Gravity waves over the Gulf of Mexico

February 26th, 2009
GOES-12 6.5 µm water vapor images

GOES-12 6.5 µm water vapor images

Kudos once again to David Zaff (National Weather Service Buffalo NY) for bringing another interesting feature to our attention. Dave sent an email mentioning a gravity wave that he saw on water vapor imagery propagating westward across the Gulf of Mexico on 26 February 2009 — and this wave  appeared to play a role in setting off some isolated convection north of Buoy 42001 around 17:00 UTC.  AWIPS images of the GOES-12 6.5 µm water vapor channel (above) showed the subtle low-altitude gravity wave feature moving from east to west (in contrast to most of the other high-altitude features moving from west to east on the imagery).

I’m usually a fan of color-enhanced water vapor imagery, but in this particular case, the low-altitude wave structure seems to show up a bit better using a simple contrast-stretched gray-scale enhancement. Utilizing the GOES-13 satellite (in orbit at 105º  West longitude), we get a slightly better view of the Gulf of Mexico region (below) — and the westward-propagating gravity wave feature is plainly seen (along with a subtle train of waves behind its leading edge). In addition, we can also see that there appeared to be a second packet of low-altitude gravity waves out ahead of the aforementioned gravity wave (which was moving from southeast to northwest). Convective initiation seems to occur around the time that the primary westward-moving wave intersects the secondary northwestward-moving wave (around 17:00 UTC).

GOES-13 6.5 µm water vapor images

GOES-13 6.5 µm water vapor images

Looking at GOES-12 3.9 µm shortwave IR imagery (below), we can see that there was a deck of patchy stratocumulus cloud present over and to the east of the area of convective initiation. The appearance of those clouds changed from light gray before sunrise to darker gray after sunrise, since the 3.9 µm shortwave IR channel is sensitive to the reflection of solar radiation off the tops of the water droplet clouds. Also note how the eastern edge of the stratocumulus cloud deck appears to erode as the westward-propagating gravity wave feature moves through that area — the passage of the wave apparently acted to mix dryer air aloft downward into the marine boundary layer (a dry layer aloft near 800 hPa was seen on both the Tampa FL and New Orleans LA rawinsonde data).

GOES-12 3.9 µm shortwave IR images

GOES-12 3.9 µm shortwave IR images

One curious feature to note on GOES-13 visible imagery (below) — which was also seen on the GOES-12 shortwave IR imagery above — was the fact that a new patch of cloudiness appeared to form in the wake of the main gravity wave feature, which exhibited a pronounced east-northeastward component of motion.

GOES-13 visible channel images

GOES-13 visible channel images

GOES-12 water vapor image + MADIS satellite winds

GOES-12 water vapor image + MADIS satellite winds

A number of satellite-derived wind targets were seen which exhibited an obvious northeasterly component: both on GOES MADIS winds around 18:00 UTC (above), and also on QuikSCAT winds several hours later as the the wave had progressed westward (below).

GOES-12 water vapor image + QuikSCAT winds

GOES-12 water vapor image + QuikSCAT winds

MODIS Sea Surface Temperature product

MODIS Sea Surface Temperature product

Two other items are worthy of mentioning, since they may have been a factor in the formation of the isolated convection: (1) a plume of warmer water (due to the Gulf of Mexico Loop Current) was evident on the MODIS Sea Surface Temperature product (above), and (2) the MIMIC Total Precipitable Water product (below) indicated that a plume of higher TPW was moving northwestward across the Gulf of Mexico, with the highest TPW values (greater than 30 mm, lighter blue colors) moving to the north of Buoy 42001 just prior to the time of convective initiation.

MIMIC Total Precipitable Water product

MIMIC Total Precipitable Water product

“Warm Ice” signature in the Nares Strait / Robeson Channel?

February 25th, 2009
NOAA-17 10.8 µm IR image

NOAA-17 10.8 µm IR 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 10.8 µm IR 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 -50º C range (cyan to dark blue colors) — and the corresponding surface report from Eureka on Ellesmere Island (station identifier CWEU) showed a temperature of -41º C (-42º F). The daily high temperature at Eureka several hours later on 25 February 2009 was only -32º C (-26º F). 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 -20º C (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)

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…

Occluding cyclone off the Pacfic Northwest coast

February 23rd, 2009
GOES-11 and GOES-13 water vapor images

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)

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

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.

NAM80 cross section

Cross section of NAM80 model fields

Aircraft “hole punch” and “distrail” cloud features over Texas

February 12th, 2009
GOES-13 visible and 3.9 µm shortwave IR images

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

NOAA-17 false color RGB image

GOES-13 visible and 10.7 µm IR images

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

Midland TX and Fort Worth TX rawinsonde data