What is a radar-confirmed tornado?
Radar can be used to track everything in the sky from massive tornadoes to flocks of birds. Here's how meteorologists use it to help save lives when severe weather strikes.
Dual-polarization radar is at the forefront of weather technology. Learn what a debris ball is and how weather forecasters use it to confirm a damaging tornado has touched down.
When rain starts to fall, people immediately look to the weather radar to see how long it will last. But when severe weather strikes, meteorologists can use this technology to pinpoint the precise location of damaging tornadoes and give more accurate warnings to those in their path.
The use of radar has expanded well beyond the circles of the meteorological community in recent years with the rise of smartphone apps, websites and social media. These allow virtually anyone to be able to look at the weather and track radar-confirmed tornadoes from almost anywhere.
For decades, the National Weather Service (NWS) has used radar to gather a wealth of meteorological data to track thunderstorms, hurricanes, blizzards, and everything in between.
An anemometer measures wind speeds near a tornado. (Ryan McGinnis/Getty Images)
Radars work by sending pulses of energy through the atmosphere. When these pulses encounter particles in the air, such as raindrops, some of the energy bounces off the object and returns to the radar. Computers then analyze this data to determine what the radar 'saw.'
However, a significant advancement in radar has unveiled even more information than ever before.
Enter: Dual-polarization weather radar.
“Dual-polarization, or dual-pol, is part of the NWS vision to build a weather-ready nation to better protect lives and livelihoods. This new technology provides 14 new radar products that will support our mission to serve our partners and customers,” the NWS explained.
A strong supercell thunderstorm drops hail behind the Doppler radar at New Underwood, South Dakota. (Photo/Eric Kurth, NOAA/NWS/ER/WFO/Sacramento)
Since dual-pol radar was implemented across the United States in the late 2000s and early 2010s, it has helped peer deeper into the heart of thunderstorms. One of the biggest breakthroughs has been with short-term tornado warnings.
As a thunderstorm develops, strengthens and begins to rotate, a hook shape can appear on the edge of the storm on radar. As the storm intensifies, the hook can become very prominent on radar in the area of the storm that can spawn a tornado.
A hook echo is seen on the AccuWeather radar near Allendale, SC, at 4 p.m. EDT on Apr. 5, 2022.
As a tornado touches down and strengthens, it lifts objects and debris off the ground and lofts them up into the atmosphere. With extremely strong twisters, some objects can be tens of thousands of feet above the ground. The cluster of airborne debris around a tornado is sometimes referred to as a "debris ball."
This debris ball can be better detected with dual-pol radar, specifically through the use of a radar product known as the correlation coefficient (CC), typically a feature on more sophisticated weather radar tools and apps.
Essentially, the CC shows the size and shape of objects in the atmosphere. This allows meteorologists to determine where it is raining, where hail is falling, and where a tornado on the ground is throwing debris into the sky.
A low CC value shows a great variety in the size and shape of objects, while a high value shows greater consistency in size and shape. Meteorologists look for low CC values within a tornado's debris ball surrounded by higher values. This often appears as a small blue circle within a larger red area.
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More than a debris ball needs to appear on radar for a tornado to be radar-confirmed.
Another crucial piece of information is how close it appears to the rotation of the storm. The storm's rotation can also be measured by using radar.
Radars measure the velocity of objects in the atmosphere relative to the radar site. Rotation can be detected when there is a couplet of winds moving toward and away from the radar located right next to each other. This often appears as a red area directly next to a green area as seen on the NWS image below.
If the radar shows a strong area of rotation and a debris ball in the same area, it is a strong indication that a tornado is occurring.
“This signature, known as the Tornadic Debris Signature (TDS), [is] a powerful tool, particularly in very rural areas where we might not otherwise receive confirmation that a tornado is doing damage for several hours. Once a TDS appears, a tornado is already doing damage,” the NWS explained.
This is how meteorologists determine where there is a radar-confirmed tornado.
A group of AccuWeather meteorologists review weather data at headquarters in State College, Pennsylvania, on March 4, 2022. (Renee Duff)
Radar can also help detect where large hail is falling in a severe thunderstorm, the dividing line between rain, sleet, and snow during a winter storm, and even animals.
When a flock of birds or colony of bats takes to the sky and the conditions are just right, they can be seen flying on radar images.
Without dual-pol, this may be incorrectly identified as rain, but the additional information gathered by the advanced technology confirms that what forecasters are seeing are animals in flight, rather than precipitation.
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