An Observational Analysis of ZDR Column Trends in Tornadic Supercells

Observations and high-resolution numerical model simulations have shown that liquid water and partially frozen hydrometeors in convective updrafts are often lofted to altitudes well above the environmental freezing level (EFL). Polarimetric observations (and retrievals from the aforementioned modeling) commonly depict upward extensions of positive differential reflectivity (e.g., Z_DR > 1 dB) above the EFL in strong convection. Therefore, the relatively recent nationwide deployment of polarimetric capability to the WSR-88D fleet offers a valuable opportunity to observe, track, and predict convective updraft trends over much of the contiguous United States. Indeed, recent work has depicted a strong correlation between the temporal evolution of these Z_DR columns and maximum updraft strength, as well as the resultant precipitation core intensity.

This new technique for observing spatiotemporal trends in convective updrafts facilitates comparison of storm-scale processes with changes in updraft characteristics. One topic of exploration is the analysis of Z_DR column trends prior to, during, and after tornadogenesis. Previous research has suggested that mid-level updraft intensity may decrease prior to tornadogenesis, owing to an increased downward-directed pressure perturbation gradient force as low-level rotation strengthens. Applying this concept to Z_DR columns, it is plausible that these signatures may tend to weaken prior to tornadogenesis. This study tests the hypothesis by gathering Z_DR column data from numerous tornadic supercell cases via the recently developed Z_DR Column Algorithm, which tracks the maximum height of detected columns. Additionally, potential prognostic capabilities as gleaned from these data will be discussed.