Extratropical transition of Hurricane Oscar

October 31st, 2018 |

Sequence of GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images [click to play MP4 animation]

Sequence of GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images [click to play MP4 animation]

A sequence of GOES-16 (GOES-East) Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) showed the transition of Category 1 Hurricane Oscar to a hurricane-force extratropical storm over the western Atlantic Ocean on 31 October 2018. The very sharp moisture gradient that developed along the western and northern edges highlighted the “”limiting streamline” of airflow through the cyclone — and a remarkably warm/dry signature (yellow to orange enhancement) was seen near the storm center late in the period.

A closer view of the GOES-16 Water Vapor imagery using AWIPS is shown below, which includes plots of buoy/ship reports and surface analyses.

Sequence of GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images [click to play MP4 animation]

Sequence of GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images [click to play MP4 animation]

Fog/stratus dissipation in southern Louisiana

October 30th, 2018 |

GOES-16

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

The topic of a conversation on Twitter, GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) revealed curious circular areas of fog/stratus dissipation across southern Louisiana on the morning of 30 October 2018. — making it a natural candidate for the “What the heck is this?” blog category.

GOES-16 GEOCAT Low IFR Probability and Fog/Low Stratus Depth products (below) indicated that this fog and low stratus had been increasing in coverage and spreading northward across Louisiana during the preceding nighttime hours (VIIRS fog/stratus Brightness Temperature Difference images) — and the fog/stratus was relatively shallow, only having a depth of about 300 feet or less. In fact, if you look closely at the Visible animation above, a few small spots of slightly brighter cloud can be seen in the vicinity of Baton Rouge KBTR which are tall objects (such as refinery stacks, and even the State Capitol building) protruding above the fog/stratus and acting as an obstacle to their flow.

GOES-16 Low Instrument Flight Rules (IFR) Probability [click to play animation | MP4]

GOES-16 Low Instrument Flight Rules (IFR) Probability [click to play animation | MP4]

GOES-16 Fog/Low Stratus Depth product [click to play animation | MP4]

GOES-16 Fog/Low Stratus Depth product [click to play animation | MP4]

A sequence of 4-panel comparisons of GOES-16 “Blue” Visible (0.47 µm), “Red” Visible (0.64 µm) and Near-Infrared “Vegetation” (0.86 µm) images with Near-Infrared “Snow/Ice” (1.61 µm), Near-Infrared “Cloud Particle Size” (2.24 µm) and Shortwave Infrared (3.9 µm) images (below) showed no indication of any substantial differences between the cloud material within the circular features and the adjacent fog/stratus. The largest “outer rings” of the dissipating fog/stratus areas had a small amount of vertical extent, which cast a shadow that was best seen in the Near-Infrared 0.86 µm and 1.61 µm images.

4-panel comparisons of GOES-16

Sequence of 4-panel comparisons of GOES-16 “Blue” Visible (0.47 µm), “Red” Visible (0.64 µm), Near-Infrared “Vegetation” (0.86 µm), “Snow/Ice” (1.61 µm), and “Cloud Particle Size” (2.24 µm), and Shortwave Infrared (3.9 µm) images [click to play animation | MP4]

The most plausible explanation for the circular dissipation features turned out to be fires that were set in sugar cane fields following harvest — particulates in the smoke could have “seeded” the fog/stratus cloud layer, either changing the particle size distribution or making the cloud more susceptible to faster dissipation after sunrise due to solar heating of black carbon nuclei within the cloud droplets.  An Aqua MODIS Shortwave Infrared (3.7 µm) image from the previous afternoon (below) did reveal a number of small thermal anomalies or fire “hot spots” (yellow to red pixels) across the region at 1909 UTC (2:09 PM local time).

Aqua MODIS Shortwave Infrared (3.7 µm) image [click to enlarge]

Aqua MODIS Shortwave Infrared (3.7 µm) image [click to enlarge]

Similarly, GOES-16 Shortwave Infrared images on 29 October (below) also showed signatures of widespread small and generally short-lived fires (darker black pixels) across southern Louisiana. Surface winds were very light across that area (KARA | KPTN | KNBG | KMSY | KNEW), minimizing smoke dispersion from any fires.

GOES-16 Shortwave Infrared (3.9 µm) images [click to play animation | MP4]

GOES-16 Shortwave Infrared (3.9 µm) images [click to play animation | MP4]

Pyrocumulonimbus cloud in South Africa

October 29th, 2018 |

Meteosat-11 Visible (0.8 µm), Shortwave Infrared (3.92 µm) and Longwave Infrared Window (10.8 µm) images [click to play animation | MP4]

Meteosat-11 Visible (0.8 µm, top), Shortwave Infrared (3.92 µm, center) and Longwave Infrared Window (10.8 µm, bottom) images [click to play animation | MP4]

The Garden Route Fires had been burning since about 24 October 2018 near George along the southern coast of South Africa (media story). On 29 October, EUMETSAT Meteosat-11 High Resolution Visible (0.8 µm), Shortwave Infrared (3.92 µm) and Longwave Infrared Window (10.8 µm) images (above) showed an elongated west-to-east oriented thermal anomaly or fire “hot spot” (red pixels) just northeast of George (station identifier FAGG) on Shortwave Infrared imagery during the hours leading up to the formation of a pyrocumulonimbus (pyroCb) cloud around 1300 UTC. The pyroCb exhibited the characteristic warm (+10 to +15ºC, darker gray enhancement) shortwave infrared cloud-top signature just off the coast at 1315 UTC, — this is due to enhanced solar reflection off ice crystals that are smaller compared to those of conventional thunderstorm tops.

