VIIRS Day Night Band imagery of Beta off the coast of Texas

September 21st, 2020 |

VIIRS Day Night Band Visible (0.7 µm) imagery and I05 11.45 µm imagery over the western Gulf of Mexico, 0813 UTC on 21 September 2020 (Click to enlarge)

Suomi NPP overflew Tropical Storm Beta off the coast of Texas shortly after 0820 UTC on 21 September (NPP orbits over North America on 21 September are shown below, taken from this site). Day Night band visible imagery shows the swirl of clouds at the center of the storm, off the coast of Texas south of Houston/Galveston and east of Corpus Christi. The 11.45 µm infrared imagery (created using CSPP software and the DB data at CIMSS, and available to NWS offices via an LDM feed) shows the convection that surrounds this center, and also the stronger convection over the Gulf of Mexico to the east.

Both visible and infrared imagery in this case show the storm center. That is not always the case. Sometimes the Day Night band alone identifies the storm center without ambiguity. The Day Night Band at 0813 observed no lightning. The light sources over the open Gulf of Mexico south of Louisiana are drilling platforms.

Suomi NPP Orbit Paths over North America, 21 September 2020 (Click to enlarge)


VIIRS Day Night Band Visible (0.7 µm) imagery over the eastern Gulf of Mexico at 0633 UTC on 21 September 2020 (Click to enlarge)

The Suomi NPP pass over the eastern Gulf of Mexico, above, from 06 UTC, shows one horizontal streak of brightness in the central Gulf that is a lighting bolt. The Geostationary Lightning Mapper on GOES-16 also observed lightning flashes — how do the two observations compare?

To make that comparison, it’s necessary to determine exactly when Suomi-NPP overflew the eastern Gulf, and that’s suggested in the orbital path figure above. The time stamp for satellite imagery is not the precise time that scanning occurred; historically, the nominal time of an image is the time of the first scan line in the image. For this descending Suomi NPP pass (the satellite is moving from north to south), that time stamp — 0633 UTC — occurred when Suomi NPP was far north of CONUS, north of Quebec.  (Similarly, the time of the 0813 UTC pass shown above has a time stamp when the Satellite was viewing north of Hudson Bay!  During the afternoon, the time stamp for those ascending passes (the satellite is moving from south to north) occurs when the satellite is far south of CONUS). The actual orbit path mapping suggests a scan time over the Gulf Coast at 0643 or 0644 UTC.  Toggles between the Day Night Band image with the lightning streak and the GLM 1-minute observations at 0643 UTC  and at 0644 UTC are shown below.   It appears that the 0644 UTC data better matches the Day Night band imagery, but the comparison is by no means obvious.  This bears further investigation!

VIIRS Day Night Band visible imagery and GLM Observations at 0643 UTC on 21 September 2020 (Click to enlarge)

VIIRS Day Night Band visible imagery and GLM Observations at 0644 UTC on 21 September 2020 (Click to enlarge)

 

Tropical Storm Beta in the Gulf of Mexico

September 20th, 2020 |

GOES-16 Imagery and Derived Motion winds at 1346 UTC on 20 September 2020 (Click to enlarge). ABI Imagery includes Visible (Band 2, 0.64 µm), the near-infrared ‘Cirrus’ (Band 4, 1.38 µm) and the upper water vapor (Band 8, 6.19 µm) and lower water vapor (Band 10, 7.34 µm) infrared imagery. Derived motion winds for 1346 UTC near 1000 mb (green), 850 mb (yellow), 700 mb (orange), 500 mb (cyan) and 300 mb (purple) are also shown

Tropical Storm Beta was in the northwest Gulf of Mexico on 20 September 2020. Visible imagery (with GLM overlain) shows two principal regions of convection, one near the center, and one in a long feeder band to the east of the storm. Derived motion winds (this image includes a legend that links vector color to level) show cyclonic low-level motion in the northwest Gulf of Mexico, divergent motion at 500 mb, and strong westerly outflow at 300 mb.

‘Cirrus Channel’ (Band 4, 1.38 µm) near-infrared imagery shows the considerable upper-level cloudiness associated with the central convection and the convective band east of the center. There is also abundant storm outflow to the east and north of the storm.

Visible imagery and low-level winds show cyclonic motion at low levels.  The convection is displaced to the east because of southwesterly shear over the storm (shown below, in an image take from this site).

200-850 mb wind shear over the Gulf of Mexico, 1400 UTC on 20 September 2020 (click to enlarge)

Both upper-level and low-level water vapor imagery show very dry mid-tropospheric air over Texas. MIMIC Total Precipitable Water, below, (source), also shows the significant dry air over the continent. (Hurricane Teddy is also apparent in the western Atlantic).

Hourly MIMIC Total Precipitable Water estimates for the 24 hours ending 1400 UTC on 20 September 2020 (Click to enlarge)

Is the dry air influencing the development of this storm? Low-level flow (850-700), below, from this site, shows weak easterly flow. Low-level flow is from regions of deep moisture. Upper-level flow (200-700 mb) shows motion from the (dryer) west. These two different airflows are influencing the development of Beta.

