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).

Pyrocumulonimbus clouds produced by the Mullen Fire in Wyoming

September 19th, 2020 |

GOES-16 “Red” Visible (0.64 µm, top), Shortwave Infrared (3.9 µm, center) and “Clean” Infrared Window (10.35 µm, bottom) images, with hourly plots of surface reports [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm, top), Shortwave Infrared (3.9 µm, center) and “Clean” Infrared Window (10.35 µm, bottom) images, with hourly plots of surface reports [click to play animation | MP4]

GOES-16 (GOES-East) “Red” Visible (0.64 µm), Shortwave Infrared (3.9 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed a series of pyrocumulonimbus (pyroCb) pulses emanating from the Mullen Fire in southeastern Wyoming on 19 September 2020. Each of the  pulses exhibited 10.35 µm brightness temperatures of -40ºC and colder  (shades of blue) — assuring the heterogeneous nucleation of all supercooled water droplets to form ice crystals, thereby meeting the criteria of a pyroCb.

The coldest cloud-top infrared brightness temperatures were around -47ºC, which corresponded to altitudes near 11 km according to rawinsonde data from Riverton, Wyoming (below).

Plot of 00 UTC rawinsonde data from Riverton, Wyoming [click to enlarge]

Plot of 00 UTC rawinsonde data from Riverton, Wyoming [click to enlarge]

Hurricane Teddy rapidly intensifies to a Category 4 storm

September 17th, 2020 |

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 Hurricane Teddy as it rapidly intensified (ADT | SATCON ) to a Category 4 storm on 17 September 2020. The coldest cloud-top infrared brightness temperatures were in the -80 to -85ºC range.

Metep-A ASCAT surface scatterometer wind speeds were as high as 74 knots in the northwestern portion  of the eyewall (below).

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

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

Microwave (85 GHz) DMSP-17 (at 1023 UTC), GMI (at 1720 UTC) and DMSP-18 (at 2034 UTC) images from the CIMSS Tropical Cyclones site are shown below.

DMSP-17 SSMI Microwave (85 GHz) image at 1023 UTC [click to enlarge]

DMSP-17 SSMI Microwave (85 GHz) image at 1023 UTC [click to enlarge]

GMI Microwave (85 GHz) image at 1720 UTC [click to enlarge]

GMI Microwave (85 GHz) image at 1720 UTC [click to enlarge]

DMSP-18 SSMI Microwave (85 GHz) image at 2034 UTC [click to enlarge]

DMSP-18 SSMI Microwave (85 GHz) image at 2034 UTC [click to enlarge]