Detection of Saharan Air Layer in the eastern Atlantic

June 15th, 2020 |

Color-enhanced GOES-16 Split Window Difference field, 1500 UTC on 15 June 2020 (Click to enlarge)

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 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 (Click to enlarge)

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 1600 UTC (Click to enlarge)

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.

GOES-16 Aerosol Optical Depth and GOES-16 Aerosol Type, 1600 UTC on 15 June 2020 (Click to enlarge)

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.

NUCAPS Sounding Locations overlain on top of Gridded NUCAPS field of 850 mb – 700 mb relative humidity (Click to enlarge)

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).

NUCAPS soundings at 1600 UTC in dry air, in the ribbon of moist air, and in dry air at the eastern edge of the NUCAPS swath.

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.

GOES-16 True Color imagery, 1730 UTC on 15 June 2020 (Click to enlarge)

Cutoff low over northern Alaska

June 12th, 2020 |

GOES-17 Mid-level Water Vapor (6.9 µm) images, with contours of PV1.5 Pressure plotted in red [click to play animation | MP4]

GOES-17 Mid-level Water Vapor (6.9 µm) images, with contours of PV1.5 Pressure plotted in red [click to play animation | MP4]

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.

GOES-17 Mid-level Water Vapor (6.9 µm) images, with contours of PV1.5 Pressure plotted in red and available NUCAPS sounding profiles denoted by green/yellow points [click to enlarge]

GOES-17 Mid-level Water Vapor (6.9 µm) images, with contours of PV1.5 Pressure plotted in red and available NUCAPS sounding profiles denoted by green/yellow points [click to enlarge]

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.

NUCAPS sounding profile [click to enlarge]

NUCAPS sounding profile [click to enlarge]

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.

GOES-17 "Red" Visible (0.64 µm) images, with hourly surface reports plotted in red [click to play animation | MP4]

GOES-17 “Red” Visible (0.64 µm) images, with hourly surface reports plotted in red [click to play animation | MP4]

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.

VIIRS True Color (RGB) and Infrared Window (11.45 µm) images from NOAA-20 and Suomi NPP [click to enlarge]

VIIRS True Color (RGB) and Infrared Window (11.45 µm) images from NOAA-20 and Suomi NPP [click to enlarge]


Shower initiation over Wisconsin

June 12th, 2020 |

GOES-16 ABI Band 2 (0.64 ) visible imagery (left) and Midwest Composite Radar (right), 1736 – 2100 UTC on 13 June 2020 (Click to animate)

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.

Total Totals index derived from NOAA-20 NUCAPS data, 1840 UTC on 12 June 2020 (Click to enlarge)

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.

900-700 mb Lapse Rates derived from NOAA-20 NUCAPS data, 1840 UTC on 12 June 2020 (Click to enlarge)

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.

GOES-16 Day Cloud Phase Distinction (left), and Day Cloud Phase Distinction overlain with Geostationary Lightning Mapper (GLM) data (Click to animate)


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.

GOES-16 Derived Stability Index (Lifted Index) in clear regions, GOES-16 ABI Band 13 (10.3 µm) infrared imagery in cloud regions, 1701-2256 UTC on 12 June 2020 (Click to animate)

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.

GOES-16 Differential Water Vapor RGB (left) and SImple Water Vapor RGB (right) from 0916 to 2116 UTC on 12 June 2020 (Click to animate).

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.

GOES-16 Air Mass RGB, 1541 to 2141 UTC on 12 June 2020 (Click to enlarge)


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.

Derecho in South Dakota

June 7th, 2020 |


GOES-16 ABI Band 13 (10.3 µm) infrared imagery, 1901 6 June 2020 – 0656 7 June 2020 (Click to play mp4 animation)

 

Portions of the High Plains and intermountain states experienced a climatologically rare Derecho event oni 6-7 June 2020. (Here is a preliminary write-up on this event from the National Weather Service in Rapid City SD;  the forecast office in Boulder discussed the event here.). The GOES-16 Clean window infrared (10.3 µm) animation, above, (Click here for the same animation as an animated gif) shows rapid development over western South Dakota late in the afternoon of 6 June. The swath of wind reports is shown in this graphic from the Storm Prediction Center.

Several satellite-based thermodynamic estimates keyed in on South Dakota as a region where instability was noteworthy. The GOES-16 All-Sky Convective Available Potential Energy (available here), shown below from 2026 UTC on 6 June when values were greatest, for example, showed a persistent corridor of instability across South Dakota.

GOES-16 ‘All-Sky’ estimates of Convective Available Potential Energy, 2026 UTC on 6 June 2020 (Click to enlarge)

NUCAPS estimates of 700-500 mb lapse rates, below (from this site), show pronounced instability upstream of South Dakota at 1945 UTC, when Suomi-NPP overflew the region. (Most of the soundings used to produce the lapse rate information were from successful infrared retrievals as shown in this graphic).

700-500 mb Lapse Rates derived from Suomi NPP NUCAPS soundings, 1945 UTC on 6 June 2020 (Click to enlarge)

Surface moisture had pooled over western South Dakota. That is shown in the plot below of surface dewpoints showing very unusual (for South Dakota) mid-60s dewpoints! Further evidence of the unusual moisture amounts over the high Plains (for early June) is in this sounding from Rapid City at 0000 UTC on 7 June (source); Precipitable Water is at 1.2″! This value is unusual for the location and time of year, as shown here (Source).

Surface Dewpoints, 2100 UTC on 6 June 2020 over South Dakota and surrounding states (Click to enlarge)

GOES-17 Full-Disk imagery (at 10-minute time-steps) captured an oblique view of the developing convection. (The ‘PACUS’ sector with 5-minute imagery terminates in west-central South Dakota so is not used here; A GOES-17 Mesoscale sector was not in place for this event, although a GOES-16 one was).

GOES-17 Visible Imagery (0.64 µm) on 7 June 2020, 0000 – 0220 UTC (Click to animate)

1-minute Mesoscale Domain Sector GOES-16 “Red” Visible images with time-matched plots of SPC Storm Reports are shown below.

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]