An intense upper-level low developed over the Canadian Arctic Archipelago on 14 July 2009, and subsequently migrated southwestward over the Northwest Territories, Yukon, and northeastern Alaska by 16 July 2009. AWIPS images of the GOES-11 + GOES-12 water vapor channel composite (above) showed a well-defined signature of the intensifying upper low during this period, with a large area of warmer/drier air (darker blue colors) within the circulation.

What was remarkable about this warm/dry signature on the water vapor imagery is the fact that such detail could be seen, in spite of the very large satellite viewing angle of the GOES satellites. Water vapor imagery is prone to the effect of “limb brightening” as the water vapor weighting function is shifted to higher, colder altitudes over the higher latitude regions — this tends to make water vapor imagery appear rather cold and “washed out” over the Arctic much of the time. However, in this case the dynamic tropopause (taken to be the pressure of the 1.5 Potential Vorticity Unit surface) was brought downward to the 850 hPa pressure level on 14 July (below) as the low intensified — and snow was even reported at Norman Wells (station identifier CYVQ) in the Northwest Territories on 16 July.

On the GOES water vapor imagery above there was a hint of some embedded vortex structure developing within the upper low circulation. These vorticies (which were likely small stratospheric intrusion vorticies) were much easier to identify on the 1-km resolution MODIS water vapor image than on the “8-km” resolution GOES-11 water vapor image (below) — this is partly due to the upward shift of the water vapor weighting function mentioned previously, and also due to the fact that the 8-km GOES-11 water vapor pixels were effectively about 30 km in size due to the very large satellite viewing angle.

Another interesting satellite signature was the appearance of a “thermal anomaly” over the Arctic Ocean north of Alaska on the GOES-11 10.7 µm IR window channel image at 09:00 UTC (below, bottom panels). At that particular time, there is a great deal of solar reflection off the water and ice surface, which appears very bright on the visible imagery (top panels), and very hot (dark black enhancement) on the 3.9 µm shortwave IR imagery (center panels). The intense solar reflection effectively causes the IR window channel brightness temperature to “roll over” from very warm to very cold values (black to white color enhancement). This IR window channel thermal anomaly does not appear if cloud cover masks the highly reflective nature of the water and ice in the Arctic Ocean.

Later in the day, GOES-11 visible imagery showed that the southern edge of the ice in the Arctic Ocean had receded a considerable distance from the northern coast of Alaska (above). The ice edge could be seen in greater detail using 1-km resolution MODIS true color imagery (below, courtesy of the GINA, University of Alaska Fairbanks SwathViewer).

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MTSAT-1R InfraRed (IR) images from the CIMSS Tropical Cyclones site (above) showed the rapid development of a cold Central Dense Overcast (CDO) as Tropical Storm 07W (Molave) intensified east of the Philippines in the West Pacific Ocean on 16 July 2009. It is interesting to note that the CDO was centered a considerable distance to the southwest of the low-level circulation of the tropical cyclone.

4-km resolution MTSAT-1R 10.3 µm IR cloud top brightness temperatures (above) were a cold as -92.4º C (purple color enhancement) at 12:30 UTC. 1 hour and 15 minutes later, a 1-km resolution MODIS 11.0 µm IR image (below) indicated that cloud top IR brightness temperatures were still as cold as -92.1º C.

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A comparison of GOES-11 and GOES-12 visible channel images (above) revealed an aerosol plume aloft that was oriented northwest-to-southeast over the western Great Lakes region on 13 July 2009. This example shows the value of forward scattering to help in the identification these kinds of aerosol plumes — note how the plume is much brighter on the GOES-11 image than the GOES-12 image, due to the morning sun’s position in relation to GOES-11 (located at 135º West longitude) versus GOES-12 (located at 75º West longitude). Also note how the hazy aerosol plume tended to “disappear” on both the GOES-11 and the GOES-12 visible images as the sun angle increased during the morning hours.

Later in the day, this aerosol plume was easily seen on AWIPS images of the MODIS near-IR 1.3 µm “cirrus detection” channel (below). The so-called “cirrus detection” channel helps to identify features that are effective scatters of light — which includes cirrus ice crystals as well as airborne aerosols (such as dust, haze, volcanic ash, or volcanic sulfates).

This aerosol plume exhibited no obvious signal on any of the other conventional MODIS channels, such as the visible, IR window, and water vapor channels (below). So was this aerosol feature due to smoke aloft from fires in Canada or Alaska, or was it a high-altitude volcanic sulfate plume (likely from the Sarychev Peak eruptions earlier in the Summer)?

As it turns out, this plume was identified as an “SO2 alert” on the IASI SO2 product (above, courtesy of Université Libre de Bruxelles). This feature also exhibited SO2 concentrations of about 6-12 Dobson Units on the Aura OMI SO2 product (below, courtesy of NOAA/NESDIS).

NOAA ARL HYSPLIT backward trajectories (below) suggest that the aerosol plume had spent some time over the Arctic region during the previous week or so, where we had seen similar evidence of high-altitude aerosol plumes on the GOES-11 visible images in early July.

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Severe convection developing over eastern South Dakota during the pre-dawn hours on 09 July 2009 exhibited an unusually large and well-defined “enhanced-v” storm top signature on AWIPS images of the GOES-12 10.7 µm IR channel (above) as it produced large hail (up to 2.50 inches in diameter) and damaging winds (gusting as high as 90 mph) across parts of South Dakota and extreme northeastern Nebraska.

An overlay of negative and positive cloud-to-ground lighting strikes (below) showed that this storm was producing a large amount of lightning in the vicinity of the overshooting top (near the vertex of the enhanced-v signature), but there was also a number of strikes located a fair distance to the northeast, far away from the coldest cloud tops.

A comparison of the 4-km resolution GOES-12 10.7 µm IR data with 1-km resolution NOAA-15 AVHRR 10.8 µm IR data (below) demonstrated the advantage of improved spatial resolution in detecting the cloud top temperature structure of the enhanced-v signature. The coldest/warmest cloud top temperatures on the NOAA-15 IR image were -80º C / -59º C (deltaT = 21º C), compared to -68º C / -56º C (deltaT = 12º C) on the GOES-12 IR image.

A NOAA-15 AVHRR Red/Green/Blue (RGB) false-color composite image (below) displayed a stunning view of the storm just after sunrise (at 11:33 UTC), about 20 minutes after it produced a wind gust to 90 mph, hail up to 1.00 inch in diameter, and brief heavy rain near Scotland in southeastern South Dakota.

Note that the overshooting top was casting a shadow onto the anvil of the storm below — and this very tall thunderstorm complex was casting an impressive shadow to the west and southwest across South Dakota and Nebraska. Also note the presence of a boundary layer gravity wave train oriented southwest-to-northeast across Nebraska, which was positioned in advance of a cold frontal boundary.

Additional radar and satellite images of this storm can be found on the AccuWeather WeatherMatrix Blog.

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