Strong gap flow into the Gulf of Tehuantepec

November 26th, 2021 |
GOES-16 True Color imagery, 1330 – 1520 UTC on 26 November 2021

GOES-16 True-Color imagery from the CSPP Geosphere site (link showing the data above) on 26 November, above, show features associated with strong flow through Chivela Pass in southern Mexico, gap winds often called Tehuano winds or Tehuantepecers. Strong descent associated with these events can often limit the presence of clouds that can be used as tracers. However, scatterometry (from this website) will show surface winds, and an MetopB overpass shortly after the end of the animation above, below, shows a core of strong winds over the ocean.

ASCAT Winds from Metop-B, 1532 UTC on 26 November 2021 (Click to enlarge)

The GOES-16 CONUS domain extends southward to the northern part of the Gulf of Tehuantepec (about 14.6 N Latitude). Visible imagery from 1516 UTC, below, is overlain with the Derived Motion Wind vectors (in the surface – 900 mb layer) at the same time. Strong northerly winds north of Chivela Pass are apparent, but the lack of clouds to track in the Gulf prevented the inference of winds there from the GOES-16 data.

GOES-16 Visible Imagery (Band 2, 0.64 µm) and Derived Motion Winds, surface-900 mb, 1516 UTC 26 November 2021 (Click to enlarge)

The strong winds are also associated with a local increase in Aerosol Optical Depth (AOD), as shown below.

GOES-16 Aerosol Optical Depth (AOD) at 1520 UTC on 26 November 2021 (click to enlarge)

Strong winds will cause significant mixing in the upper part of the ocean, which will result in cooling. Imagery from this website (shown below) shows cooling in the Gulf from previous events. Here is an animation from that website, courtesy Tim Schmit, NOAA/NESDIS/STAR

SST analysis valid at 24 November 2021 (Click to enlarge)

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GOES-17 True Color RGB images [click to play animated GIF | MP4]

In GOES-17 True Color images created using Geo2Grid (above), enhanced forward scattering during the morning hours helped to highlight the offshore transport of airborne dust.

Other blog posts discussing Tehuano wind events can be found here.

Blowing dust in Argentina

November 24th, 2021 |

GOES-16 “Red” Visible (0.64 µm), Dust RGB and Split Cloud Top Phase (11.2 µm – 8.4 µm) BTD images [click to play animated GIF | MP4]

30-second Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm), Dust RGB and Split Cloud Top Phase (11.2 µm – 8.4 µm) brightness temperature difference (BTD) images (above) revealed a plume of blowing dust propagating northward across the San Juan Province of western Argentina late in the day on 24 November 2021. The dust was being channeled through a gap in higher terrain along the foothills of the Andes (below).

GOES-16 “Red” Visible (0.64 µm) and topography images [click to enlarge]

A larger-scale view of hourly GOES-16 Visible images with plots of surface reports (below) suggested that this dust occurred in the vicinity of a strong cold front that was moving northward across Argentina.

GOES-16 “Red” Visible (0.64 µm) images, with METAR surface reports plotted in cyan [click to enlarge]

Mendoza — located south-southwest of where the dust plume first became apparent in GOES-16 imagery — reported a thunderstorm with dust at 2000 UTC (along with a southeasterly wind gust to 32 knots), followed by a reduction of surface visibility to 0.5 miles at 2215 UTC as the air temperature sharply dropped with the cold frontal passage (below). About 260 miles (420 km) east of Mendoza at Rio Cuarto, a similar sharp temperature drop was seen as the cold front passed.

Time series of surface data at Mendoza, Argentina [click to enlarge]

Hole punch cloud features over Wisconsin and Illinois

November 22nd, 2021 |

GOES-16 “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images [click to play animated GIF | MP4]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images (above) revealed the formation of several “hole punch” features across southeastern Wisconsin, northeastern Illinois and Lake Michigan on 22 November 2021 . These cloud features were caused by aircraft that were either ascending or descending through a relatively thin layer of clouds composed of supercooled water droplets — cooling from wake turbulence (reference) and/or particles from the jet engine exhaust acted as ice condensation nuclei, causing the small supercooled water droplets to turn into larger ice crystals (many of which then fall from the cloud layer, creating “fallstreak holes“). The ice crystal clouds appear as darker shades of gray on the 1.61 µm Snow/Ice images.

The GOES-16 Cloud Top Temperature derived product (below) showed that values were generally in the -30 to -35ºC range.

GOES-16 Cloud Top Temperature product [click to play animated GIF | MP4]

A toggle between 250-meter resolution Terra MODIS True Color and False Color RGB images from the MODIS Today site (below) provided a more detailed view of the numerous hole punch features at 1730 UTC, including a better depiction of the glaciated fallstreak clouds (shades of cyan) within the middle of each hole punch.

Terra MODIS True Color and False Color RGB images [click to enlarge]

Other blog posts showing examples of hole punch features can be found at this link.

Day Cloud Phase Distinction RGB and Lake Effect Snow

November 15th, 2021 |
GOES-16 Day Cloud Phase Distinction (see text for details) at 1331 UTC and NEXRAD Reflectivity at 1333 UTC, both on 15 November 2021 (Click to enlarge)

The toggle above shows NEXRAD radar (source) inset during lake-effect precipitation with two versions of the GOES-16 Day Cloud Phase Distinction RGB, one with default values, and one with ‘Green’ (ABI Band 2 Visible imagery at 0.64 ?m) and ‘Blue’ (ABI Band 5 at 1.61 ?m) reduced from default values, from 79% to 60% for Band 2, and from 59% to 50% for Band 5 (by using the composite option feature in AWIPS). It can be advantageous to alter the bounds on some RGBs during periods of low light (i.e., sunrise and sunset) to accentuate features. However, make certain at some point to reload with the default value! As the sun rises higher into the sky, features will start to look unfamiliar if the modified RGB menu remains.

The radar/satellite comparison bears comment. First, note that the location of circled features is not quite the same, due to parallax (discussed here and here): The effect of parallax is that clouds are shifted away from the sub-satellite point in AWIPS. The effect is most pronounced for towering summertime thunderstorms, but even clouds producing lake-effect snow, clouds that are far more shallow, will be shifted.

The Day Cloud Phase Distinction RGB will show clouds with more of a yellow tint when those clouds glaciate, as might be expected in more intense lake-effect snow-producing clouds. As clouds glaciate, the ‘blue’ part of this particular RGB is reduced: ice crystals within the cloud absorb (rather than reflect) solar energy at 1.61 ?m. In the toggle above, clouds with a modest yellow enhancement in the RGB align well in three circled regions where radar suggests vigorous precipitation might be falling. This is a seasonal reminder, then, to use the Day Cloud Phase Distinction to highlight regions — during the day — where lake-effect might be most impactful.


NEXRAD Reflectivity (left) and GOES-16 Day Cloud Phase Distinction RGB (right), 1551 UTC on 16 November 2021 (Click to enlarge)

A similar example is shown above from 16 November. NEXRAD Radar (on the right) shows a single band with strongest returns stretching east-southeastward from just north of Rochester. Note the distinct color change to the cloud band in the RGB that overlays the strongest radar returns. Similar colors in the RGB are also present east of eastern Lake Ontario, where radar returns are also present.