Mean 850-700 mb flow at 1200 UTC, 20 September 2020 (Click to enlarge)

Mean 700-200 mb flow at 1200 UTC, 20 September 2020 (Click to enlarge)

For the latest on Beta, refer to the pages of the National Hurricane Center (direct link for Beta).

Hurricane Sally makes landfall in Alabama

September 16th, 2020 |

GOES-16 “Clean” Infrared Window (10.35 µm) images (with and without an overlay of GLM Flash Extent Density) [click to play animation | MP4]

GOES-16 “Clean” Infrared Window (10.35 µm) images (with and without an overlay of GLM Flash Extent Density) [click to play animation | MP4]

At 0500 UTC on 16 September 2020, Hurricane Sally rapidly intensified (ADT | SATCON) to a Category 2 storm, and soon thereafter made landfall near Gulf Shores, Alabama at 0945 UTC. During that time period, 1-minute GOES-16 Infrared imagery — with and without an overlay of GLM Flash Extent Density (above) — displayed a ragged eye structure, along with a lack of GLM-detected lightning activity in the immediate vicinity of the storm center. The eye passed just to the west of Buoy 42012, where the wind gusted to 95.2 knots or 110 mph at 0510 UTC (below).

Plot of wind speed (blue), wind gusts (red) and pressure (green) at Buoy 42012

Plot of wind speed (blue), wind gusts (red) and pressure (green) at Buoy 42012


How did Sally change the Sea Surface Temperatures (SSTs) in the Northern Gulf? The toggle below of Advanced Clear Sky Processor for Ocean (ACSPO) SSTs from VIIRS, derived using CSPP (the Community Satellite Processing Package) and direct broadcast data at CIMSS, shows SSTs about 2ºF cooler (cyan in the enhancement, about 80ºF) compared to surrounding waters that are 82-83ºF..

ACSPO SSTs at 0746 and 1912 UTC on 17 September 2020 (click to enlarge)

Hurricane Sally

September 15th, 2020 |

GOES-16 Clean Window Infrared (10.3 µm) Imagery over Sally, 1113 – 1312 UTC 14 September 2020. GLM Flash Density (1-minute observations) are also plotted (Click to animate)

Hurricane Sally was moving very slowly in the north-central Gulf of Mexico at sunrise on 15 September 2020. A GOES-16 Mesoscale sector placed over the storm allowed for 1-minute imagery, and the Clean Infrared window animation, above, for the 2 hours centered on Sunrise, show a compact storm south of Mobile Bay and east of the Mississippi River delta. Geostationary Lightning Mapper (GLM) data overlain on the ABI imagery show active lightning in a feeder band to the east of the storm, but little lightning in the storm center.

GOES-16 Upper Level (6,19 µm) and Lower Level (7.34 µm) water vapor imagery, 1311 UTC on 15 September 2020 (Click to enlarge)

Intensification of Sally has been affected by dry air near the storm (as mentioned in this discussion, for example). The toggle above of GOES-16 infrared upper-level and lower-level water vapor imagery (observed at 6.19 µm and 7.34 µm, respectively), shows warm brightness temperatures to the west of the storm.  These warm temperatures (yellow in the 7.34 µm image, blue in the 6.19 µm image) are regions of mid-level dryness:  the top of the moist layer is farther down in the atmosphere in these regions.

Morphed microwave imagery, below, (from this site), shows the effect of dry air on the storm structure.  The eyewall almost forms — but it is eroded along its southern edge by dry air.

Morphed Microwave imagery following the center of Hurricane Sally for the 24 hours ending 1300 UTC on 15 September (Click to enlarge)

The 1200 UTC MIMIC Total Precipitable Water (TPW) field, below (from this site) suggests that dryer Continental air might be affecting the storm.  Advected Layer Precipitation Water from 1200 UTC (here, from this site), tells a similar tale.

Microwave-derived Total Precipitable Water, 1200 UTC on 15 September 2020 (Click to enlarge)

Visible imagery, below, at sunrise shows a thick clouds over the center of the storm.  The appearance is similar to yesterday’s. A visible animation (from 1130-1313 UTC) without the GLM overlay is available here.

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GOES-16 visible imagery (0.64 µm) with 1-minute GLM Flash Density overlain, 1145 – 1311 UTC 15 September (Click to animate)

===== 21 UTC Update =====

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (with and without an overlay of GLM Flash Extent Density) [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (with and without an overlay of GLM Flash Extent Density) [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 — with and without an overlay of GLM Flash Extent Density (above) showed a series of convective bursts that rotated around the center of Category 1 Hurricane Sally during the daylight hours. Minimal GLM-detected lightning activity was seen with these convective bursts.

A toggle between Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images (below) showed Sally at 1806 UTC. The coldest cloud-top infrared brightness temperature northwest of the eye was -86ºC.

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images at 1806 UTC [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images at 1806 UTC [click to enlarge]

For the latest information on Hurricane Sally, refer to the pages of the National Hurricane Center.  (Direct link to Sally)