A tornadic debris signature (TDS), often colloquially referred to as a debris ball, is an area of high reflectivity on weather radar caused by debris lofting into the air, usually associated with a tornado.
A hook echo is a pendant or hook-shaped weather radar signature as part of some supercell thunderstorms. It is found in the lower portions of a storm as air and precipitation flow into a mesocyclone resulting in a curved feature of reflectivity. The echo is produced by rain, hail, or even debris being wrapped around the supercell. It is one of the classic hallmarks of tornado-producing supercells.
An image of a tornadic thunderstorm near La Grange, Wyoming (USA) captured during the VORTEX2 project. In the velocity image on the left, Blues/green represent winds moving towards the radar, and reds/yellows indicate winds moving away from the radar. In the reflectivity image on the right, the main body of the storm can be seen, with the appendage on the bottom of the storm being a hook echo. (source:wikicommons; Joshua Wurman, Centre for Severe Weather Research)
Hook echoes are a reflection of the movement of air inside and around a supercell thunderstorm. Ahead of the base of the storm, the inflow from the environment is sucked in by the instability of the airmass. As it moves upward, it cools slower than the cloud environment, because it mixes very little with it, creating an echo free tube which ends at higher levels to form a bounded weak echo region (BWER).
At the same time, a mid-level flow of cool and drier air enters the thunderstorm cloud. Because it is drier than the environment, it is less dense and sinks down behind the cloud and forms the rear flank downdraft, drying the mid level portion of the back of the cloud. Those two currents have a rotation, due to the vertical windshear, and interact to form a mesocyclone. Tightening of the rotation due to the interaction of those two air currents near the surface will create the tornado.
Hook echoes are thus a relatively reliable indicator of tornadic activity, however, they merely indicate the presence of a larger mesocyclone structure in the tornadic storm rather than directly detecting a tornado.
During some destructive tornadoes, debris lofted from the surface may be detected as a "debris ball" on the end of the hook structure.
Debris balls can be a result of anthropogenic (from human activity) or biomass (plant) debris and are more likely to occur if a tornado crosses an environment such as a forest or populated area. As a result of the strong winds required to damage structures and loft debris into the air, debris balls are normally the result of EF3 or stronger tornadoes on the Enhanced Fujita Scale. Weaker tornadoes may also not cause debris balls due to their mostly short-lived nature and thus any debris may not be sampled by radar. However, not all tornadoes meeting such strength requirements exhibit debris balls, depending on their vicinity to sources of debris and distance from the radar site.
Debris balls are seen on radar reflectivity images as a small, roundish area of high reflectivity values.