PyroCumulonimbus cloud in Australia

January 25th, 2019 |

Himawari-8

Himawari-8 “Red” Visible (0.64 µm, top), Shortwave Infrared (3.7 µm, middle) and Infrared Window (10.3 µm, bottom) images [click to play to animation | MP4]

JMA Himawari-8 “Red” Visible (0.64 µm), Shortwave Infrared (3.7 µm) and Infrared Window (10.3 µm) images (above) showed the development of a pyroCumulonimbus (pyroCb) cloud from a bushfire that was burning in the eucalypt forests of eastern Victoria, Australia on 25 January 2019. A rapid-scan “Target” sector was positioned over the region beginning at 0522 UTC, providing images every 2.5 minutes (instead of the routine 10-minute interval). Cloud-top infrared brightness temperatures became colder than -40ºC (the threshold for pyroCb classification) after 0230 UTC, and eventually cooled to around -55ºC (orange enhancement). This temperature roughly corresponded to an altitude around 12 km, according to nearby Melbourne rawinsonde data (plot | text).

A closer view of Himawari-8 “Red” Visible (0.64 µm) and Shortwave Infrared (3.7 µm) images (below) revealed the rapid southeastward run of the fire, as shown by the growth of the “hot spot” (black to red pixels) on Shortwave Infrared images. The darker gray appearance of the pyroCb cloud is due to the presence of smaller ice crystals at the cloud top — these smaller ice crystals are more efficient reflectors of incoming solar radiation, making the cloud tops appear warmer than those of conventional cumulonimbus. Vigorous updrafts driven by the intense heat of the fire limit the in-cloud residence time for ice crystal growth, which leads to smaller particles being ejected at the pyroCb cloud top.

Himawari-8 "Red" Visible (0.64 µm, left) and Shortwave Infrared (3.7 µm, right) images [click to play to animation | MP4]

Himawari-8 “Red” Visible (0.64 µm, left) and Shortwave Infrared (3.7 µm, right) images [click to play to animation | MP4]

In a comparison of VIIRS True Color Red-Green-Blue (RGB) and Infrared Window (11.45 µm) images from Suomi NPP (at 0311 UTC) and NOAA-20 (at 0501 UTC) images viewed using RealEarth (below), cloud-top infrared brightness temperatures were in the -55 to -58ºC range (darker shades of orange).

VIIRS True Color RGB and Infrared Window (11.45 µm) images from Suomi NPP (0311 UTC) and NOAA-20 (0501 UTC) images [click to enlarge]

VIIRS True Color RGB and Infrared Window (11.45 µm) images from Suomi NPP (0311 UTC) and NOAA-20 (0501 UTC) images [click to enlarge]

Blowing snow across the Upper Midwest

January 24th, 2019 |

GOES-16

GOES-16 “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images [click to play MP4 animation]

1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and Near-Infrared (1.61 µm) images (above) displayed a long plume of horizontal convective roll (HCR) clouds across parts of the Upper Midwest on 24 January 2019. These HCR cloud features formed in the presence of strong northerly/northwesterly boundary layer winds in the wake of a cold frontal passage (surface analyses), and often highlight areas where significant blowing snow and ground blizzard conditions are likely occurring.

Animations of GOES-16 Visible and Snow/Ice images with plots of surface winds and weather type are shown below. ASOS sites report Haze (“H”) when the surface visibility — in this case, reduced by blowing snow — is less than 7 miles but greater than or equal to 4 miles. Some sites in Minnesota and Iowa reported a visibility between 0.5 and 2.0 miles at times (animation).

GOES-16 "Red" Visible (0.64 µm) images, with hourly plots of surface wind barbs and weather type [click to play MP4 animation]

GOES-16 “Red” Visible (0.64 µm) images, with hourly plots of surface wind barbs and weather type [click to play MP4 animation]

GOES-16 Near-Infrared "Snow/Ice" (1.61 µm) images, with hourly plots of surface wind barbs and weather type [click to play MP4 animation]

GOES-16 Near-Infrared “Snow/Ice” (1.61 µm) images, with hourly plots of surface wind barbs and weather type [click to play MP4 animation]

The signature of the HCR clouds on GOES-16 Day Cloud Phase Distinction Red-Green-Blue (RGB) images from the AOS site (below) was one that more closely resembled ice crystal clouds (light pink hues) than supercooled water droplet clouds (brighter shades of white) — suggesting a high concentration of blowing snow lofted within the boundary layer by the HRC circulations. Snow cover appears as shades of green in the RGB images.

