Microphysical differences between tornadic and nontornadic supercell rear-flank downdrafts revealed by dual-polarization radar measurements

Recent research has suggested that tornadogenesis in supercells may be significantly impacted by the thermodynamic characteristics of rear-flank downdrafts (RFDs). These characteristics are governed by microphysical processes within the storm, such as melting and evaporation of hydrometeors. Specifically, colder RFDs have been shown to inhibit tornado development, whereas warmer RFDs (perhaps indicative of less vigorous evaporative cooling) are more conducive to tornadogenesis. Polarimetric radar measurements are sensitive to such phase transitions and in theory may capture subtle differences in evaporation rates between tornadic and nontornadic RFDs. Indeed, such differences have been measured by the research polarimetric prototype WSR-88D in Central Oklahoma (KOUN). Dual-polarization data from a small sample of four tornadic and five nontornadic supercells suggest that the drop size distribution (DSD) in nontornadic storms is more skewed towards larger drops than in tornadic storms, as inferred from higher differential reflectivity Z_DR and lower cross-correlation coefficient ρ_HV for a given reflectivity factor Z_H. Physical explanations are provided describing the effect of evaporation and melting on the observed polarimetric variables with the aid of simple explicit spectral microphysical models. Theoretical and modeling results suggest that the polarimetric differences between tornadic and nontornadic RFDs may be amplified in data from C-band radars due to enhanced resonance scattering effects present at smaller wavelengths.