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Tornado Signatures in Hurricanes

David Moran
Sep 20, 2018 1:39:35 PM

When hurricanes and tropical storms make landfall, tornadoes can often become a byproduct. Usually, these tornadoes are weak. However, they can occasionally be strong enough to produce significant damage.

Tornadoes in the Plains often have distinct signatures, such as the hook echo associated with a supercell, but this is not the case for those that come from tropical systems. Sometimes they are not seen because the radar beam is overshooting the rotation, or the tornado could be so short-lived that it may not appear in a radar scan. The RadarScope reflectivity and storm relative velocity image below are from a tornado-producing storm near Roseboro, North Carolina due to Hurricane Florence.

Roseboro, NC Tornado Reflectivity and Storm Relative Velocity 9/16/18
Roseboro, NC Tornado Reflectivity and Storm Relative Velocity 9/16/18

Some weak rotation can be seen just to the northwest of Roseboro. While the rotation was relatively weak, the tornado produced a Tornado Debris Signature shown here in dual polarization data. 

Roseboro, NC Tornado Differential Reflectivity and Correlation Coefficient
Roseboro, NC Tornado Differential Reflectivity and Correlation Coefficient

Low differential reflectivity and correlation coefficient near the velocity couplet suggests that the radar may be detecting debris that is lofted by the tornado.

Not all tornadoes produced by tropical systems are weak. On the afternoon of September 17, 2018 several strong tornadoes occurred across portions of Virginia as remnants of Hurricane Florence passed through. One such tornado occurred near Midlothian, Virginia southwest of Richmond around 3:30 pm EDT. 

midlothian1
Reflectivity and Storm Relative Velocity of a Tornadic Storm near Midlothian, Virginia on September 17, 2018

Looking west-southwest of Bon Air, we see an area of rotation. Let's see if there's a debris signature associated with this storm. 

Differential Reflectivity and Correlation Coefficient of Midlothian Storm
Differential Reflectivity and Correlation Coefficient of Midlothian Storm

Looking at the differential reflectivity, the values are near zero near the rotation, and there are some low correlation values, which could be debris. 

About 10 minutes later, there were several areas of rotation west of Richmond. One especially notable signature is south-southwest of Bon Air. Here is a look at the reflectivity and storm relative velocity at 3:48 pm EDT.

Rotation Signature near Bon Air, Virginia
Rotation Signature near Bon Air, Virginia

Once again, we can look at dual polarization products to search for debris. It's important to note that the low values of both differential reflectivity and correlation coefficient must be collocated with the area of rotation.

Differential Reflectivity and Correlation Coefficient near Bon Air, Virginia
Differential Reflectivity and Correlation Coefficient near Bon Air, Virginia

Comparing the last two images, you can see the low values of differential reflectivity and correlation coefficient near the rotation signature. This is evidence that a tornado is likely occurring and lofting debris.

 

Tornadoes associated with tropical systems are usually weak, however, as seen above, they can occasionally do considerable damage. Identifying the rotation signatures can point you to areas where a tornado is developing or may have developed. Dual polarization signatures can indicate whether the tornado is lofting debris into the air.  Using these products in conjunction with each other can give you an idea of if a damaging tornado is occurring.

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