GOES-16 Day Cloud Phase Distinction RGB images [click to play animation | MP4]

GOES-16 Day Cloud Phase Distinction RGB images [click to play animation | MP4]

Dry air aloft over the western Atlantic Ocean

January 23rd, 2019 |

GOES-16 Mid-level Water Vapor (6.9 µm) images, with plots of rawinsonde sites in yellow [click to play animation | MP4]

GOES-16 Mid-level Water Vapor (6.9 µm) images, with rawinsonde sites plotted in yellow [click to play animation | MP4]

With a ridge of high pressure in place over the western Atlantic Ocean, GOES-16 (GOES-East) Mid-level Water Vapor (6.9 µm) images (above) indicated the presence of dry air within the middle troposphere off the Southeast US coast on 23 January 2019.

GOES-16 “Red” Visible (0.64 µm) images showed that marine boundary layer stratocumulus clouds covered much of this region of the Atlantic — and due to minimal absorption by mid-tropospheric water vapor, these stratocumulus clouds were also very apparent in the corresponding GOES-16 Near-Infrared “Cirrus” (1.38 µm) images (below).

GOES-16 "Red" Visible (0.64 µm) and Near-Infrared "Cirrus" (1.37 µm) images, with plots of rawinsonde sites in yellow [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm) and Near-Infrared “Cirrus” (1.38 µm) images, with rawinsonde sites plotted in yellow [click to play animation | MP4]

Terra MODIS Visible (0.65 µm) and Near-Infrared “Cirrus” (1.38 µm) images at 1513 UTC (below) also showed a clear signature of the stratocumulus clouds at 1.38 µm.

Terra MODIS Visible <em>(0.65 µm)</em> and Near-Infrared "Cirrus" <em>(1.38 µm)</em> images at 1513 UTC, with plots of rawinsonde sites in yellow [click to enlarge]

Terra MODIS Visible (0.65 µm) and Near-Infrared “Cirrus” (1.38 µm) images at 1513 UTC, with rawinsonde sites plotted in yellow [click to enlarge]

Cross sections of GFS90 model fields along Line I-I’ — oriented from Charleston, South Carolina to Bermuda — are shown below. Note the very dry air within the middle troposphere, with Specific Humidity values of less than 0.2 g/kg and Relative Humidity values less than 10% centered around the 500 hPa pressure level. In addition, the depth of the moist marine boundary layer was higher to the west at Charleston (2.6 km, at 746 hPa) than to the east at Bermuda (1.9 km, at 822 hPa).

Cross sections of GFS90 model fields along Line I-I', from Charleston, South Carolina to Bermuda [click to enlarge]

Cross sections of GFS90 model fields along Line I-I’, from Charleston, South Carolina to Bermuda [click to enlarge]

Ice accrual from freezing rain/drizzle in the Northeast US

January 22nd, 2019 |

GOES-16

GOES-16 “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images [click to play animation | MP4]

A large central/eastern US winter storm impacted much of the Northeast with rain, snow, freezing rain/drizzle and strong winds on 20 January 2019 (surface analyses) — and a post-storm comparison of GOES-16 (GOES-East)  “Red” Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images on 22 January (above) revealed a dark 1.61 µm signature (often indicative of significant ice accrual) in parts of New York, New Jersey, Connecticut, Rhode Island and Massachusetts. Since snow and ice are effective absorbers of radiation at the 1.61 µm wavelength — with ice absorbing even more strongly — those features appear as darker shades of gray in the Snow/Ice imagery.

A closer view using NOAA-20 VIIRS Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images at 1708 UTC is shown below (note: the NOAA-20 VIIRS image is incorrectly labeled as Suomi NPP). Ice accrued in thicknesses up to 0.60 inch at Meridan in central Connecticut and 0.40 inch at Newburgh in far eastern New York.

NOAA-20 VIIRS Visible (0.64 µm) and Near-Infrared "Snow/Ice" (1.61 µm) images [click to enlarge]

NOAA-20 VIIRS Visible (0.64 µm) and Near-Infrared “Snow/Ice” (1.61 µm) images [click to enlarge]