A bin-model was used to characterize the signature of dynamical microphysical processes on Z-R relationships used for radar rainfall estimation. The sensitivity analysis performed shows that coalescence is the dominant microphysical process for low to moderate rain intensity regimes (R < 20mm h-1), and that rain rate in this regime is strongly dependent on the spectral properties of the DSD (i.e. the shape). For high intensity rainfall (R > 20mm h-1), collision-breakup dynamics dominate the evolution of the raindrop spectra. Analysis of the time-dependent Z-R relationships produced by the model suggests convergence to a universal Z-R relationship for heavy intensity rainfall. Conversely, the model results show that Z-R relationships severely underestimate reflectivity in the light rainfall regime.