Surface features seen in GOES Water Vapor imagery

January 19th, 2019 |
GOES-17 Low-level Water Vapor (7.3 µm) images, plus topography [click to play animation | MP4]

GOES-17 Low-level Water Vapor (7.3 µm) images, plus topography [click to play animation | MP4]

* GOES-17 images shown here are preliminary and non-operational *

A comparison of GOES-17 Low-level Water Vapor (7.3 µm) images with topography (above) revealed that radiation being emitted by the higher elevations of the Brooks Range in northern Alaska was able to be sensed by the 7.3 µm detectors — in spite of the very large satellite viewing angle (or zenith angle) of around 75 degrees.

The GOES-17 ABI Water Vapor band weighting functions calculated using 12 UTC rawinsonde data from Fairbanks, Alaska (below) showed that the presence of cold, dry air within the middle to upper troposphere had shifted the peak pressure of the 7.3 µm weighting function downward to 753.63 hPa (corresponding to an altitude of 7053 feet) — which was at or below the elevation of much of the higher terrain of the Brooks Range. There was very little absorption of upwelling surface radiation by the small amount of water vapor that was present within the middle/upper troposphere, allowing the cold thermal signature of the higher terrain to be observed on the Water Vapor imagery.

GOES-17 Water Vapor weighting functions calculated using 12 UTC rawinsonde data from Fairbanks [click to enlarge]

GOES-17 Water Vapor weighting functions calculated using rawinsonde data from Fairbanks, Alaska [click to enlarge]

On the following day (19 January), a very cold/dry arctic air mass was moving southward across the Upper Midwest and Great Lakes — the coldest temperature in the US that morning (including Alaska) was -42ºF at Kabetogama, Minnesota — and the outline of Lake Superior was very apparent in GOES-16 (GOES-East) Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor imagery; in fact, a portion of the northwestern shoreline was even faintly visible in Upper-level (6.2 µm) Water Vapor images (below).

GOES-16 Low-level (7.3 µm), and Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images [click to play animation | MP4]

GOES-16 Low-level (7.3 µm), and Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images [click to play animation | MP4]

Plots of the GOES-16 Water Vapor band weighting functions calculated using 00 UTC rawinsonde data from International Falls, Minnesota (below) showed some radiation contribution coming from near or just above the surface. As a result, a signature of the strong surface thermal contrast — between a relatively warm Lake Superior (water surface temperatures in the 30s F) and the adjacent cold land surface temperatures (generally -10 to -20ºF) — was able to reach the satellite with minimal absorption by water vapor aloft.

GOES-16 Water Vapor band weighting functions, calculated using rawinsonde data from International Falls, Minnesota [click to enlarge

GOES-16 Water Vapor weighting functions, calculated using rawinsonde data from International Falls, Minnesota [click to enlarge]

===== 21 January Update =====

GOES-16 Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor images, with rawinsonde sites plotted in cyan [click to play animation | MP4]

GOES-16 Low-level (7.3 µm) and Mid-level (6.9 µm) Water Vapor images, with rawinsonde sites plotted in cyan [click to play animation | MP4]

In the wake of a large winter storm, arctic air spread across the eastern US on 21 January (minimum temperatures,– and the outline of the coasts of Maryland, Virginia and North Carolina could clearly be seen on GOES-16 Low-level (7.3 µm) Water Vapor images (above). In addition, the coast of the Albemarle-Pamlico Sound  and the Outer Banks of central North Carolina could even be seen for a short time on Mid-level (6.9 µm) Water Vapor imagery (for example, at 1502 UTC).

This cold/dry air mass set new daily records for lowest rawinsonde-measured Total Precipitable Water at Greensboro in central North Carolina (0.04 inch), Roanoke/Blacksburg in western Virginia (0.02 inch) and Wallops Island on the Eastern Shore of Virginia (0.05 inch). GOES-16 Total Precipitable Water product showed values in the 0.01 to 0.09 inch range in the vicinity of Roanoke and Greensboro. In plots of the GOES-16 water vapor weighting functions for those 3 rawinsonde sites (below), note the very strong contributions of radiation directly from or just above the surface for the 7.3 µm and 6.9 µm spectral bands.

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Greensboro, North Carolina [click to enlarge]

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Greensboro, North Carolina [click to enlarge]

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Roanoke/Blacksburg, Virginia [click to enlarge]

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Roanoke/Blacksburg, Virginia [click to enlarge]

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Wallops Island, Virginia [click to enlarge]

GOES-16 water vapor weighting functions, calculated using 12 UTC rawinsonde data from Wallops Island, Virginia [click to enlarge]

The first -40º temperature of the winter in Alaska

November 24th, 2018 |

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images [click to enlarge]

A sequence of Suomi NPP VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images centered over the North Slope of Alaska (above) showed a few patches of thin stratus cloud drifting westward on 24 November 2018. Ample illumination from the Moon — which was in the Waning Gibbous phase, at 98% of Full — maximized the “visible image at night” capability of the Day/Night Band. A faster animation of Infrared images helped to emphasize the westward motion of multi-year drift ice in the Beaufort Sea as it collided with the growing wedge of first-year land-fast ice off the northeast coast of Alaska.

In areas with deeper snow cover that remained generally cloud-free for long periods of time, temperatures at first-order stations dropped into the -20s and -30s F; a low of -35ºF was recorded at Nuiqsut (PAQT). A closer look at the 2314 UTC Infrared image (below) revealed surface brightness temperatures as cold as -47ºC or -53ºF (lighter shades of yellow) in the valleys near Galbraith Lake (PAGB).

