A comparison of 250-meter resolution Aqua MODIS 0.65 µm and 0.87 µm visible channel images centered on Tuscaloosa, Alabama on 28 April 2011 (above) showed signatures of a few of the larger and longer tornado damage paths from the historic tornado outbreak (SPC storm reports) that occurred on 27 April 2011. A collection of GOES, POES AVHRR, and MODIS images of the tornado outbreak are available on a separate CIMSS Satellite Blog post.

A comparison of before (17 April 2011) and after (28 April 2011) 250-meter resolution MODIS true color Red/Green/Blue (RGB) images from the SSEC MODIS Today site (below) also showed a few of the tornado damage paths — though some of the damage paths were not as evident as they were on the single-channel visible images above. In addition, another long tornado track can be seen across western Georgia.

Below is the same before/after MODIS true color image comparison, viewed using Google Earth.

A comparison of AWIPS images of 1-km resolution MODIS 0.65 µm visible channel data and the corresponding 1-km resolution Normalized Difference Vegetation Index (NDVI) product (below) revealed that a few of the larger tornado damage paths were characterized by a slightly lower NDVI value (lighter green color), due to the downed trees and damaged vegetation.

One of the larger tornado damage tracks across northwestern Alabama was also apparent on GOES-13 0.63 µm visible channel imagery (below). An animation helps to confirm that the feature is not a contrail or some other type of linear cloud feature (which would be moving in an animation, rather than stationary). While the north-south spatial resolution of the GOES imager visible detectors is 1.0 km at the satellite sub-point (over the Equator), with the larger geostationary satellite viewing angle the north-south spatial resolution over northern Alabama is about 1.38 km.

It is interesting to note that the larger tornado damage path which can been seen across northwestern Alabama on the GOES-13 visible image is very difficult to identify on the corresponding GOES-12 0.65 µm visible image (below). This is due to a change is the spectral response function on the visible channel of the GOES-13 and later series of imagery instruments — for more details, see this CIMSS Satellite Blog post.

Update 29 April 2011: A Terra MODIS pass from late morning on April 29th shows the damage paths more clearly through central Alabama. As the churned-up vegetation within the damage path slowly browns, the contrast to undamaged vegetation outside the damage path should increase, allowing for a clearer picture of the damage path. The bowtie-corrected MODIS imagery for visible channel 1 (0.65 microns, below) and visible Channel 2 show three lines of damage through central Alabama. (Here is an animation of the two visible channels).

Since this day was less cloudy than the previous, here is a better before (17 April) versus after (29 April) comparison of 250-meter resolution MODIS true color images from the SSEC MODIS Today site (below, viewed using Google Earth).

Update 30 April 2011: A few of the tornado damage paths across northern Alabama were also evident on an AWIPS image of the 1-km resolution MODIS Land Surface Temperature (LST) product (below). The LST values within the damage paths were in the low to middle 80s F (darker red color enhancement), compared to surrounding LST values in the upper 70s F (orange color enhancement), indicating that the damage paths with destroyed vegetation and tornado debris were able to heat up a few degrees more than the adjacent undisturbed vegetation. The urban areas of the cities of Tuscaloosa (KTCL) and Birmingham (KBHM) also exhibited warmer LST values (darker red) than the surrounding less urbanized, more densely forested areas.

CIMSS participation in GOES-R Proving Ground activities includes making a variety of MODIS images and products available for National Weather Service offices to add to their local AWIPS workstations.

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The tornado outbreak that affected much of the Southeast US on 27 April 2011 was one of historic proportions, in terms of the number of strong to violent tornadoes produced and the number of resulting fatalities. McIDAS images of 1-km resolution GOES-13 0.63 µm visible channel data (above; click image to play animation; also available as a QuickTime movie) showed the multiple clusters of severe thunderstorms that developed across the region during the day. The GOES-13 satellite had been placed into Rapid Scan Operations (RSO), supplying imagery as frequently as every 5-10 minutes. Zoomed-in versions of GOES-13 RSO 0.63 µm visible images covering the period of the long-track (80 mile) EF-4 Tuscaloosa (KTCL) / Birmingham (KBHM) tornado are available here, which show that the storms exhibited a number of distinct overshooting tops during the time period between 20:40 UTC and 23:25 UTC.

AWIPS images of 4-km resolution GOES-13 10.7 µm IR channel data with overlays of severe weather reports (below; click image to play animation) showed the first round of large storms with cold cloud top IR brightness temperatures (red to black to white color enhancement) that moved through the area during the pre-dawn hours (which produced mainly damaging wind reports), followed by the development later in the afternoon and early evening hours of the stronger storms that produced numerous reports of large hail and strong tornadoes ahead of an advancing cold front (SPC storm reports). Zoomed-in versions of GOES-13 RSO 10.7 µm IR images covering the period of the long-track (80 mile) EF-4 Tuscaloosa (KTCL) / Birmingham (KBHM) tornado are available here — cloud top IR brightness temperature values during the 20:40 UTC to 23:25 UTC time period were as cold as -75ºC at 22:25 UTC.

