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.

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)

Microwave rain estimates with an MCS

July 29th, 2021 |
VIIRS NPP Day Night Band visible (0.70 µm) imagery from Suomi-NPP (0740 UTC) and NOAA-20 (0831 UTC) on 29 July 2021 (Click to enlarge)

Severe thunderstorms developed (in a moderate risk region from the Storm Prediction Center; here are the Storm Reports) over northern Wisconsin late in the day on 28 July 2021 and moved to the southeast (here is an mp4 animation of GOES-16 imagery courtesy of Tim Schmit, NOAA). The Day Night Band imagery, above, from Suomi NPP (0740 UTC) and NOAA-20 (0831 UTC), shows snapshots of the storms as they moved southward into Illinois. The 0740 UTC image include more evidence of lightning — especially in southwestern Wisconsin (the horizontal streaks of light) and just southwest of Dubuque. At 0831 UTC, lightning is not detected in the Day Night Band image.

In addition to carrying the VIIRS (Visible-Infrared Imaging Radiometer Suite) instrument, Suomi-NPP and NOAA-20 also carry the Advanced Technology Microwave Sounder (ATMS), and microwave data from that instrument can be used to infer rain rates (using MIRS — Microwave Integrated Retrieval System — algorithms, that are part of Community Satellite Processing Package — CSPP — software available to use at Direct Broadcast sites). The toggles show the Day Night Band image, the ATMS-derived Rain Rate (the green region at 0742 UTC is >3″/hour!), and the base reflectivity at 0740 UTC (below) and at 0831 UTC (bottom). The MIRS Rain Rate (0742 UTC; 0833 UTC) product does outline regions where rain is likely falling, and gives credible values where the heaviest rains are falling. Note the diminishing rain rate over southwestern WI, for example, between 0740 and 0830 UTC — indicative of weaker convection — an observation echoed in the changes in lightning detection with the Day Night band. Changes in radar to reflect that difference in microwave-estimated rain rate are a little more subtle.

Because microwave estimates of rain rate are affected by background emissivity, and because water has a much lower microwave emissivity than land, you can sometimes view land/water boundaries (as in the 0742 UTC Rain Rate, below).

MIRS Rain Rate gives useful information about rains when radar observations cannot be accessed.

Suomi NPP Day Night Band visible (0.70 µm) image, ATMS estimates of Rain Rate, and 0.5-degree Reflectivity from a radar composite, all at 0740 UTC on 29 July 2021 (Click to enlarge)
NOAA-20 Day Night Band visible (0.70 µm) image, ATMS estimates of Rain Rate, and 0.5-degree Reflectivity from a radar composite, all at 0830 UTC on 29 July 2021 (Click to enlarge)

MIRS Rain Rate products are available via an LDM feed from CIMSS; they are produced using the Direct Broadcast antenna at CIMSS and are thus very timely.

Tropical disturbance off the US Southeast Coast

July 25th, 2021 |
CSPP GeoSphere animation, 1240-1550 UTC on 25 July 2021

The National Hurricane Center is monitoring an area of disturbed weather over the western Atlantic, to the north of the Bahamas and south of Cape Hatteras. The three-hour animation from the CSPP GeoSphere site (link, above), shows convection and a small low-level cyclonic circulation. This system is drifting to the east, towards Florida, and is in an environment of small values of vertical wind shear (the analysis below is from the CIMSS Tropical Weather Site) that could augur further development. Refer to the pages of the National Hurricane Center for more information.

200-850 Shear Values, 1400 UTC on 25 July 2021. The disturbance center is denoted with an I.

The Day Night Band image from Suomi NPP, below, from the early morning of 25 July 2021, (from the VIIRS Today website) shows the storm under the waning Buck Full Moon. Compare that to the NOAA-20 Day Night Band image from 24 July, the night before (link): the amount of convection decreased between 24 and 25 July.

Suomi NPP Day Night Band imagery, 25 July 2021

On the morning of 26 July 2021, the disturbance is moving into Florida/Georgia. Convection associated with the system is not over its center. (CSPP Geosphere link)

CSPP Geosphere animation, 1320-1610 UTC on 26 July 2021

Thunderstorms over the Chukchi Sea and Beaufort Sea north of Alaska

July 12th, 2021 |

Suomi NPP VIIRS Infrared Window (11.45 µm) and Visible (0.64 µm) images [click to play animation | MP4]

Suomi NPP VIIRS Infrared Window (11.45 µm) and Visible (0.64 µm) images [click to play animation | MP4]

A sequence of Suomi NPP VIIRS Infrared Window (11.45 µm) and Visible (0.64 µm) images (above) showed snapshots of thunderstorms over parts of the Chukchi Sea and the Beaufort Sea off the northern coast of Alaska on 12 July 2021. The coldest convective cloud-top infrared brightness temperatures were in the -30 to -40ºC range. Unusual aspects of these thunderstorms included their high latitude location over ice-covered waters — as far north as 75ºN latitude — and the large amount of cloud-to-surface lightning strikes that they produced.



These thunderstorms were not surface-based — instead, they were forced by an approaching cold front (surface analyses) which helped to release elevated instability within the 500-300 hPa layer (below).

Suomi NPP VIIRS Infrared Window (11.45 µm) images, with contours of NAM40 lapse rate within the 500-300 hPa layer [click to enlarge]

Suomi NPP VIIRS Infrared Window (11.45 µm) images, with contours of NAM40 lapse rate within the 500-300 hPa layer [click to enlarge]

Rawnsonde data from Utqiagvik (PABR) were not available (due to ongoing equipment malfunction at that site) — but a NUCAPS profile near the southernmost cluster of convection around 15 UTC (below) showed the layer of instability aloft.

NUCAPS profile near thunderstorms off the northern coast of Alaska [click to enlarge]

NUCAPS profile near thunderstorms off the northern coast of Alaska [click to enlarge]