VIIRS views of the surface and atmosphere, late September 2020 version

September 24th, 2020 |

VIIRS True-Color imagery on 02 and 22 September 2020 (Click to enlarge). Data from the VIIRS Today website

VIIRS True-Color imagery taken from the VIIRS Today website, above, shows the reddening/yellowing of trees over the upper midwest between 2 September and 22 September 2020. The inexorable slip into autumn and winter is ongoing.

Early in the morning on 24 September, Direct Broadcast NOAA-20 data downloaded at CIMSS and processed with CSPP showed an extensive aurora over much of Canada. (These data are available from the Direct Broadcast ftp site: link; direct link to the NOAA-20 data used below: link — note that this direct link is ephemeral and will disappear after about a week). The Day Night Band imagery is toggled with 11.45 µm infrared imagery.

NOAA-20 remapped VIIRS Day Night Band visible (0.7 µm) imagery and infrared 11.45 µm imagery, 0804 UTC on 24 September 2020

ACSPO Lake Surface Temperatures over the Great Lakes

September 23rd, 2020 |

Great Lakes lake surface temperatures, 0735 UTC on 23 September 2020, as well as zoomed-in views of the individual lakes (Click to animate)

Suomi-NPP’s descending pass at ~0730 UTC (shown here, from this site), had an ideal path — moving south from far eastern Upper Michigan to western Lower Michigan — to view all five Great Lakes.  Sprawling High Pressure and the attendant clear skies meant that a good basin wide view of the lake temperatures was observed, as shown above. (DB imagery from this pass is also available for a short time at the CIMSS DB ftp site) These ACSPO (Advanced Clear-Sky Processor for Ocean) SST fields are available to NWS offices via LDM fields. The default color bar in AWIPS has been changed so that only temperatures between 45º F and 70º F are shown

Lake Temperatures in the imagery above range, for Lake Superior from 47.5º (east of Minnesota) to 66º (just off southwest Upper Michigan); for Lake Michigan from 57º (east of Sheboygan WI) to 67º (mid-lake, east of Chicago); for Lake Huron from 55º (in northwestern Georgian Bay and also northwestern Lake Huron) to 65º (far southern Lake Huron); for Lake Erie from 63º (western Lake Erie) to 69º (central Lake Erie); for Lake Ontario from 62º (western Lake Ontario) to 67º (eastern Lake Ontario).

This view of the Great Lakes from early in September (also from the CIMSS DB site, processed with CSPP), shows — with a color bar that ranges from 41ºF to 86ºF — much warmer water, especially in Lake Superior where the coolest mid-lake temperatures were in the mid-50s. Storms and vertical mixing since early September have cooled the surface waters. GLERL has a website that shows modeled lake surface temperatures, accessible here.

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)


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.


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)