Suomi NPP VIIRS Infrared Window (11.45 µm) image at 2314 UTC [click to enlarge]

Suomi NPP VIIRS Infrared Window (11.45 µm) image at 2314 UTC [click to enlarge]

The RAWS site at Umiat Airfield (PAUM) registered a minimum temperature of -40ºF (hourly summary) at 2123 UTC on 24 November (below) — this was the first reliable -40º temperature of the 2018/2019 Winter season in Alaska. Farther to the east, the HADS site at Sagavanirktok recorded a low of -44F, but that max/min temperature data was flagged as being suspect (red) by Mesowest quality control.

Minimum and maximum temperatures for the 24-hour period ending at 20 UTC on 25 November [click to enlarge]

Minimum and maximum temperatures for the 24-hour period ending at 20 UTC on 25 November [click to enlarge]

Banner cloud in Alaska

November 7th, 2018 |

Topography + Suomi NPP VIIRS Infrared Window (11.45 µm) images, with/without overlays of NAM12 250 hPa winds [click to play animation | MP4]

Topography + Suomi NPP VIIRS Infrared Window (11.45 µm) images, with/without overlays of NAM12 250 hPa winds [click to play animation | MP4]

Suomi NPP VIIRS Infrared Window (11.45 µm) images (above) showed a well-defined banner cloud extending from the Brooks Range in northern Alaska to the Beaufort Sea on 07 November 2018. Overlays of NAM12 model 250 hPa winds revealed the presence of a branch of the polar jet stream flowing northeastward over the region. Strong southwesterly winds interacting with the topography of the Brooks Range created a standing wave which led to the formation of the banner cloud.

In a comparison of Suomi NPP VIIRS Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images (below), note the significantly warmer 3.74 µm cloud-top brightness temperatures — as much as 40 to 50ºC warmer at 2009 UTC when the sun angle was highest over Alaska — caused by enhanced solar reflectance off the very small ice crystals at the top of the banner cloud.

Suomi NPP VIIRS Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images [click to play animation | MP4]

Suomi NPP VIIRS Shortwave Infrared (3.74 µm) and Infrared Window (11.45 µm) images [click to play animation | MP4]

GOES-15 (GOES-West) Water Vapor (6.5 µm) and Infrared Window (10.7 µm) images (below) showed that a large banner cloud had persisted downwind of the Brooks Range fpr much of the day.

GOES-15 Water Vapor (6.5 µm, top) and Infrared Window (10.7 µm, bottom) images [click to play animation | MP4]

GOES-15 Water Vapor (6.5 µm, top) and Infrared Window (10.7 µm, bottom) images [click to play animation | MP4]

Alaska: a thunderstorm, single digits and a volcano

September 25th, 2018 |

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Suomi NPP VIIRS Visible (0.64 µm) and Infrared Window (11.45 µm) images (above) captured an unusually late thunderstorm that produced small hail at Anchorage PANC (surface observations) on 24 September 2018. The coldest cloud-top infrared brightness temperature was -53.8ºC, which was colder than the -46.3ºC tropopause temperature on the 00 UTC Anchorage sounding. This particular thunderstorm (Anchorage averages only 1-2 per year) even featured a wall cloud:



In far northeastern Alaska, snow cover across the North Slope and Brooks Range was evident in a sequence of Suomi NPP VIIRS Visible (0.64 µm) images (below). Since there were also areas of low cloud present (both north and south of the primary snow cover), the VIIRS Shortwave Infrared (3.74 µm) images could be used to discriminate between these low clouds — whose supercooled water droplets were effective reflectors of solar radiation, making then appear warmer or darker gray — and the cloud-free areas of snow cover.

Sequence of 4 Suomi NPP VIIRS Visible (0.64 µm) and Shortwave Infrared (3.74 µm) images [click to enlarge]

Sequence of 4 Suomi NPP VIIRS Visible (0.64 µm) and Shortwave Infrared (3.74 µm) images [click to enlarge]

The presence of this snow cover aided strong nighttime radiational cooling as a ridge of high pressure moved over the North Slope (surface analyses), and on the following morning temperatures dropped as low as 4ºF (the temperature later reached 3ºF at Toolik Lake):

Finally, along the Alaska Peninsula, Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images revealed the bright glow and hot thermal signature of the ongoing eruption of Mount Veniaminof at 1204 UTC and 1344 UTC (below).

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images at 1204 UTC [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images at 1204 UTC [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images at 1344 UTC [click to enlarge]

Suomi NPP VIIRS Day/Night Band (0.7 µm) and Shortwave Infrared (3.74 µm) images at 1344 UTC [click to enlarge]

Coincidentally, on this day GOES-17 began a test of Mode 6 operation which performs a Full Disk scan every 10 minutes. Although the Alaska Peninsula was on the extreme northwest limb of the Full Disk scan, Veniaminof’s thermal anomaly or “hot spot” (darker black pixels) could still be detected and monitored at 10 minute intervals using Shortwave Infrared (3.9 µm) imagery (below). However, an increase in layered cloud cover southeast of that area later in the day (in tandem with the extreme satellite view angle) eventually masked the warm thermal signature — a more direct view from overhead with Suomi NPP VIIRS still showed a very hot volcano summit (96.9ºC) at 2156 UTC.

GOES-17 Shortwave Infrared (3.74 µm) images [click to play animation | MP4]

GOES-17 Shortwave Infrared (3.74 µm) images [click to play animation | MP4]

Since there were no significant ash emissions from Mount Veniaminof on this day, no volcanic signature was evident on GOES-17 “Red” Visible (0.64 µm) imagery (below).

GOES-17

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

* GOES-17 images shown here are preliminary and non-operational *