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-resolutionRHIs 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, twomoment 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.