When Water Vapor Channels are Window Channels

January 2nd, 2018 |

GOES-16 Low-Level Water Vapor Imagery (7.3 µm), 1322 UTC on 2 January 2017 (Click to enlarge)

The very cold and dry airmass over the eastern half of the United States during early January 2018 is mostly devoid of water vapor, a gas that, when present, absorbs certain wavelengths of radiation that is emitted from the surface (or low clouds). That absorbed energy is then re-emitted from higher (colder) levels. Typically, surface features over the eastern United States are therefore not apparent. When water vapor amounts in the atmosphere are small, however, surface information can escape directly to space, much in the same way as occurs with (for example) the Clean Window channel (10.3 µm) on GOES-16 (water vapor does not absorb energy with a wavelength of 10.3 µm). The low-level water vapor (7.3 µm) image above, from near sunrise on 2 January 2018, shows many surface features over North and South Carolina, Kentucky, Tennessee and southern Illinois. The features are mostly lakes and rivers that are markedly warmer than adjacent land. (In fact, Kentucky Lake and Lake Barkely in southwest Kentucky are also visible in the 6.9 µm imagery!)

Weighting Functions from 1200 UTC on 2 January for Davenport IA (left), Lincoln IL (center) and Greensboro NC (right) for 6.2 µm (Green), 6.95 µm (blue) and 7.3 µm (magenta), that is, the upper-, mid- and lower-level water vapor channels, respectively, on ABI. Peak pressures for the individual weighting functions are noted, as are Total Precipitable Water values at the station (Click to enlarge)

GOES-16 Weighting Functions (Click here ) describe the location in the atmosphere from which the GOES-16 Channel is detecting energy.  The upper-level (6.2 µm) and mid-level (6.95 µm) weighting functions show information originating from above the surface.  Much surface information is available at Greensboro, with smaller proportional amounts at Davenport and Lincoln.

The “Cirrus” Channel on GOES-16’s ABI (Band 4, 1.38 µm) also occupies a spot in the electromagnetic spectrum where water vapor absorption is strong.  Thus, reflected solar radiation from the surface is rarely viewed at this wavelength.  The toggle below, between the ‘Veggie’ Channel (0.86 µm) and the Cirrus Channel (1.38 µm) shows that some surface features — for example, lakes in North Carolina — are present in the Cirrus Channel.

ABI Band 3 (0.86 µm) and ABI Band 4 (1.38 µm) (That is, Veggie and Cirrus channels) at 1502 UTC on 2 January 2018 (Click to enlarge)

Whenever the atmosphere is exceptionally dry, and skies are clear, check the water vapor channels on ABI to see if surface features can be viewed. A few examples of sensing surface features using water vapor imagery from the previous generation of GOES can be seen here.

Mixed-phase stratiform clouds in an arctic air mass

December 28th, 2017 |

AWIPS screen capture of GOES-16 Cloud Top Phase (top left), Near-Infrared

AWIPS screen capture of GOES-16 Cloud Top Phase product (top left), Near-Infrared “Snow/ice” (1.61 µm, top right), Cloud Phase brightness temperature difference (8.5 – 11.2 µm, bottom left) and “Clean” Infrared Window (10.3 µm, bottom right) images [click to enlarge]

An AWIPS screen capture showing GOES-16 (GOES-East) Cloud Top Phase, Near-Infrared “Snow/ice” (1.61 µm), Cloud Phase brightness temperature difference (8.5 µm11.2 µm) and “Clean” Infrared Window (10.3 µm) images on 28 December 2017 (above) was provided by Dan Baumgardt and Dave Schmidt (NWS La Crosse) — they were inquiring as to the why the 1.61 µm Snow/Ice imagery appeared bright across southern Minnesota (suggesting cloud tops composed primarily of supercooled water droplets), where light snow was being reported at a number of locations. Note that the Cloud Top Phase product also indicated that much of the stratus cloud deck over that same region was either Supercooled (light green) or Mixed (dark green).

An animation of GOES-16 Snow/Ice (1.61 µm) imagery (below) showed that the high reflectance (brighter white) signature of the lower-altitude stratiform cloud deck persisted across southern Minnesota into western Wisconsin and northern Iowa during the daylight hours, along with widespread surface reports of light snow. In contrast, higher-altitude clouds composed predominantly or entirely of ice crystals exhibited a darker gray appearance (since ice crystals, as well as surface snow cover and frozen lakes/rivers, are strong absorbers of radiation at the 1.61 µm wavelength).

GOES-16 Near-Infrared "Snow/Ice" (1.61 µm) images, with hourly surface-observed precipitation type plotted in yellow [click to play MP4 animation]

GOES-16 Near-Infrared “Snow/Ice” (1.61 µm) images, with hourly surface-observed precipitation type plotted in yellow [click to play MP4 animation]

In the corresponding GOES-16 “Clean” Infrared Window (10.3 µm) animation (below), much of the aforementioned lower-altitude stratiform cloud layer exhibited cloud-top infrared brightness temperatures in the -10 to -20 ºC range across far southern Minnesota into northern Iowa, with colder -20 to -30 ºC values seen in the more northern and eastern portion of the stratus cloud.

