Cold temperatures in Alaska

January 19th, 2017 |

NOAA-18 AVHRR Infrared Window (10.8 µm) image, with surface air temperatures and corresponding station identifications [click to enlarge]

NOAA-18 AVHRR Infrared Window (10.8 µm) image, with surface air temperatures and corresponding station identifications [click to enlarge]

A NOAA-18 AVHRR Infrared Window (10.8 µm) image (above) showed the signature of cold air (violet colors) settling into river valleys and other low-elevation terrain areas across the cloud-free interior of Alaska at 1916 UTC (10:16 am local time) on 18 January 2017. Note that there was a layer of clouds (warmer cyan colors) over much of the North Slope of Alaska; these clouds were acting to limit strong surface radiational cooling, with resulting surface air temperatures only as cold as the -20s F. This AVHRR image was about 1 hour before the low temperature at Fairbanks International Airport (PAFA) dropped to -51ºF (-46ºC) — the first low of -50ºF or colder at that location since 31 December 1999 (-53ºF). While these were certainly cold temperatures, in general most were several degrees warmer than the daily record lows for 18 January:

NOAA-18 AVHRR Infrared Window (10.8 µm) image centered on Bettles (PABT), with surface air temperatures and corresponding station identifications [click to enlarge]

NOAA-18 AVHRR Infrared Window (10.8 µm) image centered on Bettles (PABT), with surface air temperatures and corresponding station identifications [click to enlarge]

Closer views centered on Bettles (above) and on Tanana (below) further highlighted the influence of terrain on the pattern of surface infrared brightness temperatures.

NOAA-18 AVHRR Infrared Window (10.8 µm) image centered on Tanana (PATA), with surface air temperatures and corresponding station identifications [click to enlarge]

NOAA-18 AVHRR Infrared Window (10.8 µm) image centered on Tanana (PATA), with surface air temperatures and corresponding station identifications [click to enlarge]

A comparison of re-mapped 1-km resolution NOAA-18 and “4-km” resolution GOES-15 (GOES-West) Infrared Window imagery (below) demonstrated the spatial resolution advantage of “Low Earth Orbit” (Polar-orbiting) satellites over Geostationary satellites, especially for high-latitude regions such as Alaska. As this plot shows, the true spatial resolution of a “4-km” GOES-15 Infrared image pixel over the interior of Alaska — where that satellite’s viewing angle or “zenith angle” from the Equator is about 74 degrees — is actually closer to 16 km. For the “2-km” Infrared imagery that will be provided by the GOES-R series ABI instrument, the spatial resolution over the interior of Alaska will be closer to 8 km.

NOAA-18 vs GOES-15 Infrared Window images [click to enlarge]

NOAA-18 vs GOES-15 Infrared Window images [click to enlarge]

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NOAA-19 AVHRR Infrared Window (10.8 µm) image, with surface air temperatures and corresponding station identifications [click to enlarge]

NOAA-19 AVHRR Infrared Window (10.8 µm) image, with surface air temperatures and corresponding station identifications [click to enlarge]

The cold continued across much of Alaska on 19 January, as seen on a NOAA-19 AVHRR Infrared Window (10.8 µm) image at 1519 UTC or 4:19 am local time (above). However with a lack of cloud cover over the central portion of the North Slope, surface air temperatures were much colder (in the -40s F) compared to the -20s F that were seen there on the previous day.

NOAA-19 AVHRR Infrared Window (10.8 µm) image centered on Bettles (PABT), with surface air temperatures and corresponding station identifications [click to enlarge

NOAA-19 AVHRR Infrared Window (10.8 µm) image centered on Bettles (PABT), with surface air temperatures and corresponding station identifications [click to enlarge]

As was shown on the previous day, closer views centered on Bettles (above) and on Tanana (below) further highlighted the influence of terrain on the pattern of surface infrared brightness temperatures. On this day a layer of clouds (highlighted by the warmer cyan colors) covered the far eastern portion of the Tanana image below — note that surface temperatures in the Fairbanks area beneath these clouds were only as cold as the -30s F. Farther to the west, which remained cloud-free, the minimum temperature at Tanana was -59ºF.

