Three Body Scatter Spike
One of the assumptions of weather radar is that the size of the targets is much less than the wavelength of the radar. This assumption allows the principles of Rayleigh scattering to be used. When the targets are of a size with the wavelength of the radar Rayleigh scattering does not apply: instead, Mie scattering does.
The presence of large hail in a thunderstorm can create a so-called three body scatter spike or hail spike. In this case, the radar beam's radiation is scattered from the hail to the ground, from the ground back to the hail, and finally from the hail back to the radar. What appears on the radar scan is a false echo out the back edge of the storm along a radial. It is this false echo that is called the hail spike.
Since hail spikes can only occur if the hail is of a size with the radar's wavelength, they are signatures of large hail within the storm or of large quantities of wet hail. Lemon (1997) also points out that the downburst potential is very high due to precipitation drag in storms with hail spikes; such storms are likely to be accompanied by severe damaging winds.
References
Lemon, L. R., 1998: The radar ‘‘Three-body scatter spike’’: An operational large-hail signature. Wea. Forecasting, 13, 327–340.