Three body scatter spike

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A three body scatter spike or TBSS is an artefact 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.

[edit] Cause

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.

[edit] 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.