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.

View only this post Read Less

A sequence of GOES-16 (GOES-East) Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) showed the transition of Category 1 Hurricane Oscar to a hurricane-force extratropical storm over the western Atlantic Ocean on 31 October 2018. The very sharp moisture gradient that developed along the western and northern edges highlighted the “”limiting streamline” of airflow through the cyclone — and a remarkably warm/dry signature (yellow to orange enhancement) was seen near the storm center late in the period.

A closer view of the GOES-16 Water Vapor imagery using AWIPS is shown below, which includes plots of buoy/ship reports and surface analyses.

View only this post Read Less

The topic of a conversation on Twitter, GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) revealed curious circular areas of fog/stratus dissipation across southern Louisiana on the morning of 30 October 2018. — making it a natural candidate for the “What the heck is this?” blog category.

GOES-16 GEOCAT Low IFR Probability and Fog/Low Stratus Depth products (below) indicated that this fog and low stratus had been increasing in coverage and spreading northward across Louisiana during the preceding nighttime hours (VIIRS fog/stratus Brightness Temperature Difference images) — and the fog/stratus was relatively shallow, only having a depth of about 300 feet or less. In fact, if you look closely at the Visible animation above, a few small spots of slightly brighter cloud can be seen in the vicinity of Baton Rouge KBTR which are tall objects (such as refinery stacks, and even the State Capitol building) protruding above the fog/stratus and acting as an obstacle to their flow.

A sequence of 4-panel comparisons of GOES-16 “Blue” Visible (0.47 µm), “Red” Visible (0.64 µm) and Near-Infrared “Vegetation” (0.86 µm) images with Near-Infrared “Snow/Ice” (1.61 µm), Near-Infrared “Cloud Particle Size” (2.24 µm) and Shortwave Infrared (3.9 µm) images (below) showed no indication of any substantial differences between the cloud material within the circular features and the adjacent fog/stratus. The largest “outer rings” of the dissipating fog/stratus areas had a small amount of vertical extent, which cast a shadow that was best seen in the Near-Infrared 0.86 µm and 1.61 µm images.

The most plausible explanation for the circular dissipation features turned out to be fires that were set in sugar cane fields following harvest — particulates in the smoke could have “seeded” the fog/stratus cloud layer, either changing the particle size distribution or making the cloud more susceptible to faster dissipation after sunrise due to solar heating of black carbon nuclei within the cloud droplets.  An Aqua MODIS Shortwave Infrared (3.7 µm) image from the previous afternoon (below) did reveal a number of small thermal anomalies or fire “hot spots” (yellow to red pixels) across the region at 1909 UTC (2:09 PM local time).

Similarly, GOES-16 Shortwave Infrared images on 29 October (below) also showed signatures of widespread small and generally short-lived fires (darker black pixels) across southern Louisiana. Surface winds were very light across that area (KARA | KPTN | KNBG | KMSY | KNEW), minimizing smoke dispersion from any fires.

View only this post Read Less

The Garden Route Fires had been burning since about 24 October 2018 near George along the southern coast of South Africa (media story). On 29 October, EUMETSAT Meteosat-11 High Resolution Visible (0.8 µm), Shortwave Infrared (3.92 µm) and Longwave Infrared Window (10.8 µm) images (above) showed an elongated west-to-east oriented thermal anomaly or fire “hot spot” (red pixels) just northeast of George (station identifier FAGG) on Shortwave Infrared imagery during the hours leading up to the formation of a pyrocumulonimbus (pyroCb) cloud around 1300 UTC. The pyroCb exhibited the characteristic warm (+10 to +15ºC, darker gray enhancement) shortwave infrared cloud-top signature just off the coast at 1315 UTC, — this is due to enhanced solar reflection off ice crystals that are smaller compared to those of conventional thunderstorm tops.

Zooming out a bit to follow the southeastward drift of the pyroCb cloud (below), the coldest cloud-top 10.8 µm infrared brightness temperature (BT) was -61ºC (darker red enhancement) at 1315 UTC — then the cloud tops remained in the -55 to -59ºC range (orange enhancement) for the next 6 hours or so. Leveraging the large difference between cold 10.8 µm and warm 3.92 µm BTs, NRL calculates a pyroCb index, which classified this feature as an “intense pyroCb” (1315 UTC | animation). The coldest 10.8 µm cloud-top BT of -61ºC roughly corresponds to an altitude of 13.5 km based on 12 UTC rawinsonde data from Port Elizabeth (plot | list).

Imagery from NOAA-19 at 1420 UTC (courtesy of René Servranckx) also revealed the warm (dark gray) Shortwave Infrared pyroCb signature, along with a minimum cloud-top infrared BT of -58.1ºC (below).

A Suomi NPP VIIRS True Color Red-Green-Blue (RGB) image at 1230 UTC (below) was about a half hour before the formation of the pyroCb, but it did show a signature of smoke drifting southeastward off the coast.

On the following day (30 October), a NOAA-20 VIIRS True Color image (below) showed the classic comma cloud signature of a mid-latitude cyclone south of the coast, with the band of cold-frontal clouds extending northward across Lesotho. Note the thick plume of smoke spreading eastward within the strong post-frontal westerly winds.

A time series of of surface observations from George (below) supported the idea of a cold frontal passage: ahead of the front, temperatures rapidly rose to 104ºF/40ºC (with a dew point of 39ºF/4ºC) on 28 October about 1.5 hours prior to the formation of the pyroCb — then strong westerly winds (gusting to 40 knots/21 mps) with rising pressures and falling temperatures followed on 30 October.

The pyroCb research community believes that this is the first documented case of a pyroCb on the African continent.

 

View only this post Read Less