Cutoff low over northern Alaska

June 12th, 2020 |

GOES-17 Mid-level Water Vapor (6.9 µm) images, with contours of PV1.5 Pressure plotted in red [click to play animation | MP4]

GOES-17 Mid-level Water Vapor (6.9 µm) images, with contours of PV1.5 Pressure plotted in red [click to play animation | MP4]

GOES-17 (GOES-West) Mid-level Water Vapor (6.9 µm) images (above) showed the circulation of an anomalous middle-tropospheric cutoff low over the northwestern portion of Alaska on 12 June 2020. A Potential Vorticity (PV) anomaly associated with this low was causing the dynamic tropopause — represented by the pressure of the PV1.5 surface — to descend as low as the 500 hPa pressure level.

GOES-17 Mid-level Water Vapor (6.9 µm) images, with contours of PV1.5 Pressure plotted in red and available NUCAPS sounding profiles denoted by green/yellow points [click to enlarge]

GOES-17 Mid-level Water Vapor (6.9 µm) images, with contours of PV1.5 Pressure plotted in red and available NUCAPS sounding profiles denoted by green/yellow points [click to enlarge]

Just after 21 UTC, an overpass of the Suomi NPP satellite provided NUCAPS soundings (above) within much of the core of the cutoff low — the green NUCAPS sounding profile about 40 miles east/southeast of the 500 hPa PV1.5 pressure contour (below) displayed an apparent tropopause near the 400 hPa pressure level.

NUCAPS sounding profile [click to enlarge]

NUCAPS sounding profile [click to enlarge]

GOES-17 “Red” Visible (0.64 µm) images (below) revealed the development of numerous showers and thunderstorms across the Brooks Range and North Slope of Alaska, aided by instability beneath the cutoff low.

GOES-17 "Red" Visible (0.64 µm) images, with hourly surface reports plotted in red [click to play animation | MP4]

GOES-17 “Red” Visible (0.64 µm) images, with hourly surface reports plotted in red [click to play animation | MP4]

A higher spatial resolution view of these showers and thunderstorms was provided by a sequence of VIIRS True Color Red-Green-Blue (RGB) and Infrared Window (11.45 µm) images from NOAA-20 and Suomi NPP, as viewed using RealEarth (below). A few of these thunderstorms moved toward the Arctic Coast, with one fairly impressive storm just southwest of Katovik which exhibited cloud-top infrared brightness temperatures near -60ºC (red enhancement) around 23 UTC.

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

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


High-altitude waves over the Arctic

March 27th, 2020 |

GOES-17

GOES-17 “Ozone” (9.61 µm) images, with rawinsonde sites plotted in yellow [click to play animation | MP4]

GOES-17 (GOES-West) “Ozone” (9.61 µm) images (above) revealed waves propagating northwestward over northern Alaska, northern Yukon and the adjacent Beaufort Sea during the pre-dawn hours on 27 March 2020. That area was too illuminated by either aurora borealis or the rising sun — so Suomi NPP VIIRS Day/Night Band (0.7 µm) imagery could not confirm the presence of mesospheric airglow waves (see this blog post for some examples).

A plot of the GOES-17 “Ozone” spectral band weighting function — calculated using 12 UTC rawinsonde data from Fairbanks, Alaska — showed a peak contribution from within the stratosphere at the 39 hPa pressure level, corresponding to an altitude around 21 km (below).

Plot of GOES-17

Plot of GOES-17 “Ozone” (9.61 um) weighting function, calculated using 12 UTC rawinsonde data from Fairbanks, Alaska [click to enlarge]

The curious aspect of these waves was their northwestward propagation — rawinsonde data from 3 sites across the region (below) indicated that the winds aloft within the upper troposphere and throughout the stratosphere were strong northwesterly, which meant the waves were moving against the ambient flow. Lacking a coherent, science-based explanation for these wave features, this blog post earns its place in the “What the heck is this?” category.

Plots of rawinsonde data from Fairbanks, Alaska [click to enlarge]

Plots of rawinsonde data from Fairbanks, Alaska [click to enlarge]

Plots of rawinsonde data from Utqiagvik (formerly Barrow), Alaska [click to enlarge]

Plots of rawinsonde data from Utqiagvik (formerly Barrow), Alaska [click to enlarge]

Plots of rawinsonde data from Inuvik, Northwest Territories [click to enlarge]

Plots of rawinsonde data from Inuvik, Northwest Territories [click to enlarge]

Solstice images of Antarctica and Alaska

December 21st, 2019 |

GOES-16

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) images (above) showed that most of Antarctica experienced 24 hours of full solar illumination during the Southern Hemisphere’s Summer Solstice on 21 December 2019. Through breaks in the cloud cover, a few bright areas of sun glint were also evident, moving from west to east, due to the reflection of sunlight off ice-free water.

In spite of receiving 24 hours of sunlight, interior areas of the continent remained quite cold — due to high elevation and deep snow cover. For example, surface air temperatures at station 8927 near the center of Antarctica remained within the -25 to -35ºF range on 21 December (source).

Surface air temperatures (ºF) at automated weather stations across Antarctica [click to enlarge]

Surface air temperatures (ºF) at automatic weather stations across Antarctica [click to enlarge]

In the Northern Hemisphere, GOES-17 (GOES-West) Visible images (below) revealed a few hours of illumination of the southern summits of Denali and nearby portions of the Alaska Range. Note the presence of much colder surface air temperatures (-20s and -30s F) north of the Alaska Range.

GOES-17 "Red" Visible (0.64 µm) images [click to play animation | MP4]

GOES-17 “Red” Visible (0.64 µm) images [click to play animation | MP4]

Farther to the north across Interior Alaska, some locations reported minimum air temperatures in the -40s to -50s F. A Suomi NPP VIIRS Infrared image (below) showed surface brightness temperatures as cold as -50ºC or -58ºF (brighter yellow enhancement) in the Yukon Flats area around Fort Yukon (PFYU).

NOAA-20 VIIRS Infrared Window (11.45 µm) image at 2220 UTC [click to enlarge]

Suomi NPP VIIRS Infrared Window (11.45 µm) image at 2220 UTC [click to enlarge]

Stratus clouds affecting surface temperatures in Alaska

December 17th, 2019 |

GOES-17 Nighttime Microphysics RGB and

GOES-17 Nighttime Microphysics RGB and “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

A comparison of GOES-17 (GOES-West) Nighttime Microphysics RGB and “Clean” Infrared Window (10.35 µm) images (above) showed the formation and motion of patchy stratus clouds (RGB shades of yellow) over Interior Alaska on 17 December 2019.  Note how the clouds are difficult to detect and track on the 10.35 µm images, since the temperatures of cold land surfaces and stratus cloud tops were similar. Since these high latitudes receive little to no sufficient solar illumination to allow useful visible imagery during the winter season, the RGB product can be a helpful tool for monitoring the evolution of such low clouds.

Plots of surface data from Bettles (PABT) and Fort Yukon (PFYU) (below) showed that the stratus cloud deck — with bases in the 6,000-10,000 feet range — had an impact on surface air temperature trends, with warming occurring as radiational cooling was slowed and/or reversed as the clouds moved overhead. Temperatures continued to rise at Bettles as the cloud coverage remained broken to overcast, while the temperature briefly dropped again at Fort Yukon as the cloud coverage thinned to scattered.

Plot of surface data from Bettles, Alaska [click to enlarge]

Plot of surface data from Bettles, Alaska (PABT) [click to enlarge]

Plot of surface data from Fort Yukon, Alaska [click to enlarge]

Plot of surface data from Fort Yukon, Alaska (PFYU) [click to enlarge]