AbstractEstimating raindrop size has been a long-standing objective of polarimetric radar–based precipitation retrieval methods. The relationship between the differential reflectivity Z_dr and the median volume diameter D₀ is typically derived empirically using raindrop size distribution observations from a disdrometer, a raindrop physical model, and a radar scattering model. Because disdrometers are known to undersample large raindrops, the maximum drop diameter D_max is often an assumed parameter in the rain physical model. C-band Z_dr is sensitive to resonance scattering at drop diameters larger than 5 mm, which falls in the region of uncertainty for D_max. Prior studies have not accounted for resonance scattering at C band and D_max uncertainty in assessing potential errors in drop size retrievals. As such, a series of experiments are conducted that evaluate the effect of D_max parameterization on the retrieval error of D₀ from a fourth-order polynomial function of C-band Z_dr by varying the assumed D_max through the range of assumptions found in the literature. Normalized bias errors for estimating D₀ from C-band Z_dr range from −8% to 15%, depending on the postulated error in D_max. The absolute normalized bias error increases with C-band Z_dr, can reach 10% for Z_dr as low as 1–1.75 dB, and can increase from there to values as large as 15%–45% for larger Z_dr, which is a larger potential bias error than is found at S and X band. Uncertainty in D_max assumptions and the associated potential D₀ retrieval errors should be noted and accounted for in future C-band polarimetric radar studies.