Backscatter Differential Phase of Small Liquid-Coated Ice Particles

Observations of large co-polar backscatter differential phase (δ_co) values are observed in regions of large hail and large melting snow aggregates. The primary reasons given for the appearance of large values of δ_co in these instances is resonance scattering effects, as hailstones grow to irregular shapes and become large relative to the wavelength, and large melting aggregates maintain ice structures that have not yet collapsed into smaller liquid drops. We show that thin liquid coatings on the surface of oblate ice particles small relative to the radar wavelength can also produce substantial values of δ_co. Calculations of two-layer, liquid-coated ice spheroids show a maximum value of δ_co at small fractional depths of liquid, with δ_co decreasing as the depth of the liquid increases further. A simplified air-liquid-ice slab model shows a similar dependence of the phase shift of the reflection coefficient on the liquid slab depth, with multiple internal reflections in the liquid layer found to cause the large phase shift. As the liquid layer depth increases, increased absorption causes a reduced impact of the higher order internal reflections on the total phase shift, causing it to decrease. We suggest that similar physics are responsible for the large δ_co in small, liquid-coated ice particles. The potential to identify these particles in observations from polarimetric radars is also discussed