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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AWIPS images of the 1-km resolution MODIS visible, 11.0 µm IR window, and 3.7 µm shortwave IR channel data (above) showed some interesting differences in the appearance of convective storm tops over the northern and central Rocky Mountain region on 06 July 2009. Note that some portions of the storm tops appear significantly warmer (darker gray colors) on the 3.7 µm image — this is due to solar reflection off of smaller ice particles within the upper anvil layer. Such differences in cloud top particle size are not apparent on the visible or the standard IR window images (nor do they necessarily correspond to differences seen in the IR brightness temperature patterns on the storm tops). Storms forming with stronger updrafts may produce a higher concentration of smaller ice crystals within the anvil region of the storm top, due to less time for the ice particles to grow in size during their rapid ascent.

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