A sequence of 4 consecutive Suomi NPP VIIRS Visible (0.64 µm), Shortwave Infrared (3.74 µm) and Infrared Window (11.4 µm) images (above) showed a small swirl of clouds associated with a weak area of low pressure near the North Pole — north of Greenland (surface analyses) — on 22 August 2019.

There were Suomi NPP NUCAPS soundings available in the vicinity of the surface low (above) — profiles from the 4 squared green dot locations (green dots indicate successful sounding retrievals from both the CrIS and ATMS instruments) which were closest to both the surface low and the North Pole (below) revealed characteristically-low arctic tropopause heights of around 7-8 km, and surface temperatures dropping to below freezing at the 2 most northerly points of 88.28º and 88.57º N latitude. Note: the Suomi NPP (SNPP) CrIS anomaly that began on 24 March 2019 was resolved via a switch to the redundant Side-2 electronics on 24 June — so CrIS data once again became available for incorporation into SNPP NUCAPS soundings beginning on 01 August. Training material for NUCAPS in AWIPS is available here.

According to GCOM-W1 AMSR2 data (source), this weak surface low was over a portion of the Arctic Ocean where sea ice concentration was still high (below).

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The Split Window Difference field (10.3 µm – 12.3 µm), shown above in the south Pacific around Samoa and American Samoa (Leone is on the island of Tutuila just west of 170º W Longitude; Fitiuta is on the island of Ta’u just east of 170º W Longitude), can be used to estimate the horizontal distribution of water vapor. The Split Window Difference can give a good estimate of moisture distribution in the atmosphere over the ocean where conventional moisture measurements are limited. The image above shows greater values (3.5 – 4 K, in yellow and orange) over the northern part of the image and smaller values (2-3 K, in yellow and blue) over the southern part of the image, divided by a band of cloudiness that passes through 20º S, 170º W.

NOAA-20 overflew this region at 0056 UTC, and NUCAPS profiles were available, as shown below.

The animation below steps through soundings at different locations. Total precipitable water as determined from the sounding is indicated. In the region where the Split Window Difference field was around 4 K, precipitable water values were in the 1.5-1.7″ range; in regions where the Split Window Difference was closer to 2 K, precipitable water values were closer to 0.5-0.75″.

Microwave-only data, shown below from the MIMIC website, shows a sharp gradient at 20º S, 170º W.

At ~1200 UTC, when NUCAPS again passed over this region, profiles could again be used to discern gradients in total precipitable water.  At that time, however, the Split Window Difference field was not computed because warming of the Advanced Baseline Imager (ABI) associated with the sub-optimal performance of the Loop Heat Pipe meant that Band 15 data were not available.  (Baseline Level 2 Products, such as total precipitable water, are also unavailable from GOES-17 because of the Loop Heat Pipe issue) The Split Window Difference field could be computed from Himawari-8 data however.

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GOES-16 (GOES-East) “Red” Visible (0.64 µm), Shortwave Infrared (3.9 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed the formation of a pyrocumulonimbus (pyroCb) cloud over far southeastern Bolivia on 18 August 2019. The small anvil cloud briefly surpassed the -40ºC pyroCb threshold from 1800-1820 UTC, attaining a minimum cloud-top infrared brightness temperature of -45.2ºC along the Bolivia/Paraguay border at 1800 UTC. This pyroCb formed over the hottest southern portion of an elongated fire line, as seen in the Shortwave Infrared imagery.

A 1.5-day animation of GOES-16 Shortwave Infrared images (from 12 UTC on 17 August to 2350 UTC on 18 August) revealed the rapid southeastward run of the fire to the Bolivia/Paraguay border on 17 August, followed by the eastward expansion of the fire line on 18 August (below).

A toggle between Suomi NPP VIIRS True Color Red-Green-Blue (RGB) and Infrared Window (11.45 µm) images as viewed using RealEarth (below) showed the large and dense smoke plume streaming southeastward, with the small pyroCb along the Bolivia/Paraguay border at 1745 UTC — the brighter white tops of the pyrocumulus and pyrocumulonimbus clouds reached higher altitudes than the tan-colored smoke plume. The coldest cloud-top infrared brightness temperature was about -55ºC (orange enhancement), which corresponded to an altitude around 9 km according to rawinsonde data from Corumbá, Bolivia.

 

Looking more closely at yesterday’s smoke over South America, the aerosol index from #NOAA20 #OMPS was in double digits! Values on the right refer to the UTC of the measurements. pic.twitter.com/3XBqoEVs4M

— Colin Seftor (@colin_seftor) August 19, 2019

Strong northerly to northwesterly surface winds were blowing across the region, in advance of an approaching cold front (surface analyses) — at Robore, Bolivia (located just north-northwest of the fires), winds were gusting to 25-28 knots during much of the day (below).

This is likely the second confirmed case of a South American pyroCb (the first being on 29 January 2018) — in addition, it’s the second pyroCb documented in the tropics and the first pyroCb documented during a winter season. Thanks to Mike Fromm (NRL) for bringing this case to our attention!

===== 25 August Update =====

GOES-16 Visible, Shortwave Infrared and “Clean” Infrared Window images (above) showed that another pyroCb developed from that same general fire complex, southwest of Robore, Bolivia (SLRB), on 25 August.

A toggle between Suomi NPP and NOAA-20 VIIRS True Color RGB and Infrared Window images as viewed using RealEarth (below) showed the large and dense smoke plume streaming southeastward, with the small pyroCb just north of the Bolivia/Paraguay border — the brighter white tops of the pyrocumulus and pyrocumulonimbus clouds reached higher altitudes than the tan-colored smoke-rich clouds at lower altitudes.

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1-minute Mesoscale Domain Sector GOES-17 (GOES-West) “Red” Visible (0.64 µm) and Shortwave Infrared (3.9 µm) images (above) revealed thick smoke and a pronounced thermal anomaly (hot pixels, darker black) associated with the Swan Lake Fire on the Kenai Peninsula in south-central Alaska on 17 August 2019. Later in the day, a few pyrocumulus jumps could be seen in Visible imagery over the fire source region, as fire behavior increased (another day when pyrocumulus jumps were apparent with this fire was 30 June, during a period when southerly winds were transporting dense smoke to the Anchorage area).

Strong northerly-northwesterly winds were transporting smoke from the Swan Lake Fire southward across the Kenai Peninsula and the Seward area — a time series of surface report data from Seward (below) showed that this smoke had reduced the visibility to less than 1 mile by 03 UTC (7 PM local time). South-central Alaska was experiencing drought conditions, which had worsened from the preceding week; the strong winds on this day acted to dry fuels even further, leading to a re-invigoration of the long-lived fire.

The PM2.5 Air Quality Index reached 427 at Cooper Landing, and 358 farther downwind at Seward (below).

The southward transport of smoke across the Seward area and out over the adjacent offshore waters of the Gulf of Alaska was evident in VIIRS True Color Red-Green-Blue (RGB) images from NOAA-20 and Suomi NPP, as viewed using RealEarth (below).

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