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 (https://ftp.ssec.wisc.edu/pub/eosdb/j01/viirs/, from this ephemeral site — https://ftp.ssec.wisc.edu/pub/eosdb/j01/viirs/2021_08_02_214_1837/images/ — 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]

Tropical Invest 90L becomes Subtropical Storm Ana in the Atlantic

May 21st, 2021 |

GOES-16 Visible and Infrared images [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (above) displayed the exposed low-level circulation of Invest 90L, in the Atlantic Ocean (about 150 miles northeast of Bermuda, TXKF) on 21 May 2021.

GOES-16 Visible images with plots of Visible Derived Motion Winds (below) revealed a few wind speeds of 50 knots or greater (red wind barbs) within its northwest quadrant — but since no organized and sustained deep convection remained in close proximity to the low-level circulation, Invest 90L was not yet considered to be a tropical cyclone.

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, with plots of Visible GOES-16 Derived Motion Winds [click to play animation | MP4]

===== 22 May Update =====

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]

During the overnight hours, convection organized and intensified around the low-level circulation of Invest 90L — and as of 0900 UTC it was classified as Subtropical Storm Ana. GOES-16 Infrared images of Ana are shown above.

A closer view of GOES-16 Visible images (below) indicated that the center of Ana eventually stopped moving southwestward toward Bermuda, performed a counterclockwise loop, then began moving to the northeast.

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 with plots of Derived Motion Winds (below) indicated that the maximum near-surface wind speeds were 39 knots.

GOES-16 "Red" Visible (0.64 µm) images, with plots of Derived Motion Winds [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm) images, with plots of Visible GOES-16 Derived Motion Winds [click to play animation | MP4]

GOES-16 "Red" Visible (0.64 µm) images, with plots of Metop-A ASCAT winds [click to enlarge]

GOES-16 “Red” Visible (0.64 µm) image at 1219 UTC, with plots of Metop-A ASCAT winds [click to enlarge]

An overpass of Metop-A at 1219 UTC provided ASCAT surface scatterometer winds centered on Ana (above), with a maximum speed value of 31 knots just northwest of the storm center. In general, Visible GOES-16 Derived Motion Wind speed values were around 5 knots faster than nearby ASCAT winds (below) — since the former are computed by tracking cloud targets that are above the surface, where winds speeds are greater (due to a lack of surface friction).

GOES-16 "Red" Visible (0.64 µm) image at 1219 UTC, with plots of Visible GOES-16 Derived Motion Winds and Metop-A ASCAT winds [click to enlarge]

GOES-16 “Red” Visible (0.64 µm) image at 1219 UTC, with plots of Visible GOES-16 Derived Motion Winds and Metop-A ASCAT winds [click to enlarge]