A dual-polarization radar signature of hydrometeor refreezing in winter storms

Polarimetric radar measurements in winter storms that produce ice pellets have revealed a unique signature that is indicative of ongoing hydrometeor refreezing. This refreezing signature is observed within the lowlevel subfreezing air as an enhancement of differential reflectivity Z_DR and specific differential phase K_DP and a decrease of radar reflectivity factor at horizontal polarization Z_H and copolar correlation coefficient p_hv. It is distinct from the overlying melting-layer "brightband" signature and suggests that unique microphysical processes are occurring within the layer of hydrometeor refreezing. The signature is analyzed for four icepellet cases in central Oklahoma as observed by two polarimetric radars.Astatistical analysis is performed on the characteristics of the refreezing signature for a case of particularly long duration. Several hypotheses are presented to explain the appearance of the signature, along with a summary of the pros and cons for each. It is suggested that preferential freezing of small drops and local ice generation are plausible mechanisms for the appearance of the Z_DR and K_DP enhancements. Polarimetric measurements and scattering calculations are used to retrieve microphysical information to explore the validity of the hypotheses. The persistence and repetitiveness of the signature suggest its potential use in operational settings to diagnose the transition between freezing rain and ice pellets.