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.
As intimidating as the term "Correlation Coefficient" sounds (typically abbreviated to 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.
On April 28th, 2014, a significant tornado struck near Graysville, Alabama, just on the outskirts of Birmingham. Luckily, nobody was killed, but the tornado inflicted notable damage on nearby homes and structures. On radar, the presentation was clear - a large hook wrapped into the supercell, and velocities indicated high winds. At the time, CC was relatively new, however, and 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 does that? Tornadoes. Sure enough, the data proved to us that a tornado was ongoing, and this helped confirm the tornado which was rain-wrapped, moving fast, and occurring at night. Without this tool, the proper high-impact warnings may not have been issued, and indeed to this day may still elude us when tornadoes like this come around. Here in the southern US, they are often extremely difficult to see, so it goes without saying - CC can be a meteorologists best friend when confirmation is scant on the ground.