Polarimetric Signatures above the Melting Layer in Winter Storms: An Observational and Modeling Study

Polarimetric radar observations above the melting layer in winter storms reveal enhanced differential reflectivity Z DR and specific differential phase shift K DP , collocated with reduced copolar correlation coefficient ρ hv ; these signatures often appear as isolated “pockets.” High-resolution RHIs and vertical profiles of polarimetric variables were analyzed for a winter storm that occurred in Oklahoma on 27 January 2009, observed with the polarimetric Weather Surveillance Radar-1988 Doppler (WSR-88D) in Norman. The Z DR maximum and ρ hv minimum are located within the temperature range between −10° and −15°C, whereas the K DP maximum is located just below the Z DR maximum. These signatures are coincident with reflectivity factor Z H that increases toward the ground. A simple kinematical, one-dimensional, two-moment bulk microphysical model is developed and coupled with electromagnetic scattering calculations to explain the nature of the observed polarimetric signature. The microphysics model includes nucleation, deposition, and aggregation and considers only ice-phase hydrometeors. Vertical profiles of the polarimetric radar variables ( Z H , Z DR , K DP , and ρ hv ) were calculated using the output from the microphysical model. The base model run reproduces the general profile and magnitude of the observed Z H and ρ hv and the correct shape (but not magnitude) of Z DR and K DP . Several sensitivity experiments were conducted to determine if the modeled signatures of all variables can match the observed ones. The model was incapable of matching both the observed magnitude and shape of all polarimetric variables, however. This implies that some processes not included in the model (such as secondary ice generation) are important in producing the signature.