NOAA-19 AVHRR Infrared Window (10.8 µm) images centered on Tanana (PATA), with surface air temperatures and corresponding station identifications [click to enlarge]

NOAA-19 AVHRR Infrared Window (10.8 µm) images centered on Tanana (PATA), with surface air temperatures and corresponding station identifications [click to enlarge]

Time series plots of surface weather conditions at Fairbanks, Tanana and Bettles during the 18-19 January period are shown below. Note that the surface visibility was periodically restricted 1 statute mile or less, due to ice fog, at all 3 locations.

Surface weather conditions at Fairbanks [click to enlarge]

Surface weather conditions at Fairbanks [click to enlarge]

Surface weather conditions at Tanana [click to enlarge]

Surface weather conditions at Tanana [click to enlarge]

Surface weather conditions at Bettles [click to enlarge]

Surface weather conditions at Bettles [click to enlarge]

First full day of Summer: snow in the Brooks Range of Alaska

June 22nd, 2016 |

GOES-15 Water Vapor (6.5 µm) images [click to play animation]

GOES-15 Water Vapor (6.5 µm) images [click to play animation]

GOES-15 (GOES-West) Water Vapor (6.5 µm) images (above) showed the southeastward migration of an upper-level low across the North Slope and the eastern Brooks Range of Alaska during the 21 June – 22 June 2016 period. A potential vorticity (PV) anomaly was associated with this disturbance, which brought the dynamic tropopause — taken to be the pressure of the PV 1.5 surface — downward to below the 600 hPa pressure level over northern Alaska. Several inches of snow were forecast to fall in higher elevations of the eastern portion of the Brooks Range.

With the very large satellite viewing angle (or “zenith angle”) associated with GOES-15 imagery over Alaska  — which turns out to be 73.8 degrees for Fairbanks — the altitude of the peak of the Imager 6.5 µm water vapor weighting function (below) was shifted to higher altitudes (in this case, calculated using rawinsonde data from 12 UTC on 22 June, near the 300 hPa pressure level).

GOES-15 Imager water vapor (Band 3, 6.5 µm) weighting function [click to enlarge]

GOES-15 Imager water vapor (Band 3, 6.5 µm) weighting function [click to enlarge]

The ABI instrument on GOES-R will have 3 water vapor bands, roughly comparable to the 3 water vapor bands on the GOES-15 Sounder — the weighting functions for those 3 GOES-15 Sounder water vapor bands (calculated using the same Fairbanks rawinsonde data) are shown below. Assuming a similar spatial resolution as the Imager, the GOES-15 Sounder bands 11 (7.0 µm, green) and 12 (7.4 µm, red) would have allowed better sampling and visualization of the lower-altitude portion of this particular storm system. The 3 ABI water vapor bands are nearly identical to those on the Himawari-8 AHI instrument; an example of AHI water vapor imagery over part of Alaska can be seen here.

GOES-15 Sounder water vapor weighting function plots [click to enlarge]

GOES-15 Sounder water vapor weighting function plots [click to enlarge]

As the system departed and the clouds began to dissipate on 22 June, GOES-13 Visible (0.63 µm) images (below) did indeed show evidence of bright white snow-covered terrain on the northern slopes and highest elevations of the Brooks Range.

GOES-15 Visible (0.63 µm) images [click to play animation]

GOES-15 Visible (0.63 µm) images [click to play animation]

A sequence of 1-km resolution POES AVHRR Visible (0.86 µm) images (below) showed a view of the storm during the 21-22 June period, along with the resultant snow cover on 22 June. However, the snow quickly began to melt as the surface air temperature rebounded into the 50’s and 60’s F at some locations.

POES AVHRR Visible (0.86 µm) images [click to play animation]

POES AVHRR Visible (0.86 µm) images [click to play animation]

The increase in fresh snow cover along the northern slopes and the highest elevations of the central and northeastern Brooks Range — most notably from Anaktuvuk Pass to Fort Yukon to Sagwon — was evident in a comparison of Suomi NPP VIIRS true-color Red/Green/Blue (RGB) images from 17 June and 22 June, as viewed using RealEarth (below). The actual time of the satellite overpass on 22 June was 2134 UTC.

