C-band Dual-Polarization Radar Estimation of the Maximum Rain Drop Diameter and the Concentration of Large Rain Drops

Accurate estimation of rainfall integral parameters, including radar observables, from drop size distribution (DSD) data is sensitive to the choice of the maximum rain drop diameter (D_max). This sensitivity is exacerbated at C-band due to resonance effects for large drop diameters in excess of 5 mm. Due to sampling limitations, it is often a challenge to reliably estimate D_max with a disdrometer. The resulting uncertainties in D_max increase errors in general radar-rainfall retrieval methods, which rely on disdrometer observations for DSD input to radar models. Although the sample size is significantly increased, radar methods for estimating D_max are relatively unproven. In this study, I explore the possibility of constraining the large drop tail of the DSD with radar. More specifically, I estimate both D_max and the concentration of large (D > 5 mm) rain drops (N_T5) from C-band dual-polarization (DP) radar observations of the horizontal reflectivity (Z_h), differential reflectivity (Z_dr), and correlation coefficient (ρ_hv). Using simulations, I assess the likely theoretical error associated with the C-band DP radar retrieval of D_max and N_T5. C-band DP radar observations of rain from the University of Alabama in Huntsville (UAH) Advanced Radar for Meteorological and Operational Research (ARMOR) are then utilized to estimate D_max and N_T5 in a wide variety of precipitation regimes in Northern Alabama. When available, the temporal evolution and overall statistical nature of the ARMOR and two-dimensional video disdrometer (2DVD) estimates of D_max and N_T5 are inter-compared to provide an additional empirical consistency check of the method. Among other potential applications, better characterization of D_max and the large drop tail of the DSD can be used to improve radar-rainfall algorithms for NASA's Global Precipitation Measurement (GPM) Mission and GPM Ground Validation (GV) efforts.