GOES-16 Split Window difference (SWD) fields, above, and Meteosat Dust RGB imagery (both from 1500 UTC on 15 June 2020, and available at this site) suggest that a Saharan Air Layer (SAL) event is developing in the eastern Atlantic Ocean.  (Click here to see an animation of the Split Window Difference; AWIPS note: the default SWD enhancement has been replaced in the animation by the Grid/Low-Range Enhanced field) The dry, dusty air associated with SALs has an impact on air quality in the Caribbean, and it also suppresses tropical cyclone activity.  What other satellite products can be used to track this feature?

The 4-panel images below includes GOES-16 Aerosol Detection (upper left), GOES-16 Dust RGB (upper right), GOES-16 Split-Window Difference (10.3 µm – 12.3 µm) (lower left), and gridded NOAA-Unique Combined Atmospheric Processing System (NUCAPS) 850-700 mb Relative Humidity at 1550 UTC`and 1600 UTC. GOES-16 Aerosol Detection can detect SAL layers because of the suspended dust in the atmosphere. Both the Dust RGB and Split Window difference fields detect the SAL because of the differential absorption of infrared radiation at 10.3 µm and 12.3 µm by silicates within the dust. 

The bright pink in the Dust RGB is characteristic of dust detection with that product, and it shows a strong feature emerging from Africa.  The Split Window difference (SWD) field shows the SAL region to be blue to brown;  note that SAL air is indicated over the central Atlantic as well.  The yellow/orange/red enhancement in the SWD (bracketed by blue regions in the enhancement that suggest dry air) between 30º and 40º W Longitude, and 10º and 20º N Latitude, suggests more moisture in the air there.

Aerosol Optical Depth is higher in the SAL air because of the suspended dust.  Relatively cleaner air is north and south of the feature.  The low-level Relative Humidity in the SAL air is low.  This is more easily seen in the later image — 1600 UTC with this ascending NOAA-20 pass — than in the earlier (1550 UTC) image.  (Click here to see a toggle of both gridded NUCAPS fields).

GOES-16 Aerosol Products, below, identify both the region of higher Aerosol Optical Depth (AOD), and the type of Aerosol (in this case:  Dust) that is responsible for the higher AOD values.

The plot below shows the location of the NUCAPS points for the early afternoon swath in the eastern Atlantic. The points overlay the 850 mb – 700 mb relative humidity fields. Note the dryness at the eastern edge of the image, near 30º W; a skinny tongue of moisture (cyan in the moisture enhancement) extends to the north (corresponding to the region in the SWD where yellow/orange/red colors suggest more moisture), with dry air north and west of that moisture, near 45º W.

The three soundings below show the dry air to the north and west (13º N, 42.5º W), the relative moisture in the middle (13.5º N, 36.5º W), and the dry air to the east (13.5º N, 31º W).

True-color imagery (an example computed using CSPP-Geo is shown below from 1710 UTC) can also be used to track dust.  Dust in true color has a much different presentation than adjacent clear(er) skies.

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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) showed the smoke plume and thermal anomaly (cluster of hot pixels) associated with the Magnum Fire in northern Arizona on 12 June 2020. The hottest Shortwave Infrared brightness temperatures observed were 138.7ºC (411.9 K), which is the saturation temperature for those ABI detectors. Near and immediately downwind of the fire source region, brighter-white pyrocumulus clouds were seen penetrating the top of the darker gray smoke plume. About 50-60 miles north of the fire, the smoke plume drifted over Bryce Canyon, Utah (KBYC) — but the surface visibility there remained at 10 miles, indicating that the smoke remained aloft (and automated hourly reports listed an overcast layer at 9-12 kft from 03-05 UTC).

At 2112 UTC, the Suomi NPP VIIRS Fire Radiative Power product as viewed using RealEarth (below) revealed a maximum FRP value of 142.3 MW, and a band I4 (3.74 µm) infrared brightness temperature of 367 K.

On the following day (13 June), a veil of broken to overcast cirrus moved over the Magnum Fire for much of the day — but in 1-minute GOES-17 3.9 µm imagery, the fire’thermal anomaly was only completely masked for very brief periods when the clouds were at their maximum thickness (below).

Another view of the fire using 5-minute imagery from GOES-16 (GOES-East) provided quantitative products such as Fire Power, Fire Temperature and Fire Area (below) — these 3 products are components of the GOES Fire Detection and Characterization Algorithm (FDCA). These FDCA products are still being tested and evaluated using GOES-17 data before being released.

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GOES-17 (GOES-West) Mid-level Water Vapor (6.9 µm) images (above) showed the circulation of an anomalous middle-tropospheric cutoff low over the northwestern portion of Alaska on 12 June 2020. A Potential Vorticity (PV) anomaly associated with this low was causing the dynamic tropopause — represented by the pressure of the PV1.5 surface — to descend as low as the 500 hPa pressure level.

Just after 21 UTC, an overpass of the Suomi NPP satellite provided NUCAPS soundings (above) within much of the core of the cutoff low — the green NUCAPS sounding profile about 40 miles east/southeast of the 500 hPa PV1.5 pressure contour (below) displayed an apparent tropopause near the 400 hPa pressure level.

