NOAA-17 Infrared (10.8 µm) image

One reason the “What the heck is this?” blog category was created was for situations when we struggle to provide a scientifically-sound explanation of interesting features seen on satellite imagery. Take for example the NOAA-17 AVHRR Infrared (10.8 µm) image (above): it shows what appears to be an elongated “warm ice” signature over the northern half of the Nares Strait (which includes the Robeson Channel) that separates Canada’s Ellesmere Island (which covers much of the left half of the image) and Greenland (which covers much of the right half of the image). There are some cirrus clouds present over the far northwestern part of the image, but for the most part this is a cloud-free satellite scene. Most of the region exhibited very cold surface IR brightness temperatures in the -30º to -50ºC range (cyan to dark blue colors) — and the corresponding surface report from Eureka on Ellesmere Island (station identifier CWEU) showed a temperature of -41ºC (-42ºF). The daily high temperature at Eureka several hours later on 25 February 2009 was only -32ºC (-26ºF). Dress warmly if you have plans to visit Eureka over Spring Break.

What is most striking about the image above is the elongated area of significantly warmer IR brightness temperatures over the (supposedly) ice-covered Nares Strait / Robeson Channel — IR values of -10º to -20ºC (orange to yellow colors) were seen over a large portion of that particular feature. With surface air temperatures so cold, it seems highly unlikely that the waters in that channel were becoming ice-free. The Canadian Ice Service regional ice analysis from 16 February (below) indicated that the ice coverage was 9/10 or greater in the area of the “warm signature” feature seen on satellite imagery — however, their ice analysis did indicate that anywhere from 2/10 to 6/10 of the total ice coverage consisted of relatively “young” ice (having a thickness of only 10-30 cm).

Canadian Ice Service regional ice analysis (16 February 2009)

So what would be the source such a warm signal on the IR satellite image: was there some sort of warm current of water beneath ice that would cause a significant amount of thermal energy to “bleed through” the (relatively thin) ice and be detected as warmer temperatures on the IR imagery? Was the ice thickness less than normal due to abnormally warm temperatures during the previous summer months, which increased the water temperatures enough to then slow the rate of ice formation during the winter?

In addition, it should be noted that the North Water Polynya is a seasonally-recurring area of ice-free or limited-ice water that forms in Baffin Bay (which connects to the far southern portion of the Nares Strait) — but would that polynya extend as far north into the Nares Strait as the Robeson Channel?

If you have any ideas, hypotheses, or conspiracy theories that would help to explain this curious satellite feature, we’d love to hear them — send us an email!

— 03 MARCH UPDATE —

I received an email from Trudy Wohlleben of the Canadian Ice Service, with a fantastic explanation of why we’re seeing such a “warm ice signature” on the satellite imagery:

This year (and the last two years) have been quite unusual. The ice in Nares Strait usually consolidates around the beginning of February, and normally you would not see an elongated warm signature extending right up the strait as you are seeing this winter. The ice then usually breaks up in the last week of July.

The normal (1971-2000) winter situation is shown on this map:
http://ice.ec.gc.ca/IA_NWCA_MCSI/ar_ctmed0301.gif , where black is 10/10 consolidated ice coverage and where red is mobile 9-10/10 ice coverage.

Nares Strait did not consolidate in 2007, allowing for a continuous flow of thick multi-year ice from the Arctic Ocean down to Baffin Bay. It did consolidate in 2008, but further north than normal (with the ice bridge starting at the north end of Kane Basin instead of in Smith Sound). This year, 2009, it hasn’t consolidated yet (again). So far (unlike 2007) an ice bridge HAS formed, preventing a flow of Arctic multi-year sea ice into the Strait. But (unlike 2008) it has formed at the far north end of Robeson Channel.

Because of the location of the ice bridge at the north end of Robeson Channel (instead of in the normal position in Smith Sound) and because of the prevailing northerly winds through the Strait, a polynya has formed extending the length of Nares Strait. The winds continuously push newly formed ice southwards, away from the ice bridge, keeping the ice at the north end of the Strait permanently broken and thin. Normally this polynya (The North Open Water or NOW polynya) forms at Smith Sound, with open water first appearing in mid-May to early-June (see: http://ice.ec.gc.ca/IA_NWCA_MCSI/ar_ctmed0611.gif).

