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!

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