Variability of rain rate estimators due to rainfall types: assessment based on hydrometeorology testbed data

S-band profiling (S-PROF) radar measurements conducted at the southeastern United States Hydrometeorology Testbed coastal and mountain sites indicated a frequent occurrence of rain which did not exhibit radar bright band (BB) and was observed outside the convective core precipitation. This common nonbrightband (NBB) rain contributes about 15-20% of total accumulation and is not considered as separate rain type by current WSR-88D precipitation segregation radar-based schemes, which separate rain into stratiform, convective, and sometimes, tropical types. Measurements from disdrometer collocated with S-PROF based observations of rain types showed that drop size distributions (DSDs) characteristic of NBB rain have a much larger relative fraction of smaller drops compared to those of BB and convective core rains. A year-long combined DSD and rain type observations were used to derive scanning radar rain rate (R) estimators including those based on traditional reflectivity (Ze) and as well as the ones which also use such polarimetric variables as differential reflectivity (ZDR) and specific differential phase shift (KDP). While differences among same type estimators for mostly stratiform BB and convective core rain were relatively minor, estimators derived for the NBB rain type were quite distinct. Underestimations in NBB rain rate estimates derived using wrong rain type estimators (e.g., those for BB or convective rain or default operational radar estimators) for the same values of radar observables can reach a factor of about 2, though the phase-based estimators are somewhat less susceptible to DSD details. No significant differences among the estimators for the same rain type derived using DSD from different observational sites were present despite significant separation and differing terrain. This study presents conventional and polarimetric rain rate estimators for different rain types and suggests an approach to identify areas of common NBB rain using polarimetric radar Ze and ZDR measurements.