Thanks, Trudy! So the mystery is solved — the “warm signature” seen on the 25 February IR image is due to the following: while there was indeed ice in the Nares Strait, much of that ice was fairly thin and broken, allowing a significant amount of thermal energy from the underlying water to pass upward through the ice and be detected by the satellite IR sensors. In a “normal” winter, we would not be seeing such a strong “warm signature” in the Nares Strait as early as the end of February. Something to think about for the Climate Change naysayers, I suppose…

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GOES-11 and GOES-13 water vapor images

A cyclone off the Pacific Northwest coast was maturing and entering the occluded stage on 23 February 2009. An animation of  GOES-11 and  GOES-13 water vapor channel imagery (above) showed a very compact “dry swirl” signature that often signals a cyclone’s transition to the occluded stage. Note that the subtle features and gradients are more clear on the 4-km resolution GOES-13 water vapor channel data (compared to the 8-km resolution of the GOES-11 water vapor channel data).

An AWIPS image of the GOES-11 water vapor image with surface reports and HPC-analyzed surface fronts  (below) displayed the relationship between the satellite features and the surface features. Note that ship 3FMH7 (located to the north of the occluded front) reported blowing spray (group 70722) at 18 UTC — the present weather symbol on the station model plot that is used for “blowing dust/sand” is also used for blowing spray at sea.

GOES-11 water vapor image (with surface fronts and surface data)

A comparison of the GOES-11 water vapor image and the GOES-11 sounder Total Column Ozone product at 16:00 UTC (below) indicated that ozone values were quite high (in excess of 450 Dobson Units, lighter red color enhancement) over the region of the occluding cyclone.

GOES-11 water vapor + GOES-11 sounder Total Column Ozone

A west-to-east cross section along Line G-G’ using 18:00 UTC NAM80 model fields (below) showed that the dynamic tropopause (taken to be the height of the PV1.5 potential vorticity surface) had descended to below the 500 hPa pressure level in the vicinity of the occluding cyclone.

Cross section of NAM80 model fields

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GOES-13 visible and 3.9 µm shortwave IR images

We received the following in an email from the  National Weather Service forecast office at Fort Worth, Texas:

Some of our forecasters noted an interesting feature on visible satellite imagery on Thursday, Feb 12, 2009. I would like to get an expert opinion on what was causing the observed features. There was a layer of ~ 15kft altocumulus along with some scattered-broken areas of cirrus.

Excellent question…and we appreciate the heads-up on this event. An animation of GOES-13 visible and 3.9 µm “shortwave IR” images (above; QuickTime animation) showed the evolution of two “hole punch” cloud features that were drifting eastward across northern Texas on 12 February 2009. The first hole punch cloud feature moved just to the north of Dallas/Fort Worth (DFW) around 14:45 UTC, while the second feature moved just south of DFW about an hour later (around 15:45 UTC). In addition, an elongated aircraft dissipation trail (or “distrail”) could be seen to the west of the first hole punch feature (oriented west-to-east on the 14:15 and 14:32 UTC visible images), with a second distrail forming about an hour later (oriented northwest-to-southeast on the 15:15 and 15:32 UTC visible images).

These aircraft “hole punch” and “distrail” cloud features form when an aircraft ascends or descends through a layer of supercooled water droplet cloud, with the engine exhaust causing the droplets to glaciate — the resulting ice crystals then fall toward the ground, creating a visible hole or trail in the cloud layer. Note that there is a subtle “brighter white” signal evident on the GOES-13 3.9 µm shortwave IR images in the area of the hole punch features — this colder signal confirms the idea that the aircraft engine exhaust was causing the supercooled water droplets to glaciate.

