How Many Tropical Cyclones Are There In This Picture?

(Last updated May 12, 2002)



[Small TC]  As you might guess, this is a trick question. (And no fair cheating by checking the information line at the bottom of the image!) This is an infrared image from the GOES-8 satellite taken at 1132 UTC July 18. This image has been enhanced so thunderstorms show up in various shades of orange and red. Since thunderstorms are an essential part of a tropical cyclone, it would figure that any tropical cyclones in this image would show up as an orange or red blob. (177K GIF)

This picture shows four rather interesting areas of thunderstorms. (I don't include the area of orange over the Atlantic east of the Mid-Atlantic states because it is too linear to be a tropical cyclone. It looks like a frontal system.) The first is over the northern Gulf of Mexico near the Mouth of the Mississippi River. This system has a good cloud swirl suggesting a circulation. The second is in the Atlantic east of Florida. This system has strong thunderstorms, but doesn't seem to be swirling. A third system is over Minnesota and South Dakota. Finally, is that an area of swirling thunderstorms over Oklahoma? Is that a tropical cyclone over land?

First, the easy answer: The system along the Gulf Coast is Hurricane Danny having just crossed the mouth of the Mississippi River enroute to landfall at Mobile Bay, Alabama.

The thunderstorms east of Florida are associated with an upper level trough of low pressure. Such troughs normally include cold air at the mid and upper levels (4-12 miles above the surface) of the troposphere, which makes the atmosphere more unstable and increases thunderstorm activity. Upper level troughs and lows occasionally trigger tropical cyclones, but not in this case! The thunderstorms later dissipated without threatening to become anything more serious.

The thunderstorms over Minnesota and South Dakota are a mesoscale convective complex (or MCC). This is an group of thunderstorms that develops a generally round area of cold cloud tops. This system has roughly the same size and cloud tops of Danny, but it lacks the circulation. Thus, it is not a tropical cyclone.

Circulation doesn't seem to be a problem for the system over Oklahoma. The cloud pattern suggests not only a cyclonic circulation, but also an upper level anticyclonic circulation like those seen in tropical cyclones. This system is actually a mesoscale convective vortex (or MCV). Thunderstorm complexes (such as the one over Minnesota and South Dakota) can develop middle level (3-6 miles above the surface) circulations if they are strong enough and last long enough. At the same time, they develop an upper level ridge of high pressure 10-12 miles up. MCVs resemble tropical cyclones, except they are usually missing the strong low level (below 3 miles above the surface) circulation of a tropical cyclone.

The MCV over Oklahoma had developed a few days before from a large group of thunderstorms. It then got trapped in an area of light wind flow that caused slow motion. This allowed the system to keep its structure and develop several more bursts of thunderstorms. (It persisted over Oklahoma for at least the period July 17-19.) It caused rainfalls in excess of 10 inches over parts of Oklahoma along with some flooding. Given the satellite appearance (and other data indicating the circulation was only 1-2 miles above the surface), it is quite likely that this system would have developed into a tropical cyclone had it been over warm water.

So, the answer to the question: One tropical cyclone in this picture.

This image is from the Images/Movies of Hurricanes and Special Events from the National Climatic Data Center. This site features images of tropical cyclones all over the world, as well as images of winter storms, flooding, volcanoes, and other interesting events. Also check out their Satellite's Eye Art Galleries for case studies of interesting events.


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Interesting Image Page #5 - Tropical Terrors in Twins and Triplets

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