Rope cloud feeding into Tropical Invest 98L

September 22nd, 2021 |

GOES-16 True Color RGB images [click to play animation | MP4]

GOES-16 (GOES-East) True Color RGB images created using Geo2Grid (above) showed a long rope cloud feeding into the western portion of Tropical Invest 98L in the eastern Atlantic Ocean on 22 September 2021. The rope cloud was located near the African Monsoon Trough (12 UTC surface analysis), and north of the rope cloud the hazy appearance of a dust-laden Saharan Air Layer was apparent — so this rope cloud feature likely marked the boundary between dry SAL air to the north and moist tropical air to the south (as seen in the MIMIC Total Precipitable Water product). In addition, Metop-A ASCAT surface scatterometer winds at 0956 UTC and 2104 UTC showed speed and/or directional convergence in the vicinity of the rope cloud (between 10-15 N latitude). Also of interest was the effect of the Cabo Verde islands on marine boundary layer clouds beneath the SAL. On the following day, Invest 98L intensified to become Tropical Storm Sam, as discussed in this blog post.

The hazy Saharan Air Layer was also seen in a Suomi NPP VIIRS True Color image at 1439 UTC, as visualized using RealEarth (below). The west-to-east oriented rope cloud was located around 11-12 N latitude.

VIIRS True Color RGB image from Suomi NPP [click to enlarge]

The “Saharan Air Layer” (SAL) Split Window Difference product from the CIMSS Tropical Cyclones site (below) depicted the westward advance of the dusty SAL air off northwestern Africa.

GOES-16 “Saharan Air Layer” Split Window Difference product [click to play animation]

Hurricane Nicholas makes landfall in Texas

September 14th, 2021 |
GOES-16 Mesoscale Sector infrared imagery (Band 13, 10.3), 0321-0844 UTC on 14 September 2021

Hurricane Nicholas made landfall on the Gulf Coast shortly after 0530 UTC on 14 September. It is a challenge to determine the storm center from the animation above, although deep convection is suggestive of its location. Note the collapse of deep convection as well at the end of the animation. Radar imagery, below (from this site), is helpful in placing the storm center.

NEXRAD Composite Reflectivity, 0524, 0534, 0544, 0554 UTC on 14 September 2021 (Click to enlarge)

MetOp-B overflew Nicholas at 0315 UTC shortly before landfall. ASCAT winds (from this site) at 0315 UTC show the circulation center very close to the shoreline. A similar image from the OSI SAF Multiplatform viewer is here.

Metop-B ASCAT winds, 0314 UTC on 14 September 2021 (Click to enlarge)

Nicholas is embedded in very moist air. A 24-hour MIMIC Total Precipitable Water animation, below, shows the moisture plume. Heavy rain is forecast for the central Gulf Coast.

Total Precipitable Water, 1000 UTC on 13 September – 0900 UTC 14 September (click to enlarge)

Nicholas has weakened to a Tropical Storm as of 0900 UTC on 14 September. Refer to the National Hurricane Center website for more information.

Sun glint and calm winds

August 2nd, 2021 |

True-color imagery from the VIIRS Today website, below (click here for a direct link to the image below at the VIIRS Today site), shows an unusual sunglint pattern over the eastern Gulf of Mexico, to the southwest of the Florida peninsula. Typically, sunglint features are fairly wide in VIIRS imagery, as evidenced from this Suomi-NPP image, also from 2 August, but to the east of Florida. However, the winds over the eastern Gulf were very light on 2 August, so surface wave action was reduced. When the ocean approaches glassy calm, solar reflection becomes more unidirectional, and a brighter spot becomes visible in True-Color imagery. When seas are choppier (as was the case to the east of Florida), solar reflection off the ocean is diffuse, and a less concentrated region of brightness results. An good analogy might be reflection off flat aluminum foil (representing a flat sea state) or very crinkled aluminum foil (representing an agitated sea state).

NOAA-20 True Color Imagery over the eastern Gulf of Mexico from 2 August 2021 (Click to enlarge)

What evidence is present of light winds? Consider the Metop-A Scatterometry image, below, from this site. Both the ascending pass (about 0145 UTC) and descending pass, below (about 1410 UTC; orbit imagery available here, from this site) show very weak winds over the Gulf to the southwest of Florida.

Metop-A Scatterometer winds from the ca. 1410 UTC descending pass, 2 August 2021 (Click to enlarge)

The effect of the sun glint occurs in both visible and shortwave infrared channels. The imagery below, downloaded from the CIMSS Direct Broadcast ftp site (, from this ephemeral site — — in particular), shows the five Image bands from VIIRS, a prominent signal is apparent in all but the longest wavelength.

NOAA-20 VIIRS I-Bands 01-05 over the southeastern Gulf of Mexico, 1842 UTC on 2 August 2021 (Click to animate)

Does the sunglint affect products? The Cloud Mask product is tricked by the large reflectance into believing a cloud is present, as highlighted by the box below. The image also includes Cloud Type and Cloud Phase. Because liquid water clouds are believed present, the ACSPO Sea-surface temperature algorithm (at bottom) produces no values in that region. Careful inspection of the image, however, shows marginally warmer waters (a lighter pink color is apparent) adjacent to the sunglint where the lack of wind means surface mixing of waters is suppressed and a thin layer of very warm water can develop on the top of the ocean.

CLAVR-X Cloud Mask product, 1842 UTC on 2 August 2021, along with Cloud Type (upper right) and Cloud Phase (lower right).
ACSPO SSTs at 1842 UTC on 4 August 2021

This animation steps through all the VIIRS M-bands. The sunglint is apparent in nearly all of them, except for wavelengths longer than 4 µm. The sunglint is also missing from the 1.38 µm (M09) imagery because of strong absorption of energy at that wavelength by water vapor.

Other examples of diagnosing areas of light winds over water (via a lack of sunglint or moonglint) can be found here, here, here and here.

Subtropical storm Raoni off the coast of South America

June 28th, 2021 |

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]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) showed the circulation of Subtropical Storm Raoni (discussion issued by MARINHA) off the coast of Argentina/Uruguay on 28 June 2021.

In the corresponding GOES-16 “Clean” Infrared Window (10.35 µm) images (below), the coldest cloud tops exhibited infrared brightness temperatures around -50ºC (shades of yellow).

GOES-16 “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

GOES-16 “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

===== 29 June Update =====

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]

GOES-16 Visible images (above) showed that Subtropical storm Raoni continued its northeastward motion, and was located off the coast of far southeastern Brazil on 29 June (12 UTC surface analysis | discussion).

GOES-16 Infrared images (below) indicated that Raoni was unable to maintain a persistent closed ring of cold clouds around its center.

GOES-16 “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

GOES-16 “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

A sequence of ASCAT surface scatterometer winds (source) from Metop-A and Metop-C (below) revealed swaths of wind speeds of 40 knots or greater within the northeastern and southwestern sectors of the system.

ASCAT winds from Metop-A and Metop-C [click to enlarge]

ASCAT winds from Metop-A and Metop-C [click to enlarge]