(when you see a "finger" jutting out of a thunderstorm on Doppler radar, there's a good chance that particular cell is producing large, potentially damaging hail, ping-pong to softball size. This artifact, this electronic signature is produced when the radar beam interacts with large hailstones within a thunderstorm updraft, creating a surge of energy, a "spike" on the edge of the thunderstorm. The top image shows VIL Density, a measure of hail volume within the cell. The lower image is Storm Relativy Velocity, showing motion of raindrops and hail stones. Green returns are moving away from the radar site, the red returns are moving toward the site, indicating strong rotation within this severe "mesocyclone". These violent, spinning thunderheads are most likely to spawn large, damaging hail and on occasion: tornadoes.)
A three body scatter spike or TBSS is an artifact on a weather radar display indicative of large hail. They are identified by a spike of weak reflectivity echos that extend out from a thunderstorm, and away from the radar site.
Also known as hail spikes, these are the result of energy from the radar hitting hail and being deflected to the ground, where they deflect back to the hail and then to the radar. Because of the energy hitting the ground at least once and the hail multiple times, it has a weaker return echo than the energy that went from the radar to the hail and back to the radar. The spike occurs where the energy took more time to go from the hail to the ground and back as opposed to the energy that went direct from the hail to the radar. This results in the radar picking up the energy at a later time which puts the echo further away from the radar than the actual location of the hail on the same radial path. Since hail cores are most intense at higher elevations, hail spikes only appear at the levels aloft that accompany the most intense hail. Because of this, hail spikes are usually not seen at lower elevations.
Use in forecasting
Because of their observed accuracy in indicating large hail aloft, TBSS's are used operationally by the National Weather Service to identify thunderstorms that could likely produce large, severe hail. This would warrant the issuance of a severe thunderstorm warning or mention of large hail in a tornado warning.