When severe weather strikes, you often hear meteorologists use phrases like “radar-indicated” rotation. That’s because we use tools on radar that can show us the storm’s relative velocity, meaning winds moving toward and away from the radar site.
They look much like this map. When the red and green are close together, that is called a couplet, which tells us that the winds are rotating in the atmosphere and capable of producing a tornado.
The keyword here is “capable” though. This is one of many tools that the National Weather Service uses to issue tornado warnings.
But without a trained spotter or law enforcement seeing a tornado touchdown with their own two eyes, how can we confirm that those rotating winds aloft materialize into a tornado at the surface? Let me introduce you to Dual-polarization. “Dual-pol” radar provides technology beyond tracking rain or lightning. I’ll explain further…
As a tornado touches down and strengthens, it lifts objects and debris off the ground tossing them into the atmosphere. The cluster of debris around a tornado is sometimes referred to as a “debris ball.” Here is a classic debris ball we picked up on Exact Track 4D radar last night. The dark shade of blue verifies that debris is in the atmosphere and there is likely a tornado touching down. If we want to get really scientific, this radar product that we conversationally call our “debris detector” is better known as the correlation coefficient (CC) which shows the size and shape of objects in the atmosphere.
But before we can call it a radar-confirmed tornado, there is another vital tool we have to cross check on radar. How does this apparent debris ball compare to the location of the rotating winds we see on that red and green velocity map? Here is another look at those images side-by-side.
If our radar shows a strong area of rotation and a debris ball in the same area, that is a strong signature that there is a tornado occurring allowing meteorologists to confirm that there is a tornado using radar-based products. This is helpful in rural areas where people might not otherwise see a tornado happen, or at night, like the one that happened overnight in Grand Blanc, when the skies are dark and you can’t see what is happening around you.