Zooming out a bit to follow the southeastward drift of the pyroCb cloud (below), the coldest cloud-top 10.8 µm infrared brightness temperature (BT) was -61ºC (darker red enhancement) at 1315 UTC — then the cloud tops remained in the -55 to -59ºC range (orange enhancement) for the next 6 hours or so. Leveraging the large difference between cold 10.8 µm and warm 3.92 µm BTs, NRL calculates a pyroCb index, which classified this feature as an “intense pyroCb” (1315 UTC | animation). The coldest 10.8 µm cloud-top BT of -61ºC roughly corresponds to an altitude of 13.5 km based on 12 UTC rawinsonde data from Port Elizabeth (plot | list).

Meteosat-11 Shortwave Infrared (3.92 µm, left) and Longwave Infrared Window (10.8 µm, right) images [click to play animation | MP4]

Meteosat-11 Shortwave Infrared (3.92 µm, left) and Longwave Infrared Window (10.8 µm, right) images [click to play animation | MP4]

Imagery from NOAA-19 at 1420 UTC (courtesy of René Servranckx) also revealed the warm (dark gray) Shortwave Infrared pyroCb signature, along with a minimum cloud-top infrared BT of -58.1ºC (below).

NOAA-19 AVHRR imagery at 1420 UTC [click to enlarge]

NOAA-19 AVHRR imagery at 1420 UTC [click to enlarge]

A Suomi NPP VIIRS True Color Red-Green-Blue (RGB) image at 1230 UTC (below) was about a half hour before the formation of the pyroCb, but it did show a signature of smoke drifting southeastward off the coast.

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

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

On the following day (30 October), a NOAA-20 VIIRS True Color image (below) showed the classic comma cloud signature of a mid-latitude cyclone south of the coast, with the band of cold-frontal clouds extending northward across Lesotho. Note the thick plume of smoke spreading eastward within the strong post-frontal westerly winds.

NOAA-20 VIIRS True Color RGB image [click to enlarge]

NOAA-20 VIIRS True Color RGB image [click to enlarge]

A time series of of surface observations from George (below) supported the idea of a cold frontal passage: ahead of the front, temperatures rapidly rose to 104ºF/40ºC (with a dew point of 39ºF/4ºC) on 28 October about 1.5 hours prior to the formation of the pyroCb — then strong westerly winds (gusting to 40 knots/21 mps) with rising pressures and falling temperatures followed on 30 October.

Time series plot of of surface observations from George [click to enlarge]

Time series plot of of surface observations from George [click to enlarge]

The pyroCb research community believes that this is the first documented case of a pyroCb on the African continent.

 

Flood wave along the Nueces River in Texas

October 27th, 2018 |

As pointed out by NWS Corpus Christi (above), GOES-16 (GOES-East) Near-Infrared “Vegetation” (0.86 µm) images revealed an interesting flood wave moving along the Nueces River on 27 October 2018 (following a recent period of heavy rainfall).

A toggle between before (10 October) and after (27 October) Aqua MODIS False Color Red-Green-Blue (RGB) images from the MODIS Today site (below) showed dramatic differences between the amount of water (darker shades of blue) flowing along portions of the Nueces River on those 2 days.

Before (10 October) and after (27 October) Aqua MODIS False Color RGB images [click to enlarge]

Before (10 October) and after (27 October) Aqua MODIS False Color RGB images [click to enlarge]

A comparison of Suomi NPP VIIRS Visible (0.64 µm), Near-Infrared Vegetation (0.86 µm) and Near-Infrared Snow/Ice (1.61 µm) images from 27 October (below) demonstrated the improved land/water contrast of the Near-Infrared imagery, which makes it helpful for diagnosing certain types of flooding signatures.

Suomi NPP VIIRS Visible (0.64 µm), Near-Infrared Vegetation (0.86 µm) and Near-Infrared Snow/Ice (1.61 µm) images [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm), Near-Infrared Vegetation (0.86 µm) and Near-Infrared Snow/Ice (1.61 µm) images [click to enlarge]

===== 28 October Update =====

GOES-16 Near-Infrared

GOES-16 Near-Infrared “Vegetation” (0.86 µm) images at 1552 UTC on 27 and 28 October [click to enlarge]

A toggle between GOES-16 Near-Infrared “Vegetation” (0.86 µm) images at 1552 UTC on 27 and 28 October (above) showed the advance of the flood wave during that 24-hour period.

A comparison of Suomi NPP VIIRS Near-Infrared “Vegetation (0.86 µm) and “Snow/Ice” (1.61 µm) images from the early afternoon hours on 27 and 28 October (below) displayed these 24-hour changes at a higher spatial resolution (375 meters, vs 1 km at satellite subpoint with GOES-16). The rear edge of the flood wave (located about 25 miles southeast of Cotulla) appeared to show up a bit better in the 0.86 µm images than the 1.61 µm.

Suomi NPP VIIRS Near-Infrared

Suomi NPP VIIRS Near-Infrared “Vegetation (0.86 µm) and “Snow/Ice” (1.61 µm) images from 27 and 28 October [click to enlarge]

Finally, in a toggle between 250-meter resolution Aqua MODIS False Color RGB images from 27 and 28 October (below), the advance of the leading edge of the flood wave can clearly be seen.

Aqua MODIS False Color RGB images from 27 and 28 October [click to enlarge]

Aqua MODIS False Color RGB images from 27 and 28 October [click to enlarge]

===== 29 October Update =====

GOES-16 Near-Infrared

GOES-16 Near-Infrared “Vegetation” (0.86 µm) images from 1552 UTC on 27, 28 and 29 October [click to enlarge]

GOES-16 Near-Infrared “Vegetation” images from 1552 UTC on 27, 28 and 29 October (above) showed the continued eastward movement of the flood wave down the Nueces River.