Suomi NPP VIIRS true-color RGB images, 17 June and 22 June [click to enlarge]

Suomi NPP VIIRS true-color RGB images, 17 June and 22 June [click to enlarge]

Intense hurricane-force storm in the Bering Sea

December 13th, 2015 |

Himawari-8 Water Vapor (6.9 µm) images [click to play animation

Himawari-8 Water Vapor (6.9 µm) images [click to play animation

Japanese Meteorological Agency Himawari-8 Water Vapor (6.9 µm, 2-km resolution) images (above) showed the rapid intensification of a hurricane-force extratropical cyclone over the North Pacific Ocean and Bering Sea during the 12 December – 13 December 2015 period. The 6.9 µm is one of 3 water vapor spectral bands on the Himawari AHI instrument — GOES-R will feature 3 nearly identical water vapor bands on the ABI instrument.

According to surface analyses from the Ocean Prediction Center, the storm was centered over Japan at 00 UTC on 11 December, and began rapidly intensifying later that day as it continued moving northeastward; it eventually deepened to a minimum central pressure of 924 hPa (27.29 inches of mercury) over the far southern Bering Sea at 06 UTC on 13 December. This equaled the analyzed minimum central pressure of Post-Tropical Cyclone Nuri in November 2014, which was one of the strongest storms on record in the Bering Sea.

Corresponding GOES-15 Water Vapor (6.5 µm, 4-km resolution) images (below) offered a slightly closer view of the intensifying storm. The unique satellite signature — resembling a curved scorpion tail — of a phenomenon known as a sting jet was seen to begin developing around 20 UTC on 12 December south of the Aleutian Islands. Several hours after the middle-tropospheric sting jet feature on water vapor imagery moved over Adak Island (PADK on the images) around 0130 UTC, sustained surface winds of 82 knots (94 mph) with gusts to 106 knots (122 mph) were recorded just after 09 UTC. According a Tweet from the Ocean Prediction Center, winds from the storm also produced wave heights of 63 feet.

GOES-15 Water Vapor (6.5 µm) images [click to play animation]

GOES-15 Water Vapor (6.5 µm) images [click to play animation]

A time series of surface observations at Adak Island (below) indicated that the minimum station pressure of 939.0 hPa (27.73 inches of mercury) was recorded just after 04 UTC.

Time series of Adak Island, Alaska surface observation [click to enlarge]

Time series of Adak Island, Alaska surface observation [click to enlarge]

Additional imagery from this event can be found on the RAMMB GOES-R Proving Ground Blog.

Ice growth in Hudson Bay

November 24th, 2015 |

GOES-13 Visible (0.63 µm) images [click to play animation]

GOES-13 Visible (0.63 µm) images [click to play animation]

GOES-13 (GOES-East) Visible (0.63 µm) images (above) showed the growth of offshore ice in the western and northwestern portions of Hudson Bay on 24 November 2015. Also evident on the imagery was cloud streets aligned with the northerly/northwesterly flow of cold arctic air over the water, as well as the presence of a mesoscale low moving southeastward. Apparently this mesoscale low was behind the primary low (with its associated trailing occluded front), which was depicted to be over the eastern portion of Hudson Bay (surface analyses) during the daylight hours of the visible imagery.

A better view of the offshore ice (as well as the ice in central Hudson Bay, northeast of the aforementioned mesoscale low) was provided by Suomi NPP VIIRS true-color and false-color images, visulized using the SSEC RealEarth web map server (below). In the false-color image, snow cover and ice appear as darker shades of cyan.

Suomi NPP VIIRS true-color and false-color images [click to enlarge]

Suomi NPP VIIRS true-color and false-color images [click to enlarge]

A comparison of Canadian Ice Service analyses from 16 November and 23 November (below) showed the growth of the offshore ice along the western and northwestern edges of Hudson Bay, as well as the larger area of ice growing southward in the central portion of Hudson Bay during that 1-week period. The departure from normal images at the bottom indicated that ice concentration along the western and northwestern edges was well below normal (red), while the concentration of the large area of ice in central Hudson Bay was greater than normal (blue).

Hudson Bay ice concentration on 16 and 23 November 2015 [click to enlarge]

Hudson Bay ice concentration on 16 and 23 November 2015 [click to enlarge]

Hudson Bay ice concentration departure from normal on 16 and 23 November [click to enlarge]

Hudson Bay ice concentration departure from normal on 16 and 23 November [click to enlarge]