GOES-16 "Clean" Infrared Window (10.3 µm) images, with hourly surface-observed precipitation type plotted in yellow [click to play MP4 animation]

GOES-16 “Clean” Infrared Window (10.3 µm) images, with hourly surface-observed precipitation type plotted in yellow [click to play MP4 animation]

Plots of rawinsonde data (at 12 UTC on 28 December) from Aberdeen, South Dakota and Chanhassen, Minnesota (below) showed that the temperature profiles within the low-altitude cloud layers were close to isothermal, with air temperatures generally in the -16 to -22 ºC range.

Rawinsonde data from Aberdeen, South Dakota [click to enlarge]

Rawinsonde data from Aberdeen, South Dakota [click to enlarge]

Rawinsonde data from Chanhassen, Minnesota [click to enlarge]

Rawinsonde data from Chanhassen, Minnesota [click to enlarge]

So how could snow be falling from stratus clouds whose tops appeared be be composed of supercooled water droplets? A journal article titled “Vertical Motions in Arctic Mixed-Phase Stratiform Clouds” demonstrated that in-cloud glaciation can and does occur below the supercooled liquid cloud top in an arctic air mass. This example certainly shows that in an arctic air mass, mixed/supercooled cloud above snow or ice cloud is possible, particularly in temperatures between -20 ºC and -30 ºC — and cloud phase classification for operational decisions must sometimes look beyond the examination of single-band satellite imagery (or even derived products such as Cloud Phase).

Thanks to Mike Pavolonis (NOAA/NESDIS/CIMSS) and Jordan Gerth (CIMSS) for their insightful explanations regarding cloud phase — and thanks to the NWS La Crosse staff for bringing this interesting case to our attention!

GOES-16 is now the operational GOES-East satellite

December 18th, 2017 |

All 16 Bands from GOES-16 at 2102 UTC on 18 December 2017 [click to enlarge]

All 16 Bands from GOES-16 at 2102 UTC on 18 December 2017 [click to enlarge]

GOES-16, which has been sending data from the GOES-East position since 14 December, became the operational GOES-East satellite at 2100 UTC on 18 December, succeeding GOES-13 [which itself became GOES-East, succeeding GOES-12, in April 2010; (this post, from April 2003, is the first one with GOES-12 as the operational GOES-East, it took over for GOES-8 that month!) ].  The animation above shows all 16 Bands of the first operational CONUS image from GOES-16.

 

Hurricane Gert

August 15th, 2017 |

GOES-16 imagery (all 16 ABI Bands) from 1912-2132 UTC, 15 August 2017 [click to play animation]

GOES-16 imagery (all 16 ABI Bands) from 1912-2132 UTC, 15 August 2017 [click to play animation]

GOES-16 data posted on this page are preliminary, non-operational and are undergoing testing

Hurricane Gert, a Category-1 storm on the Saffir-Simpson scale, is over the open Atlantic Ocean east of Cape Hatteras. It is close enough to the USA, however, that it is within GOES-16’s CONUS domain where 5-minute sampling is routine. The animation above shows all 16 channels from GOES-16 ABI, every five minutes from 1912-2132 UTC on 15 August 2017. A distinct eye is not apparent in the visible or infrared satellite imagery, but microwave data (from here) suggests an eye is present, at least at times. A comparison of 2035 UTC DMSP-16 SSMIS Microwave (85 GHz) and 2045 UTC GOES-13 Infrared Window (10.7 µm) images can be seen here.

The low-level Water Vapor imagery, below, shows that Gert is south and east of a front along the East Coast. This front should steer the storm to the north and east. Swells from the storm will affect the East Coast however.

GOES-16 imagery Low-Level Water Vapor (7.34 µm) Infrared Imagery from 1832-2137 UTC, 15 August 2017 [click to play animation]

GOES-16 Low-Level Water Vapor (7.34 µm) Infrared Imagery from 1832-2137 UTC, 15 August 2017 [click to play animation]

For more information on Gert, consult the website of the National Hurricane Center, or the CIMSS Tropical Weather Website.

GOES-16 ABI Imagery from the morning of 16 August 2017, below, shows that an eye has appeared in visible and infrared imagery.

GOES-16 imagery (all 16 ABI Bands) from 1117-1337 UTC, 16 August 2017 [click to play animation]

GOES-16 imagery (all 16 ABI Bands) from 1117-1337 UTC, 16 August 2017 [click to play animation]

A closer view using 1-minute interval GOES-16 Mesoscale Sector “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.3 µm) images, below, showed that  the most vigorous areas of deep convection were generally confined to the northern semicircle of the eyewall region — cloud-top infrared brightness temperatures were as cold as -80º C (violet color enhancement) at times.

GOES-16 Visible (0,64 µm, top) and Infrared Window (10.3 µm, bottom) images [click to play MP4 animation]

GOES-16 Visible (0.64 µm, top) and Infrared Window (10.3 µm, bottom) images [click to play MP4 animation]