GOES-13 10.7 µm IR channel images (click image to play animation)

Automated detection of overshooting tops allows identification of regions of intense convective updrafts that penetrate into the stratosphere. Overshooting tops correlate well with regions of intense rainfall and with severe weather. In tropical systems, overshooting tops are most frequent during intensification. The loop above shows overshooting tops (identified as the yellow thunderstorm symbol) superimposed on GOES-13 10.7 µm IR channel imagery as Hurricane Irene moved over the southern Bahamas. This is during a time when the hurricane was slowly strengthening.

The automatic detection of overshooting tops is done using a GOES-R Proving Ground algorithm, developed in preparation for GOES-R that is scheduled to be launched in late 2015 or early 2016. Overshooting tops that are detected over the Continental United States using data from GOES-East (GOES-13) are available at this website. As Irene moves northward over the Atlantic south of Cape Hatteras and east of Georgia and South Carolina on August 25th through the 27th, overshooting tops, if they are present, will be indicated; their presence suggests a vigorous hurricane. This website shows overshooting tops (automatically detected using Meteosat data) over the tropical Atlantic west of Africa; there is a correlation between tropical wave intensification and OT frequency.

GOES-13 IR/WV Difference image

GOES-13 Overshooting Tops

Overshooting tops can also be detected by subtracting the GOES-13 10.7 µm and 6.5 µm imagery (that is, IR – Water Vapor brightness temperatures). Overshooting tops inject water vapor into the stratosphere where it is more readily detected and therefore produces a larger signal on the difference product. For example, this image, also above, left, (taken from the CIMSS Tropical Cyclones site) shows the WV/IR difference at 1945 UTC on 24 August. Compare it to this image (also above, right) that shows the auto-detected tops at 1932 UTC (None were detected at 1945 UTC). (Click for Visible imagery for 1932 UTC and 1945 UTC) Note that the difference field shows a very large region where overshooting tops are indicated to the west of the hurricane eye — this is more likely a cirrus shield than a region of many overshoots which are brief in nature — and a region to the northeast of the hurricane eye is in a region where an overshoot was detected at 1932 UTC. By 1945 UTC, the initial overshoot may be collapsing, but the high cirrus shield will still be present, and the difference field detects it.

Overshooting tops continue to be detected in Irene’s eyewall at 2115 UTC, 2125 UTC and 2132 UTC. The burst of overshoot production seems to end by 2140 UTC, however.

Some of the convective bursts that were producing the overshooting tops can be seen in an animation of GOES-13 0.63 µm visible channel images (below; click image to play animation; also available as a QuickTime movie). The GOES-13 satellite continued to be in Rapid Scan Operations mode, providing images as frequently as every 5-10 minutes. It is interesting to note the “trochoidal motion” (or wobble) of the eye of Hurricane Irene as it moved northwestward across the Bahamas during the day.

GOES-13 0.63 µm visible channel images (click image to play animation)

A sequence of 1-km resolutiion POES AVHRR 12.0 µm IR images (below) revealed a number of areas which exhibited cloud top IR brightness temperatures of -80ºC or colder (purple color enhancement) — the coldest seen was -87ºC at 12:30 UTC.

POES AVHRR 12.0 µm IR images

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GOES-13 0.63 µm visible channel images (click image to play animation)

On 22 August 2011 Tropical Storm Irene intensified to become the first Atlantic basin hurricane of the 2011 season. McIDAS images of GOES-13 0.63 µm visible channel data (above; click image to play animation) showed that while no eye was yet apparent, a number of discrete convective bursts could be seen during the day. The GOES-13 satellite had been placed into Rapid Scan Operations (RSO) mode, providing images as frequently as every 5-10 minutes.

An animation of GOES-13 10.7 µm IR images from the CIMSS Tropical Cyclones site (below) showed that large convective bursts continued into the night-time hours, as Hurricane Irene reached Category 2 intensity.

GOES-13 10.7 µm IR images

The presence of a central dense overcast meant that no eye was apparent on the GOES IR imagery — but a DMSP SSMIS 85 GHz microwave brightness temperature image (below) showed that an eye was indeed in the process of forming at 00:48 UTC on 23 August.

DMSP SSMIS 85 GHz microwave brightness temperature image

===== 23 August Update =====

GOES-13 0.63 µm visible images (click image to play animation)

On 23 August 2011, some hints of an eye began to appear early in day on GOES-13 0.63 µm visible channel images (above), but the view of an eye structure became obscured by a series of large convective bursts later in the day. As requested by the National Hurricane Center, the GOES-13 satellite remained in Rapid Scan Operations (RSO) mode, providing images as frequently as every 5-10 minutes.

In a comparison of AWIPS images of 1-km resolution POES AVHRR 0.65 µm visible and 10.8 µm IR data (below), the hints of an eye structure could be more easily seen on the visible image, while widespread “transverse banding” within the upper level cirrus outflow was very evident on the IR image.

