Multi-Radar Analysis of the 20 May 2013 Moore, Oklahoma Supercell through Tornadogenesis and Intensification

Abstract

A multi-radar analysis of the 20 May 2013 Moore, Oklahoma, U.S. supercell is presented using three Weather Surveillance Radars 1988 Doppler (WSR-88Ds) and PX-1000, a rapid-scan, polarimetric, X-band radar, with a focus on the period between 1930 and 2008 UTC, encompassing supercell maturation through rapid tornado intensification. Owing to the 20-s temporal resolution of PX-1000, a detailed radar analysis of the hook echo is performed on (1) the microphysical characteristics through a hydrometeor classification algorithm (HCA)—inter-compared between X- and S-band for performance evaluation—including a hail and debris class and (2) kinematic properties of the low-level mesocyclone (LLM) assessed through $\Delta V_r$ analyses. Four transient intensifications in $\Delta V_r$ prior to tornadogenesis are documented and found to be associated with two prevalent internal rear-flank downdraft (RFD) momentum surges, the latter surge coincident with tornadogenesis. The momentum surges are marked by a rapidly advancing reflectivity ($Z_H$) gradient traversing around the LLM, descending reflectivity cores (DRCs), a drop in differential reflectivity ($Z_{DR}$) due to the advection of smaller drops into the hook echo, a decrease in correlation coefficient (${\rho }_{hv}$), and the detection of debris from the HCA. Additionally, volumetric analyses of $Z_{DR}$ and specific differential phase ($K_{DP}$) signatures show general diffusivity of the $Z_{DR}$ arc even after tornadogenesis in contrast with explosive deepening of the $K_{DP}$ foot downshear of the updraft. Similarly, while the vertical extent of the $Z_{DR}$ and $K_{DP}$ columns decrease leading up to tornadogenesis, the phasing of these signatures are offset after tornadogenesis, with the $Z_{DR}$ column deepening the lagging of $K_{DP}$.