Rope Cloud over the northwest Gulf of Mexico

January 22nd, 2018 |

GOES-16 “Red Visible” 0.64 µm imagery from 1402-2142 UTC on 22 January 2018. (Click to animate)

Visible GOES-16 Satellite Imagery over the northeastern Gulf of Mexico on 22 January 2018 showed the development of a Rope Cloud. Such features have been discussed before on the CIMSS Blog — here, here, here and here! Rope Clouds are handy features in satellite analysis over the ocean because they indicate distinctly where the surface cold front exists. Note that the WPC surface analysis, shown here for 1500 UTC, has the front in the same location as the rope cloud, with convection noted out in advance of the surface cold front. The hourly animation below, showing surface observations and the GOES-16 Red Visible (0.64 µm) Imagery, confirms the windshifts that were observed when the Rope Cloud/Cold Front passed any station.

Hourly Surface Observations and GOES-16 “Red Visible” 0.64 µm imagery from 1400-2200 UTC on 22 January 2018. (Click to enlarge)

Explosive cyclogenesis off the East Coast of the United States

January 4th, 2018 |

GOES-16 Clean Window (10.3 µm) Imagery, 0102-1337 UTC on 4 January 2018 (Click to animate)

A strong extratropical cyclone that deposited snow in the deep south developed explosively during the early morning hours of 4 January 2018. The GOES-16 Clean Window (10.3 µm) animation, above, from 0102 – 1337 UTC on 4 January, brackets the explosive development: from 993 hPa at 0000 UTC to 968 mb at 0900 UTC, a strengthening that easily meets the “Bomb” criteria set forth by Sanders and Gyakum (1980). The Clean Window animation shows the strong surface circulation with well-defined conveyor belts. Convection develops at the leading edge of the dry slot that is approaching southern New England at the end of the animation. The Low-Level Water Vapor (7.3 µm) animation for the same time, below, suggests very strong descent behind the storm, where brightness temperatures warmer than -10º C (orange in the enhancement used) are widespread.

GOES-16 Low-Level Water Vapor (7.3 µm) Infrared Imagery, 0102-1332 UTC on 4 January 2018 (Click to animate)

This storm can also be viewed using Red-Green-Blue composites (in addition to the single-channel animations shown above). The Airmass RGB, below, combines the Split Water Vapor Difference (6.2 µm – 7.3 µm) as Red, Split Ozone (9.6 µm – 10.3 µm) as Green, and Upper level Water Vapor (6.2 µm) as Blue. (Other storms analyzed with the Airmass RGB can be seen here, here, and here). The strong red signal in the Airmass RGB south of the storm suggests very strong sinking motion.

GOES-16 AirMass RGB Product, 0102-1332 UTC (Click to animate)

ASCAT Scatterometer winds over the system at 0205 UTC showed an elongated surface circulation with multiple observations of winds exceeding 50 knots (in red), and a large region (in yellow) of winds exceeding 35 knots.

GOES-16 ABI Clean Window (10.3 µm) and ASCAT Scatterometer winds, 0205 UTC on 4 January 2018 (Click to enlarge)

GOES-16 ABI Red Visible (0.64 µm) and ASCAT Scatterometer winds, 1520 UTC on 4 January 2018 (Click to enlarge)

The 1520 UTC ASCAT pass, above, sampled half the storm, and hurricane-force winds were indicated.

The snow that was deposited in the Deep South by this storm (also discussed here) persisted through a cold night and was visible in the GOES-16 Visible (0.64 µm) imagery, below. Highly reflective snow can be difficult in a still image to distinguish from clouds — but the Snow/Ice Channel on GOES-16 (1.61 µm) detects energy at a wavelength that is strongly absorbed by ice. Thus, snow (and ice) on the ground (or in clouds), has a different representation. (Here are toggles between the two images, with and without a map). The snow cover over coastal Georgia, South and North Carolina appears dark in the Snow/Ice channel because the snow is absorbing, not reflecting, the 1.61 µm radiation.  It is noteworthy that the 1.61 µm image is especially dark over far southeastern Georgia northeastward along the immediate coastline of South Carolina.  These are regions where freezing rain and sleet fell, versus predominantly snow to the north and west (as also noted here; The National Weather Service in Tallahassee tweeted out an ice/snow accumulation map that also agrees with the 1.61 µm image).  Ice in the cirrus clouds northeast of North Carolina is also apparent in the Snow/Ice 1.61 µm imagery.

GOES-16 Band 2 Visible (0.64 µm) Imagery, 1412 UTC on 4 January 2018 (Click to enlarge)

GOES-16 ABI Band 5 Snow/Ice (1.61 µm) Imagery, 1412 UTC on 4 January 2018 (Click to enlarge)

Suomi NPP overflew the storm shortly after midnight on 4 January; Day Night band visible imagery (courtesy Kathleen Strabala, CIMSS), below, shows a well-developed cyclone covering much of the northeast Atlantic Ocean. Snow cover is apparent over the deep south of the United States.

Suomi NPP Day Night Band Visible (0.7 µm) Imagery, 0614 UTC on 4 January 2018 (Click to enlarge)

(Added, 5 January 2018: This website shows a during-the-day CIMSS True Color Image animation of the storm on 4 January 2018. Animation courtesy Dave Stettner, CIMSS).

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!