GOES-17 “Red” Visible (0.64 µm) images (below) revealed the development of numerous showers and thunderstorms across the Brooks Range and North Slope of Alaska, aided by instability beneath the cutoff low.

A higher spatial resolution view of these showers and thunderstorms was provided by a sequence of VIIRS True Color Red-Green-Blue (RGB) and Infrared Window (11.45 µm) images from NOAA-20 and Suomi NPP, as viewed using RealEarth (below). A few of these thunderstorms moved toward the Arctic Coast, with one fairly impressive storm just southwest of Katovik which exhibited cloud-top infrared brightness temperatures near -60ºC (red enhancement) around 23 UTC.

Summer thunderstorms stretched across the North Slope and near the Arctic Coast today. Data from the GLD360 depicted over 800 Cloud-to-Ground lightning events in the area! Here is the scene at Atqasuk, Utqiagvik, Nuiqsut, and Deadhorse this afternoon and evening. #akwx #lightning pic.twitter.com/tGeqswONW5

— NWS Fairbanks (@NWSFairbanks) June 13, 2020

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Showers developed over southern Wisconsin late in the day on 12 June 2020. What satellite products could be used to anticipate where the showers would develop? The animation of visible and radar, above, shows that the storms initiated near a boundary (mostly stationary) that separated Lake Michigan-influenced air with less stable air (based on cumuliform cloud development) to the south and west. Showers develop near the lake breeze front starting around 2000 UTC; a parallax shift is obvious between the radar and satellite (2100 UTC example) A parallax correction on the satellite imagery would shift the cloud locations towards the sub-satellite point (0, 75.2 W for GOES-East).

NUCAPS (NOAA-Unique Combined Atmospheric Processing System) soundings combine infrared and microwave information from the high spectral resolution CrIS (Cross-track Infrared Sounder) and ATMS (Advanced Technology Microwave Sounder) instruments on NOAA-20 to yield estimates of the thermodynamic structure of the atmosphere. NOAA-20 overflew the western Great Lakes shortly after 1800 UTC on 12 June, and clear skies at the time means the infrared information was complete. (In cloudy skies, NUCAPS soundings are more typically driven by ATMS data, which has coarser spectral and horizontal resolution).

The Total Totals index shown below was derived from the NUCAPS thermodynamic information. A gradient in stability exists between the most unstable air in western Wisconsin and the more stable lake-influenced air over eastern Wisconsin.

The low-level lapse rate, below, (from 900-700 mb), also shows a gradient in stability in the region where shower development occurred. It is not unusual for shower initiation to occur in gradients of stability (Example 1, Example 2,…), so that is a region on which to focus when waiting for convection to start.

Once the shower development occurs, when will lightning occur?  As noted in this blog post, the Day Cloud Phase Distinction Red-Green-Blue imagery that includes the 1.61 µm band (at which wavelength reflectance is greatly affected by the presence of ice) gives a visual clue to when glaciation occurs, and cloud-top glaciation commonly precedes lightning development.  The animation below shows the Day Cloud Phase Distinction on the left, and the Day Cloud Phase Distinction overlain with Geostationary Lightning Mapper (GLM) Flash Extent Density.

There are only two detected lightning flashes in this animation — and in both cases, the Day Cloud Phase Distinction has become more orange/yellow and less green/blue before the lightning strike. This color change occurs as the 1.61 µm imagery becomes darker: ice in the cloud top increases the absorption (and reduces the reflectance) of 1.61 µm solar energy. Compare the 2111, 2116 and 2121 imagery for the lightning strike near Madison in Dane County; Similarly, compare the 2051, 2056, 2101 and 2106 imagery for the 2101 UTC lightning strike in Waukesha county). There are subtle color changes (on other days the changes are more obvious!) in the Day Cloud Phase Distinction RGB that preceded lightning events.

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GOES-16 Level 2 Products include Derived Stability Products (these can be found online here as well), and the mostly clear skies on 12 June meant a good signal.  The Baseline Lifted Index, shown below from 1701 through 2256 UTC,  shows convection developing along the eastern edge of less stable air.

Is there an easily identifiable trigger that spawned these storms? Water Vapor imagery often shows impulses in clear skies. The two RGB products below combine different water vapor channels.  There is a subtle increase in the amount of orange in the Differential Water Vapor before the convection starts.  This increase in the red component is an increase in the brightness temperature difference between upper and lower water vapor channels, a difference that can be associated with upper-tropospheric forcing.  The simple water vapor RGB (that includes the upper and lower water vapor channels, but not the difference between them) on the right shows no obvious signal.

The Air Mass RGB (described here) also has the split water vapor difference as its red component. The animation below (from this site), shows a subtle change in air mass (cooler, dryer air moving southward from Canada) that could have provided an additional triggering mechanism for the convection.

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Webcams in Madison, WI, that capture the evolution of these storms, and also show the GOES-16 imagery (derived from this site), are available at this tweet from @GOESGuy.

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