Sensitivity of C-Band Polarimetric Radar–Based Drop Size Estimates to Maximum Diameter

Abstract

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