Resonance Scattering Effects in Wet Hail Observed by a Dual-Frequency, Dual-Polarization DOW Radar

Traditional dual-frequency weather radar techniques employ two widely spaced frequency bands, selected such that one produces scattering that can be described by the Rayleigh approximation, and one for which hydrometeors are electromagnetically large. Recent studies, however, have suggested that closely spaced frequencies within the same band may produce a differential response in hydrometeors that cause resonance scattering effects. The recent upgrade of the Doppler on Wheels (DOW) 7 radar to dual-polarization with dual-X-band frequencies (9.5 GHz and 9.35 GHz) with the same antenna allows us to explore the possibilities of hail detection and/or sizing using matched beams.

Data were obtained from a storm that produced dime-sized hail during the DOW-7 visit to Penn State in late 2014. High-resolution RHI scans through the storm reveal dual-frequency differences in radar variables (reflectivity factor, differential reflectivity, etc.). T-matrix scattering calculations for wet hail reveal that the observations are consistent with resonance scattering off wet hail 1.5-1.8 cm in diameter. Additional dual-frequency differences are consistent with wavelength-dependent propagation effects (or artifacts) such as differential attenuation, depolarization, and nonuniform beam filling. The implications of these observations for hail detection and sizing and for validating theoretical models of propagation effects will be discussed.