POES AVHRR 0.65 µm visible and 10.8 µm IR images

Later, during the evening hours, the signature of an eye began to appear on GOES-13 10.7 µm IR images from the CIMSS Tropical Cylones site (below).

GOES-13 10.7 µm IR images

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GOES-13 10.7 µm IR images

TROPICAL STORM IRENE SPECIAL DISCUSSION NUMBER 1
NWS NATIONAL HURRICANE CENTER MIAMI FL AL092011
700 PM AST SAT AUG 20 2011

AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INVESTIGATING THE TROPICAL WAVE EAST OF THE LESSER ANTILLES FOUND A SMALL LOW-LEVEL CIRCULATION CENTER JUST SOUTHWEST OF A LARGE CONVECTIVE BURST AND A MINIMUM PRESSURE OF ABOUT 1006 MB. THE PLANE ALSO MEASURED A MAXIMUM WIND OF 53 KT AT 1400 FT AND BELIEVABLE WINDS OF ABOUT 45 KT FROM THE SFMR. THUS ADVISORIES ARE BEING INITIATED ON TROPICAL STORM IRENE WITH AN INITIAL INTENSITY OF 45 KT.

GOES-13 10.7 µm IR images from the CIMSS Tropical Cyclones site (above) showed the large convective burst, along with the location and initial NHC forecast track of the center of Irene.

Within the large convective burst feature, GOES-13 IR / Water Vapor brightness temperature difference product images (below) displayed negative values (green to yellow to red color enhancement) indicative of intense overshooting tops — a signature that is favorable for continued intensification.

GOES-13 IR / Water Vapor brightness temperature difference images

===== 21 August Update =====

The GOES-13 satellite was placed into Rapid Scan Operations (RSO) beginning at 10:15 UTC on 21 August (providing images as frequently as every 5-10 minutes), to monitor Tropical Storm Irene as it approached Puerto Rico. McIDAS images of GOES-13 0.63 µm visible channel data (below; click image to play animation) showed the development of a well-defined cyclonic circulation, as well as a few low-level outflow boundaries propagating outward along the southwestern quadrant of the storm.

GOES-13 0.63 µm visible channel images (click image to play animation)

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GOES-13 0.63 µm visible channel images

A tornado produced EF1 damage and was responsible for one fatality (NWS Green Bay summary) as it moved though the town of Wausaukee in far northeastern Wisconsin during the late afternoon hours on 19 August 2011. This event brought the number of tornado deaths to 550 so far in 2011, making this the 4th deadliest year on record so far in terms of tornado-related fatalities.

McIDAS images of 1-km resolution GOES-13 0.63 µm visible channel data (above) showed the development of the thunderstorm along the Wisconsin/Michigan border region around 18:00 UTC (1:00 pm local time), which then moved southeastward ahead of an advancing cold frontal boundary. The “W” overlaid on the images indicates the location of Wausaukee. Some features to note on the GOES visible imagery include: (1) the formation of a series of northwest-to-southeast oriented boundary layer horizontal convective roll clouds in the vicinity of Iron Mountain (station identifier KIMT), which marked the location of a residual convective outflow boundary from a squall line which moved eastward across the Upper Peninsula of Michigan during the early morning hours on 19 August, (2) subtle storm top shadowing indicating the presence of vigorous overshooting tops, and (3) the development of a well-defined back-sheared storm top anvil along the western edge of the storm toward the end of the animation. A photo of the back-sheared anvil was taken from the Green Bay, Wisconsin area around 00:40 UTC, looking northeast toward the storm (below, courtesy of Peg Zenko).

Photo of back-sheared anvil (courtesy of Peg Zenko)

AWIPS images of 4-km resolution GOES-13 10.7 µm IR channel data (below) showed that cloud top IR brightness temperatures associated with this storm cooled to about -60ºC (darker red color enhancement), but no distinct “enhanced-v” or other severe storm top signatures were apparent.

GOES-13 10.7 µm IR channel images

However, about an hour and 45 minutes before the tornado moved through Wausaukee, a comparison of a 1-km resolution POES AVHRR 10.8 µm IR image with the corresponding 4-km resolution GOES-13 10.7 µm IR image around 20 UTC (below) illustrated the distinct advantage of higher spatial resolution for detecting the presence of colder overshooting cloud tops (-69ºC on POES AVHRR, compared to -57ºC on GOES) as well as important cold/warm thermal couplets (the POES AVHRR image displayed a well-defined cold/warm thermal couplet of -69ºC/-49ºC).

POES AVHRR 10.8 µm IR image + GOES-13 10.7 µm IR image

The GOES-13 sounder Convective Available Potential Energy (CAPE) derived product image at 19:00 UTC (below) revealed that there were pockets of CAPE in the 2300-2900 J/kg range (brighter yellow color enhancement) just ahead of the developing storm, which at that time was centered just west-northwest of Land O Lakes, Wisconsin (station identifier KLNL).

GOES-13 sounder Convective Available Potential Energy (CAPE)

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