On the differential phase in the melting layer

In this paper, we examine differential phase (Φ_DP) in the melting layer in winter clouds. Our analysis of the Φ_DP profiles obtained with the polarimetric WSR-88D radar shows two characteristic features: Φ_DP exhibits a positive shift (or perturbation) in the melting layer and slight gradual increase due to a low specific differential phase (K_DP) within the layer. The drop of the cross-correlation coefficient ρ_hv within the melting layer causes enhanced statistical fluctuations in the Φ_DP estimates. However, the depth of the Φ_DP “perturbations” is considerably larger than the one attributed to the sole decrease of ρ_hv. Two factors are known that can cause the positive bias of Φ_DP: a) the differential phase upon scattering by large “spongy” melting particles and b) nonuniform beam filling.

We also consider another factor that can increase Φ_DP: the differential phase upon scattering by small (few mm) and highly nonspherical particles. It is demonstrated that such particles (spongy or water-coated) can produce backscatter differential phases of 5..7 deg. Water content in such particles can be as low as 2..3%.

We evaluate and compare possible contributions to Φ_DP from all three factors, i.e., backscattering by large “spongy” snowflakes and /or smaller-size nonspherical particles, and nonuniform beam filling. The impact of beam broadening is estimated from the vertical profiles of the polarimetric variables measured at close distances from the radar. Results of our calculations are compared with radar data collected with the polarimetric WSR-88D radar.