Final answer:
Precipitation moving in a circle around the radar, perpendicular to the radar beam, will result in a zero radial velocity detection by the radar due to the lack of a radial component in its motion.
Step-by-step explanation:
With base radial velocity, precipitation moving perpendicular to the radar beam (in a circle around the radar) will have a zero radial velocity relative to the radar. The Doppler effect, which is used by radar systems to determine the velocity of objects, only applies to motion toward or away from the observer. Therefore, if the precipitation is moving in a manner that is perpendicular to the radar's line of sight, it won't cause a Doppler shift, which means the radial velocity, as detected by the radar, is zero. This is because only the radial component of the object's velocity can be detected, which, in this specific case, is nonexistent.