During the overnight hours Monday, November 5 into Tuesday, November 6, 2018, a tornado was confirmed via radar imagery which showed correlating signs of rotation across various radar products.
Shear rate and velocity products showed strong signs of rotation, and another radar product called correlation coefficient showed signs of debris being picked up by the storm.
The correlation coefficient essentially measures the shape and size of objects being detected by radar, which can indicate if debris is present. The spot of higher correlation coefficient values correlating with rotation is sometimes called a tornado debris signature, which was present while a tornado formed just west of Aspel, which is located 10 miles west of Scottsoro.
How are tornado debris signatures (TDS) used to detect tornadoes?
You may be more familiar with other radar signatures, like base reflectivity (the green moving objects that denote rain, denoted in the image below as “Z”) or doppler velocity (the red/green signatures that denote changes in wind direction as well as speed, denoted in the image below as “SRM”).
The correlation coefficient (denoted below as “CC) is newer dual-pol doppler radar product that allows meteorologists to detect objects that are different in size. This change of size within the objects shows up as a brighter orange or even blue/green within the pink areas — the pink signifies objects of roughly the same size and shape, but the orange/blue/green areas signify areas where objects are different sizes and shapes.
This means the radar is detecting objects that are not rain that may be embedded within the rain itself. These objects could be broken tree limbs, leaves, pine needles, and possibly remnants of houses or buildings in the event that a tornado hits these structures.
However, just because a signature shows up on correlation coefficient means a tornado debris signature is present. The correlation coefficient signature needs to be co-located with both the reflectivity signature as well as the velocity signature in order for the tornado debris signature to be considered valid (as seen by the three products in the image above; the ZDR is a topic for another day).
A tornado debris signature is often what is used by the National Weather Service when they issue “radar confirmed” tornado warnings. These signatures are very helpful at night when tornadoes are normally difficult to detect, and the TDS is especially helpful for determining rain-wrapped tornadoes embedded within quasi-linear convective systems (QLCS’s, or “squall lines”).
What’s interesting to note about tornado debris signatures detected in the fall is that they can be more “enhanced” due to the larger amount of leaves that are present on trees (and on the ground) compared to the spring time severe weather months. When meteorologists observe TDS on radar products during autumn months, the intensity of the TDS may be enhanced due to the extra leaves and pine needles present during the fall. It is during the post-storm damage survey that the true nature of the storm damage will be realized.