Height product (above) showed that the vortex and surrounding stratus clouds had tops generally in the 12,000-14,000 feet range (darker shades of blue).

Comparisons of Terra and Aqua MODIS Visible (0.65 µm), Cirrus (1.37 µm), Snow/Ice (1.61 µm) and Infrared Window (11.0 µm) at 1816 UTC and 1955 UTC are shown above — and comparisons of VIIRS Visible (0.64 µm), Snow/Ice (1.61 µm) and Infrared Window (11.45 µm) images from Suomi NPP at 1841 UTC and 2020 UTC along with NOAA-20 (incorrectly labeled as Suomi NPP) at 1941 UTC are shown below. Infrared Window brightness temperatures from both MODIS and VIIRS were in the -20º to -25ºC range (cyan to light blue enhancement) within and adjacent to the vortex.

A closer view was provided by a sequence of True Color and False Color Red-Green-Blue (RGB) images from Terra/Aqua MODIS and Suomi NPP VIIRS as visualized using RealEarth (below).

The mesoscale vortex — whose diameter was only about 20-30 miles — formed within cyclonic boundary layer flow about 100 miles south of an advancing cold front (below).

One potential forcing mechanism could have been a lobe of 700 hPa vorticity which was about 30 miles upstream of the vortex at 12 UTC and 18 UTC, according to the NAM12 model (below).

[Note: AWIPS color enhancements that differ from the defaults were used for some of the GOES-16 images and products shown here, to better highlight the subtle vortex feature]

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Strong winds across southern Iran and Pakistan were lofting plumes of blowing sand/dust offshore over the Gulf of Oman and the Arabian Sea during 01 November, 02 November and 03 November 2018 — a sequence of daily composites of True Color Red-Green-Blue (RGB) images from Terra MODIS, Aqua MODIS and Suomi NPP VIIRS from RealEarth (above) showed the increase in dust transport during that 3-day period.

A comparison of True Color RGB images from Terra MODIS, NOAA-20 VIIRS, Suomi NPP VIIRS and Aqua MODIS on 03 November is shown below.

Metop-A and Metop-B ASCAT data (source) showed surface wind speeds in the 20-25 knot range emerging from the coast where plumes of blowing dust were located (below).

EUMETSAT Meteosat-11 High Resolution Visible (0.8 µm) images from 02 November and 03 November (below) showed the daily evolution of the dust plumes.

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We are down for @RELAMPAGO2018, but #ARMCACTI just captured dual frequency radar observations of above anvil cirrus plume associated with an overshooting top @krisbedka @AtmoScientist @murillomojo pic.twitter.com/lC3GOPWCMu

— Steve Nesbitt (@70_dbz) November 3, 2018

An Above-Anvil Cirrus Plume (AACP) was observed over northern Argentina on 03 November 2018 during the RELAMPAGO-CACTI field experiment — radar indicated that the plume was 2-3 km above the top of the main thunderstorm anvil. A comparison of GOES-16 (GOES-East) “Red” Visible (0.64 µm), Near-Infrared “Snow/Ice” (1.61 µm) and “Clean” Infrared Window (10.3 µm) images (below) showed that the plume exhibited a colder appearance compared to the underlying anvil (which is explained by the temperature profile from 12 UTC Cordoba rawinsonde data). GOES-16 was actually scanning the AACP at 14:38:41 UTC — very close to the time of the radar image. The plume-producing thunderstorm was located south of Cordoba (identifier SACO).

Animations of GOES-16 Visible vs Snow/Ice and Visible vs Infrared are shown below. The southern storm also produced a smaller AACP at 1445 UTC.

Views of the convection across that region were provided by Terra MODIS (1457 UTC), Aqua MODIS (1736 UTC), Suomi NPP VIIRS (1742 UTC) and NOAA-20 VIIRS (1832 UTC) True Color Red-Green-Blue (RGB) images from RealEarth (below). 

True Color RGB images from Terra MODIS (1457 UTC), Aqua MODIS (1736 UTC), Suomi NPP VIIRS (1742 UTC) and NOAA-20 VIIRS (1832 UTC) [click to enlarge]

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Strong gap winds accelerating out of the Copper River Valley along the southern coast of Alaska were lofting fine particles of glacial silt/sand and transporting those aerosols southwestward across the Gulf of Alaska on 31 October and 01 November 2018. A sequence of NOAA-20 VIIRS True Color Red-Green-Blue (RGB) images viewed using RealEarth (above) showed that the plume was more widespread on 01 November.

A comparison of Suomi NPP VIIRS Visible (0.64 µm), Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images (below) showed the plume at 2022 UTC on 01 November. The map overlay has been removed from one set of images, to better reveal the dust plume source region. Note that the plume appeared much warmer (darker shades of red)  in the Shortwave Infrared image — this is due to enhanced solar reflectance off the small dust particles. Since airborne dust is generally transparent at longer infrared wavelengths, only the thickest portion of the plume exhibited a subtle signature on the 11.45 µm image.

The surface visibility briefly dropped to 3 miles at Middleton Island (PAMD) around the time of the Suomi NPP VIIRS images. as gusty north-northeasterly winds carried the plume over that location (below). Although Cordova (station identifier PACV) is only about 20 miles northwest of the Copper River Delta, the localized gap winds did not affect that site (where wind speeds were 3 knots or less the entire day).

ASCAT surface scatterometer winds (source) from Metop-A and Metop-B (below) showed speeds in the 25-30 knot range where the gap winds were exiting the Copper River Delta.

A toggle between Suomi NPP VIIRS Visible (0.64 µm) and Infrared Brightness Temperature Difference (11-12 µm) images (source) at 2204 UTC on 01 November (below) showed a subtle BTD signal within the more dense center portion of the plume, due to the silicate composition of some of the airborne particulate matter.

VIIRS Aerosol Optical Thickness (AOT) products from the eIDEA site (below) revealed larger AOT values on 01 November.

The gap winds were caused by a strong gradient between cold high pressure over Interior Alaska/Yukon and an occluding gale force low pressure system in the Gulf of Alaska (surface analyses: WPC)| OPC). GOES-15 (GOES-West) Visible (0.63 µm) images (below) showed the circulation of the low, and surface observations highlighted the cold air over snow-covered inland areas. While the dust plume was faintly apparent, it did not show up as well with the lower spatial resolution and large viewing angle of GOES-15.

A similar — though more prolonged and intense — event was noted in October 2016.

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