Correlation Coefficient: How Radar Tracks Debris in Tornadoes

We’ve discussed several times before just how many various ways we have at our disposal to investigate radar data to find possibly dangerous areas of rotation – be it from analyzing the structure of storms on radar, or their doppler velocities, these tools have proven invaluable since their nationwide rollout some 35 years ago. In more recent years, however, new tools have entered the radar users toolkit and have opened up a new dimension of radar analysis – literally. With the advent of what we call “dual-pol” in recent years, NEXRAD radars can now shoot out two different beams at two different angles, enabling the radar to process the three-dimensional shape of the objects it hits, from birds to bees to rain and hail, and interestingly enough, tornadic debris.

We call the output of these variables “Correlation Coefficient” – now, as intimidating as the term “Correlation Coefficient” sounds (typically abbreviated as just CC), it’s actually a fairly simple principle – think of it as a 0 to 100 scale, with closer to 100 (in red) indicating very similarly shaped objects being reflected by the radar, and closer to 0 (in greens and blues) indicating very peculiar, unusual objects being reflected by the radar. Early uses of this proved it useful in finding hail, but an interesting phenomena began occurring during tornado events; debris – be it glass, wood, or trees – actually was visible in the data. How exactly can you tell this, though? Let’s look at an example.

Just Yesterday, December 9th, 2023, a significant tornado struck near Hendersonville, TN, just on the outskirts of the Nashville Metro. Surveys and investigations are ongoing, as this tornado inflicted severe damage on nearby homes and structures – of which there were unfortunately many in such a populated area. On radar, the presentation was a pretty trademark presentation – a large hook wrapped into the supercell, and velocities indicated high winds (at times in excess of 130mph, per the data!). At this same time, CC showed us what exactly was going on – an area of low CC (blue) surrounded by high CC (red). This indicated that, within the very similar, consistent mass of rain the supercell was producing, there were some very atypical, non-weather objects being lifted INTO the storm… and what causes that? Tornadoes. Sure enough, the data proved to us beyond a reasonable that a tornado was ongoing, and this helped confirm the tornado which was partially rain-wrapped and touched down quickly. Without this tool, the proper high-impact warnings may have taken longer to be issued, which is especially exacerbated by how fast storms in this region can move – they can be in and out before you can react.

It’s life-saving technology like this that can make or break the publics awareness of an emergency situation, and it’s for this reason we installed a dual-polarimetric radar of our own here in Southern Middle Tennessee to have the sharpest and closest view of whatever Mother Nature throws at the Tennessee Valley – which we know all too well can be some of the most impactful weather on the planet.

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