VIIRS Infrared Window (11.45 µm) images from NOAA-20 and Suomi NPP as viewed using RealEarth (above) showed thunderstorms beginning to increase in areal coverage north of and in the vicinity of American Samoa on 20 June 2020, as a surface trough north of the islands began to move southward.

To monitor the flash flooding potential of these thunderstorms, a GOES-17 (GOES-West) Mesoscale Domain Sector was positioned over the American Samoa region — which provided “Clean” Infrared Window (10.35 µm) images at 1-minute intervals (below). Some of these storms exhibited minimum cloud-top infrared brightness temperatures around -80ºC (black enhancement), producing heavy rainfall (over 6 inches in 9 hours) and strong winds (gusting to 60 mph) according to the NWS Pago Pago compilation of local storm reports.

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CIMSS can now supply 88.2 GHz imagery from the Direct Broadcast antennas in Madison. Data from the Advanced Technology Microwave Sounder (ATMS) on Suomi NPP and NOAA-20 is processed by CSPP and is provided via an LDM feed. The toggle above shows GOES-16 ABI Clean Window Infrared (10.3 µm) imagery and the 88.2 GHz imagery from ATMS and the morning pass on 18 June 2020.

There is a stark contrast between land and water in clear skies because of the low emissivity of water in the microwave.  Convective clouds over northern Plains and central Canada also have a big impact on the microwave signal.  In the Gulf of Mexico, the colder region shown (yellow in the color enhancement) has a brightness temperature around -48º to -55º C, and it is surrounded by regions (green) with brightness temperatures in the -25º to -40º C range.  Emissivity can be affected by wind speeds (that generate small waves); low clouds can also affect (warm) the emissions detected.

 

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As a follow-up to this 15 June blog post, GOES-16 (GOES-East) Split Window Difference (10.3 µm – 12.3 µm) and Dust RGB (Red-Green-Blue) images (above) displayed signatures of another dense plume of Saharan Air Layer dust — which appeared as shades of yellow in the Split Window Difference images, and shades of magenta in the Dust RGB images — that was streaming westward off the coast of Africa and moving over the Cape Verde Islands and the eastern Atlantic Ocean from 0600 UTC on 17 June to 0020 UTC on 18 June 2020. This renewed pulse of dust was caused by an anomalously strong easterly wind burst within the lower troposphere.

GOES-16 True Color RGB images created using Geo2Grid (below) showed the characteristic tan hues of the dust plume during daylight hours (0800-1850 UTC).

True Color RGB images from NOAA-20 and Suomi NPP as viewed using RealEarth (below) provided views of the dust plume at 14 UTC and 15 UTC. Note that the core of the dust plume moved directly over the Cape Verde Islands.

Plots of surface report data from Sal, Cape Verde (GVAC) and Nauackchott, Mauritania (GQNO) are shown below. The surface visibility dropped below 1 mile at Sal, Cape Verde from 16-18 UTC — and along the coast of Africa at Nauackchott, Mauritania the arrival of the dry easterly winds was very evident in the sharp drop of dewpoint temperatures after 09 UTC.

===== 18 June Update =====

On the following day, GOES-16 True Color RGB images (above) showed that the dust plume had moved a bit farther west and northwest. A longer 2-day (17-18 June) animation of GOES-16 Split Window Difference and Dust RGB images is shown below.

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The animation above shows Advanced Clear-Sky Processor for Ocean (ACSPO) sea-surface temperatures at three different times on 16 June: 1811 UTC (using data from Suomi NPP), 1903 UTC (using data from NOAA-20) and 1955 UTC (using data, again from Suomi NPP). (Orbit paths for the satellites can be viewed here). Note that the default color map bounds for these images has been changed to be from 50º F to 90º F.

Compare the 3 images above to the 0727 UTC 17 June SST analysis, below, or to the SST analysis from 1924 UTC on 15 June 2020, at bottom. In all the analyses, mid-Gulf SSTs are fairly constant around 82º F. Shoal waters south of Louisiana or off the coast of southwest Florida show very warm temperatures on the 16th. This kind of near-shore warming can occur during the day when winds are weak and wave action is small. (The 3 surface charts from 1800 UTC on 14, 15 and 16 June, shown here, show a weakening in the winds with time over the northern Gulf of Mexico.) As winds and wind-driven waves slacken, the amount of turbulent mixing at the ocean surface decreases, allowing for the surface skin of the ocean to become very warm; that warmth is detected by the satellite. Winds and waves do not slacken in the central Gulf; vertical mixing in the top of the ocean in that region does not change. (The relationship between winds and sea surface temperatures has been discussed on this blog in the past; see here, for example.)

Large diurnal changes in near-shore sea-surface temperatures very often indicate slack winds and small waves.

ACSPO Sea-Surface Temperatures are available via an LDM feed from CIMSS. They are computed from Direct-Broadcast data downloaded via antennas in Madison.

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