A NOAA-17 false color Red/Green/Blue (RGB) composite image using channels 01/02/04 (below) showed the second hole punch cloud as it was moving to the south of DFW at 15:58 UTC. Similar aircraft hole punch and distrail cloud features have been seen in the past: for example, over the southcentral US and also over Wisconsin.

NOAA-17 false color RGB image

GOES-13 visible and 10.7 µm IR images

GOES-13 10.7 µm IR data (above) showed cloud top brightness temperatures in the -20 to -30º C range (cyan to dark blue color enhancement) over much of the cloud patch where the initial hole punch feature was seen. Rawinsonde data from both Midland and Fort Worth in Texas (below) displayed a moist layer centered around 425 hPa that corresponded to those temperatures — this indicates that the hole punch and distrail features were at a fairly high altitude (around 20,000 feet or so). Dallas/Fort Worth METAR reports listed cloud bases at 15,000 feet during the period.

Midland TX and Fort Worth TX rawinsonde data

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GOES-12 10.7 µm IR images

The first tornado-related fatalities of the 2009 severe weather season occurred during a tornado outbreak that affected parts of Texas and Oklahoma (SPC storm reports) on 10 February – 11 February 2009. GOES-12 10.7 µm IR images (above; QuickTime animation) showed the development of multiple lines of severe convection during the afternoon and evening hours of 10 February, with large areas of cloud top temperatures in the -55º to -65º C range (orange to darker red color enhancement).

The tornado fatalities occurred in the town of Lone Grove  — located just west of Ardmore in far southern Oklahoma — around 01:30 UTC (7:30 pm local time). A comparison of GOES-11, GOES-12, and GOES-13 IR images around that time (below) demonstrated the effect of “parallax”, which leads to the apparent displacement of the cold “overshooting top”  cloud feature due to varying satellite view angles. Using GOES-11 (positioned over the Pacific Ocean at 135º West longitude), the coldest overshooting top IR pixel (with an IR brightness temperature of -63º C, darker red color enhancement) was located about 5 km southeast of Ardmore (KADM). Using GOES-12 (positioned over the Atlantic Ocean at 75º West longitude), the coldest IR pixel (also -63º C) was located about 22 km west of Ardmore. Finally, the GOES-13 satellite (positioned at 105º West longitude) had a more direct viewing angle, and therefore less of a parallax error: the coldest IR pixel (-64º C) was located 12 km northwest of Ardmore — this places the “overshooting top” a few km to the north of Lone Grove, which would be the expected location.

GOES-11, GOES-12, and GOES-13 10.7 µm IR images

A closer view using the GOES-12 visible channel images (below; QuickTime animation) shows the development of the initial line of storms over Oklahoma. The storm that produced the fatal tornado at Lone Grove formed  to the east of the main line of storms, and began to appear  just south of the Oklahoma/Texas border near the end of the animation.

GOES-12 visible images

AWIPS images of the GOES sounder Lifted Index (LI) derived product (below) showed that the atmosphere was destabilizing during the afternoon hour, with LI values over Oklahoma as low as -8º C by 19:00 UTC and -13º C by 21:00 UTC.

GOES sounder Lifted Index derived product images

In addition, the GOES sounder Total Precipitable Water (TPW) derived product images (below) indicated  that there was a northward surge of moisture across Oklahoma during the hours leading up to convective development, with TPW values  exceeding 20 mm (0.8 inch) by 18:00 UTC, and TPW values exceeding 30 mm (1.2 inches) by 21:00 UTC.

GOES sounder Total Precipitable Water derived product images

A strong upper-level trough was moving eastward across the southern Rocky Mountains region, and the GOES sounder Total Column Ozone derived product images (below) depicted elevated values of ozone associated with this trough. Total Column Ozone values of 350-425 Dobson Units (green to red color enhancement) often correspond to a lowering of the dynamic tropopause and/or the presence of a potenial vorticity (PV) anomaly — and the approach of the trough and the associated PV anomaly likely helped to produce an environment that favored  upward vertical motions on a synoptic scale across the southern Plains region late in the day on 10 February.

GOES sounder Total Column Ozone derived product images

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