Matthew Van Den Broeke
Date of this Version
Supercell thunderstorms produce unique polarimetric radar signatures that are not often observed in unorganized deep convection. Repetitive signatures include deep and persistent differential reflectivity (ZDR) columns and the ZDR arc signature, which are both indicative of thermodynamic and microphysical processes intrinsic to supercells. Prior investigations of supercell polarimetric signatures, both those observed by operational and research radars, and those simulated numerically, reveal positive correlations between the ZDR column depth and cross-sectional area and quantitative characteristics of the radar reflectivity field. This study expands upon prior work by incorporating a dataset of discrete, right moving supercells from across the continental United States, as observed by the operational, Weather Surveillance Radar 1988-Doppler (WSR-88D) network. Several quantitative metrics from ZDR and ZHH signatures are compared against characteristics of ZDR columns, including the depth of the column, and the cross-sectional area of the column within ~1 km of the environmental freezing level. Sample statistics including median, mean, and maximum metric values were compared and tested using non-parametric similarity tests, including a Mann Whitney U-test and the two-sample Kolmogorov–Smirnov test. Cross-correlation coefficients were calculated between ZDR column metrics and the remaining polarimetric signature metrics with increasing positive and negative lag of up to 45 minutes, for both individual storm observation periods, and as whole tornadic and nontornadic samples. A bootstrapping method (i = 5000) was conducted on the observed data, where bootstrapped distributions of metric median, mean, and maximum values were obtained, and the tornadic – nontornadic difference in the 95th percentile median values were compared against the respective observed statistic value differences. Paired metric comparison datapoints were also bootstrapped over all offset values, and the cross-correlation coefficients were compared against the observed values. After completing the analysis, the results reveal: 1) Significant (95% confidence level) differences exist between most of the tornadic and nontornadic sample metrics including larger max ZHH storm-core and mean ZDR arc values and larger inferred hail areal extent among the nontornadic sample, and deeper and broader ZDR columns within the tornadic sample; 2) Significant correlation values between metric comparisons from the tornadic sample involving ZDR arc characteristics indicative of polarimetric associations unique to pretornadic and tornadic supercells; 3) Significant correlation values between ZDR column metrics and inferred hail radar metrics supportive of prior observations indicative of cyclical processes in both tornadic and nontornadic supercells.
Advisor: Matthew Van Den Broeke