Final answer:
Precipitation moving away from the radar is displayed as having a positive radial velocity, which is seen when detecting motion through the Doppler effect.
Step-by-step explanation:
With base radial velocity, precipitation moving away from the radar has a positive velocity, indicating an increase in distance from the observation point. According to the Doppler effect, when an object moves away from an observer, the waves it emits (whether they're sound waves, light waves, or in this case, radar waves) are stretched, causing a shift to lower frequencies in the case of sound or towards the red end of the spectrum in light. Conversely, when an object approaches the observer, the waves are compressed, leading to a higher frequency sound or a blue shift in light. This concept also applies to detecting precipitation with radar, where the reflected radio waves elongate when the precipitation moves away, resulting in this positive velocity.
The illustration of the Doppler effect and its application to radial velocities of stars in a binary system indicates that a positive radial velocity signifies the star, or in the case of weather radar, precipitation, is moving away from the observer. These principles also underpin measurements of recessional velocities of galaxies, as astronomically larger scales of the same effects point to the universe's expansion. Hence, when a weather radar picks up this positive velocity, it's a clear indication that the precipitation is receding from the radar's location.