This is a schematic of a Doppler power spectra from a vertically pointing beam.  Two spectral peaks are indicated.  One is due to rayleigh scattering from precipitation.   The terminal fall speed of the precip. is Vt.  The other peak is due to clear-air return from Bragg Scattering.   Va is the Doppler velocity of the clear-air return.   Vr is the measured radial velocity, and S is the measured Signal power.   Note that Va=Vr-Vt.   N is the background noise level.