With the higher 1-km spatial resolution of the POES AVHRR IR imagery (below), more detail could be seen in the cloud top IR brightness temperature structure, and much colder cloud top temperatures could be detected in the vicinity of the strongest overshooting tops (as cold as -83ºC, violet color enhancement). Other similar 1-km resolution POES AVHRR IR and MODIS IR image examples (with overlays of storm reports) are available at 16:28 UTC, 18:10 UTC, 18:12 UTC, 18:35 UTC, 19:48 UTC, 19:52 UTC, 20:13 UTC, 00:01 UTC, and 03:34 UTC.

Although there was widespread cloudiness across much of the Southeast US, hourly GOES-13 Sounder Convective Available Potential Energy (CAPE) derived product images (below) were still able to provide some indication as to the instability of the air mass that was feeding northward into the region that morning.

Another important ingredient was the approach of a strong trough aloft, along with an associated strong mid-level jet streak as seen in a comparison of 1-km resolution MODIS 6.7 µm water vapor imagery and CRAS model 500 millibar (hPa) wind speeds (below).

CIMSS participation in GOES-R Proving Ground activities includes making a variety of POES AVHRR, MODIS, and additional GOES Sounder images and products available for National Weather Service offices to add to their local AWIPS workstations. The VISIT training lessons “POES and AVHRR Satellite Products in AWIPS”, “MODIS Products in AWIPS“, and “Water Vapor Imagery and Potential Vorticity Analysis” are available to help users understand these products and their applications to weather analysis and forecasting.

To prepare for the upcoming GOES-R era, new products are being developed and tested at CIMSS using the current generation of satellite data — in fact, some of these new products are now being distributed to and evaluated by a few NWS Offices. Specially-tailored products such as Convective Initiation, Overshooting Tops, and Enhanced-V will allow for the automatic detection of the various developmental stages of convection.

Slide the “Set Fade Level” button located under examples of these images (above) to fade between the CIMSS Convective Initiation (CI) and CIMSS Overshooting Tops (OT) products (derived from satellite observations), along with Cloud-to-Ground (CG) lightning strikes observed from ground-based sensors. Note that there is good agreement between the locations of the satellite-derived CI and OT products and the SPC storm reports for the day (below).

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McIDAS images of GOES-13 0.63 µm visible channel data (above; click image to play animation) showed an excellent example of an undular bore propagating southeastward off the coast of Texas and out over the Gulf of Mexico on 27 April 2011. This bore exhibited multiple wave fronts, with as many as 10-15 being visible at various times. Also note the presence of thick haze both ahead of and behind the bore: this was due to smoke from fires burning in southern Mexico and Central America (which was moving northward across the Gulf of Mexico ahead of the bore) and also from fires burning in Texas and northern Mexico (which was moving southeastward behind a cold frontal boundary that was trailing the bore).

A comparison of AWIPS images of POES AVHRR and MODIS visible channel data with overlays of hourly MADIS atmospheric motion vectors (or cloud-tracked winds) showed that the undular bore was moving southeastward at a speed of 20-30 knots.

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We received the following in an email message from Paul Fuentes at the National Weather Service forecast office at Key West, Florida:

“Just though you might be interested in a peculiar little feature we noticed at the WFO Key West on AWIPS from GOES-13 IR imagery on 4/23/11 starting at about 22:301Z over Andros Island (24.21N, 77.7W) and persisted to into the afternoon on 4/24/11. The feature looked almost like a smoke ring (several miles across) that was drifting off to the Northeast that emanated off of Andros Island and was also apparent in VIS/WV.”

McIDAS images of 4-km resolution GOES-13 6.5 µm “water vapor channel” data (above) showed the feature in question, as it first appeared along the east coast of Andros Island (the large island in the lower left corner of the images) at 22:45 UTC on 24 April 2011 — and then propagated northeastward, grew in size, exhibited progressively colder brightness temperatures, and at times took on a ring-like shape.

A closer look using 1-km resolution GOES-13 0.63 µm visible channel images (below) again revealed the ring-like structure that was evident at various times during the day on 25 April 2011.

The feature (as seen on 10.8 µm POES AVHRR IR imagery, below) seemed to follow the CRAS model 300 hPa wind flow — and southwesterly winds were not found until the 375 hPa pressure level on the 12 UTC rawinsonde report from Nassau in the Bahamas.

A comparison of the MODIS 0.65 µm visible channel, the 11.0 µm IR window channel, the 6.7 µm water vapor channel, and the 1.3 µm cirrus detection channel images at 15:57 UTC (below) seem to support the idea that this was a high ice cloud feature.

So what exactly was this interesting satellite feature? Until an explanation is found, this blog posting shall remain in the “What the heck is this?” Category…

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